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In this study, available large ensemble datasets in the Coupled Model Intercomparison Phase 6 (CMIP6) archive were used to provide the first multi-variate overview of the evolution of extreme seasons over Antarctica and the Southern Ocean during the 20th and 21st centuries following medium-to-high radiative forcing scenarios. The results show significant differences between simulated changes in background mean climate and changes in low (10th percentile) and high (90th percentile) extreme seasons. Regional winter warming is most pronounced for cold extremes. In summer, there are more pronounced increases in high extremes in precipitation and westerly wind during the ozone hole formation period (late 20th century), affecting coastal regions and, in particular, the Antarctic Peninsula. At midlatitudes, there is a reduction in the range of summer season wind extremes. Suggested mechanisms for these differences are provided relating to sea ice retreat and westerly jet position. [ABSTRACT FROM AUTHOR]

Stellar evolution theory predicts that the Sun was fainter in the past, which can pose difficulties for understanding Earth's climate history. One proposed solution to this Faint Young Sun (FYS) problem is a more luminous Sun in the past. In this paper, we address the robustness of the solar luminosity history using the yrec code to compute solar models including rotation, magnetized winds, and the associated mass-loss. We present detailed solar models, including their evolutionary history, which are in excellent agreement with solar observables. Consistent with prior standard models, we infer a high solar metal content. We provide predicted X-ray luminosities and rotation histories for usage in climate reconstructions and activity studies. We find that the Sun's luminosity deviates from the standard solar model trajectory by at most 0.5 per cent during the Archean (corresponding to a radiative forcing of 0.849 W m |$^{-2}$|⁠). The total mass-loss experienced by solar models is modest because of strong feedback between mass and angular momentum loss. We find a maximum mass-loss of |$1.35 \times 10^{-3} \,{\rm M}_\odot$| since birth, at or below the level predicted by empirical estimates. The associated maximum luminosity increase falls well short of the level necessary to solve the FYS problem. We present compilations of paleotemperature and CO |$_2$| reconstructions. One-dimensional 'inverse' climate models demonstrate a mismatch between the solar constant needed to reach high temperatures (e.g. 60–80 |$^{\circ }$| C) and the narrow range of plausible solar luminosities determined in this study. Maintaining a temperate Earth, however, is plausible given these conditions. [ABSTRACT FROM AUTHOR]

Recent studies suggest the observed seasonal delay of rainfall over the Sahel is mainly driven by anthropogenic aerosol forcing, which features a robust east–west contrasting changes in recent decades, with negative and positive top-of-the-atmosphere shortwave radiative forcing in South and East Asia (SA&EA) and in North America and Europe (NA&EU), respectively. Their individual effects on the Sahel rainfall annual cycle remain unclear. Here, by designing idealized sensitivity experiments based on Community Earth System Model (CESM1.2), we show that both negative radiative forcing over SA&EA and positive radiative forcing over NA&EU contribute to the delayed phase and enhanced amplitude of Sahel rainfall annual cycle. To understand the underlying physical processes, both the convective quasi-equilibrium (CQE) and atmospheric energetic frameworks are utilized. Both frameworks can well explain the seasonal delay in the rainfall annual cycle over the Sahel under the regional radiative forcings, with the CQE framework showing stronger explanatory power. Based on the insights from both frameworks, we conducted a moisture budget analysis and found that the negative radiative forcing over SA&EA causes an anomalous low-level anticyclonic circulation, which transports more moisture to the Sahel. As the anomalous anticyclone is stronger in summer and fall compared to spring, rainfall is delayed in the Sahel. Under the influence of the positive radiative forcing over NA&EU, more warming in the northern hemisphere causes northward transport of moisture to the northern tropics associated with the cross-equatorial transport of energy. The moisture transport increases relative humidity over the Sahel and enhances the effective atmospheric heat capacity that delays the seasonal rainfall. [ABSTRACT FROM AUTHOR]

Under global warming, extreme events have been increasing in the last decade and are projected to increase in the future with every increment of global warming. The potential increase in compound drought and hot events may induce a complex web of impacts on societies, ecosystems, and economies, including crop failure, wildfires, and water scarcity. This is particularly concerning for Brazil, where it has been demonstrated to be vulnerable to recent extreme climate events. Using an ensemble of CORDEX-CORE simulations over Tropical Brazil, we investigate changes in compound events in response to changes in radiative forcing and their impact on climate extreme events, including drought and extreme heat. The simulations are conducted at a 25 km horizontal grid spacing using lateral and lower boundary forcing from three Coupled Model Intercomparison Project Phase 5 (CMIP5) climate models. Each model covers the period from 1980 to 2100 under two Radiative Concentration Pathways (RCP2.6 and RCP8.5) in the 21st-century projection period. We used observed data from the Brazilian Daily Weather Gridded Data (BR-DWGD) to evaluate the simulations and perform a quantitative assessment of areas affected by these compound events during the present day. The study finds a generally good agreement between RCM simulations and observed data, with moderate to high correlation coefficients for precipitation, though the strength of these correlations varies across different regions and seasons. The analysis emphasizes the prevalence of compound climate events during the Austral summer season and projects a significant increase in both extreme heat and drought events in the coming decades. These findings underscore Brazil's vulnerability to compound climate events, highlighting the need for adaptive strategies and policy interventions to mitigate the socio-economic and environmental impacts across various sectors. [ABSTRACT FROM AUTHOR]

Developing countries, especially Africa, contribute the least to greenhouse gas (GHG) emissions while facing disproportionate impacts of climate change, including environmental and economic. Climate change cost projection studies using integrated assessment models (IAMs) are mostly global and often make policy recommendations for specific regions. This study aims to quantify the abatement costs of implementing four climate policies to limit global warming to 2 degreesCelsius above the pre-industrial era in the Africa and Middle East (AME) region. Representative concentration pathways (RCPs) from 2010 to 2100 were employed to forecast these costs. The RCPs were assessed using PAGE 09, an IAM called the policy analysis for greenhouse effects, which evaluates the costs and benefits of climate actions or inactions. The findings indicate that in 2030 at a 90% confidence level, RCP 2.6 is projected to result in a 1.63% GDP loss, equivalent to a potential cost of up to $1,698,829.95 million. Conversely, RCP 6.0 could lead to a loss of as much as 12.22% of GDP. However, the study reveals that as the policies are consistently implemented, the percentage loss of GDP gradually declines and approaches zero by 2100 for RCPs 2.6, 4.5, and 8.5. In the case of RCP 6.0, the decline begins earlier, from 2030. These findings suggest that the economic impact of the RCP climate policies varies during the early stages of their implementation but can become beneficial to the AME region, progressively leading to 2100 with sustained efforts. However, using the RCP profiles directly for a multicultural region like AME is challenging due to the limitations of employing the shared socio-economic pathways (SSPs) for different emission scenarios and the issue of inconsistent baselines. It is arguable that a hybrid approach combining elements from the four climate pathways could be pursued in the AME region, specifically Africa, to minimize associated economic costs. The AME is essentially diverse. Therefore, the study recommends improving the regional representation of the SSPs and, perhaps, the RCPs, to enhance the accuracy of future assessments. This study is the first to quantify climate change mitigation costs for the AME region by leveraging RCP projections up to 2100, emphasizing Africa, which has been historically neglected despite its high vulnerability index. [ABSTRACT FROM AUTHOR]

SuCCESs is a bottom-up Integrated Assessment Model (IAM) that represents energy production and use, materials production, land-use and climate globally. The primary use-case for SuCCESs is to calculate long-term scenarios until 2100 that consider the interactions between these systems, for example the greenhouse gas emissions from energy, materials and land-use and their impact on climate change. The four systems are hard-linked in SuCCESs and scenarios are solved through intertemporal optimization by minimizing discounted system costs to satisfy projected demand and other constraints, e.g. climate targets. This yields a long-term equilibrium solution between the modelled systems. This article introduces the model logic and structure, describes the overall representation of each system, and provides an evaluation by comparing the scenarios produced by SuCCESs with different end-of-century radiative forcing targets to those from other IAMs. Towards this end, and to demonstrate the capability of SuCCESs for large-scale scenario exploration, we also conduct a sensitivity analysis employing Monte Carlo sampling with a 1000-member scenario ensemble for each radiative forcing target. Last, we discuss some practical aspects and different ways of using the model in long-term scenario analyses. [ABSTRACT FROM AUTHOR]

Sustained near‐surface ozone pollution despite stringent emission controls in China has drawn wide attention. However, how such rapid regional emission reductions affect global tropospheric ozone remains unexplored. Here, using an ensemble of measurements and global model simulations, we find Chinese emission reductions during 2013–2020 have increased domestic surface ozone; particularly in North China, they have decreased the global tropospheric ozone burden by 2.7 Tg (54% of China's anthropogenic contribution and 7% of global anthropogenic contribution in 2019) and ozone radiative forcing by 8.8 mW m−2 (64% of China's anthropogenic contribution and 9% of global anthropogenic contribution in 2019). Nitrogen oxides (NOx) associated nonlinear chemistry and height‐dependent atmospheric transport patterns primarily cause the contrasting impacts. Future Chinese NOx emission controls would continue to decrease global tropospheric ozone, while regional surface ozone may still increase unless NOx reductions exceed ∼53% of the 2019 level. Our results reveal the substantial benefit of regional emission reductions in China on global tropospheric ozone mitigation. Plain Language Summary: Tropospheric ozone is an important air pollutant and greenhouse gas and mainly produced by atmospheric photochemical reactions. Its levels are largely impacted by anthropogenic emission sources. Over the past decade, anthropogenic emissions in China have greatly declined, which may alter tropospheric ozone on a global scale. Using numerical simulations from a global chemical transport model, we show that emission reductions in China have substantially decreased the global tropospheric ozone burden over 2013–2020. The emission reductions have reduced the global anthropogenic tropospheric ozone radiative forcing and the Chinese contributions by 9% and 64%, respectively, although the emission reductions increased domestic surface ozone over North China. The differences in the nitrogen oxides (NOx) associated ozone formation regimes and atmospheric transport mechanisms over the surface of North China and the free troposphere primarily cause the contrasting impacts. We estimate that further emission reductions of NOx in China would cause nonlinear domestic and global tropospheric ozone changes. Our assessment demonstrates that emission reductions in China help global tropospheric ozone mitigation. Deep emission reductions are needed to further reduce domestic ozone pollution in China. Key Points: Emission reductions in China have substantially decreased the global tropospheric ozone burden and its radiative forcingDifferences in ozone chemistry and atmospheric transport mechanisms cause contrasting domestic and global impactsFuture NOx reductions in China would further mitigate global tropospheric ozone despite short‐term domestic surface ozone increases [ABSTRACT FROM AUTHOR]

Stratospheric volcanic aerosols can affect the global radiative balance and stratospheric composition. In this study, we analyze ensemble experiments with an interactive stratospheric aerosol microphysical general circulation model, designed to assess the climate forcing from large‐magnitude explosive eruptions in the tropics and northern extratropics. Previous studies have generally identified a lower radiative forcing from extratropical eruptions from the shorter stratospheric lifetime of volcanic sulfate aerosols. However, our study finds that both the shorter lifetime and lower effective radiative forcing (ERF) efficacy contribute to the lower ERF in the northern extratropical eruptions. The simulated 2‐year averaged ERF efficacy in northern extratropical eruptions is 22% lower than that in the equatorial eruptions due to the seasonal mismatch of peak stratospheric aerosol optical depth and solar radiation in the first year after a northern extratropical volcanic eruption. Additionally, equatorial eruptions accelerate the Brewer‐Dobson circulation (BDC), while northern hemispheric (NH) extratropical eruptions decelerate the BDC branch in NH and accelerate the BDC branch in southern hemisphere (SH), leading to different spatio‐temporal pattern of ozone anomalies. Consequently, dominated by the dynamical processes, equatorial eruption leads to ozone loss in tropics and increase in midlatitudes, while both northern extratropical summer and winter eruptions trigger ozone decrease in NH and increase in SH. Plain Language Summary: Volcanic eruption can introduce a large amount of SO2 into stratosphere, leading to perturbations of radiation and stratospheric compositions. Interactive model simulations show northern extratropical summer eruptions tend to produce ∼40% less effective radiative forcing (ERF) than equatorial eruptions. The differences tend to be attributed to the shorter lifetime of volcanic aerosols in the northern extratropics. We further find the comparable contribution of ERF efficacy (per unit aerosol optical depth), which is 22% less in northern extratropical eruptions than equatorial eruptions. This is attributed to the sulfate burden peak in the first winter with lower solar radiation in northern extratropical summer eruptions. The uneven distributions of sulfate aerosols also lead to varied responses of Brewer‐Dobson circulation (BDC) and ozone, and BDC transportation contributes more than chemical processes. Consequently, ozone loss in tropics and increase in midlatitudes for equatorial eruptions, while northern extratropical eruptions trigger ozone decrease in the injected hemisphere and increase in the opposite hemisphere. Key Points: Modeled shorter aerosol lifetime and less radiative forcing efficacy lead to ∼40% less radiative forcing in extratropical eruptionsSimulated effective radiative forcing efficacy in northern extratropical summer eruptions is 22% lower than that in equatorial eruptionsNorthern extratropical eruptions lead to distinct pattern of Brewer‐Dobson circulation and ozone change compared to equatorial eruptions [ABSTRACT FROM AUTHOR]

ABSTRACT Mediterranean climate‐type regions (MCRs) are characterised by warm‐to‐hot dry summers and mild‐wet winters. These regions are typically found on the western or southern edges of continents, for example, in the Mediterranean Basin, the west coast of North and South America, southern Africa and southwest Australia. The MCRs are vulnerable to climate variability and change related to their unique characteristics, such as pronounced rainfall seasonality and prolonged hot and dry summers. Based on historical observations and CMIP6 climate projections, we apply an empirical bio‐climatic assessment of how the geographic distribution of MCRs has changed during the last century and how these zones will be further impacted under continued warming. Results indicate a poleward and eastward expansion of MCRs in the Mediterranean Basin, North America‐California and South America‐Central Chile regions. For parts of Southern Africa and Southern Australia, a retreat of the MCR margins and an expansion of more arid climate zones are projected. These shifts are particularly profound according to high emission and radiative forcing pathways and future scenarios. The warming in MCRs is projected to accelerate (e.g., mean regional warming of up to 5.5°C under a 4°C global warming scenario), and precipitation will decrease by about 5%–10% for every additional degree of global warming. One exception is the California MCR, where rainfall will likely increase. Such changes can challenge water resources, food security and other aspects of human livelihood and ecosystems in these unique geographical zones. [ABSTRACT FROM AUTHOR]

In this study, we investigated the temporal variation of organic and inorganic aerosol with its optical properties in Mumbai (India), an urban coastal region. Mean PM2.5 concentrations during the sampling period were 175 μg/m3 (winter) and 90 μg/m3 (summer). During winter, the average concentrations of organic (OC), elemental (EC), and water-soluble organic carbon (WSOC) were three times higher than in summer. Secondary organic carbon (SOC) contribution in OC was higher in summer (78%) than in winter (53%), and strong solar radiation in summer likely caused this outcome. Aerosols were slightly acidic in both seasons, with an average pH of 5.7 (winter) and 6.0 (summer). A correlation was observed between SOC and the acidity of particles in summer (R2 = 0.6), indicating some amount of acid-catalysed SOC formation. In both seasons, the sulphate oxidation ratio (SOR) was higher than the nitrate oxidation ratio (NOR), which may reflect a preference for SO2 oxidation over NO2 or the difference in partitioning ammonium nitrate into ammonium sulphate under high RH. The dominant mechanism of SOC formation (gas vs aqueous phase oxidation) also showed seasonal variation. In winter, a relatively steep reduced major axis (RMA) slope of O3/CO suggests gas phase oxidation was the dominant mechanism of SOC production. Winter has more BrC fraction than summer, indicating higher absorbing aerosols, though the efficiency of absorbing the light was higher in summer. To assess the radiative forcing of PM2.5 on a local scale, an effective carbon ratio (ECR) was computed. The findings pointed to a local radiative heating impact caused by PM2.5. The spectral slope ratio and MAE at 250 to 300 nm ratio (E2/E3) revealed a higher abundance of high molecular weight species in WSOC during summer than in winter. [ABSTRACT FROM AUTHOR]

Recent studies have suggested that stratospheric aerosol injection (SAI) of solid particles for climate intervention could reduce stratospheric warming compared to injection of SO2 ${\text{SO}}_{2}$. However, interactions of microphysical processes, such as settling and coagulation of solid particles, with stratospheric dynamics have not been considered. Using a global chemistry‐climate model with interactive solid particle microphysics, we show that agglomeration significantly reduces the backscatter efficiency per unit of injected material compared to mono‐disperse particles, partly due to faster settling of the agglomerates, but mainly due to increased forward‐ over backscattering with increasing agglomerate size. Despite these effects, some materials substantially reduce required injection rates as well as perturbation of stratospheric winds, age of air and stratospheric warming compared to injection of SO2 ${\text{SO}}_{2}$, with the most promising results being shown by 150 nm diamond particles. Uncertainties remain as to whether stratospheric dispersion of solid particles is feasible without formation of agglomerates. Plain Language Summary: [Stratospheric warming is an undesired side effect of climate intervention via SAI. Recent studies have shown that stratospheric warming could be reduced when injecting solid particles instead of gaseous SO2 ${\text{SO}}_{2}$. However, most of these studies looked at the stratospheric particle mass required for a given radiative forcing (RF), without accounting for gravitational settling of particles or the effect of particles sticking together after mutual collision. We show that accounting for these effects significantly reduces the amount of backward reflected radiation per unit of stratospheric particle mass decreasing the radiative efficiency of the injected material. This is due to the combined effect of faster gravitational settling and the larger fraction of forward reflected radiation over backward reflected radiation with increasing agglomerate size. However, we show that injection of diamond particles at a radius of 150 nm instead of SO2 ${\text{SO}}_{2}$ significantly reduces required stratospheric injection rates as well as perturbation of stratospheric winds, age of stratospheric air and stratospheric water vapor concentrations due to small stratospheric warming per unit of RF. However, large uncertainties remain as to whether it will be feasible to inject solid particles into the stratosphere at concentrations low enough to prohibit that the particles stick together.] Key Points: We explore stratospheric injections of six solid particles and gaseous SO2 within a climate model with comprehensive aerosol microphysicsAccounting for settling and agglomeration of solid particles can substantially reduce the radiative forcing (RF) per unit of injected materialInjection of diamond particles (r = 150 nm) instead of SO2 largely reduces stratospheric temperature, circulation and water vapor anomalies [ABSTRACT FROM AUTHOR]

In the Southern Hemisphere, Earth system models project an intensification of winter storm tracks by the end of the twenty-first century. Previous studies using idealized models showed that storm track intensity saturates with increasing temperatures, suggesting that the intensification of the winter storm tracks might not continue further with increasing greenhouse gases. Here, we examine the response of midlatitude winter storm tracks in the Southern Hemisphere to increasing CO2 from two to eight times preindustrial concentrations in more realistic Earth system models. We find that at high CO2 levels (beyond 4×CO2), winter storm tracks no longer exhibit an intensification across the extratropics. Instead, they shift poleward, weakening the storm tracks at lower midlatitudes and strengthening at higher midlatitudes. By analyzing the eddy kinetic energy (EKE) budget, the nonlinear storm-track response to an increase in CO2 levels in the lower midlatitudes is found to stem from a scale-dependent conversion of eddy available potential energy to EKE. Specifically, in the lower midlatitudes, this energy conversion acts to oppositely change the EKE of long and short scales at low CO2 levels, but at high CO2 levels, it mostly reduces the EKE of shorter scales, resulting in a poleward shift of the storms. Furthermore, we identify a "tug of war" between the upper and lower temperature changes as the primary driver of the nonlinear-scale-dependent EKE response in the lower midlatitudes. Our results suggest that in the highest emission scenarios beyond the twenty-first century, the storm tracks' response may differ in magnitude and latitudinal distribution from projected changes by 2100. Significance Statement: The Southern Hemisphere winter storm track is projected to intensify by the end of the century, with the most significant intensification occurring in the higher midlatitudes. However, we show that the intensification is not a linear function of the radiative forcing associated with increasing CO2 levels. In fact, our study shows a poleward shift at very high CO2 levels, with the storm track moving southward. This suggests that the Southern Hemisphere winter storm track may require time-sensitive adaptation strategies, as the impacts of global warming on the storm track may not be a linear function of CO2 concentration in the atmosphere. [ABSTRACT FROM AUTHOR]

Background: Vector-borne diseases, such as malaria, pose a significant global threat, and climatological factors greatly influence their intensity. Tropical countries, like India, are particularly vulnerable to such diseases, making accurate estimation of malaria risk crucial. Methods: This study utilized the well-known Vector-borne Disease Community Model, VECTRI, developed by the International Centre for Theoretical Physics in Trieste. The model was implemented to estimate malaria's Entomological Inoculation Rate (EIR). Future climatic prediction datasets, including CMIP 5 and population data sets, were used as inputs for the analysis. Three RCP scenarios are considered (Representative Concentration Pathways are climate change scenarios that project radiative forcing to 2100 due to future greenhouse gas concentrations). The projections covered the period from 1 Jan, 2020, to 31 Dec, 2029. Results: The estimated mean EIR for the years 2020–2029 ranged, and a significant decline in malaria risk was observed with all RCP 2.6, 4.5, and 8.5 scenarios. Each year 0.3 to 2.6 [min–max] EIR/person/day decline is observed with a strong decline in man rainfall ranging from 5 to 17 [min–max] mm/year and associated high temperatures ranging from 0.03 to 0.06 [min–max] °C/year. During the post-monsoon period, August to November were identified as highly prone to malaria transmission. Spatial analysis revealed that the east coast of India faced a higher vulnerability to malaria risk, which kept increasing through RCP scenarios. Thus, it is essential to exercise caution, especially in areas with heavy rainfall. Conclusion: This research provides valuable insights for policy-makers, highlighting the need to implement future strategies to mitigate malaria risk effectively. By utilizing these findings, appropriate measures can be taken to combat the threat posed by malaria and protect public health. [ABSTRACT FROM AUTHOR]

Earth system models (ESMs) are essential for understanding the interaction between human activities and the Earth's climate. However, the computational demands of ESMs often limit the number of simulations that can be run, hindering the robust analysis of risks associated with extreme weather events. While low‐cost climate emulators have emerged as an alternative to emulate ESMs and enable rapid analysis of future climate, many of these emulators only provide output on at most a monthly frequency. This temporal resolution is insufficient for analyzing events that require daily characterization, such as heat waves or heavy precipitation. We propose using diffusion models, a class of generative deep learning models, to effectively downscale ESM output from a monthly to a daily frequency. Trained on a handful of ESM realizations, reflecting a wide range of radiative forcings, our DiffESM model takes monthly mean precipitation or temperature as input, and is capable of producing daily values with statistical characteristics close to ESM output. Combined with a low‐cost emulator providing monthly means, this approach requires only a small fraction of the computational resources needed to run a large ensemble. We evaluate model behavior using a number of extreme metrics, showing that DiffESM closely matches the spatio‐temporal behavior of the ESM output it emulates in terms of the frequency and spatial characteristics of phenomena such as heat waves, dry spells, or rainfall intensity. Plain Language Summary: Ideally, to study how damaging phenomena like heatwaves, droughts and downpours will change in the future under global warming, we would want a large number of climate model runs producing many realizations of climate futures that we can analyze and from which the new characteristics of climate extremes can be quantified. Currently, emulators can rapidly generate simulations of future climate, but often to relatively low frequencies, as decadal, annual or monthly output at best in most cases, which is insufficient for studying extreme events that occur on a daily timescale. We show how it is possible to train a machine learning model to produce daily series of temperature or precipitation from monthly averages, thus facilitating a more robust investigation into how extreme events will change in the future. Key Points: Earth system models (ESMs) are key devices for understanding how human actions will affect the future global climateComputational demands prevent us from running them for more than a handful of scenarios. ESM emulators are most commonly limited to the monthly frequencyWe present DiffESM as a data‐driven emulator of ESMs that closely matches the spatiotemporal distributions of ESMs at daily frequency [ABSTRACT FROM AUTHOR]

Rainfall over mainland Southeast Asia experiences variability on seasonal to decadal timescales in response to a multitude of climate phenomena. Historical records and paleoclimate archives that span the last millennium reveal extreme multi-year rainfall variations that significantly affected the societies of mainland Southeast Asia. Here we utilize the Community Earth System Model Last Millennium Ensemble (CESM-LME) to quantify the contributions of internal and external drivers to decadal-scale rainfall extremes in the Southeast Asia region. We find that internal variability was dominant in driving both Southeast Asian drought and pluvial extremes on decadal timescales although external forcing impacts are also detectable. Specifically, rainfall extremes are more sensitive to Pacific Ocean internal variability than the state of the Indian Ocean. This discrepancy is greater for droughts than pluvials which we suggest is attributable to external forcing impacts that counteract the forced Indian Ocean teleconnections to Southeast Asia. Volcanic aerosols, the most effective radiative forcing during the last millennium, contributed to both the Ming Dynasty Drought (1637–1643) and the Strange Parallels Drought (1756–1768). From the Medieval Climate Anomaly to the Little Ice Age, we observe a shift in Indo-Pacific teleconnection strength to Southeast Asia consistent with enhanced volcanism during the latter interval. This work not only highlights asymmetries in the drivers of rainfall extremes but also presents a framework for quantifying multivariate drivers of decadal-scale variability and hydroclimatic extremes. [ABSTRACT FROM AUTHOR]

All clouds influence the Earth's radiative budget, with their net radiative forcing being negative. However, high-level clouds warrant special attention due to their atmospheric warming effects. A comprehensive characterization of cirrus requires information on cloud coverage, obtainable from various data types. Active satellite sensors are presently the most accurate source for cirrus data, but their usefulness in climatological studies is limited. On the contrary, passive data, available for the past 40 years with sufficient temporal resolution for climatological research, were not specifically designed for cirrus detection. In this study, we assessed the utility of MODIS standard products for creating a cirrus mask by validating them against CALIOP data. Our objective was to determine if a MODIS product exists that detects cirrus with the same accuracy as CALIOP. Using CALIOP data as the reference, we evaluated six tests for cirrus detection considered in MODIS cloud masking algorithm and their combination (ALL TESTS CONSOLIDATION, ATC). Additionally we applied two ISCCP-originating tests: ISCCP3.6 and ISCCP23 tests. All tests have been applied to MODIS radiances. Study revealed that ATC test was the most effective resulting with the overall accuracy of 72.98 % during daytime and 59.50 % at night (probability of detection: 80.87 % and 25.46 %, false alarm rate of 34.86 % and 6.90 %, and Cohen's kapppa coefficient of 0.46 and 0.19 respectively). However, its effectiveness was notably reduced during nighttime compared to daytime. We conclude that the test is suitable for creating a mask of high-level clouds. [ABSTRACT FROM AUTHOR]

This study analyzes the vertical distribution, optical properties, radiative forcing, and several perturbation events of stratospheric aerosols using observations from a ground-based polarization lidar in Wuhan (30.5° N, 114.4° E) from 2010 to 2021. The background stratospheric aerosol optical depth (sAOD) was 0.0044 ± 0.0019 at 532 nm , as calculated during a stratosphere-quiescent period from January 2013 to August 2017. In addition, several cases of volcanic aerosol and wildfire-induced smoke were observed. Volcanic aerosols from the Nabro (2011) and Raikoke (2019) eruptions (both in boreal summer) increased the sAOD to 2.9 times the background level. Tracers of smoke from the Canadian wildfire in the summer of 2017 were observed twice, at 19–21 km on 14–17 September and at 20–23 km on 28–31 October, with a plume-isolated aerosol optical depth (AOD) of 0.002–0.010 and a particle linear depolarization ratio δp of 0.14–0.18, indicating the dominance of non-aged smoke particles. During these summertime events, the injected stratospheric aerosols were captured by the large-scale Asian monsoon anticyclone (AMA), confining the transport pathway to mid-latitude Asia. On 8–9 November 2020, smoke plumes originating from the California wildfire in October 2020 appeared at 16–17 km , with a mean δp of 0.13. Regarding seasonal variation, the sAOD in the cold half-year (0.0054) is 69 % larger than in the warm half-year (0.0032) due to stronger meridional transport of stratospheric aerosols from the tropics to middle latitudes. The stratospheric radiative forcing was - 0.11 Wm-2 during the stratosphere-quiescent period and increased to - 0.31 Wm-2 when volcanic aerosols were largely injected. These findings contribute to our understanding of the sources and transport patterns of stratospheric aerosols over mid-latitude Asia and serve as an important database for the validation of model outputs. [ABSTRACT FROM AUTHOR]

Recent analyses show the importance of methane shortwave absorption, which many climate models lack. In particular, Allen et al. (2023) used idealized climate model simulations to show that methane shortwave absorption mutes up to 30 % of the surface warming and 60 % of the precipitation increase associated with its longwave radiative effects. Here, we explicitly quantify the radiative and climate impacts due to shortwave absorption of the present-day methane perturbation. Our results corroborate the hypothesis that present-day methane shortwave absorption mutes the warming effects of longwave absorption. For example, the global mean cooling in response to the present-day methane shortwave absorption is -0.10±0.07 K, which offsets 28 % (7 %–55 %) of the surface warming associated with present-day methane longwave radiative effects. The precipitation increase associated with the longwave radiative effects of the present-day methane perturbation (0.012±0.006 mm d -1) is also muted by shortwave absorption but not significantly so (-0.008±0.009 mm d -1). The unique responses to methane shortwave absorption are related to its negative top-of-the-atmosphere effective radiative forcing but positive atmospheric heating and in part to methane's distinctive vertical atmospheric solar heating profile. We also find that the present-day methane shortwave radiative effects, relative to its longwave radiative effects, are about 5 times larger than those under idealized carbon dioxide perturbations. Additional analyses show consistent but non-significant differences between the longwave versus shortwave radiative effects for both methane and carbon dioxide, including a stronger (negative) climate feedback when shortwave radiative effects are included (particularly for methane). We conclude by reiterating that methane remains a potent greenhouse gas. [ABSTRACT FROM AUTHOR]

Heterogeneous radiative forcing in mid-latitudes, such as that exerted by aerosols, has been found to affect the Arctic climate, though the mechanisms remain debated. In this study, we leverage deep learning (DL) techniques to explore the complex response of the Arctic climate system to local radiative forcing over Europe. We conducted sensitivity experiments using the Max Planck Institute Earth System Model (MPI-ESM1.2) coupled with atmosphere–ocean–land-surface components. Large-scale circulation patterns can mediate the impact of the forcing on Arctic climate dynamics. We employed a DL-based clustering approach to classify large-scale atmospheric circulation patterns. To enhance the analysis of how these patterns impact the Arctic climate, the poleward moist static energy transport (PMSET) associated with the atmospheric circulation patterns was incorporated as an additional similarity metric in the clustering process. Furthermore, we developed a novel method to analyze the circulation patterns' contributions to various climatic parameter anomalies. Our findings indicate that the negative radiative forcing over Europe alters existing circulation patterns and their occurrence frequency without introducing new ones. Specifically, our analysis revealed that while the regional radiative forcing alters the occurrence frequencies of the circulation patterns, these changes are not the primary drivers of the forcing's impact on the Arctic parameters. Instead, it is the shifts in the mean spatial characteristics of the atmospheric circulation patterns, induced by the forcing, that predominantly determine the effects on the Arctic climate. Our methodology facilitates the uncovering of complex, nonlinear interactions within the climate system, capturing nuances that are often obscured in broader seasonal anomaly analyses. This approach enables a deeper understanding of the dynamics driving observed climatic anomalies and their links to specific atmospheric circulation patterns. [ABSTRACT FROM AUTHOR]

The concept of equivalence is widely employed to aggregate different types of resource depletion or emissions. The practice offers convenience for reporting and can assist policymaking. However, equivalence is typically established using a model based only on selected criteria. If alternative criteria are used, the relative importance of different types of resource depletion or emissions can change. This communication addresses the subject of greenhouse gas (GHG) emissions and carbon dioxide equivalence (CO2e). Using the Australian beef cattle and sheep meat sectors as a case study, radiative forcing (RF) footprints were quantified, and a method is presented to express these as CO2e emissions rather than the units W/m2. RF footprints are an alternative way of aggregating GHG emissions. They incorporate RF from current-year emissions and RF from historical emissions that remain in the atmosphere, avoiding the need to choose an arbitrary time horizon. There is simplicity and familiarity associated with the CO2e unit. However, it is concluded that whenever GHG emissions are reported as CO2e emissions, there is a need for transparency about the basis for equivalence, justification of the approach, as well interpretation of the results, and potential implications of selecting other bases of equivalency. [ABSTRACT FROM AUTHOR]

Light‐absorbing carbonaceous aerosols that dominate atmospheric aerosol warming over India remain poorly characterized. Here, we delve into UV‐visible‐IR spectral aerosol absorption properties at nine PAN‐India COALESCE network sites (Venkataraman et al., 2020, https://doi.org/10.1175/bams‐d‐19‐0030.1). Absorption properties were estimated from aerosol‐laden polytetrafluoroethylene filters using a well‐constrained technique incorporating filter‐to‐particle correction factors. The measurements revealed spatiotemporal heterogeneity in spectral intrinsic and extrinsic absorption properties. Absorption analysis at near‐UV wavelengths from carbonaceous aerosols at these regional sites revealed large near‐ultraviolet brown carbon absorption contributions from 21% to 68%—emphasizing the need to include these particles in climate models. Further, satellite‐retrieved column‐integrated absorption was dominated by surface absorption, which opens possibilities of using satellite measurements to model surface‐layer optical properties (limited to specific sites) at a higher spatial resolution. Both the satellite‐modeled and direct in‐situ absorption measurements can aid in validating and constraining climate modeling efforts that suffer from absorption underestimations and high uncertainties in radiative forcing estimates. Plain Language Summary: Particulate pollution in the atmosphere scatter and absorb incoming solar energy, thus cooling or warming Earth's atmosphere. In developing countries and especially in India, one of the most polluted regions of the world, the extent to which particles can absorb solar energy and warm the atmosphere is not well understood. Here, for the first time, we measure particle absorption simultaneously at nine ground sites across India, in diverse geographical regions with different levels and types of particulate pollution. We find that organic carbon particles exert large absorption at near‐ultraviolet wavelengths, which contain significant solar energy. These light absorbing organic carbon particles, called brown carbon, are emitted in large quantities from biomass burning (e.g., burning crop residue and cooking on wood‐fired stoves). Comparing ground measurements of absorption with satellite‐retrieved measurements that are representative of the entire atmospheric column, we find that near‐surface atmospheric particles can exert significant warming. This study highlights the need to improve climate model simulations of particulate pollution's impact on the climate by incorporating spatiotemporal surface‐level absorption measurements, including absorption by brown carbon particles. Key Points: Measurements at nine regional PAN‐India sites reveal several regions with large aerosol absorption strengthBrown carbon contributes significantly (21%–68%) to near‐ultraviolet absorption, indicating its importance in shortwave light absorptionStrong correlations observed between satellite data and surface absorption indicate future potential in modeling surface absorption [ABSTRACT FROM AUTHOR]

The downwelling longwave radiation at the surface (DLR) is a key component of the Earth's surface energy budget. We present a novel set of equations that explicitly account for both clouds and the CO2 $\mathrm{C}{\mathrm{O}}_{\mathrm{2}}$ effect to calculate the all‐sky DLR. This paper first extends the clear‐sky DLR model of Shakespeare and Roderick (2021, https://doi.org/10.1002/qj.4176) to include temperature inversions and clouds. We parameterize relevant cloud properties through theoretical and empirical considerations to formulate an all‐sky model. Our model is more accurate than existing methods (reduces Root Mean Squared Error by 2.1–8.7 W/m2 $\mathrm{W}/{\mathrm{m}}^{\mathrm{2}}$ and 1.2–10.1 W/m2 $\mathrm{W}/{\mathrm{m}}^{\mathrm{2}}$ compared to ERA5 reanalysis and in‐situ data respectively), and provides a strong physical basis for the estimation of the downwelling longwave from near‐surface information. We highlight the important role of CO2 $\mathrm{C}{\mathrm{O}}_{\mathrm{2}}$ dependence by showing our model largely captures the change in atmospheric emissivity purely due to CO2 $\mathrm{C}{\mathrm{O}}_{\mathrm{2}}$ (i.e., the instantaneous radiative forcing) in CMIP6 models. Plain Language Summary: The downwelling longwave radiation (DLR) at the surface is a key component of the energy balance at the Earth's surface. Understanding how the DLR will change under future climate conditions is vital. For the first time, we explicitly write a set of equations to calculate the DLR that sufficiently account for the impact of CO2 $\mathrm{C}{\mathrm{O}}_{\mathrm{2}}$ and clouds simultaneously. Our model is more accurate than existing methods, and provides a much stronger physical basis for the estimation of the downwelling longwave from near‐surface information. In this paper, we extend an existing method for estimating the DLR under clear‐sky conditions (i.e., no clouds) to operate under all sky conditions. This method can be used to inform models where the DLR is needed, but only basic observations are available. Key Points: Downwelling longwave radiation (DLR) is a poorly estimated element of the surface energy budget by existing analytical modelsExplicitly accounting for temperature inversions and cloud emissivities improves the accuracy of DLR estimationConsidering the radiative forcing from increasing CO2 $\mathrm{C}{\mathrm{O}}_{\mathrm{2}}$ is necessary to produce unbiased future estimates of DLR [ABSTRACT FROM AUTHOR]

There is debate about the use of wetlands as natural climate solutions due to their ability to act as a "double-edged sword" with respect to climate impacts by both sequestering CO2 while emitting CH4. Here, we used a process-based greenhouse gas (GHG) perturbation model to simulate wetland radiative forcing and temperature change associated with wetland state conversion over 500 years based on empirical carbon flux measurements, and CO2-equivalent (CO2-e.q.) metrics to assess the net flux of GHGs from wetlands on a comparable basis. Three CO2-e.q. metrics were used to describe the relative radiative impact of CO2 and CH4—the conventional global warming potential (GWP) that looks at pulse GHG emissions over a fixed timeframe, the sustained-flux GWP (SGWP) that looks at sustained GHG emissions over a fixed timeframe, and GWP* that explicitly accounts for changes in the radiative forcing of CH4 over time (initially more potent but then diminishing after about a decade)—against model-derived mean temperature profiles. GWP* most closely estimated the mean temperature profiles associated with net wetland GHG emissions. Using the GWP*, intact wetlands serve as net CO2-e.q. carbon sinks and deliver net cooling effects on the climate. Prioritizing the conservation of intact wetlands is a cost-effective approach with immediate climate benefits that align with the Paris Agreement and the Intergovernmental Panel on Climate Change timeline of net-zero GHG emissions by 2050. Restoration of wetlands also has immediate climate benefits (reduced warming), but with the majority of climate benefits (cooling) occurring over longer timescales, making it an effective short and long-term natural climate solution with additional co-benefits. [ABSTRACT FROM AUTHOR]

Developing future climate projections begins with choosing future emissions scenarios. While scenarios are often based on storylines, here instead we produce a probabilistic multi-million-member ensemble of radiative forcing trajectories to assess the relevance of future forcing thresholds. We coupled a probabilistic database of future greenhouse gas emission scenarios with a probabilistically calibrated reduced complexity climate model. In 2100, we project median forcings of 5.1 watt per square meters (5th to 95th percentiles of 3.3 to 7.1), with roughly 0.5% probability of exceeding 8.5 watt per square meters, and a 1% probability of being lower than 2.6 watt per square meters. Although the probability of 8.5 watt per square meters scenarios is low, our results support their continued utility for calibrating damage functions, characterizing climate in the 22nd century (the probability of exceeding 8.5 watt per square meters increases to about 7% by 2150), and assessing low-probability/high-impact futures. The probability of exceeding various climate thresholds has been calculated. While the warmest future scenario has less than a 1 percent chance of being exceeded this century, the paper discusses why such warm scenarios are still relevant. [ABSTRACT FROM AUTHOR]

Anthropogenic aerosol particles partially mask global warming driven by greenhouse gases, both directly by reflecting sunlight back to space and indirectly by increasing cloud reflectivity. In recent decades, emissions of anthropogenic aerosols have declined globally and at the same time shifted from the North American and European regions, foremost to Southeast Asia. Using simulations with the Max Planck Institute Earth System Model version 1.2 (MPI-ESM1.2), we find that the direct effect of aerosols has continued to increase despite declining emissions. Concurrently, the indirect effect has diminished in approximate proportion to the emissions. In this model, which employs parameterized aerosol effects with constant regional direct effect efficiency, the enhanced efficiency of aerosol radiative forcing in emissions is associated with less cloud masking, longer atmospheric residence times, and differences in aerosol optical properties. [ABSTRACT FROM AUTHOR]

As an important source of sub-micrometer particles, atmospheric new particle formation (NPF) has been observed in various environments. However, most studies provide little more than snapshots of the NPF process due to their underlying observations being limited in space and time. To obtain statistically relevant evidence on NPF across various environments, we investigated the characteristics of NPF based on a 5-year dataset of the German Ultrafine Aerosol Network (GUAN). The results were also compared with observations in previous studies, with the aim to depict a relatively complete picture of NPF in central Europe. The highest NPF occurrence frequency was observed in regional background sites, with an average of about 19 %, followed by urban background (15 %), low-mountain-range (7 %), and high Alpine (3 %) sites. The annual mean growth rate between 10 and 25 nm varied from 3.7–4.7 nm h -1 , while the formation rate with same size range 10–25 nm from 0.4 to 2.9 cm -3 s -1. The contribution of NPF to ultrafine particles (UFPs) was about 13 %, 21 %, and 7 % for the urban background, regional background, and low mountain range, respectively. The influence of NPF on cloud condensation nuclei (CCN) number concentration and the aerosol extinction coefficient for NPF days was the highest in mountainous areas. These findings underscore the importance of local environments when assessing the potential impact of NPF on regional climate in models, and they also emphasize the usefulness of a long-term aerosol measurement network for understanding the variation in NPF features and their influencing factors over a regional scale. [ABSTRACT FROM AUTHOR]

Wildfires and agricultural burning generate seemingly increasing smoke aerosol emissions, impacting societal and natural ecosystems. To understand smoke's effects on climate and public health, we analyzed the spatiotemporal distribution of smoke aerosols, focusing on two major light-absorbing components, namely black carbon (BC) and brown carbon (BrC) aerosols. Using NASA's Earth Polychromatic Imaging Camera (EPIC) instrument aboard NOAA's Deep Space Climate Observatory (DSCOVR) spacecraft, we inferred BC and BrC volume fractions and particle mass concentrations based on spectral absorption provided by the Multi-Angle Implementation of Atmospheric Correction (MAIAC) algorithm with 1–2 h temporal resolution and ∼ 10 km spatial resolution over North America and central Africa. Our analyses of regional smoke properties reveal distinct characteristics for aerosol optical depth (AOD) at 443 nm, spectral single-scattering albedo (SSA), aerosol layer height (ALH), and BC and BrC amounts. Smoke aerosols in North America showed extremely high AOD up to 6, with elevated ALH (6–7 km) and significant BrC components up to 250 mg m -2 along the transport paths, whereas the smoke aerosols in central Africa exhibited stronger light absorption (i.e., lower SSA) and lower AOD, resulting in higher-BC mass concentrations and similar BrC mass concentrations than the cases in North America. Seasonal burning source locations in central Africa, following the seasonal shift in the Intertropical Convergence Zone and diurnal variations in smoke amounts, were also captured. A comparison of retrieved AOD 443 , SSA 443 , SSA 680 , and ALH with collocated AERONET and CALIOP measurements shows agreement with RMSE values of 0.2, 0.03–0.04, 0.02–0.04, and 0.8–1.3 km, respectively. An analysis of the spatiotemporal average reveals distinct geographical characteristics in smoke properties closely linked to burning types and meteorological conditions. Forest wildfires over western North America generated smoke with a small-BC volume fraction of 0.011 and a high ALH with large variability (2.2 ± 1.2 km), whereas smoke from wildfires and agricultural burning over Mexico region shows more absorption and low ALH. Smoke from savanna fires over central Africa had the most absorption, with a high-BC volume fraction (0.015) and low ALH with a small variation (1.8 ± 0.6 km) among the analyzed regions. Tropical forest smoke was less absorbing and had a high variance in ALH. We also quantify the estimation uncertainties related to the assumptions of BC and BrC refractive indices. The MAIAC EPIC smoke properties with BC and BrC volume and mass fractions and assessment of the layer height provide observational constraints for radiative forcing modeling and air quality and health studies. [ABSTRACT FROM AUTHOR]

Marine low-level clouds are key to the Earth's energy budget due to their expansive coverage over global oceans and their high reflectance of incoming solar radiation. Their responses to anthropogenic aerosol perturbations remain the largest source of uncertainty in estimating the anthropogenic radiative forcing of climate. A major challenge is the quantification of the cloud water response to aerosol perturbations. In particular, the presence of feedbacks through microphysical, dynamical, and thermodynamical pathways at various spatial and temporal scales could augment or weaken the response. Central to this problem is the temporal evolution in cloud adjustment, governed by entangled feedback mechanisms. We apply an innovative conditional Monte Carlo subsampling approach to a large ensemble of diurnal large-eddy simulation of non-precipitating marine stratocumulus to study the role of solar heating in governing the evolution in the relationship between droplet number and cloud water. We find a persistent negative trend in this relationship at night, confirming that the role of microphysically enhanced cloud-top entrainment. After sunrise, the evolution in this relationship appears buffered and converges to ∼-0.2 in the late afternoon. This buffering effect is attributed to a strong dependence of cloud-layer shortwave absorption on cloud liquid water path. These diurnal cycle characteristics further demonstrate a tight connection between cloud brightening potential and the relationship between cloud water and droplet number at sunrise, which has implications for the impact of the timing of advertent aerosol perturbations. [ABSTRACT FROM AUTHOR]

Solar climate intervention refers to a group of methods for reducing climate risks associated with anthropogenic warming by reflecting sunlight. Marine cloud brightening (MCB), one such approach, proposes to inject sea‐salt aerosol into one or more regional marine boundary layer to increase marine cloud reflectivity. Here, we assess the potential influence of various MCB experiments on Africa's climate using simulations from the Community Earth System Model (CESM2) with the Community Atmosphere Model (CAM6) as its atmospheric component. We analyzed four idealized MCB experiments under a medium‐range background forcing scenario (SSP2‐4.5), which brighten clouds over three subtropical ocean regions: (a) Northeast Pacific (MCBNEP); (b) Southeast Pacific (MCBSEP); (c) Southeast Atlantic (MCBSEA); and (d) these three regions simultaneously (MCBALL). Our results suggest that the climate impacts of MCB in Africa are highly sensitive to the deployment region. MCBSEP would produce the strongest global cooling effect and thus could be the most effective in decreasing temperatures, increasing precipitation, and reducing the intensity and frequency of temperature and precipitation extremes across most parts of Africa, especially West Africa, in the future (2035–2054) compared to the historical climate (1995–2014). MCB in other regions produces less cooling and wetting despite similar radiative forcings. While the projected changes under MCBALL are similar to those of MCBSEP, MCBNEP and MCBSEA could see more residual warming and induce a warmer future than under SSP2‐4.5 in some regions across Africa. All MCB experiments are more effective in cooling maximum temperature and related extremes than minimum temperature and related extremes. Plain Language Summary: We investigate the potential impact of artificially increasing marine cloud reflectivity on Africa's climate. Human influence on the climate is projected to increase the risk of damaging extreme events across the globe. In Africa, one of the most vulnerable regions to climate change, these impacts are already being felt, especially among communities least able to adapt. It is possible to increase the reflectivity of marine clouds by spraying them with sea salt particulates. This approach was proposed as one possible way of reducing warming by reducing the amount of sunlight reaching the earth's surface. The potential implications of such initiatives for the climate system remain uncertain, especially in Africa. In this study, we analyzed a climate model, that simulates increased reflectivity of clouds to assess the potential impacts of artificially brightening clouds over different subtropical marine regions on the African climate, focusing on mean and extreme precipitation and temperature events. Our results suggest that the impact of artificially increasing marine cloud reflectivity depends on the marine region of intervention. Key Points: The climate impacts of Marine Cloud Brightening (MCB) in Africa are highly sensitive to the deployment regionMCB in the Southeast Pacific could effectively cool Africa and reduces temperature and precipitation extremesIn this study, MCB is more effective in cooling maximum temperature and related extremes than minimum temperature and related extremes [ABSTRACT FROM AUTHOR]

Tropical oceans are the main global water vapor and latent heat sources, but their responses to radiative forcing remain unclear. Here, we investigate oceanic moisture dynamics of the western tropical Pacific (WTP) over the past 210,000 years through an approach of planktonic foraminiferal triple oxygen isotope (Δ′17O). The Δ′17O record is dominated by the precession cycles (~23,000 years), with lower values reflecting higher humidity in concert with higher Northern Hemisphere summer insolation. Our empirical and modeling results, combined with other geological archives, suggest that the enhanced moisture convergence over the WTP largely intensifies changes in the meridional and zonal hydrological cycles, affecting rainfall patterns in East Asia and northern South America. We propose that the insolation-driven WTP moisture dynamics play a pivotal role in regulating tropical hydroclimate. [ABSTRACT FROM AUTHOR]

The oceanic emission of dimethyl sulfide (DMS) plays a vital role in the Earth's climate system and constitutes a substantial source of uncertainty when evaluating aerosol radiative forcing. Currently, the widely used monthly climatology of sea surface DMS concentration falls short of meeting the requirement for accurately simulating DMS-derived aerosols with chemical transport models. Hence, there is an urgent need for a high-resolution, multi-year global sea surface DMS dataset. Here we develop an artificial neural network ensemble model that uses nine environmental factors as input features and captures the variability of the DMS concentration across different oceanic regions well. Subsequently, a global sea surface DMS concentration and flux dataset (1° × 1°) with daily resolution spanning from 1998 to 2017 is established. According to this dataset, the global annual average concentration was ∼ 1.71 nM, and the annual total emissions were ∼ 17.2 Tg S yr -1 , with ∼ 60 % originating from the Southern Hemisphere. While overall seasonal variations are consistent with previous DMS climatologies, notable differences exist in regional-scale spatial distributions. The new dataset enables further investigations into daily and decadal variations. Throughout the period 1998–2017, the global annual average concentration exhibited a slight decrease, while total emissions showed no significant trend. The DMS flux from our dataset showed a stronger correlation with the observed atmospheric methanesulfonic acid concentration compared to those from previous monthly climatologies. Therefore, it can serve as an improved emission inventory of oceanic DMS and has the potential to enhance the simulation of DMS-derived aerosols and associated radiative effects. The new DMS gridded products are available at https://doi.org/10.5281/zenodo.11879900 (Zhou et al., 2024). [ABSTRACT FROM AUTHOR]

Methane is the second largest contributor to global surface air temperature rise. Reducing atmospheric methane will mitigate climate change and improve air quality. Since the main sink of methane is the hydroxyl radical (OH) in the atmosphere, increasing OH concentration will accelerate the methane oxidation process and reduce methane concentration. Because the primary source of OH is the reaction between water vapor and ozone, scientists have proposed a water vapor enhancement strategy to raise OH concentrations in the atmosphere and remove methane. We use a two-box model to evaluate interactions between OH and methane and a radiative kernel method to calculate radiative responses to water vapor content growth. This proves that increasing OH concentration does have a positive effect on methane reduction. If the concentration of OH is increased by 10% from its current value and maintained for 50 (100) years, 45 Tg yr−1 (67.5 Tg yr−1) more methane will be oxidized by OH, and the cumulative effects of the oxidation are equivalent to a 120.5 Gt (219.6 Gt) reduction in CO2 and will lower the global surface air temperature by 0.054 °C (0.099 °C). Our study also provides insights into a mixed picture of global and regional radiative responses to the growth of water vapor content. The reduced radiative forcing by methane removal cannot overpower the increased radiative forcing by water vapor from the global average point of view. However, due to OH's greater sensitivity to water vapor and weaker radiative response at higher latitudes, this perspective may be reversed if abrupt CH4 emissions from permafrost thaw occur. [ABSTRACT FROM AUTHOR]

The partitioning of cross-equatorial heat transport between the atmosphere and the ocean is investigated through a series of fully coupled experiments, where external radiative forcing is applied over different latitudinal bands. The modeling results reveal a first-order collaborative relationship between the ocean and the atmosphere. In response to low-latitude perturbations, the cross-equatorial heat transport is realized mainly through the atmosphere, as the total ocean heat transport (OHT) is largely offsets by its horizontal gyre component. However, as perturbations shift to higher latitudes in both hemispheres, the OHT responses become more important. We also find a significant interhemispheric asymmetry in the cross-equatorial heat transport, with the OHT contribution being more important in the Southern Hemisphere. This asymmetry is mainly attributed to different responses in the changes of the ocean circulation. When the radiative forcing is placed over the Southern Ocean, a buoyancy-driven clockwise cell is generated that extends from the Southern Ocean into the tropics in both the Indo-Pacific and Atlantic basins, effectively transporting energy to the northern hemisphere. Conversely, when the radiative forcing is placed over the northern high latitudes, the weakening of the Atlantic Meridional Overturning Circulation is responsible for the anomalous southward heat transport. [ABSTRACT FROM AUTHOR]

Since 2000, Clouds and the Earth's Radiant Energy System (CERES) instruments on Aqua and Terra satellites have recorded top-of-atmosphere (TOA) clear-sky outgoing radiation fluxes. Over northwest India, analysis of these two decades of data revealed significant negative trends in reflected shortwave radiation, indicating positive shortwave radiative forcing. To understand the drivers of this trend, we investigate changes in the surface reflectance, aerosol optical depth (AOD), Normalized Difference Vegetation Index (NDVI), and land surface temperature from MODIS observations during the same period. We find decreasing trends in surface reflectance and AOD, while NDVI shows a pronounced increase. Notably, despite positive shortwave radiative forcing, daytime land surface temperature exhibits a significant decrease, with only a slight nighttime increase. Sensitivity analysis using a radiative transfer model identifies decrease in the surface reflectance as the primary contributor to the decrease in TOA shortwave fluxes. This suggests that the observed greening, potentially linked to expanded irrigation projects, has played a crucial role by reducing both surface reflectance and aerosol production. Our findings highlight the potential for increasing vegetation as a locally effective strategy to mitigate the effects of global warming through enhanced evapotranspiration process, even in the presence of positive shortwave radiative forcing. Investigating regional climate trends is vital for understanding and quantifying climate change; these results highlight the need for future climate change impact studies to consider interplay between vegetation dynamics and regional climate. [ABSTRACT FROM AUTHOR]

Climate models enable an analysis of observation data related to global warming and climate changes, and they allow making predictions for near the future progress of climate changes. In this study, basics of the climate model, which is used in the analysis of global warming and climate change topics, and the analyses that can be carried out by using these models, are reviewed. Simplified energy balance model considers the balance state between the incoming solar energy from the sun and the radiation energy outgoing from the earth to space. In this perspective, the model allows considering the factors, which can influence global warming, as radiative forces that are acting on the balance state. Thus enabling simplified analyzes of climate changes based on the energy budget even though the climate changes depend on very complex and dynamically changing meteorological and geological factors. In this study, within the framework of basic radiation laws, such as Stefan-Boltzmann and Wien Displacement, the absorption and reflection rates of the solar radiation by the earth, and the atmospheric window concepts, which provide energy escape from the earth's atmosphere to the space within the long-wavelength of the electromagnetic spectrum, were reviewed. In this context, the radiative forcing due to the Albedo effect and the radiative forcing effects due to the accumulation of CO2 molecules in the atmosphere, which have the potential to alter depending on human activities, were explained. Illustrative examples and observation data-based calculations and analyzes are presented to make basic concepts and presented models easy-to-understand. This article is intended to be a tutorial that explains some basic concepts on the topics of global warming and climate change. [ABSTRACT FROM AUTHOR]

Peatland restoration is seen as a key nature‐based solution to tackle climate change and biodiversity loss. In Europe, nearly 50% of peatlands have been drained during the last decades, which have shifted their soils to carbon dioxide (CO2) sources. Soils of forestry‐drained peatlands are known to vary from CO2 sources to small sinks depending on their fertility and wetness. When peatlands are restored, it can be expected that rates of CO2 and methane exchange will vary depending on site fertility and wetness. We generated seven restoration pathways with different starting and end points and assessed the climate impacts of them. The GHG emission coefficients were compiled from literature, and radiative forcing was calculated for a 500‐year time period since restoration. All seven restoration pathways improved carbon sink capacity; however, the climate impact differed from cooling to warming. The highest cooling impact occurred in a pathway leading from nutrient‐rich drained peatlands toward tree‐covered spruce or pine mires. Warming impacts occurred in a pathway leading from nutrient‐poor drained peatlands toward open peatlands. The results of this study can be used to help identify peatland sites and restoration targets to maximize climate change mitigation from restoration. In practice, however, restoration has to fulfill other targets, such as biodiversity safeguarding, improvement of hydrological conditions, and socio‐economic aspects. Fulfilling all targets simultaneously requires compromises on all targets. [ABSTRACT FROM AUTHOR]

Light-absorbing carbonaceous aerosols are important contributors to both air pollution and radiative forcing. However, their abundances and sources remain poorly constrained, as can be seen from the frequently identified discrepancies between the observed and modeled results. In this study, we focused on elemental carbon (EC ; as a measure of black carbon) and light-absorbing organic carbon (i.e., BrC) in Northeast China, a new targeted region of the latest clean-air actions in China. Three campaigns were conducted during 2018–2021 in Harbin, covering distinct meteorological conditions and emission features. Various analytical methods were first evaluated, and the mass concentrations of both BrC and EC were validated. The validated BrC and EC measurement results were then used for source apportionment, together with other species including tracers (e.g., levoglucosan). The observation-based results suggested that despite the frigid winter in Harbin, the formation of secondary organic carbon (SOC) was enhanced at high levels of relative humidity (RH). This enhancement could also be captured by an air quality model incorporating heterogeneous chemistry. However, the model failed to reproduce the observed abundances of SOC, with significant underestimations regardless of RH levels. In addition, agricultural fires effectively increased the observation-based primary organic carbon (POC) concentrations and POC to EC ratios. Such roles of agricultural fires were not captured by the model, pointing to substantial underestimation of open burning emissions by the inventory. This problem merits particular attention for Northeast China, given its massive agricultural sector. [ABSTRACT FROM AUTHOR]

Ruminant livestock industries can support the climate stabilization ambitions of the Paris Agreement through interventions that reduce GHG emissions (predominantly biogenic methane) and sequester carbon in landscapes. This study explored pathways for the Australian red meat industry (grazing, feedlot finishing, and domestic processing) to become climate neutral, whereby the radiative forcing (RF) footprint is plateaued and there is no additional forcing contribution. Emissions timeseries (CO2, N2O, CH4) were compiled for 1990 to 2020 and projected to 2030 under a business-as-usual scenario (including an 18% increase in sheep and 13% increase in beef cattle) and with a range of production system and vegetation management interventions. The RF footprint peaked in 2018 at 7.13 mW/m2 and decreased to 7.07 mW/m2 in 2020. With the future expansion of the herd/flock and under business-as-usual conditions, the RF footprint is projected to increase by 2.8% by 2030. However, with a combination of interventions, production has the potential to increase with a decreasing RF footprint, a condition that can be described as climate neutral. The Australian red meat industry has made an historical contribution to global RF increase. However, with ongoing RF management, it is possible to increase food production within climate-neutral limits. [ABSTRACT FROM AUTHOR]

In this study, we examined the impact of climate change on the virtual water content of key crops in Kerman province for future periods. Specifically, we utilized the climatic data from the HadCM3 model under the RCP4.5 radiative forcing scenario. The model was calibrated and validated for the base period of 1991 -2011. We predicted the precipitation levels, as well as the maximum and minimum temperatures, for selected stations from 2011 to 2070 using data from LARS -WG. These predictions were then compared to the base period. The virtual water content was calculated for three selected crops: alfalfa, barley, and wheat. Our findings indicate that climate change has a significant impact on evapotranspiration and the performance of these crops, consequently affecting future agricultural water productivity. As we project an increase in average temperature during the growing season due to climate change, it is worth noting that the maximum temperature parameter will be more affected by this phenomenon than the minimum temperature. This, in turn, will lead to increased water requirements and plant evaporation - transpiration during this period. Our research also reveals a decrease in precipitation during hot seasons and an increase during cold seasons across all study stations. Notably, the virtual water content for all crops studied demonstrates an upward trend, with barley and wheat showing the greatest average increase in the future period. Specifically, the Kerman station exhibits a substantial increase in virtual water content for barley and alfalfa products, at a minimum of 30% higher than the base period. [ABSTRACT FROM AUTHOR]

Solar irradiance is a component of the climate system that plays a major role in the global climate change phenomenon. Top of the atmosphere (TOA) radiative flux affects the temperature of the Earth's surface and the atmosphere. In this paper, the impact of CO2 removal on radiative flux at the TOA and surface over West Africa is investigated. A decrease in TOA downwelling shortwave radiations across the periods is simulated, signifying a reduction in warming at the TOA within latitude 14°N and above. Reduction in warming is projected in Niger, Burkina Faso, and Mali in the period 2020–2049 as TOA upwelling longwave radiation increased in these locations. Significant changes are projected more under clear sky conditions than cloudy. Removal of 1.13 ppm/yr (8.81 Gigatonnes of CO 2 per year), 2.23 ppm/yr (17.39 Gigatonnes of CO 2 per year), and 3.50 ppm/yr (27.30 Gigatonnes of CO 2 per year) is projected for the periods 2020–2049, 2040–2069, and 2071–2100, respectively. Clear sky condition simulation shows a greater reduced level of warming before the end of the year 2100. Radiative flux reduction is projected far more at the surface than at TOA. Climate sensitivity and radiative forcing of −2.2 ± 0.1°CW/m2 and −0.4 ± 0.1 W/m2 are obtained, respectively. In contrast, a climate system's response to forcing of Δ T = 0.88°C and Δ T = 3.76°C per unit decrease in radiative forcing are projected on global and West African scales, respectively. Research highlights: Most radiative parameters were reduced in each period compared to the reference period under both cloudy and clear sky conditions at the TOA. Significant changes are projected more under clear sky conditions than cloudy. The projection of decreased radiative flux over West Africa is more at the surface than at TOA. Simulated impacts are from latitude 14°N and below as warming is reduced in this area, while moderate warming is expected above it. Reduction in atmospheric CO2 concentrations enhanced negative climate forcing. The implementation of carbon dioxide removal is capable of reversing climate in the long run. [ABSTRACT FROM AUTHOR]

Over the past 30 years, the Arctic Dipole Anomaly (DA) has repeatedly led to record lows in summer sea ice extent, with cloud radiative effects (CRE) playing a crucial regulatory role. Here, we reveal the CRE variations between positive and negative DA events and elucidate the slowing impacts of CRE on sea ice thickness (SIT) changes. The DA triggers robust meridional winds and transpolar drift, markedly reducing SIT in the Beaufort Sea (BeS), Chukchi Sea (CS), and East Siberian Sea (ESS), while increasing it in the Greenland Sea (GS). CRE significantly slow SIT changes, contributing +14.4, +4.4, +16.4, and −26.7 cm to changes from June to August, against total changes of −55.9, −29.4, −39.8, and +42.8 cm in September over BeS, CS, ESS, and GS, respectively. This study underscores the key impacts of CRE on sea ice variation, emphasizing their significance in the polar climate system. Plain Language Summary: The Arctic Dipole Anomaly is one of the principal atmospheric circulation patterns in the Arctic region, significantly influencing the spatial distribution of sea ice through induced changes in polar wind directions. These wind changes also affect the fraction of Arctic clouds and, by modifying cloud‐surface radiative forcing, alter the surface energy balance. Utilizing 20 years of multi‐source observation data along with reanalysis meteorology, we observed that during the summer, a positive Arctic Dipole Anomaly significantly decreases the thickness of sea ice in the Pacific sector and increases it in the Atlantic sector. During this period, variations in cloud‐surface radiative forcing substantially slow the extent of sea ice thickness reduction/increase. In other words, without considering the impact of cloud radiative forcing, the changes in sea ice thickness would be more severe. Key Points: There is a significant reduction in both the sea ice concentration and thickness across the Pacific sector of the Arctic with Arctic Dipole Anomaly phases in summerThe Arctic Dipole Anomaly induces alterations in the wind and temperature fields, which subsequently influence the spatial distribution of Arctic cloudsVariations in clouds and surface albedo alter the surface energy balance and slow the sea ice changes caused by the Dipole Anomaly [ABSTRACT FROM AUTHOR]

Deposition of wildfire smoke on snow contributes to its darkening and accelerated snowmelt. Recent field studies have identified dark brown carbon (d-BrC) to contribute 50–75% of shortwave absorption in wildfire smoke. d-BrC is a distinct class of water-insoluble, light-absorbing organic carbon that co-exists in abundance with black carbon (BC) in snow across the world. However, the importance of d-BrC as a snow warming agent relative to BC remains unexplored. We address this gap using aerosol-snow radiative transfer calculations on datasets from laboratory and field measurement. We show d-BrC increases the annual mean snow radiative forcing between 0.6 and 17.9 W m−2, corresponding to different wildfire smoke deposition scenarios. This is a 1.6 to 2.1-fold enhancement when compared with BC-only deposition on snow. This study suggests d-BrC is an important contributor to snowmelt in midlatitude glaciers, where ~40% of the world's glacier surface area resides. [ABSTRACT FROM AUTHOR]

Biophysical processes of forests affect climate through the regulation of surface water and heat fluxes, which leads to further effects through the adjustment of clouds and water cycles. These indirect biophysical effects of forests on clouds and their radiative forcing are poorly understood but highly relevant in the context of large-scale deforestation or afforestation, respectively. Here, we provide evidence for local decreases in global low-level clouds and tropical high-level clouds from deforestation through both idealized deforestation simulations with climate models and from observations-driven reanalysis using space-for-time substitution. The decreased cloud cover can be explained by alterations in surface turbulent heat flux, which diminishes uplift and moisture to varying extents. Deforestation-induced reduction in cloud cover warms the climate, partially counteracting the cooling effects of increased surface albedo. The findings from idealized deforestation experiments and space-for-time substitution exhibit disparities, with global average offsets of, respectively, approximately 44% and 26%, suggesting the necessity for further constraints. This study shows that deforestation locally reduces global low-level and tropical high-level clouds, with the resulting warming partially counteracting the cooling of increased surface albedo. [ABSTRACT FROM AUTHOR]

Global warming during the Last Glacial Termination was interrupted by millennial-scale cool intervals such as the Younger Dryas and the Antarctic Cold Reversal (ACR). Although these events are well characterized at high latitudes, their impacts at low latitudes are less well known. We present high-resolution temperature and hydroclimate records from the tropical Andes spanning the past ~16,800 y using organic geochemical proxies applied to a sediment core from Laguna Llaviucu, Ecuador. Our hydroclimate record aligns with records from the western Amazon and eastern and central Andes and indicates a dominant long-term influence of changing austral summer insolation on the intensity of the South American Summer Monsoon. Our temperature record indicates a ~4 °C warming during the glacial termination, stable temperatures in the early to mid-Holocene, and slight, gradual warming since ~6,000 y ago. Importantly, we observe a ~1.5 °C cold reversal coincident with the ACR. These data document a temperature change pattern during the deglaciation in the tropical Andes that resembles temperatures at high southern latitudes, which are thought to be controlled by radiative forcing from atmospheric greenhouse gases and changes in ocean heat transport by the Atlantic meridional overturning circulation. [ABSTRACT FROM AUTHOR]

Climatic impacts of historical volcanism are principally tied to the eruption size, while observation versus model discrepancies have been commonly attributed to the uncertainties in paleo‐reconstruction or malpresentation of volcanic aerosols in models. Here we present convergent evidence for significant compensation effect of ocean latent heat (LH) in balancing the tropical volcanic‐induced heat loss, by introducing an effective perturbation ratio which is found to decrease with increasing eruption magnitude. Four LH compensation hot spots overlapping with the trade wind regions are identified, together with three western boundary currents regions with intensified LH loss. Comparison between the 1258 Samalas and 1452 Unidentified eruptions suggests considerable modulation of the concurring El Nino‐Southern Oscillation on LH anomaly, which is further verified by CESM large ensemble sensitivity experiments. This study depicts how the interplay between the ocean and the atmosphere could contribute to the overall resilience of the climate system in the face of volcanic disturbances. Plain Language Summary: Large volcanic eruptions drive significant climate perturbation and the resulting cooling generally decreases with increasing eruption size, especially over the surface and upper ocean. In this study we introduce an effective volcanic perturbation ratio, defined as the ratio between the surface net flux anomaly and the volcanic‐induced shortwave flux anomaly, and analyze its relationship with the eruption magnitude. Our results, based on tree‐ring reconstructions and the CESM‐LME simulations, suggest that the effective ratio gradually decreases with the increasing shortwave perturbation, and the reduction of latent heat (LH) flux from ocean to atmosphere balances a significant portion of the volcanic‐induced net surface heat loss. LH varies with wind speeds and vertical specific humidity gradient. We identify four LH compensation effect hot spots which highly overlap with the trade wind regions, and three western boundary currents regions with intensified ocean latent heat loss. We run sensitivity experiments using the 1815 eruption of Mount Tambora as a test case, and find that the ENSO responses accompanying the eruption could modify the LH compensation effect. Key Points: Volcanic‐induced temperature responses do not scale linearly with the eruption sizes in both reconstruction and model simulations with same aerosol ‐ radiative treatmentOcean latent heat (LH) flux plays an important role in compensating volcanic‐induced radiative heat loss which enhance with growing eruption sizeInternal climate variability such as ENSO inserts significant influence on LH's compensation role through the wind‐evaporation‐SST feedback [ABSTRACT FROM AUTHOR]

Ships brighten low marine clouds from emissions of sulfur and aerosols, resulting in visible "ship tracks". In 2020, new shipping regulations mandated an ∼80% reduction in the allowed fuel sulfur content. Recent observations indicate that visible ship tracks have decreased. Model simulations indicate that since 2020 shipping regulations have induced a net radiative forcing of +0.12 Wm−2. Analysis of recent temperature anomalies indicates Northern Hemisphere surface temperature anomalies in 2022–2023 are correlated with observed cloud radiative forcing and the cloud radiative forcing is spatially correlated with the simulated radiative forcing from the 2020 shipping emission changes. Shipping emissions changes could be accelerating global warming. To better constrain these estimates, better access to ship position data and understanding of ship aerosol emissions are needed. Understanding the risks and benefits of emissions reductions and the difficultly in robust attribution highlights the large uncertainty in attributing proposed deliberate climate intervention. Plain Language Summary: Ships have a unique climate effect due to brightening of low marine clouds, resulting in visible "ship tracks". These ship tracks are due to clouds interacting with ship emissions, particularly sulfur. Recently, regulations have drastically reduced allowable ship sulfur emissions. This has resulted in a decrease in observable ship tracks. Modeling and observations indicate that the reduction in ship sulfur emissions could have slightly warmed the planet starting in 2020. These changes are remarkably co‐incident with observed patterns of cloud changes and may have accelerated global warming. Key Points: Recent regulations on ship sulfur emissions have decreased ship tracks and resulted in +0.12 Wm−2 of radiative forcingObserved cloud anomalies are correlated with observed ocean temperature anomalies and shipping radiative forcingReduced ship emissions may have accelerated global warming contributing to recent warm Northern Hemisphere surface temperatures [ABSTRACT FROM AUTHOR]

Global warming has increased the probability of extreme climate events, with compound extreme events having more severe impacts on socioeconomics and the environment than individual extremes. Utilizing the Coupled Model Intercomparison Project Phase 6 (CMIP6), we predicted the spatiotemporal variations of compound extreme precipitation-high temperature events in China under three Shared Socioeconomic Pathways (SSPs) across two future periods, and analyzed the changes in exposed populations and identified influencing factors. From the result, we can see that, the CMIP6 effectively reproduces precipitation patterns but exhibits biases. The frequency of compound event rises across SSPs, especially under high radiative forcing, with a stronger long-term upward trend. Furthermore, the economically developed areas, notably China's southeastern coast and North China Plain, will be hotspots for frequent and intense compound extreme events, while other regions will see reduced exposure. Finally, in the long-term future (2070–2100), there is a noteworthy shift in population exposure to compound events, emphasizing the increasing influence of population factors over climate factors. This highlights the growing importance of interactions between population and climate in shaping exposure patterns. [ABSTRACT FROM AUTHOR]

Aerosol effective radiative forcing (ERF) has persisted as the most uncertain aspect of anthropogenic forcing over the industrial period, limiting our ability to constrain estimates of climate sensitivity and to confidently predict 21st century climate change. Aerosol–cloud interactions are the most uncertain component of aerosol ERF. The 2014–2015 Holuhraun volcanic eruption acted as a large source of sulfur dioxide, providing an opportunistic experiment for studying aerosol–cloud interactions at a climatically relevant scale. We evaluate the observed aerosol-induced perturbation to marine liquid cloud properties inside the volcanic plume in the first month of the eruption and compare the results to those from UKESM1 (UK Earth System Model). In the first 2 weeks, as expected, we find an in-plume shift to smaller and more numerous cloud droplets in both the observations and the simulations. We find an observed increase in liquid water path (LWP) values inside the plume that is not captured in UKESM1. However, in the third week, the in-plume shift to smaller and more numerous cloud droplets is neither observed nor modelled, and there are discrepancies between the observed and modelled response in the fourth week. An analysis of the model simulations and trajectory modelling reveals that air mass history and background meteorological factors can strongly influence aerosol–cloud interactions between the weeks of our analysis. Overall, our study supports the findings of many previous studies: the aerosol impact on cloud effective radius is significant, with differences in the observed and modelled response for in-cloud LWP. [ABSTRACT FROM AUTHOR]

Globally, emissions from aviation affect Earth's climate via complex processes. Contrail cirrus and carbon dioxide emissions are the largest factors contributing to aviation's radiative forcing on climate. Contrail cirrus, like natural cirrus clouds, impacts Earth's climate. Even with the extensive ongoing research, the relative importance of the climate effects of contrails compared to other aviation effects on climate still has major uncertainties requiring further research. Contrail cirrus encompasses linear contrails and the associated cirrus clouds; these are characterized by ice particle properties, e.g., size, concentration, mixing, extinction, ice water content, optical depth, geometrical depth, and cloud coverage. The climate impact of contrails may intensify due to projected increases in air traffic. The radiative forcing from global contrail cirrus has the potential to triple and could reach as much as 160 mWm-2 by 2050. This projection is based on anticipated growth in air traffic and a potential shift to higher altitudes. The future climate impact of contrail cirrus is influenced by factors like the magnitude and geographical spread in air traffic, advancements in fuel efficiency, the effects of the use of alternative fuels, and the effects of the changing climate on the background atmosphere. This study reviews the microphysical processes affecting contrail formation and the aging of contrails and contrail cirrus. Furthermore, the study explores global observational datasets for contrails, current analyses, and future projections and will aid in evaluating the effectiveness and tradeoffs associated with various mitigation strategies. The research highlights gaps in knowledge and uncertainties while outlining research priorities for the future. [ABSTRACT FROM AUTHOR]

The warming radiative forcing effect (RFE) derived from atmospheric amines attracts a lot of attention because of their contribution to brown carbon. Herein, the enhanced influence of amines (methyl-, dimethyl-, and trimethylamine) on cooling RFE of mineral particles is first confirmed at visible wavelengths. Present results state heterogeneous uptake and oxidation reactions of atmospheric amines are feasible on mineral particles under clean and polluted conditions, which are proven by related thermodynamics and kinetics data obtained using combined classical molecular dynamics and density function theory methods. Based on mineral particles, simple forcing efficiency (SFE) results explain that amine uptake induces at least 11.8 %–29.5 % enhancement on cooling RFE of amine–mineral particles at visible wavelengths. After amine heterogeneous oxidation, oxidized amine–mineral particle cooling RFEs are furthermore enhanced due to increased oxygen contents. Moreover, oxidized amine–mineral particles under clean conditions show 27.1 %–47.1 % SFE increments at 400–600 nm, which is at least 11.3 % higher than that of itself under polluted conditions, due to high-oxygen-content product formation through amine autoxidation. Our results suggest cooling RFE derived from atmospheric amines can be equally important to their warming RFE on the atmosphere. It is necessary to update the heterogeneous oxidation mechanism and kinetics data of amines in atmospheric models in order to accurately evaluate the whole RFE caused by amines on the atmosphere. [ABSTRACT FROM AUTHOR]