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Time series of radiative cooling of the upper mesosphere and lower thermosphere (UMLT) by carbon dioxide (CO2) are examined for evidence of trends over 20 years. Radiative cooling rates in K day−1 provided by the SABER instrument are converted to time series of infrared power radiated from three distinct layers between 0.1 hPa and 0.0001 hPa (65–105 km). Linear regression against time and a predictor for solar variability provides estimates of the trend in exiting longwave radiation (ELR) from these layers. Trends in ELR are not significantly different from zero at 95% or 99% confidence in each layer. These results demonstrate energy conservation in the UMLT on decadal time scales and show that the UMLT continues to radiate the same amount of energy it receives despite cooling and contracting over two decades. These results are enabled by the long‐term stability of the SABER instrument calibration. Plain Language Summary: The Earth's upper mesosphere and lower thermosphere (UMLT) is the region between approximately 65 and 105 km in altitude. Infrared radiation emitted by CO2 is a fundamental component of the energy budget of the UMLT. Carbon dioxide (CO2) is increasing in this region. The amount of infrared energy emitted by CO2, over time, must balance the amount of energy from sunlight that is absorbed in the UMLT. Observations from orbiting satellites over the past two decades have shown that the temperature in the UMLT is decreasing due to the increasing CO2. A decreasing temperature implies a decrease in infrared energy emitted by an object. Energy conservation, however, requires that the amount of infrared energy radiated from the UMLT balance the amount of energy received, regardless of the temperature. In this paper we show that there is no significant long‐term change with time (i.e., zero trend) in the infrared energy emitted from the UMLT even though the UMLT temperature has been decreasing for the last 20 years (and longer) due to increasing CO2. This result confirms that energy is conserved in a cooling and contracting UMLT. Key Points: Time series of infrared power (W) radiated by CO2 from the upper mesosphere & lower thermosphere (UMLT) are developedTrends in radiated power are not different from zero at 95% or 99% confidence in the three layers examined between 0.1 and 0.0001 hPaIncreasing CO2 reduces the UMLT temperature while radiating the same amount of energy over time, thus conserving energy [ABSTRACT FROM AUTHOR]

The ability of satellite instruments to accurately observe long‐term changes in atmospheric temperature depends on many factors including the absolute accuracy of the measurement, the stability of the calibration of the instrument, the stability of the satellite orbit, and the stability of the numerical algorithm that produces the temperature data. We present an example of algorithm instability recently discovered in the temperature data set from the SABER instrument on the NASA TIMED satellite. The instability resulted in derived temperatures that were substantially colder than anticipated from mid‐December 2019 to mid‐2022. This algorithm‐induced change in temperature over 1 to 2 years corresponded to the expected temperature change over several decades from increasing anthropogenic CO2. This paper highlights the importance of algorithm stability in developing Geospace Data Records (GDRs) for Earth's mesosphere and lower thermosphere. A corrected version (Version 2.08) of the temperatures from SABER is described. Plain Language Summary: Instruments on Earth orbiting satellites offer the opportunity to detect long‐term changes in atmospheric temperature. Many factors may affect the ability to identify actual long‐term changes in the temperature and to distinguish these from changes in the instrument or from unintended changes in the algorithm that produces the temperature data from the instrument observations. SABER is an instrument on a NASA satellite that has been observing temperatures from 15 to 110 km (10–68 miles) in altitude for over 20 years. An "instability" in the scientific algorithm used to derive temperature from the instrument observations was recently discovered, beginning in late 2019. An unintended change was made in a parameter central to the derivation of temperature from SABER measurements. The consequence was that the atmospheric temperatures between 85 and 110 km (51–68 miles) from 2020 onward were several degrees colder than they would have been without the unintended change. This has been corrected and an updated version of the SABER temperatures and all other SABER data products, called Version 2.08, is now publicly available. Key Points: The concept of Geospace Data Records (GDRs) and their relevance to accurate detection of long‐term change is introducedAlgorithm instability in a GDR of the 20‐year record of SABER temperatures between 85 and 110 km is described and correctedThe field of Geospace Climate is emerging as a frontier with scientific and economic relevance. Accurate GDRs are essential to both [ABSTRACT FROM AUTHOR]

The ability of global transport models to go up in resolution becomes discriminating for greenhouse gas atmospheric inversions. This paper describes the porting on Graphics Processing Units of the global transport model currently used in the European operational Copernicus Atmosphere Monitoring Service (CAMS) for CO2 and N2O inversions. It represents an important milestone to achieve sub‐degree resolution. The code includes not only the direct model but also its tangent‐linear and its adjoint versions which are needed in variational inversions. Tests were carried out for CO2 at a resolution of 2.50° in longitude, 1.27° in latitude and 79 layers in the vertical, corresponding to 1,626,768 3D cells, 4.5 times more than the current standard resolution of the model used in the CAMS reanalyzes. A month's worth of computation of the tangent‐linear and of the adjoint versions now takes 2.5 min, including 50 s for reading meteorological data. Plain Language Summary: Atmospheric transport models are intensively used to infer global greenhouse gas emissions and removals, from atmospheric measurements: a single global analysis involves repeated and long transport simulations. This intensive use has limited the spatial resolution of such analyses despite an increasing need for national greenhouse gas budgets, despite an increasing number of corresponding space‐based observations at kilometer resolution, and despite an increasing number of high‐quality surface measurements made in sites marked by strong local influences. For the global transport model currently used in the European operational atmosphere monitoring service, our objective within the next 5 years is to make it reach a resolution of about 50 km over the whole globe. This paper describes an important milestone in this direction with the porting of this transport model on hardware components initially developed for video display but now used for high performance computing. Tests were carried out at a resolution of 2.50° in longitude, 1.27° in latitude and 79 layers in the vertical, corresponding to 4.5 times more 3D cells than the current standard resolution of the model. The direct model itself now takes less time than reading the input meteorological data. Key Points: The workload in a Eulerian transport model on a longitude‐latitude grid can be embarrassingly parallel enough to run efficiently on a Graphics Processing UnitThe transport model of the Laboratoire de Météorologie Dynamique was run in a new grid of 1,626,768 3D cellsThe calculation time in the direct version of the model is now less than the time to read the input meteorological data [ABSTRACT FROM AUTHOR]

Earth experienced the strongest geomagnetic storm in 20 years over 10–13 May 2024. The Ap and Dst geomagnetic indices were 273 and −291.94 nT on 11 May. The Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument on the Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics satellite observed significant enhancement in thermospheric infrared emission at 15, 5.3, 4.3, and 1.27 μm. On 11 May the daily global power radiated by nitric oxide (NO) at 5.3 μm was 1.41 TW and by carbon dioxide (CO2) at 15 μm was 1.35 TW. These are the largest single day power values observed by SABER in 22 years and the first time the daily power radiated by NO exceeded that of CO2. The total infrared power (above background) radiated during the storm was 2.64 TW (2.28 × 1017 J). Significant enhancement in limb radiance observed at 4.3 μm (to 250 km tangent height) is likely indicative of NO + formation during the storm. Plain Language Summary: The layer of Earth's atmosphere above 100 km is referred to as the "thermosphere" and can be thought of as the boundary between the space environment above it and the atmosphere below it. In early May 2024, an active sunspot ejected a significant amount of charged particles (protons and electrons) within a strong magnetic field toward Earth. This "coronal mass ejection" or CME encountered Earth's high atmosphere 10–13 May, causing a significant "geomagnetic storm" that significantly heated the thermosphere and modified its chemical composition. The effects of this storm were observed by the SABER instrument which is flying on the NASA Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics satellite. Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) observes infrared light emitted by carbon dioxide, nitric oxide, molecular oxygen, and ionized nitric oxide produced during the storm. The 10–13 May storm caused the global average infrared radiation levels to increase by nearly a factor of 10 at some wavelengths in 1 day. The storm and the infrared radiation levels dissipated back to pre‐storm levels over the next 2 days. This storm was the third strongest in terms the power (2.64 trillion watts) radiated by the thermosphere in the past 22 years observed by SABER. The observed enhancements in infrared radiation at multiple wavelengths offer the opportunity to more fully understand how the Sun interacts with and influences Earth's highest atmosphere. Key Points: The daily Ap (273 nT) and Dst (−291.94 nT) indexes on 11 May 2024 were the largest since the launch of the Thermosphere‐Ionosphere‐Mesosphere Energetics and Dynamics (TIMED) satellite in December 2001The power radiated 11 May by NO and CO2 exceeded 1 TW each and the NO power (1.41 TW) exceeded the CO2 power (1.35 TW) for the first timeTotal storm radiated power, above background, is 2.64 TW (2.28 × 1017 J), making it the third strongest storm during the TIMED era [ABSTRACT FROM AUTHOR]

Twenty‐two years (2002–2023) of infrared radiative cooling rate data derived from the SABER instrument on the NASA TIMED satellite are presented. Global daily and global annual infrared power (Watts, W) emitted by nitric oxide (NO) and carbon dioxide (CO2) illustrate the variability of the geospace environment on timescales from days to decades. The 11‐year solar cycle (SC) is evident in the global power data and in vertical profiles of infrared cooling rates (nW/m3). The global annual power radiated by NO and CO2 are larger in 2023 than at any time since 2003 and 2002, respectively. The to‐date peak in NO infrared power in SC 25 is larger than in SC 24, is comparable to SC 20, but is less than in SCs 18–19 and 21–23. Two geomagnetic storms in 2023 radiated more than 1 TW and are in the top 10 strongest storms observed by SABER. Plain Language Summary: SABER is an instrument on the NASA TIMED satellite, launched in December 2001, and is still operating nominally in 2024. SABER measures the amount of infrared energy emitted by the nitric oxide (NO) molecule and the carbon dioxide (CO2) molecule from Earth's thermosphere, the region of atmosphere above 100 km altitude. These measurements are used to derive the amount of power (Watts) radiated globally by these two molecules on daily to annual timescales. The infrared power data is then analyzed to determine its variability in the lower thermosphere from daily to decadal time scales. Of particular interest is the influence of the well‐known 11‐year cycle of solar activity which is readily evident in the infrared power data. SABER also observes the effects of geomagnetic storms, identifying two strong storms in 2023 that each produced over 1 TW of infrared power. Key Points: SABER on TIMED has observed infrared radiation from the thermosphere since January 2002, for 22 years, the equivalent of two solar cyclesGlobal annual infrared power radiated by NO and CO2 are larger in 2023 than any time since 2003 and 2002, respectivelyTwo geomagnetic storms in 2023 exceeded 1 TW of radiated energy, above background, and are in the top 10 strongest storms observed [ABSTRACT FROM AUTHOR]

Geologic storage of CO2 and H2 are climate‐positive techniques for meeting the energy transition. While similar formations could be considered for both gases, the flow dynamics could differ due to differences in their thermophysical properties. We conduct a rigorous pore‐scale study of water/CO2 and water/H2 systems at relevant reservoir conditions in a Bentheimer rock sample using the lattice Boltzmann method to quantify the effects of capillary, viscous, inertial, and wetting forces during gas invasion. At similar conditions, H2 invasion is weaker compared to CO2 due to unfavorable viscosity ratios. Increasing flow rate, however, increases the breakthrough saturation for both gas systems in the range of capillary numbers studied. At isolated conditions of flow rate, viscosity ratio, and wettability, local inertial effects are found to be critical and show consistent increase in the invaded gas saturation. The effect of inertial forces persits for both gases across all field conditions tested. Plain Language Summary: We present a numerical investigation at the pore‐scale to contrast the dynamics of fluid transport for water/CO2 and water/H2 systems at reservoir conditions. Understanding such dynamics is critical to evaluate the feasibility of the geologic storage of these gases (long‐term storage of CO2 and short‐term, periodic storage of H2), which are being researched as green techniques for mitigating climate change. Through a systematic analysis, we demonstrate the importance of pore‐scale mechanisms like local inertial effects which are not well studied in the literature. These effects cause fast fluid motion and consistently allow more fluid to be injected in the rock independently from the other physical effects. Such effects should be carefully accounted for when modeling these systems for large‐scale gas storage. Key Points: At similar reservoir conditions, invasion of H2 is found to be weaker as opposed to CO2 due to unfavorable viscosity ratiosAmong the tested reservoir scenarios, P/T conditions for the saline aquifer provided the largest amount of invaded gas until breakthroughLocal inertial effects control the dynamics of the invading gas across different wettability, viscosity ratios, and capillary numbers [ABSTRACT FROM AUTHOR]

The equilibrium climate sensitivity estimated from different sources is inconsistent due to its dependence on the surface warming pattern. Cloud feedbacks have been identified as the major contributor to this so‐called pattern effect. We find a large unforced pattern effect in CERES data, with cloud feedback estimated from two consecutive 125‐month periods (March 2000–July 2010 and August 2010–December 2020) changing from −0.45 ± 0.85 to +1.2 ± 0.78 W/m2/K. When comparing to models, 27% of consecutive 10‐year segments in CMIP6 control runs have differences similar to the observations. We also compare the spatial patterns in the CERES data to those in climate models and find they are similar, with the East Pacific playing a key role. This suggests that the impact of the unforced pattern effect can be significant and that models are capable of reproducing its global‐average magnitude. Plain Language Summary: Climate sensitivity, the amount of surface warming in response to increasing carbon dioxide, is often used as a metric to quantify the severity of climate change. The estimates of climate sensitivity from different sources, however, are not consistent. The main reason is that the response of clouds to surface temperature changes, called cloud feedback, depends on the spatial pattern of surface warming. We investigate the impact of this so‐called pattern effect on cloud feedback in observations and climate models. Here we find that the observed cloud feedbacks calculated from two consecutive decades have significant differences in both global mean value and spatial pattern, and the climate models produce similar results. This highlights the fact that differences of surface warming patterns can have large influences on cloud feedbacks, and climate models have the ability to reproduce what happened in the real world. Key Points: The difference in cloud feedback between two ∼10‐year periods inferred from CERES is 1.6 ± 1.1 W/m2/KThe distinct difference in cloud feedback verifies the existence of a large unforced pattern effectCMIP6 models produce unforced pattern effects of similar magnitude and spatial structure to those seen in the observations [ABSTRACT FROM AUTHOR]

Ocean mesoscale flows significantly influence nutrient distribution and biological productivity, yet the scarcity of eddy‐permitting observational data sets and climate modeling hinders understanding their role in carbon sequestration. Using an eddy‐resolving global simulation, this study investigates the significance of ocean mesoscales in air‐sea carbon dioxide (CO2) exchange. Results show over 30% of CO2 flux variance in energetic regions attributed to flows with horizontal scales smaller than 2°. Mesoscale flows can drive a cumulative CO2 flux that is either a net carbon sink or source depending on region, with magnitudes on the order of 105 tonnes of carbon per year. Variations in this mesoscale‐related CO2 flux are correlated with local relative vorticity and the background gradient of ocean partial pressure of CO2. This analysis underscores the importance of considering ocean mesoscales in monitoring carbon flux, highlighting the need to explore the influence of increasing eddy activity on carbon uptake in a changing climate. Plain Language Summary: This study investigates the influence of ocean flows with horizontal scales of tens to hundreds of kilometers (called mesoscale flows) on the exchange of carbon dioxide between the ocean and the atmosphere using a state‐of‐the‐art high resolution global simulation. It is shown that in certain parts of the ocean, more than 30% of the variability in carbon dioxide exchange at the ocean surface is linked to these relatively small‐scale ocean motions. It is further revealed that mesoscale flows can lead to a significant gain or loss of carbon from the ocean at the regional level, and that depends on how the flow is oriented with respect to the background distribution of carbon in the ocean. Gaining a better understanding of how the mesoscale flow features influence air‐sea carbon dioxide exchange is crucial for understanding the ocean's role in the carbon cycle and its ability to absorb and store carbon in a changing climate. Key Points: Over 30% of air‐sea CO2 flux variability is attributed to ocean spatial scales smaller than 2° in eddy‐rich regionsThe cumulative mesoscale‐induced CO2 flux is on the order of 105 tonnes of carbon per yearMesoscale‐related CO2 flux is governed by both relative vorticity and the background gradient of partial pressure of CO2 [ABSTRACT FROM AUTHOR]

Each year, the biological carbon pump (BCP) transports large quantities of carbon from the ocean surface to the interior. The efficiency of this transfer varies geographically, and is a key determinant of the atmosphere‐ocean carbon dioxide balance. Traditionally, the attention has been focused on explaining perceived geographical variations in this transfer efficiency (TE) in an attempt to understand it, an approach that has led to conflicting results. Here we combine observations and modeling to show that the spatial variability in TE can instead be explained by the seasonal variability in carbon flux attenuation. We also show that seasonality can explain the contrast between known global estimates of TE, due to differences in the date and duration of sampling. Our results suggest caution in the mechanistic interpretation of annual‐mean patterns in TE and demonstrates that seasonally and spatially resolved data sets and models might be required to generate accurate evaluations of the BCP. Plain Language Summary: Each year, marine phytoplankton convert carbon dioxide into millions of tonnes of organic carbon with a fraction of it reaching the deep ocean, where it can remain for hundreds of years. The efficiency of this surface‐to‐depth carbon transfer is therefore a key determinant of the atmosphere‐ocean carbon dioxide balance. However, the variability of this transfer efficiency (TE) and its underlying causes are poorly understood, to the extent that different studies report contradicting results. We show that the existence of seasonal variability in the attenuation of sinking carbon particles can explain the observed spatial variability in annual TE and reconcile with the literature. Our findings suggest caution in interpreting results from sparse but time‐varying data sets, highlighting that seasonal variability should be considered when studying the oceanic carbon cycle. Key Points: Spatial variability in carbon transfer efficiency (TE) can be generated solely by the seasonality of flux attenuation informed by observationsSeasonality in flux attenuation can reconcile contrasting TE maps reported in the literatureResolving and understanding seasonality are key for an accurate evaluation of the biological carbon pump under climate change [ABSTRACT FROM AUTHOR]

The eastern equatorial Pacific (EEP) is a source of atmospheric CO2 during the last deglaciation, but the associated oceanic dynamics in the broader low‐latitude Pacific is not fully understood. Here, we report 30,000‐year‐long surface and subsurface pCO2 records for the western equatorial Pacific (WEP), based on boron isotopes in two planktonic foraminiferal species from core MD10‐3340. Our results show that the WEP surface became a significant atmospheric CO2 sink despite that its subsurface waters were enriched by CO2 during the last deglaciation to early Holocene. Combined with EEP proxy data and model results, we suggest that a deglacial‐early Holocene zonal seesaw of sea‐air CO2 exchange across the equatorial Pacific led to a net CO2 outgassing much greater than the modern situation. This can be ascribed to strengthened Subtropical‐Tropical Circulation, resulting in stronger upper ocean stratification in the WEP concurrent with enhanced upwelling of CO2‐rich subsurface waters in the EEP. Plain Language Summary: The modern equatorial Pacific is one of the main sources of CO2 released from the ocean into the atmosphere, with the strongest outgassing in the central‐eastern part and nearly neutral state in the western part. However, according to our boron isotope data of planktonic foraminifera, the western equatorial Pacific was turned into a significant atmospheric CO2 sink rather than a source between 16,000 and 7,000 years ago. At the same time, the eastern equatorial Pacific released more CO2, resulting in a zonal seesaw of sea‐air CO2 exchange across the equatorial Pacific that generally accelerated the CO2 outgassing and favored global warming. We further argue that these deglacial‐early Holocene CO2 changes were ascribed to the strengthened Subtropical‐Tropical Circulation, in which the accelerated eastward Equatorial Undercurrent leads to stronger upper ocean stratification in the WEP, and also converges seawater CO2 into the upper thermocline and finally releases at the surface EEP. This finding helps us understand and quantitatively assess future global carbon budget, particularly how the upper ocean circulation modulates the sea‐air CO2 flux in a manner similar to the past. Key Points: Strengthened upper ocean stratification led to significant atmospheric CO2 sink in the western equatorial Pacific during ∼16–7 kaThe zonal seesaw of sea‐air CO2 exchange in the equatorial Pacific resulted in a net CO2 outgassing much greater than the modern situationThe sea‐air CO2 exchange across the equatorial Pacific was modulated by the Subtropical‐Tropical Circulation on orbital timescales [ABSTRACT FROM AUTHOR]

The impact of rising atmospheric CO2 (pCO2atm) from anthropogenic activities on pH, dissolved inorganic carbon, carbonate mineral saturation, and aluminum (Al) speciation is evaluated for 18 northeastern USA lakes using polythermal, sliding activity reaction path models. pCO2atm was forced using two scenarios from the IPCC's Sixth Assessment Report in which pCO2atm attains either 600 or 1,100 ppm in 2,100. Results suggest pH will decrease 0.15 and 0.32 pH units, aCO32− ${a}_{\mathrm{C}{\mathrm{O}}_{3}^{2-}}$ will decrease 24% and 49%, and Ωaragonite will decrease 21% and 45%, respectively. These changes are of the same magnitude as those expected for the oceans. The effects of rising pCO2atm on sub‐lethal 20% effect concentrations (i.e., EC20) of Al for brook trout (Salvelinus fontinalis) are evaluated with the biotic ligand model, which indicates Al toxicity effects will increase as pH decreases. These changes could reverse gains in water quality and fisheries health achieved since implementation of the Clean Air Act. Plain Language Summary: Fossil fuel consumption, deforestation, and changing land use are all contributing to rising CO2 concentrations in the atmosphere. In addition to atmospheric warming and climate impacts, rising atmospheric CO2 will also promote acidification of ocean surface waters, which is expected to have detrimental effects on marine ecosystems. Much less is known about how rising atmospheric CO2 will impact terrestrial freshwaters such as lakes and streams. We developed reaction path models for 18 lakes from northeastern USA to investigate possible impacts of rising atmospheric CO2 and temperature on these systems. The models indicate that acidification of these lakes could be of similar magnitude to ocean acidification. The saturation state of these lakes with respect to the mineral aragonite, which is important for calcifying organisms, will also decrease. Finally, acidification will alter aluminum (Al) speciation, which could augment toxic effects from this metal on important sport fish like brook trout, possibly reversing gains made in water quality since implementation of the Clean Air Act decreased the input of strong acids via acid rain. Key Points: Rising atmospheric CO2 from anthropogenic activities will lead to acidification of lakes in the northeastern USA during the 21st centuryAcidification will alter Al speciation and lower concentrations required to cause a sub‐lethal response in 20% of the resident brook troutAnthropogenic‐sourced CO2 could reverse gains in lake water quality realized since implementation of the Clean Air Act [ABSTRACT FROM AUTHOR]

Ocean ventilation is a key mechanism for transporting anthropogenic CO2 (CANTH) from the ocean surface toward its interior. We investigated the link between ocean ventilation and CANTH increase in the East Sea using data from surveys conducted in 1992, 1999, 2007, and 2019. Between 1992 and 1999, the East Sea Intermediate Water (300−1,500 m) accumulated CANTH at a rate of 0.3 ± 0.1 mol C m−2 yr−1. However, in the subsequent period (1999−2007) this rate decreased to <0.1 ± 0.1 mol C m−2 yr−1. There was a resurgence in the CANTH increase rate between 2007 and 2019, reaching 0.4 ± 0.1 mol C m−2 yr−1. The East Sea Intermediate Water ventilation changes, inferred from the changes in water column O2 level and the Arctic Oscillation‐driven winter surface temperature in the deep water formation region, were responsible for the periodic decline and recovery in CANTH increase. Plain Language Summary: Ocean ventilation plays a crucial role in transporting anthropogenic CO2 from the surface toward the deep ocean interior. In the East Sea, demonstrating a direct causal relationship between ocean ventilation and anthropogenic CO2 uptake has remained elusive because of limited observational evidence. To address this knowledge gap, we conducted direct measurements of carbon parameters during four surveys in the East Sea that together spanned nearly three decades (1992, 1999, 2007, and 2019). Our investigation revealed a cyclical pattern of waxing and waning in the rate of anthropogenic CO2 increase within the East Sea over the past three decades. Significantly, we demonstrated that fluctuations in the ventilation of the East Sea were the primary driving force behind the decadal changes in anthropogenic CO2 increase. Furthermore, our research unveiled a potential connection between the periodic fluctuations in surface temperature linked to the Arctic Oscillation and variations in East Sea ventilation. Our findings shed light on the intricate dynamics between ocean ventilation, winter sea surface temperature patterns, and CO2 uptake in the East Sea. Key Points: The rate of anthropogenic CO2 increase in the East Sea showed cyclic trendsThe cyclic trends of anthropogenic CO2 increase were linked to the Arctic OscillationOur findings shed light on changes in anthropogenic CO2 uptake and transport by the global ocean in response to climate variations [ABSTRACT FROM AUTHOR]

Aqueous fluids are essential to extract slab‐trapped carbon at forearc to subarc depths and interpret the decarbonation efficiency of global subduction zones. A large amount of carbon survives beyond subarc depths. The behavior of such carbon, however, remains unclear owing to a lack of experimental studies. Here, we investigate the decarbonation behavior of carbonated oceanic crust containing different amounts of H2O at pressures from subarc depths to the top of the mantle transition zone. We find that calcium‐rich carbonatitic liquids can form at temperatures of ∼950–1,150°C at depths below ∼150–300 km, corresponding to warm/cold thermal regimes in most subduction zones. Therefore, hydrous carbonatitic liquids should be pervasive in subduction zones, while extensive dehydration reactions and fluid activities are critical for creating carbonatitic liquids and preventing most of the surviving carbon from being subducted into the deep Earth. Plain Language Summary: The deep recycling of CO2 and H2O in subduction zones is key for understanding the habitability and climate change of Earth. Here, we use a high‐pressure and high‐temperature instrument, the multi‐anvil press, to investigate the fate of subducted carbon in oceanic crust. We find that H2O can depress the decarbonation temperature of subducted oceanic crust by hundreds of °C. Most of the surviving carbon that descends to depths below ∼150 km may be released into the upper mantle by hydrous carbonatitic liquids, rather than entering the lower mantle. The fate of such carbon has not been well constrained, but it is important to understand this process because CO2 released into the convecting upper mantle is key to constraining the origin of the deep carbon reservoir. Key Points: Effect of H2O on the decarbonation behavior of subducted oceanic crust was investigatedHydrous carbonatitic liquids can form at ∼950–1,150°C at depths below ∼150 kmFluid activity is critical for removing most of the surviving carbon beyond subarc depths [ABSTRACT FROM AUTHOR]

Air‐sea exchange of carbon dioxide (CO2) in the Southern Ocean plays an important role in the global carbon budget. Previous studies have suggested that flow around topographic features of the Southern Ocean enhances the upward supply of carbon from the deep to the surface, influencing air‐sea CO2 exchange. Here, we investigate the role of seafloor topography on the transport of carbon and associated air‐sea CO2 flux in an idealized channel model. We find elevated CO2 outgassing upstream of a seafloor ridge, driven by anomalous advection of dissolved inorganic carbon. Argo‐like Lagrangian particles in our channel model sample heterogeneously in the vicinity of the seafloor ridge, which could impact float‐based estimates of CO2 flux. Plain Language Summary: The Southern Ocean, the ocean surrounding Antarctica, contributes significantly to carbon exchange between the global ocean and the atmosphere, which in turn matters for climate change. Here, we use a simplified model of the Southern Ocean to see how mountain ranges on the sea floor influence the carbon exchange at the ocean‐atmosphere interface. We find that the seafloor mountain ranges lead to more carbon exchange. Floating carbon sensors in our model ocean may under or over sample the water near the mountains and this can affect the carbon exchange that they report. Key Points: We examine the localized patterns of air‐sea carbon dioxide (CO2) fluxes in an idealized Southern Ocean‐like model with simple biogeochemistryWe find intense sea‐air CO2 fluxes upstream of seafloor topography driven by anomalous advection of inorganic carbonDue to the topography, uncertainty in the flux is highly sensitive to sampling network design [ABSTRACT FROM AUTHOR]

Subantarctic Mode Water (SAMW) is one of the most important water masses globally in taking up anthropogenic heat and carbon dioxide. However, its long‐term changes in response to varying climatic conditions are not well understood. We use an ocean state estimate to analyze SAMW volume budgets for the period 1992 to 2017. They reveal a decadal SAMW volume reorganization comparable to the long‐term trend in Indian Ocean, and a multi‐decadal volume reorganization exceeding the long‐term trend in the Pacific. In both sectors, the SAMW reorganization exhibits a two‐layer density structure, with compensating volume changes of lighter and denser SAMW, driven by heat flux changes in the Indian sector and central Pacific and freshwater flux changes in the southeast Pacific. This variability is governed by a cumulative effect of surface flux anomalies associated with the Interdecadal Pacific Oscillation. Shorter‐term trends observed during the Argo period are largely explained by this variability. Plain Language Summary: Subantarctic Mode Water (SAMW) is a voluminous water mass that forms on the equatorward side of the Antarctic Circumpolar Current, and plays a fundamental role in the uptake and storage of anthropogenic heat and carbon dioxide. Measurements taken by a fleet of robotic instruments, that drift in the ocean, documented a volume increase of lighter SAMW and a volume decrease of denser SAMW since the mid‐2000s. However, the drivers of this variability are still not completely understood. Using the results of a model that assimilates all observations, also prior to this period, we show that the change in SAMW volume primarily arises from fluctuations on timescales of a decade or longer. These decadal to multidecadal fluctuations exhibit a reorganization of volume from lighter to denser SAMW and vice versa, with nearly compensating changes. They are primarily driven by atmospheric forcing variations governed by teleconnections from the tropical Pacific. We find that the SAMW variability does not only respond to the instantaneous atmospheric forcing, but the ocean acts as an integrator of the atmospheric forcing and has a memory of atmospheric conditions in previous years. Key Points: Decadal fluctuations of Subantarctic Mode Water volume regionally exceed long‐term trends and exhibit compensating volume changes of lighter and denser waterThis two‐layer density reorganization is imposed by surface heat (Indian, central Pacific) and freshwater (southeast Pacific) fluxesThe reorganization is significantly impacted by the Interdecadal Pacific Oscillation, inducing a variability dipole in each basin [ABSTRACT FROM AUTHOR]

Mesoscale eddies are abundant in the global oceans and known to affect oceanic and atmospheric conditions. Understanding their cumulative impact on the air‐sea carbon dioxide (CO2) flux may have significant implications for the ocean carbon sink. Observations and Lagrangian tracking were used to estimate the air‐sea CO2 flux of 67 long lived (>1 year) mesoscale eddies in the South Atlantic Ocean over a 16 year period. Both anticyclonic eddies originating from the Agulhas retroflection and cyclonic eddies originating from the Benguela upwelling act as net CO2 sinks over their lifetimes. Anticyclonic eddies displayed an exponential decrease in the net CO2 sink, whereas cyclonic eddies showed a linear increase. Combined, these eddies significantly enhanced the CO2 sink into the South Atlantic Ocean by 0.08 ± 0.04%. The studied eddies constitute a fraction of global eddies, and eddy activity is increasing; therefore, explicitly resolving eddies appears critical when assessing the ocean carbon sink. Plain Language Summary: Ocean mesoscale eddies can form when part of a main current becomes separated or through internal ocean instabilities which form circular rotating currents that propagate across the oceans. These eddies last from weeks to years and can modify the ocean properties of the water captured within them, which in turn affects the net exchange of carbon between this water and the atmosphere. Little is known about how these eddies modify the absorption of carbon across the global ocean, collectively referred to as the ocean carbon sink, despite them being ubiquitous features of the global oceans. Using in situ and satellite‐based observations, we show that eddies in the South Atlantic Ocean enhance the absorption of carbon from the atmosphere, thus modifying the ocean to be a stronger net sink of carbon. These results are important as they quantify how much eddies contribute to the absorption of carbon from the atmosphere to the ocean, and highlight the need to include eddies when assessing ocean carbon budgets. Key Points: Satellite and in situ observations with Lagrangian tracking were used to estimate the cumulative CO2 flux of long lived mesoscale eddiesDrivers of the pCO2(sw) variability significantly changed over the anticyclonic eddies lifetime but did not in cyclonic eddiesBoth anticyclonic and cyclonic eddies enhance the CO2 sink into the South Atlantic Ocean [ABSTRACT FROM AUTHOR]

Arctic warming alters land‐to‐sea fluxes of nutrients and organic matter, which impact air‐sea carbon exchange. Here we use an ocean‐biogeochemical model of the southeastern Beaufort Sea (SBS) to investigate the role of Mackenzie River biogeochemical discharge in modulating air‐sea CO2 fluxes during 2000–2019. The contribution of six biogeochemical discharge constituents leads to a net CO2 outgassing of 0.13 TgC yr−1, with a decrease in the coastal SBS carbon sink of 0.23 and 0.4 TgC yr−1 due to riverine dissolved organic and inorganic carbon, respectively. Years with high (low) discharge promote more CO2 outgassing (uptake) from the river plume. These results demonstrate that the Mackenzie River modulates the capacity of the SBS to act as a sink or source of atmospheric CO2. Our work suggests that accurate model representation of land‐to‐sea biogeochemical coupling can be critical for assessing present‐day Arctic coastal ocean response to the rapidly changing environment. Plain Language Summary: We modeled the discharge of freshwater and six biogeochemical constituents from the Mackenzie River into the southeastern Beaufort Sea to study their impact on ocean‐atmosphere carbon dioxide (CO2) fluxes during 2000–2019. We find that biogeochemical constituents from river runoff promote CO2 outgassing to the atmosphere in the river plume region. Dissolved inorganic carbon is the main contributor to this phenomenon, with river discharge events driving pulses of intense CO2 outgassing during ice‐free periods. Our results show that the capacity of the SBS to act as an atmospheric CO2 sink or source is strongly related to interannual variability in biogeochemical river discharge. Our results highlight the increased coupling of Arctic land‐ocean biogeochemical systems under rapid environmental changes. Key Points: Mackenzie River biogeochemical discharge decreases the southeastern Beaufort Sea carbon sinkTerrestrial dissolved inorganic carbon (DIC) is the primary driver of outgassing events in the SBS, followed by terrestrial DOCInterannual variability in river discharge modulates localized air‐sea CO2 flux [ABSTRACT FROM AUTHOR]

Satellite XCO2 retrievals could improve the estimates of surface carbon fluxes, but it remains unknown on what scales these estimates are robust. Here, we use the time‐dependent Bayesian synthesis top‐down method and prior net ecosystem exchanges (NEEs) from 12 terrestrial biosphere models (TBMs) to infer the monthly carbon fluxes of 51 land regions with constraints by GOSAT XCO2 retrievals. We find that the uncertainty (standard deviation of 12 TBMs) reduction rates (uncertainty reduction rate (URR)) decrease significantly at decreasing spatial scales. On the continental‐scale, the mean URR is about 57%, and the annual and seasonal cycle estimates of NEE are rather robust. The evaluation shows that the posterior CO2 concentrations are significantly improved at the continental scale. Our study suggests that the GOSAT XCO2 can only promise a robust continental‐scale NEE estimate, and improving the XCO2 accuracy is an effective way to achieve robust estimates on smaller scales under current spatial coverage. Plain Language Summary: Remotesensing‐based CO2 measurement can improve the estimates of surface carbon fluxes due to its relatively well global coverage, but it remains unknown on what spatial scales the satellite observation could provide a robust estimate. Here, net ecosystem exchanges (NEEs) from 12 terrestrial biosphere models (TBMs) of 51 land regions for the period of 2011–2014 are constrained using GOSAT XCO2 retrievals, and the uncertainty (standard deviation of 12 TBMs) reduction rates (uncertainty reduction rate (URR)) at different spatial scales are analyzed. We find that (a) from the whole globe to the mean of 51 regions, the URR decreases from 95% to 16%. (b) On the continental‐scale, the mean URR is about 57%, and the annual NEEs in Asia, N. America, Europe, S. America, Africa, and Australia are estimated to be −2.07 ± 0.25, −0.85 ± 0.09, −0.76 ± 0.21, −0.36 ± 0.17, −0.36 ± 0.30, and −0.15 ± 0.12 PgC yr−1, respectively. Our study suggests that the GOSAT XCO2 can only promise a robust continental‐scale NEE estimate, and improving the XCO2 accuracy is an effective way to achieve robust estimates on smaller scales under current satellite observing capacity. Key Points: Terrestrial carbon sinks estimated based on GOSAT XCO2 and net ecosystem exchanges (NEE) from 12 terrestrial biosphere models using atmospheric inverse methodThe uncertainty reduction rates decrease significantly at decreasing spatial scalesThe GOSAT XCO2 can only promise a robust continental‐scale NEE estimate [ABSTRACT FROM AUTHOR]

Although the Coupled Model Intercomparison Project 6 (CMIP6) protocol provides an experiment to estimate effective radiative forcing (ERF), it is only quantified for few models. We present new estimates of ERF for models participating in CMIP6 by applying the method developed in Fredriksen et al. (2021, https://doi.org/10.1029/2020JD034145), and validate our approach with available fixed‐SST forcing estimates. We estimate ERF for experiments with abrupt changes of CO2, 1% increase of CO2, historical forcings, and future scenarios, and demonstrate that CMIP6 ERF is lower than CMIP5 ERF at the end of the historical period, but grows faster than CMIP5 in the future scenarios, ending up at higher levels than CMIP5 at the end of the 21st century. The simulated radiative efficiency of CO2 has not changed much, suggesting that the larger future increase in CO2 concentrations in CMIP6 compared to CMIP5 is important for explaining the forcing difference. Plain Language Summary: To understand climate model responses, it is useful to separate between the drivers of climate change and their responses. We present new estimates of the drivers, called the effective radiative forcing, for the latest generation of climate models (Coupled Model Intercomparison Project 6 (CMIP6)). This estimates the energy imbalance at the top of the atmosphere and is a measure of human and natural influences on climate. Normally this requires additional climate model experiments to make these estimates, but since these have only been run for a few models, we are here aiming to make the best alternative estimates based on more widely available data, following the method in Fredriksen et al. (2021, https://doi.org/10.1029/2020JD034145). We show that our forcing estimates are growing faster during the 21st century for the new CMIP6 models than for the previous generation of models (CMIP5), and suggest this can be attributed to the higher CO2 concentrations in future scenarios for CMIP6 compared to CMIP5. Key Points: Our method to estimate effective radiative forcing based on common model diagnostics is consistent with Coupled Model Intercomparison Project 6 (CMIP6) fixed‐SST estimatesWe present coherent estimates for abrupt CO2, 1% CO2, historical, and future scenario experimentsForcing estimates for the 21st century grow faster for CMIP6 than for CMIP5 models [ABSTRACT FROM AUTHOR]

The current narrative of artificial upwelling (AU) is to translocate nutrient rich deep water to the ocean surface, thereby stimulating the biological carbon pump (BCP). Our refined narrative takes the response of the solubility pump and the CO2 emission scenario into account. Using global ocean‐atmosphere model experiments we show that the effectiveness of a hypothetical maximum AU deployment in all ocean areas where AU is predicted to lower surface pCO2, the draw down of CO2 from the atmosphere during years 2020–2100 depends strongly on the CO2 emission scenario and ranges from 1.01 Pg C/year (3.70 Pg CO2/year) under RCP 8.5 to 0.32 Pg C/year (1.17 Pg CO2/year) under RCP 2.6. The solubility pump becomes equally effective compared to the BCP under the highest emission scenario (RCP 8.5), but responds with CO2 outgassing under low CO2 emission scenarios. Plain Language Summary: Artificial upwelling (AU) is a proposed marine carbon dioxide removal (CDR) method, which suggests deploying pipes in the ocean to pump deep water to the ocean's surface. This process theoretically has several different impacts on the surface layer including an increase in the nutrient concentration, as well as a decrease in surface water temperature. Changes in the carbon cycle and associated with biological components are covered by the biological carbon pump (BCP), while changes via physical‐chemical processes are covered by the solubility pump. Using numerical ocean modeling and simulating almost globally applied AU between the years 2020 and 2100 under several different atmospheric CO2 emission scenarios, we show that AU leads under every simulated emission scenario to an additional CO2‐uptake of the ocean, but the potential increases under higher emission scenarios (up to 1.01 Pg C/year (3.70 Pg CO2/year) under the high CO2‐emission scenario RCP 8.5). The individual contribution via the BCP is under every emission scenario positive, while the processes associated with the solubility pump can lead to CO2‐uptake under higher emission scenarios and CO2 outgassing under lower emission scenarios. Key Points: Artificial upwelling (AU) effectiveness to draw down CO2 from the atmosphere is strongly dependent on the future CO2 emission scenarioThe solubility pump becomes as effective as the biological carbon pump under high emission scenariosOrganic matter transfer efficiency decreases under AU, likely due to higher water temperatures below the ocean's surface [ABSTRACT FROM AUTHOR]

El Niño Southern Oscillation (ENSO) influence on carbon dioxide (CO2) growth rates are not spatially uniform or simultaneous around the globe. Using atmospheric CO2 observations and atmospheric chemistry‐transport model (ACTM), we show that anomalies in CO2 fluxes and growth rate originate in the tropics, as an effect of ENSO. The CO2 anomalies are then propagated from the equator toward the poles with asymmetric delays. A maximum delay of about 8 and 4 months are found at the northern hemisphere (NH) and southern hemisphere (SH) high latitudes, respectively. This asymmetric time delay is because of the CO2 flux anomaly mainly originate in the tropical SH land and transport of SH air into the NH is slower than that for NH air into the SH. The poleward increase of time delay is more homogeneous in the upper troposphere, as per the ACTM simulations, but observational evidence suffers from gaps in long‐term measurements. Plain Language Summary: El Niño Southern Oscillation (ENSO) is a dominant tropical climate mode that has uneven impact on carbon dioxide (CO2) growth rates in different parts of the globe. In this study, we show an asymmetric meridional evolution of ENSO signal imprinted on CO2 growth rate using observations and chemistry‐transport model simulations. A time delay of 8 and 4 months between ENSO and CO2 growth rate are found near northern and southern high latitudes. This propagation asymmetry near the surface is linked to asymmetry in poleward transport of airmass and dominance of southern tropical flux variability to ENSO. The propagation is more homogenous in the upper atmosphere, where the flux signals are mixed. The terrestrial biosphere flux and fire emissions are reiterated as the underlying biophysical process in CO2 anomaly influenced by ENSO. Key Points: El Niño Southern Oscillation (ENSO) events strongly influence tropical land carbon dioxide (CO2) flux variability, and global CO2 variability both at surface and upper atmosphereAsymmetrical meridional propagation of the ENSO‐CO2 flux signal is imprinted on CO2 growth rate near the surfaceThe terrestrial biosphere fluxes and fire induced emissions from the tropical southern hemisphere show dominant CO2 flux variability [ABSTRACT FROM AUTHOR]

The ocean reduces human impact on the climate by absorbing and sequestering CO2. From 1950s to the 1980s, observations of pCO2 and related ocean carbon variables were sparse and uncertain. Thus, global ocean biogeochemical models (GOBMs) have been the basis for quantifying the ocean carbon sink. The LDEO‐Hybrid Physics Data product (LDEO‐HPD) interpolates sparse surface ocean pCO2 data to global coverage by using GOBMs as priors, and applying machine learning to estimate full‐coverage corrections. The largest component of the GOBM corrections are climatological. This is consistent with recent findings of large seasonal discrepancies in GOBMs, but contrasts the long‐held view that interannual variability is a major source of GOBM error. This supports extension of the LDEO‐HPD pCO2 product back to 1959, using a climatology of model‐observation misfits prior to 1982. Consistent with previous studies for 1980 onward, air‐sea CO2 fluxes for 1959–2020 demonstrate response to atmospheric pCO2 growth and volcanic eruptions. Plain Language Summary: The ocean removes carbon dioxide (CO2) from the atmosphere and reduces climate change caused by humans. The magnitude of this removal can be estimated using computer models of ocean physics, chemistry, and biology, as well as statistical extrapolations of observations. The observational record is too sparse to directly reconstruct air‐sea fluxes prior to 1982, but by combining models and a statistical approach, we make an estimate for 1959‐present that is substantially informed by observations. The LDEO‐Hybrid Physics Data product (LDEO‐HPD) product for air‐sea CO2 exchange includes two periods, with the first previously published for 1982–2018 and extended here to end in 2020, and the second being this extension back in time. For 1959–1981, LDEO‐HPD corrects models using the monthly average of data‐based corrections derived from the observed period, a choice justified by our finding that these monthly means are the largest component of the needed corrections during the observed period. The LDEO‐HPD product agrees much better with independent observations than the models alone, and can be used to understand what controls year to year changes in the ocean carbon sink. Key Points: Climatological deviations of ocean hindcast models from observations are substantially larger than interannual deviationsClimatological correction extends machine learning based hybrid model‐observation product temporally for surface ocean pCO2 (LDEO‐Hybrid Physics Data product)The ocean carbon sink from 1959 to 2020 has responded to atmospheric pCO2 growth and volcanic eruptions [ABSTRACT FROM AUTHOR]

While the annual mean Arctic Amplification (AA) index varied between two and three during the 1970–2000 period, it reached values exceeding four during the first two decades of the 21st century. The AA did not change in a continuous fashion but rather in two sharp increases around 1986 and 1999. During those steps the mean global surface air temperature trend remained almost constant, while the Arctic trend increased. Although the "best" CMIP6 models reproduce the increasing trend of the AA in 1980s they do not capture the sharply increasing trend of the AA after 1999 including its rapid step‐like increase. We propose that the first sharp AA increase around 1986 is due to external forcing, while the second step close to 1999 is due to internal climate variability, which models cannot reproduce in the observed time. Plain Language Summary: The rate of Arctic warming varied in time, while the rate of global warming was nearly constant over the period 1960–2020. This led to a variable Arctic Amplification (AA) defined as a ratio of the Arctic warming trend to mean global warming trend. In addition, the Arctic experienced sudden changes in the rate of warming with step‐like changes in annual mean Arctic Amplification around the years 1986 and 1999. Climate models do not capture the second observed sharp increase of the AA. We suggest that the first AA increase around 1986 is primarily due to external forcing (increasing concentration of carbon dioxide) while the second sharp increase around 1999 is dominated by internal climate variability which current models cannot capture accurately within the time scale of the physical phenomena. Key Points: Annual mean Arctic Amplification (AA) within the period 1970–2020 changed in steep steps around 1986 and 1999. It reached values over 4.0Even those CMIP6 models best at reproducing the AA did not reproduce the second sudden increase near 1999We conjecture that the first AA sharp increase is due to external forcing, while the second is due to internal climate variability [ABSTRACT FROM AUTHOR]

The Subpolar North Atlantic is prone to recurrent extreme freshening events called Great Salinity Anomalies (GSAs). Here, we combine hydrographic ocean analyses and moored observations to document the arrival, spreading, and impacts of the most recent GSA in the Irminger Sea. This GSA is associated with a rapid freshening of the upper Irminger Sea between 2015 and 2020, culminating in annually averaged salinities as low as the freshest years of the 1990s and possibly since 1960. Upon the GSA propagation into the Irminger Sea over the Reykjanes Ridge, the boundary currents rapidly advected its signal around the basin within months while fresher waters slowly spread and accumulated into the interior. The anomalies in the interior freshened waters produced by deep convection during the 2017–2018 winter and actively contributed to the suppression of deep convection in the following two winters. Plain Language Summary: The Subpolar North Atlantic is prone to recurrent extreme freshening events (i.e., when the salinity decreases abruptly) called Great Salinity Anomalies (GSAs). As the circulation spreads them around the subpolar region, they indirectly contribute to the reduction of deepwater formation and weakening of the vertical transport of heat and anthropogenic carbon dioxide in the ocean. Here, we combine historical and new moored temperature and salinity records to document the arrival, spreading, and impacts of the most recent GSA in the Irminger Sea. This GSA led to a rapid freshening of the Irminger Sea between 2015 and 2020, 2–4 times faster than the freshening rates registered in the previous 10 years. This event culminated in annually averaged salinities as low as the freshest years of the 1990s and possibly since 1960. Tracking the GSA's path in the subpolar gyre, we found that the GSA flows with the boundary currents along the basin's rim within months while other processes slowly spread the fresher waters to the central Irminger Sea. The accumulation of fresher waters in the basin's center freshened the deep water formed during the 2017–2018 winter and actively contributed to the halt of deepwater formation in the following two winters. Key Points: Fresh anomaly identified in the Iceland Basin exhibits characteristics of a Great Salinity Anomaly (GSA) as it spreads into the Irminger SeaWithin the Irminger Sea, the GSA was rapidly advected by the boundary currents while slowly spreading to the basin centerExtreme freshening played a significant role in suppressing deep convection in the Irminger Sea during the 2018–2019 and 2019–2020 winters [ABSTRACT FROM AUTHOR]

Disturbance of marine sediments results in the remineralization of sedimentary organic matter (OM) and impacts upon natural burial processes. Management interventions which restrict or remove activities that cause seabed disturbance may offer effective strategies to protect the most vulnerable of these shelf sea OM stores, offering new opportunities to deliver climate mitigation actions. While the largest quantities of OM are often stored in the expansive offshore regions of continental shelves and might therefore suggest appropriate zones for management interventions to protect vulnerable OM stores, our results highlight that these offshore regions generally contain OM of low reactivity. Conversely, inshore and coastal sediments store significant quantities of highly reactive OM that is at greater risk of remineralization when disturbed. The marked spatial disparities between OM reactivity across shelf sea sedimentary environments highlights the need to focus emergent policy and future management interventions toward the protection of inshore and coastal sediments. Plain Language Summary: If disturbed, the organic carbon within seafloor sediments can potentially be converted to carbon dioxide and a portion lost to the atmosphere, where it potentially contributes to climate change. Our results highlight that the reactivity (biodegradability) of sedimentary carbon across shelf seas is highly variable with low reactivity organic matter found in the offshore region, while organic matter within inshore and coastal sediments can be highly reactive and potentially vulnerable to break‐down when disturbed. These differences in organic matter reactivity highlight the need to focus emergent management interventions that can deliver protection toward the most vulnerable organic matter stores in inshore and coastal sediments. Key Points: Within most shelf sediments organic matter (OM) is generally of low reactivity (i.e., low labile OM content) and resistant to remineralizationMuddy nearshore sediments hold large quantities of high reactivity OM which is at risk of remineralization when disturbedPolicy interventions that seek to protect reactive OM should prioritize near‐shore sediment OM over the resilient OM in many shelf seas [ABSTRACT FROM AUTHOR]

Air‐sea carbon dioxide (CO2) flux is generally estimated by the bulk method using upper ocean CO2 fugacity measurements. In the summertime Arctic, sea‐ice melt results in stratification within the upper ocean (top ∼10 m), which can bias bulk CO2 flux estimates when the seawater CO2 fugacity is taken from a ship's seawater inlet at ∼6 m depth (fCO2w_bulk). Direct flux measurements by eddy covariance are unaffected by near‐surface stratification. We use eddy covariance CO2 flux measurements to infer sea surface CO2 fugacity (fCO2w_surface) in the Arctic Ocean. In sea‐ice melt regions, fCO2w_surface values are consistently lower than fCO2w_bulk by an average of 39 μatm. Lower fCO2w_surface can be partially accounted for by fresher (≥27%) and colder (17%) melt waters. A back‐of‐the‐envelope calculation shows that neglecting the summertime sea‐ice melt could lead to a 6%–17% underestimate of the annual Arctic Ocean CO2 uptake. Plain Language Summary: The Arctic Ocean is considered to be a strong sink for atmospheric CO2. The air‐sea CO2 flux is almost always estimated indirectly using bulk seawater CO2 fugacity measured from the ship's seawater inlet at typically ∼6 m depth. However, sea‐ice melt results in near‐surface stratification and can cause a bias in air‐sea CO2 flux estimates if the bulk water CO2 fugacity is used. The micrometeorological eddy covariance flux technique is not affected by stratification. Here for the first time, we employ eddy covariance measurements to assess the impact of sea‐ice melt on Arctic Ocean CO2 uptake estimates. The results show that the summertime near‐surface stratification due to sea‐ice melt could lead to an ∼10% (with high uncertainty) underestimation of the annual Arctic Ocean CO2 uptake. Key Points: Seawater CO2 fugacity (fCO2w) vertical gradients are generated by fresh and cold sea‐ice melt water, which lowers surface fCO2wAir‐sea CO2 fluxes are biased when estimated using fCO2w observations from the sub‐surface (6 m depth) in sea‐ice melt areasSummertime sea‐ice melt potentially results in a 6%–17% (with high uncertainty) underestimate of annual Arctic Ocean CO2 uptake [ABSTRACT FROM AUTHOR]

The strength of the biological soft tissue pump in the ocean critically depends on how much organic carbon is produced via photosynthesis and how efficiently the carbon is transferred to the ocean interior. For a given amount of limiting nutrient, phosphate, soft tissue pump would be strengthened if the carbon (C) to phosphorus (P) ratio of sinking organic matter increases as the remineralization length scale of C increases. Here, we present a new data compilation of particle flux stoichiometry and show that C:P of sinking particulate organic matter (POM) in the ocean twilight zone on average is likely to be higher than the C:P ratio of surface suspended POM. We further demonstrate using a physics‐biology coupled global ocean model combined with a theory from first principles that an increase in C:P export flux ratio in the ocean's twilight zone can lead to a considerable drawdown of atmospheric pCO2. Plain Language Summary: The ocean's twilight zone, located below the ocean's sunlit zone, is a region where many critical biogeochemical processes occur but are not well characterized. Most notably, this is the zone where microbes and animals consume organic matter produced by primary producers in the surface ocean. How efficiently this organic matter is degraded exerts essential controls on atmospheric carbon dioxide levels and energy transfer in the marine food web. Here we show using a new global data compilation that organic carbon and phosphorus particles are degraded at different rates. This leads to a considerable change in carbon to phosphorus ratio at depth. We incorporate this variability into the 3D ocean model to show that such change can substantially increase the ocean's ability to absorb carbon dioxide from the atmosphere. The study ultimately highlights the need to accurately characterize the ocean's role in climate change by studying the particle dynamics in the twilight zone. Key Points: Global particle flux data suggests a systematic increase in carbon (C) to phosphorus (P) flux stoichiometry in the ocean twilight zoneIncrease in the C:P export flux ratio through the twilight zone can substantially increase atmospheric pCO2 drawdownFurther studies are required to elucidate mechanisms leading to spatiotemporal C:P export flux variability in the twilight zone [ABSTRACT FROM AUTHOR]

As the abyssal oceans warm, stratification is also expected to change in response. This change may impact mixing and vertical transport by altering the buoyancy flux, internal wave generation, and turbulent dissipation. In this study, repeated surveys of three hydrographic sections in the Southwest Pacific Basin between the 1990s and 2010s are used to estimate the change in buoyancy frequency N2. We find that below the θ=0.8°C isotherm, N2 is on average reduced by a scaling factor of s=0.88±0.06, a 12% reduction, per decade that intensifies with depth. At Θ=0.63°C, we observe the biggest change: s=0.71±0.07, or a 29% reduction per decade. Within the same period, the magnitude of vertical diffusive heat flux is also reduced by about 0.01Wm−2, although this estimate is sensitive to the choice of estimated diffusivity. Finally, implications of these results for the heat budget and global ocean circulation are qualitatively discussed. Plain Language Summary: The large‐scale circulation of the ocean is primarily driven by density differences. As dense, heavy water sinks, it fills the deep ocean basins and aids in pushing water around the globe, cycling around the world over many centuries. A key location where this happens is around Antarctica. The ice and cold winds cool the water, making it denser. This cooled water sinks, displacing the deep water and pushing it northwards. As Antarctica warms, this water carries the extra heat into the rest of the world, causing the deep ocean to rapidly warm. In the Southwest Pacific Basin, we find that this bottom intensified warming has caused a significant reduction in the stratification of the deepest layer over the past three decades. This change can disrupt the global ocean conveyor belt, impacting the transport of heat, carbon dioxide, nutrients, and other dissolved matter around the world. Key Points: A decadal stratification decrease is estimated from 25 years of repeat hydrography in the abyssal Southwest Pacific BasinThis change is significant below Θ = 0.75°C and intensifies with depth, reaching a 22%–36% decrease by Θ = 0.65°CVertical diffusive heat flux is also reduced during the same time period by ∼0.01 Wm−2/decade [ABSTRACT FROM AUTHOR]

Peatlands play a critical role in the global carbon (C) cycle, encompassing ∼30% of the 1,500 Pg of C stored in soils worldwide. However, this C is vulnerable to climate and land‐use change. Ecosystem models predict the impact of perturbation on C fluxes based on soil C pools, yet responses could vary markedly depending on soil organic matter (SOM) chemistry. Here, we show that one SOM functional group responds strongly to environmental factors and predicts the risk of carbon dioxide (CO2) release from peatlands. The molecular composition of SOM in 125 peatlands differed markedly at the global scale due to variation in temperature, land‐use, vegetation, and nutrient status. Despite this variation, incubation of peat from a subset of 11 sites revealed that O‐alkyl C (i.e., carbohydrates) was the strongest predictor of aerobic CO2 production. This explicit link provides a simple parameter that can improve models of peatland CO2 fluxes. Plain Language Summary: Peatlands influence climate, because their soils contain the equivalent of two thirds of the Earth's atmospheric carbon. However, the carbon stored in peatlands is at risk due to climate and land‐use changes, which may increase microbial production of carbon dioxide (CO2) and promote future warming. We found that the chemical composition of soil organic carbon determines the amount of carbon loss from peatlands following drainage. Further, the chemical composition of organic carbon varied among global peatlands based on site characteristics. Therefore, CO2 emissions from soil due to climate and land‐use change vary globally depending on peat carbon chemistry. Information on the chemical composition of soil organic matter will help earth system models improve estimates of CO2 production from peatlands. Key Points: The global variation in soil organic matter chemistry (i.e., specifically the proportion of O‐alkyl carbon) in peatlands is determined primarily by mean annual temperatureO‐alkyl carbon concentration predicts CO2 production at the global scale across peatlands with contrasting environment and chemistry [ABSTRACT FROM AUTHOR]

The future carbon balance of boreal ecosystems under increasing temperatures is highly uncertain. In particular, the net effects of a longer growing season versus enhanced respiration are poorly understood. Here, we use a geostatistical inverse model from 2012 to 2014 to determine temperature sensitivity in Alaskan biomes throughout the growing season, in order to identify the relative effects of these competing phenomena. We find that temperature explains a large portion of the disparities in autumn carbon flux between 2013 and 2014. Boreal forests experienced a growing season extension during the warm October of 2013 that offset increased respiration into autumn in years with high temperatures. In contrast, increased temperatures in the tundra and shrublands led to a large respiration signal during October 2013, producing a greater net carbon release. These results suggest a greater vulnerability of Alaskan tundra and shrubland carbon stocks compared to boreal forest carbon stocks under warming. Plain Language Summary: Increasing atmospheric concentrations of greenhouse gases are driving climate change, which is happening particularly rapidly near the poles of the Earth. This rapid warming may help plants in these high‐latitude regions take up more carbon through photosynthesis, which could decrease the amount of carbon dioxide (CO2) in the atmosphere, slowing climate change. However, rapid warming is also speeding up soil microbial activity and other processes that release more CO2 into the atmosphere at high latitudes, possibly accelerating climate change. Therefore, the balance of these different processes will determine the net impact of high‐latitude ecosystems on climate change. We assess how much warming and associated conditions influence the total CO2 exchange across Alaska. We find that during our study period, warming was very important for autumn CO2 exchange. Forests in Alaska continued their photosynthesis later into autumn during the warmest year of our study period, offsetting enhanced CO2 release from these regions. In contrast, under the same conditions, areas covered by shrubs and grasses simply lost large amounts of carbon. These results suggest that under warming, high‐latitude grasslands and shrublands are particularly susceptible to releasing large amounts of CO2, which could potentially accelerate climate change. Key Points: Autumn warming is associated with a growing season extension in the boreal forests in the interior of AlaskaThese same conditions are associated with a strong release of carbon from shrublands and some tundra regions in AlaskaShrubland and tundra regions of Alaska may be more vulnerable to net carbon losses because of a lack of a growing season extension [ABSTRACT FROM AUTHOR]

Greenhouse gases (GHGs), including carbon dioxide (CO2), impact global and local outgoing longwave radiation (OLR). The Antarctic is known for its near‐surface temperature inversion, where the addition of GHGs can lead to increased OLR during all but the winter months. These changes in OLR, however, are unable to explain modeled surface warming due to changes in GHGs across central Antarctica. Here we develop a simple explanation showing why adding CO2 always warms the surface, and allowing an estimation of the change in surface temperature due to a change in CO2 concentration based on the initial surface temperature. We develop a radiative‐advective‐turbulent single‐column model based on observed temperatures for explicit comparisons between our estimations and model equilibrium behavior. We confirm that Antarctic surface temperatures warm as GHG concentrations increase, and find that this response is best explained through the surface greenhouse effect rather than that of the top of atmosphere (TOA). Plain Language Summary: Greenhouse gases (GHGs), such as carbon dioxide (CO2), warm the earth. Typically the temperature of the earth's atmosphere cools with altitude, but in the Antarctic, the ground is so cold that temperatures with height up to a few kilometers altitude are warmer than the surface. This phenomenon, called a temperature inversion, as well as low levels of observed warming in central Antarctica, has led to questions of whether or not CO2 warms or cools the surface of the Antarctic. Here, we develop a model of a single vertical column of the atmosphere and use it to inform a simple explanation connecting the CO2 concentration to surface temperature changes. This allows us to estimate the temperature change due to a change in CO2 based on the initial surface temperature alone, and confirms that the Antarctic warms in response to increased GHG concentrations. Key Points: We derive an expression for surface temperature change due to CO2 concentration based on an effective bandwidth and the initial temperatureWe develop a single‐column radiative‐advective‐turbulent model for Antarctic equilibria under varying CO2 concentrationsAntarctic surface temperature responses are best understood by a surface greenhouse effect [ABSTRACT FROM AUTHOR]

During the summer 2017 ASCENDS/ABoVE airborne science campaign, the NASA Goddard CO2 Sounder lidar overflew smoke plumes from wildfires in the British Columbia, Canada. In the flight path over Vancouver Island on 8 August 2017, the column XCO2 retrievals from the lidar measurements at flight altitudes around 9 km showed an average enhancement of 4 ppm from the wildfires. A comparison of these enhancements with those from the Goddard Global Chemistry Transport model suggested that the modeled CO2 emissions from wildfires were underestimated by more than a factor of 2. A spiral‐down validation performed at Moses Lake airport, Washington showed a bias of 0.1 ppm relative to in situ measurements and a standard deviation of 1 ppm in lidar XCO2 retrievals. The results show that future airborne campaigns and spaceborne missions with this type of lidar can improve estimates of CO2 emissions from wildfires and estimates of carbon fluxes globally. Plain Language Summary: Wildfires are a major source of greenhouse gases. However, there are large uncertainties in the estimated CO2 emissions from wildfires in global emissions inventories. The estimates of column‐averaged CO2 (XCO2) from satellite measurements using passive remote sensing techniques are significantly degraded or screened out by the scattering from smoke in the scene. NASA Goddard Space Flight Center has developed an integrated‐path, differential absorption lidar approach to measure XCO2 from space. Measurements of time‐resolved laser backscatter profiles from the atmosphere allow this technique to accurately estimate XCO2 and range to terrain and water surfaces even in the presence of wildfire smoke. We demonstrate this capability over Vancouver Island through the dense smoke plumes from wildfires in the Canadian Rockies during the summer 2017 ASCENDS/ABoVE airborne science campaign. To our knowledge this is the first use of lidar to remotely sense CO2 enhancements from large wildfires. Future airborne campaigns and spaceborne missions with this capability can improve estimates of CO2 emissions from wildfires and help estimates of carbon fluxes globally. Key Points: NASA Goddard CO2 Sounder Lidar can accurately measure CO2 enhancements from wildfires through dense smoke plumesThis is the first use of lidar to remotely sense CO2 enhancements from large wildfiresThese types of lidar measurements can be used to validate estimates of CO2 emissions from wildfires and improve estimates of carbon fluxes [ABSTRACT FROM AUTHOR]