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An observationally constrained time series of historical aerosol effective radiative forcing (ERF) from 1750 to 2019 is developed in this study. We find that the time history of aerosol ERFs diagnosed in CMIP6 models exhibits considerable variation and explore how the time history of aerosol forcing influences the probability distributions of present‐day aerosol forcing and emergent metrics such as climate sensitivity. Using a simple energy balance model, trained on CMIP6 climate models and constrained by observed near‐surface warming and ocean heat uptake, we derive estimates for the historical aerosol forcing. We find 2005–2014 mean aerosol ERF to be −1.1 (−1.8 to −0.5) W m−2 relative to 1750. Assuming recently published historical emissions from fossil fuel and industrial sectors and biomass burning emissions from SSP2‐4.5, aerosol ERF in 2019 is −0.9 (−1.5 to −0.4) W m−2. There is a modest recovery in aerosol forcing (+0.025 W m−2 decade−1) between 1980 and 2014. This analysis also gives a 5%–95% range of equilibrium climate sensitivity of 1.8°C –5.1°C (best estimate 3.1°C) with a transient climate response of 1.2°C –2.6°C (best estimate 1.8°C). Plain Language Summary: There are two main human drivers of climate change: (a) Greenhouse gas emissions, which warm the planet; and (b) air pollution (aerosols) that offset some of this warming. Unfortunately, disentangling the effects of historical aerosol cooling is difficult based on the available observations. Therefore, we often use climate models to estimate how much aerosols have cooled the Earth since the start of the Industrial Revolution. Over the mid‐to‐late 20th Century, some climate models simulate less warming compared to 1850 than has been observed. This may be because aerosol cooling in some climate models is too strong. Our approach combines the relationships between aerosol emissions and their cooling effects on temperature from 11 climate models with simpler representations of the underlying physics. This simpler mathematical framework allows us to more fully account for uncertainty in both aerosol cooling and its effects on surface temperature and ocean heat uptake by running a much larger set of simulations. Our results suggest that the effect of aerosol cooling has only unwound slowly since 1980, and that it is difficult to determine how sensitive the climate is from this method. Key Points: We determine the most plausible time history of aerosol forcing that matches surface temperature and Earth energy uptake constraintsConstrained aerosol forcing shows a modest recovery between 1980 and 2014, slower than the rate simulated by many CMIP6 modelsThe best estimate aerosol forcing using this method is −1.10 W m−2 for 2005–2014 relative to 1750 [ABSTRACT FROM AUTHOR]

Human activity has led to increased atmospheric concentrations of many gases, including halocarbons, and may lead to emissions of many more gases. Many of these gases are, on a per molecule basis, powerful greenhouse gases, although at present‐day concentrations their climate effect is in the so‐called weak limit (i.e., their effect scales linearly with concentration). We published a comprehensive review of the radiative efficiencies (RE) and global warming potentials (GWP) for around 200 such compounds in 2013 (Hodnebrog et al., 2013, https://doi.org/10.1002/rog.20013). Here we present updated RE and GWP values for compounds where experimental infrared absorption spectra are available. Updated numbers are based on a revised “Pinnock curve”, which gives RE as a function of wave number, and now also accounts for stratospheric temperature adjustment (Shine & Myhre, 2020, https://doi.org/10.1029/2019MS001951). Further updates include the implementation of around 500 absorption spectra additional to those in the 2013 review and new atmospheric lifetimes from the literature (mainly from WMO (2019)). In total, values for 60 of the compounds previously assessed are based on additional absorption spectra, and 42 compounds have REs which differ by >10% from our previous assessment. New RE calculations are presented for more than 400 compounds in addition to the previously assessed compounds, and GWP calculations are presented for a total of around 250 compounds. Present‐day radiative forcing due to halocarbons and other weak absorbers is 0.38 [0.33–0.43] W m−2, compared to 0.36 [0.32–0.40] W m−2 in IPCC AR5 (Myhre et al., 2013, https://doi.org/10.1017/CBO9781107415324.018), which is about 18% of the current CO2 forcing.Plain Language Summary: Human activity has led to increased atmospheric concentrations of many gases, including halocarbons (used, e.g., in refrigeration and air conditioning), and may lead to emissions of many other gases. While some halocarbons, such as chlorofluorocarbons (CFCs), are known to deplete stratospheric ozone, they are also powerful greenhouse gases contributing to radiative forcing (the net change in the energy balance of the Earth system) and hence climate change. We find that the present‐day contribution from halocarbons and related compounds to radiative forcing is about 18% of the forcing due to increased concentrations of CO2. By using established methods and available laboratory measurements of absorption of infrared radiation for each gas, we quantify the radiative efficiency (i.e., a compound's strength as a greenhouse gas) for a total of around 600 compounds. For around 250 compounds we provide so‐called global warming potentials (GWP), which are used to compare the climate impact of emissions of different gases and are commonly used to inform policy decisions. Results presented here can be used to derive values for emission metrics other than GWP. The present work is the most comprehensive review of the radiative efficiency and GWP of halocarbons and other weak absorbers performed to date.Key Points: Radiative efficiencies are reassessed for more than 600 compounds and global warming potentials calculated for around 250 of theseForty‐two compounds have >10% different radiative efficiency compared to a comprehensive review in 2013Present‐day radiative forcing due to halocarbons and other weak absorbers is 0.38 [0.33–0.43] W m−2, which is ~18% of the CO2 forcing [ABSTRACT FROM AUTHOR]

Black carbon (BC) aerosols influence precipitation through a range of processes. The climate response to the presence of BC is however highly dependent on its vertical distribution. Here, we analyze the changes in the energy budget and precipitation impacts of adding a layer of BC at a range of altitudes in two independent global climate models. The models are run with atmosphere‐only and slab ocean model setup to analyze both fast and slow responses, respectively. Globally, precipitation changes are tightly coupled to the energy budget. We decompose the precipitation change into contributions from absorption of solar radiation, atmospheric longwave radiative cooling, and sensible heat flux at the surface. We find that for atmosphere‐only simulations, BC rapidly suppresses precipitation, independent of altitude, mainly because of strong atmospheric absorption. This reduction is offset by increased atmospheric radiative longwave cooling and reduced sensible heat flux at the surface, but not of sufficient magnitude to prevent reduced precipitation. On longer timescales, when the surface temperature is allowed to respond, we find that the precipitation increase associated with surface warming can compensate for the initial reduction, particularly for BC in the lower atmosphere. Even though the underlying processes are strikingly similar in the two models, the resulting change in precipitation and temperature by BC differ quite substantially. Plain Language Summary: Soot particles change precipitation by absorbing solar radiation and heating the surrounding air. The atmosphere rapidly adjusts to this added warming by changing relative humidity, clouds, and precipitation. We use two climate models to investigate these rapid adjustments in the atmosphere caused by soot particles. We insert soot particles in different vertical layers in the models and find that soot particles quickly warm the atmosphere and reduce precipitation. Soot particles at higher altitudes stabilize the atmosphere and increase cloud cover located below. Given all the processes soot particles influence in the atmosphere, the similarities in underlying processes by the two climate models are striking. The resulting change in precipitation and temperature differ quite substantially. Key Points: The strong atmospheric shortwave absorption by black carbon suppresses precipitationRapid adjustments decrease the direct radiative effect of black carbon in two independent modelsEven though the underlying processes are strikingly similar in the models, the resulting change in precipitation by black carbon differs [ABSTRACT FROM AUTHOR]

Stratospheric temperature adjustment (STA) is often a significant component of greenhouse gas radiative forcing (RF), for both the most widely used definition of RF and effective radiative forcing. It is well established that the magnitude and sign of STA differs among greenhouse gases, being negative (at the tropopause) for CO2 increases (because it induces a cooling of the stratosphere) and positive for many halocarbons (because they induce a warming of the stratosphere); this effect is strongly related to the opacity (and hence the effective emitting temperature) of the troposphere at the wavelengths at which gases absorb. Here the spectral variation of STA is examined for the first time by systematically imposing a weak absorber in each of 300 bands of a narrow‐band radiation code. For this weak absorber, STA is negative for wavelengths greater than about 13 μm and positive in the 8‐ to 13‐μm "window" except in the vicinity of the 9.6‐μm ozone band. By combining narrow‐band and line‐by‐line model results, these findings are used to improve a widely used fast method to estimate radiative efficiency (RE, the RF per unit change in concentration) directly from laboratory measurements or theoretical calculations of halocarbon absorption cross sections; the new method reproduces detailed RE calculations to better than 1.4%. This is a significant improvement over a cruder method used to account for STA in RE tabulations in the Intergovernmental Panel on Climate Change's Fifth Assessment Report, which were used to calculate metrics such as the Global Warming Potential, for halocarbons and related substances. Plain Language Summary: Increased emissions and concentrations of greenhouse gases, due to human activity, are a major driver of climate change. Carbon dioxide is the most important such gas, but many other gases are emitted which, collectively, contribute significantly to climate change. This paper focuses on methods to calculate the climate impact of halocarbons which are used in refrigeration, insulation, air conditioning, etc. This includes chlorofluorocarbons (CFCs), whose usage has been phased out because they deplete the ozone layer; hydrochlorofluorocarbons (HCFCs) which were regarded as transitional substances to replace the CFCs; and hydrofluorocarbons, which are widely used replacements for CFCs and HCFCs. It is important to quantify the climate impacts of halocarbons and to compare the relative impact of different halocarbons; this information is used in international agreements aimed at reducing greenhouse gas emissions. This paper presents an important refinement of a widely used simple method for calculating and comparing the climate impact of different halocarbons. This refinement considers the impact of these gases on temperatures in the stratosphere (i.e., altitudes of about 10–50 km); to achieve this, the paper first develops a general understanding of the way such gases influence stratospheric temperatures. The new technique is a significant improvement over previous methods. Key Points: Stratospheric temperature adjustment is an important component of the effective radiative forcing for many greenhouse gasesThe variation of the magnitude and sign of this adjustment with the wavelength at which greenhouse gases absorb is investigatedThe results are used to enhance a widely used simple method for estimating halocarbon radiative efficiency [ABSTRACT FROM AUTHOR]

Quantifying the efficacy of different climate forcings is important for understanding the real‐world climate sensitivity. This study presents a systematic multimodel analysis of different climate driver efficacies using simulations from the Precipitation Driver and Response Model Intercomparison Project (PDRMIP). Efficacies calculated from instantaneous radiative forcing deviate considerably from unity across forcing agents and models. Effective radiative forcing (ERF) is a better predictor of global mean near‐surface air temperature (GSAT) change. Efficacies are closest to one when ERF is computed using fixed sea surface temperature experiments and adjusted for land surface temperature changes using radiative kernels. Multimodel mean efficacies based on ERF are close to one for global perturbations of methane, sulfate, black carbon, and insolation, but there is notable intermodel spread. We do not find robust evidence that the geographic location of sulfate aerosol affects its efficacy. GSAT is found to respond more slowly to aerosol forcing than CO2 in the early stages of simulations. Despite these differences, we find that there is no evidence for an efficacy effect on historical GSAT trend estimates based on simulations with an impulse response model, nor on the resulting estimates of climate sensitivity derived from the historical period. However, the considerable intermodel spread in the computed efficacies means that we cannot rule out an efficacy‐induced bias of ±0.4 K in equilibrium climate sensitivity to CO2 doubling when estimated using the historical GSAT trend. Plain Language Summary: Does the climate respond in the same way to carbon dioxide as it does to methane or aerosol changes? The simple way of thinking about forcing and response in the Earth system assumes that it does, such that, a Watt per square meter forcing from CO2 has the same response as an equivalent forcing from aerosols. Recent work has suggested that this might not be true and that differences in how effective different forcings are at increasing surface temperature (their efficacy) may account for a low estimate of climate sensitivity when examining historical change. We show this all depends on how you estimate your Watts per meter squared forcing in the first place. Using the effective radiative forcing concept to estimate forcing strength makes temperature changes far more predictable, and a lot of these issues with efficacy variation are not as pronounced as they were with earlier definitions. Key Points: Multimodel mean efficacies computed using effective radiative forcing are close to one for major anthropogenic drivers, but there is notable intermodel spreadSurface temperature‐driven radiative feedbacks are generally not constant through time across forcing experimentsPDRMIP results suggest that the efficacy impact on equilibrium climate sensitivity derived from the historical period is limited to ±0.4 °C or better [ABSTRACT FROM AUTHOR]

We compare six methods of estimating effective radiative forcing (ERF) using a set of atmosphere‐ocean general circulation models. This is the first multiforcing agent, multimodel evaluation of ERF values calculated using different methods. We demonstrate that previously reported apparent consistency between the ERF values derived from fixed sea surface temperature simulations and linear regression holds for most climate forcings, excluding black carbon (BC). When land adjustment is accounted for, however, the fixed sea surface temperature ERF values are generally 10–30% larger than ERFs derived using linear regression across all forcing agents, with a much larger (~70–100%) discrepancy for BC. Except for BC, this difference can be largely reduced by either using radiative kernel techniques or by exponential regression. Responses of clouds and their effects on shortwave radiation show the strongest variability in all experiments, limiting the application of regression‐based ERF in small forcing simulations. Plain Language Summary: Climate drivers such as greenhouse gases and aerosols influence the Earth's climate by perturbing the Earth's energy budget at the top of the atmosphere, which is referred to as effective radiative forcing (ERF) when the atmospheric response is included in the calculation. ERF plays a crucial role in understanding the climate response to these drivers and predicting long‐term climate change. Previously, ERFs have been estimated for greenhouse gases using two techniques that generally lead to similar values. Here we show that such consistency holds for most climate drivers. ERF values estimated from different methods may differ by 10–50%, and this difference may reach 70–100% for black carbon. Regression techniques do not work well in some models when imposed forcings are relatively small. Key Points: ERF estimated using fixed SST simulations and linear regression are fairly consistent for most climate forcingsMultimodel mean ERF values vary by 10–50% with different methods, and this difference may reach 70–100% for black carbonInternal variability limits the application of linear regression technique in small‐forcing experiments [ABSTRACT FROM AUTHOR]

Atmospheric methane grew very rapidly in 2014 (12.7 ± 0.5 ppb/year), 2015 (10.1 ± 0.7 ppb/year), 2016 (7.0 ± 0.7 ppb/year), and 2017 (7.7 ± 0.7 ppb/year), at rates not observed since the 1980s. The increase in the methane burden began in 2007, with the mean global mole fraction in remote surface background air rising from about 1,775 ppb in 2006 to 1,850 ppb in 2017. Simultaneously the 13C/12C isotopic ratio (expressed as δ13CCH4) has shifted, has shifted, now trending negative for more than a decade. The causes of methane's recent mole fraction increase are therefore either a change in the relative proportions (and totals) of emissions from biogenic and thermogenic and pyrogenic sources, especially in the tropics and subtropics, or a decline in the atmospheric sink of methane, or both. Unfortunately, with limited measurement data sets, it is not currently possible to be more definitive. The climate warming impact of the observed methane increase over the past decade, if continued at >5 ppb/year in the coming decades, is sufficient to challenge the Paris Agreement, which requires sharp cuts in the atmospheric methane burden. However, anthropogenic methane emissions are relatively very large and thus offer attractive targets for rapid reduction, which are essential if the Paris Agreement aims are to be attained. Plain Language Summary: The rise in atmospheric methane (CH4), which began in 2007, accelerated in the past 4 years. The growth has been worldwide, especially in the tropics and northern midlatitudes. With the rise has come a shift in the carbon isotope ratio of the methane. The causes of the rise are not fully understood, and may include increased emissions and perhaps a decline in the destruction of methane in the air. Methane's increase since 2007 was not expected in future greenhouse gas scenarios compliant with the targets of the Paris Agreement, and if the increase continues at the same rates it may become very difficult to meet the Paris goals. There is now urgent need to reduce methane emissions, especially from the fossil fuel industry. Key Points: Atmospheric methane is rising; its carbon isotopic ratio has become more depleted in C‐13The possible causes of the change include an increase in emissions, with changing relative proportions of source inputs, or a decline in methane destruction, or bothIf this rise continues, there are significant consequences for the UN Paris Agreement [ABSTRACT FROM AUTHOR]

Abstract: Future projections of east Amazonian precipitation indicate drying, but they are uncertain and poorly understood. In this study we analyze the Amazonian precipitation response to individual atmospheric forcings using a number of global climate models. Black carbon is found to drive reduced precipitation over the Amazon due to temperature‐driven circulation changes, but the magnitude is uncertain. CO2 drives reductions in precipitation concentrated in the east, mainly due to a robustly negative, but highly variable in magnitude, fast response. We find that the physiological effect of CO2 on plant stomata is the dominant driver of the fast response due to reduced latent heating and also contributes to the large model spread. Using a simple model, we show that CO2 physiological effects dominate future multimodel mean precipitation projections over the Amazon. However, in individual models temperature‐driven changes can be large, but due to little agreement, they largely cancel out in the model mean. [ABSTRACT FROM AUTHOR]

Summary: This report describes a case of sudden lameness in an 18‐year‐old Percheron cross mare after presumably being kicked by another horse, and the subsequent formation of a ganglion cyst originating from the lateral femorotibial joint. Physical examination, radiographic and ultrasonographic investigation identified a soft round 5 cm diameter mass attached to the left lateral femorotibial joint. After surgical removal, histopathological examination confirmed a ganglion cyst. The horse responded well to the surgical removal of the cyst, and 4 weeks after the surgery, the mare has returned back to her same athletic performance. According to the authors' knowledge, this is the first reported case that describes a peri‐articular ganglion cyst originating from the stifle joint. [ABSTRACT FROM AUTHOR]

Summary Background: The rate of metabolic inactivation of proton-pump inhibitors is determined by polymorphisms of CYP2C19 . It is not known if CYP2C19 variant alleles affect responses to proton-pump inhibitor therapy in gastro-oesophageal reflux disease (GERD). Aim: To determine if the CYP2C19 genotype is associated with clinical effectiveness of proton-pump inhibitors during GERD therapy. Methods: GERD patients undergoing ambulatory gastric and oesophageal pH monitoring were genotyped for CYP2C19 polymorphisms. Results: Sixty subjects were enrolled. Forty-four subjects had two wild-type alleles, 15 had one variant, and one had two variant CYP2C19 alleles. The presence of a variant allele was significantly associated with a lower odds of gastric acid breakthrough during proton-pump inhibitor therapy [odds ratio 5.14, 95% confidence interval (CI) 1.17–22.61]. The presence of a variant allele was not associated with a lower odds of significant oesophageal acid exposure (odds ratio 2.50, 95% CI 0.60–10.52), or the occurrence of symptoms (incidence rate ratio 1.06, 95% CI 0.54–2.06). Conclusions: These results indicate that factors other than gastric acid secretion are important determinants of reflux in GERD patients. This suggests that CYP2C19 genotype testing will not be useful in proton-pump inhibitor therapy of GERD, except perhaps in identifying patients at risk for hypochlorhydria and consequent hypergastrinemia. [ABSTRACT FROM AUTHOR]

Rapid adjustments are responses to forcing agents that cause a perturbation to the top of atmosphere energy budget but are uncoupled to changes in surface warming. Different mechanisms are responsible for these adjustments for a variety of climate drivers. These remain to be quantified in detail. It is shown that rapid adjustments reduce the effective radiative forcing (ERF) of black carbon by half of the instantaneous forcing, but for CO2 forcing, rapid adjustments increase ERF. Competing tropospheric adjustments for CO2 forcing are individually significant but sum to zero, such that the ERF equals the stratospherically adjusted radiative forcing, but this is not true for other forcing agents. Additional experiments of increase in the solar constant and increase in CH4 are used to show that a key factor of the rapid adjustment for an individual climate driver is changes in temperature in the upper troposphere and lower stratosphere. Plain Language Summary: Long‐term global warming can be estimated with knowledge of how climate forcing agents affect the Earth's top‐of‐atmosphere energy imbalance or effective radiative forcing. Changes in climate forcers, such as greenhouse gases, the Sun's intensity, or emission of aerosol particles, typically impose a direct change in the energy budget, termed an instantaneous radiative forcing. Further to this, a climate forcer may induce changes in the atmosphere, such as a change in thermal structure, clouds, or humidity. These changes themselves, termed rapid adjustments, contribute to the top‐of‐atmosphere energy budget. Together, the instantaneous radiative forcing plus rapid adjustments equals the effective radiative forcing. We show that for different climate forcing agents, the rapid adjustments behave very differently and are driven by different atmospheric mechanisms. For example, rapid adjustments add to the instantaneous forcing for a carbon dioxide increase, due to a cooling of the stratosphere, but oppose instantaneous forcing for black carbon, driven by a warming troposphere and lowering of cloud height. Understanding rapid adjustments gives a more complete picture of the climate effects of different climate forcers. Key Points: Rapid adjustments affect the Earth's energy balance in different ways for greenhouse gas, aerosol, and solar forcingRadiative kernels and partial radiative perturbations are used to diagnose rapid adjustments from atmospheric and cloud changesNoncloud adjustments agree well between models, whereas cloud adjustments exhibit more spread [ABSTRACT FROM AUTHOR]

Different climate drivers influence precipitation in different ways. Here we use radiative kernels to understand the influence of rapid adjustment processes on precipitation in climate models. Rapid adjustments are generally triggered by the initial heating or cooling of the atmosphere from an external climate driver. For precipitation changes, rapid adjustments due to changes in temperature, water vapor, and clouds are most important. In this study we have investigated five climate drivers (CO2, CH4, solar irradiance, black carbon, and sulfate aerosols). The fast precipitation responses to a doubling of CO2 and a 10‐fold increase in black carbon are found to be similar, despite very different instantaneous changes in the radiative cooling, individual rapid adjustments, and sensible heating. The model diversity in rapid adjustments is smaller for the experiment involving an increase in the solar irradiance compared to the other climate driver perturbations, and this is also seen in the precipitation changes. Plain Language Summary: Future projections of precipitation changes are uncertain, both on regional and global scales. Understanding the climate models' diversity of precipitation change and how these models respond to various climate drivers, such as greenhouse gases and aerosols, is a key topic in climate research. Using sophisticated techniques, we quantify the processes altering precipitation changes on a short time scale and show that changes in the vertical profile of temperature, water vapor, and clouds contribute very differently to precipitation changes for various climate drivers. Our results show that model diversity in precipitation changes varies strongly between the climate drivers. Key Points: Separation of instantaneous and rapid adjustment contributions to precipitation changesContributions of rapid adjustments to precipitation changes differ substantially between climate driversRadiative kernels are applied to understand individual rapid adjustment terms [ABSTRACT FROM AUTHOR]