Chemistry, Aerosols and Radiative Forcing in CMIP6-era models

The Aerosol Chemistry Model Intercomparison Project (AerChemMIP), part of the Coupled-Model Intercomparison Project 6 (CMIP6), builds on earlier Chemistry-Climate Model Intercomparison(CCMI), and Hemispheric Transport of Air Pollution (HTAP) experiments to quantify the climate and air quality impacts of aerosols and chemically reactive gases from the pre-industrial to present day, and projected into the future. Linked projects, the Radiative Forcing Model Intercomparison Project (RFMIP) and the Precipitation Driver Response Model Intercomparison Project (PDRMIP), examine radiative forcing estimates across the CMIP6 models. This session, with a focus on global scale atmospheric chemistry and aerosol modelling and radiative forcing in AerChemMIP, RFMIP and other CMIP6-era simulations, aims to bring together scientists with an interest in:

- quantifying the impact of emissions changes on atmospheric composition and radiative forcing over the period 1850-2100.

- evaluating reactive gases and aerosols in CMIP6 models against observations.

- exploring chemistry-climate interactions in CMIP6 models, with a focus on climate response to changes in trace gas and aerosol concentrations.

- understanding components of radiative forcing.

We invite submission of abstracts in the following areas:

- Global and regional evaluation of CMIP, CCMI and HTAP models and variability across different spatial and temporal scales.

- Modelling studies to understand the cause and impact of changes in atmospheric composition in CMIP6 experiments.

- Quantification of radiative forcing in CMIP6 models, including the roles of chemistry and aerosols.

We particularly welcome contributions from the AerChemMIP, RFMIP and PDRMIP modelling communities to these discussions.

Public information:
At 1542 CEST our highlight abstracts will be presented. These include Robert Allen on the efficacy of control of methane to mitigate climate change, Gill Thornhill on a keystone analysis of the radiative forcing of aerosols and gases, and Steven Turnock speaking about the impact of emissions control on air quality.
Convener: William Collins | Co-conveners: Piers M. Forster, Paul Griffiths, James Keeble, Fiona O'Connor
vPICO presentations
| Fri, 30 Apr, 15:30–17:00 (CEST)
Public information:
At 1542 CEST our highlight abstracts will be presented. These include Robert Allen on the efficacy of control of methane to mitigate climate change, Gill Thornhill on a keystone analysis of the radiative forcing of aerosols and gases, and Steven Turnock speaking about the impact of emissions control on air quality.

vPICO presentations: Fri, 30 Apr

Chairpersons: William Collins, James Keeble, Paul Griffiths
Nadir Jeevanjee, Jacob Seeley, David Paynter, and Stephan Fueglistaler

Instantaneous clear-sky CO2 forcing is known to vary significantly over the globe, but the climate factors which control this are not well understood. Building upon the work of Wilson (2012), we build a first-principles, analytical model for CO2 forcing which requires as input only the temperatures at the surface and roughly 20 hPa, as well as column relative humidity. This model quantitatively captures global variations in clear-sky CO2 forcing, and shows that the meridional forcing gradient is predominantly due to the meridional surface temperature gradient, with modulation by water vapor. In particular, the Simpsonian behavior of water vapor emission implies an upper bound on CO2 forcing (with respect to surface temperature) which is realized in the present day tropics.

How to cite: Jeevanjee, N., Seeley, J., Paynter, D., and Fueglistaler, S.:  An analytical model for spatially varying clear-sky CO2 forcing, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-217,, 2021.

James Weber, Scott Archer-Nicholls, N. Luke Abraham, Youngsub M. Shin, Thomas Bannan, Rebecca Schwantes, Michael Jenkin, Anwar Khan, and Alexander T. Archibald

We present the first incorporation and evaluation of the Common Representative Intermediates version 2.2 chemistry mechanism, CRI v2.2, for use in the United Kingdom Earth System Model (UKESM1). Tuned against the MCM v3.3.1, the CRI v2.2 mechanism builds on the previous CRI version, CRI v2.1, in UKESM1 (Archer-Nicholls et al., 2020) by updating isoprene chemistry and offers a more comprehensive description of tropospheric chemistry than the standard chemistry mechanism STRAT-TROP (ST).

CRI v2.2 adds state-of-the-art isoprene chemistry with the introduction of HOx-recycling via the isoprene peroxy radical isomerisation pathway, making UKESM1 one of the first CMIP6 models to include this important chemistry. HOx-recycling has noticeable effects on oxidants in regions with large emissions of biogenic volatile organic compounds (BVOCs). Low altitude OH in tropical forested regions increases by 75-150% relative to ST, reducing the existing model low bias compared to observations. Consequently, isoprene surface mixing ratios decrease considerably (25-40%), significantly improving the model high bias relative to ST. Methane lifetime decreases by 2% and tropospheric ozone burden increases by 4%.

Aerosol processes also differ between CRI v2.2 and ST, resulting in changes to the size and number distributions. Relative to ST, CRI v2.2 simulates an 8% decrease in the sulphate aerosol burden with 20% decreases in the nucleation and Aitken modes. By contrast, the secondary organic aerosol (SOA) nucleation mode burden increases by 11%. Globally, the average nucleation and Aitken mode aerosol number concentrations decrease by 20%.

The differences in aerosol and gas phase chemistry between CRI v2.2 and ST are likely to have impacts on the radiation budget. We plan to use CRI v2.2 and ST to investigate the influence that the chemical mechanism has on the simulated chemistry-climate feedbacks from BVOCs. In addition, CRI v2.2 will serve as the basis for the addition of a scheme describing the formation of highly oxygenated organic molecules (HOMs) from BVOCs, facilitating a semi-explicit mechanism for new particle formation from organic species.

How to cite: Weber, J., Archer-Nicholls, S., Abraham, N. L., Shin, Y. M., Bannan, T., Schwantes, R., Jenkin, M., Khan, A., and Archibald, A. T.: Incorporation and evaluation of the CRI v2.2 chemical mechanism in UKESM1: An alternative mechanism with updated isoprene chemistry for investigating the influence of BVOCs on atmospheric composition and climate. , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1236,, 2021.

Tianyi Fan, Xiaohong Liu, Chenglai Wu, Yi Gao, Qiang Zhang, Chuanfeng Zhao, Xin Yang, and Yanglian Li

          Biases of aerosol simulation by models participating the Aerosol and Chemistry Model Intercomparison Project (AerChemMIP) were identified over China. Although the yearly trend of simulated aerosol optical depth (AOD) agrees with the MODIS satellite retrievals for the country-wide averages, this agreement is an offset between the underestimation of AOD over eastern China and the overestimation of AOD over western China. The AODs were underestimated over the Northeastern China Plain and the North China Plain all year along and overestimated over Sichuan Basin in the winter. These model biases were persistent over multiple years from 2002 to 2015. We attempt to evaluate the impact of emission uncertainties on model simulated aerosol properties and aerosol radiative forcing by comparing the simulations by the Community Earth System Model version 2 (CESM2) with the default inventory developed by the Community Emission Data System (CEDS) and with a country-level inventory (Multi-resolution Emission Inventory for China, MEIC). It turns out that the differences between simulations with the two emission inventories are much smaller than the differences between simulations and observations. Low-bias of precursor gases (e.g., SO2), too strong convergence of wind field, too strong dilution and transport by summer monsoon circulation, too much wet scavenging by precipitation, and too weak aerosol swelling due to low-biased relative humidity are suggested to be responsible for the biased AOD in eastern China. This indicates that the influence of emission inventory uncertainties on aerosol radiative forcing can be overwhelmed by influences of biased meteorology and aerosol processes. Therefore, it is necessary for climate models to perform reasonably well in the dynamical, physical and chemical processes in order to estimate the aerosol radiative forcing.   

How to cite: Fan, T., Liu, X., Wu, C., Gao, Y., Zhang, Q., Zhao, C., Yang, X., and Li, Y.: Biases of aerosol simulation in the AerChemMIP models over China and impact of emission uncertainties, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1689,, 2021.

xiao li, minghuai wang, yawen liu, yiquan jiang, and xinyi dong

Knowledge of aerosol concentration, type, and physical and chemical properties is necessary to understand their role in Earth’s climate system. However, CMIP6 models’ performance of AOD simulation in China lacks a comprehensive evaluation and the potential improvement for CMIP6 models is still unclear. Here, we assess the performance of CMIP6 models in simulating annual mean AOD climatology and its seasonality over China from 2000 to 2014 and explore the underlying reasons for its performance. Compared with CMIP5, CMIP6 models can better capture the annual mean AOD climatology magnitude over Eastern Central China (ECC) with a notable enhancement of 52.98% due to a significant increase in the dominate sulfate aerosol. However, the majority of CMIP6 models fail to capture the observed inverted “V-like” pattern that depicts two centers of maximum AOD in spring over northeast China (NEC) and in summer over southeast China (SEC), respectively. The deficiency of two maximums by CMIP6 models is separately due to the negative bias in the simulation of organic aerosol (OA) AOD and sulfate AOD. Our analysis suggests that the deviation of simulated precipitation, relative humidity (RH), and liquid water path (LWP) in CMIP6 models contributes to the deviation of simulated sulfate AOD through affecting sulfate aerosol concentration by wet deposition and aqueous-phase production. Therefore, this study illustrates the urgent need to improve AOD simulation in global climate models.

How to cite: li, X., wang, M., liu, Y., jiang, Y., and dong, X.: Assessment of AOD by 16 CMIP6 Models Based on Satellite-Derived Dataset from 2000-2014 over Eastern Center China, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1708,, 2021.

Ulas Im, Kostas Tsigaridis, Gregory S. Faluvegi, Peter L. Langen, Joshua P. French, Rashed Mahmood, Thomas Manu, Knut von Salzen, Daniel C. Thomas, Cynthia H. Whaley, Zbigniew Klimont, Henrik Skov, and Jørgen Brandt

In order to study the future aerosol burdens and their radiative and climate impacts over the Arctic (>60 °N), future (2015-2050) simulations have been carried out using the GISS-E2.1 Earth system model. Different future anthrpogenic emission projections have been used from the Eclipse V6b and the Coupled Model Intercomparison Project Phase 6 (CMIP6) databases. Results showed that Arctic BC, OC and SO42- burdens decrease significantly in all simulations following the emission projections, with the CMIP6 ensemble showing larger reductions in Arctic aerosol burdens compared to the Eclipse ensemble. For the 2030-2050 period, both the Eclipse Current Legislation (CLE) and the Maximum Feasible Reduction (MFR) ensembles simulated an aerosol top of the atmosphere (TOA) forcing of -0.39±0.01 W m-2, of which -0.24±0.01 W m-2 were attributed to the anthropogenic aerosols. The CMIP6 SSP3-7.0 scenario simulated a TOA aerosol forcing of -0.35 W m-2 for the same period, while SSP1-2.6 and SSP2-4.5 scenarios simulated a slightly more negative TOA forcing (-0.40 W m-2), of which the anthropogenic aerosols accounted for -0.26 W m-2. The 2030-2050 mean surface air temperatures are projected to increase by 2.1 °C and 2.4 °C compared to the 1990-2010 mean temperature according to the Eclipse CLE and MFR ensembles, respectively, while the CMIP6 simulation calculated an increase of 1.9 °C (SSP1-2.6) to 2.2 °C (SSP3-7.0). Overall, results show that even the scenarios with largest emission reductions lead to similar impact on the future Arctic surface air temperatures compared to scenarios with smaller emission reductions, while scenarios with no or little mitigation leads to much larger sea-ice loss, implying that even though the magnitude of aerosol reductions lead to similar responses in surface air temperatures, high mitigation of aerosols are still necessary to limit sea-ice loss. 

How to cite: Im, U., Tsigaridis, K., Faluvegi, G. S., Langen, P. L., French, J. P., Mahmood, R., Manu, T., von Salzen, K., Thomas, D. C., Whaley, C. H., Klimont, Z., Skov, H., and Brandt, J.: Aerosols and their impacts on future Arctic climate change under different emission projections in the GISS-E2.1 Earth system model, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2566,, 2021.

Christopher Smith, Glen Harris, Matthew Palmer, Nicolas Bellouin, William Collins, Michael Schulz, Gunnar Myhre, Jean-Christophe Golaz, Mark Ringer, Trude Storelvmo, and Piers Forster

Aerosol forcing remains the most uncertain component of the total climate forcing on the Earth system. RFMIP and AerChemMIP contained experiments that allow us to determine time-slice present day (2014 minus 1850) from 17 CMIP6 models, and transient (1850 to 2014, or 2100) aerosol forcing from 11 models. In CMIP6, aerosol present-day aerosol forcing is -1.01 (full range -1.37 to -0.63) W m-2, a range considerably narrower than comprehensive assessments of aerosol forcing from multiple lines of evidence such as AR5 (-1.9 to -0.1 W m-2) or Bellouin et al. 2020 (-2.0 to -0.35 W m-2). The transient experiments also show a diversity in time histories, with most models showing a peak negative aerosol forcing at some time between 1975 and 2010, and recent trends varying from strongly recovering to slightly strengthening aerosol forcing. Models that were run to 2100 under SSP2-4.5 all show a projected weakening aerosol forcing.

By fitting a simple relationship of how globally integrated emissions of black carbon, organic carbon and SO2 relate to effective radiative forcing from aerosol-radiation interactions (ERFari) and aerosol-cloud interactions (ERFaci), an emissions to forcing relationship can be determined for these 11 RFMIP and AerChemMIP models. Using a 100,000 member Monte Carlo ensemble of historical aerosol time series, where coefficients are drawn from these model-derived distributions, and total 1850 to 2014 aerosol forcing is taken from the wider distributions of Bellouin et al. (2020), we create a best estimate historical time series for aerosol forcing (with uncertainty) that is constrained to historical warming and observed ocean heat uptake using a simple climate model. This method can also be used to predict aerosol forcing from future emissions scenarios, such as the SSPs and those derived from integrated assessment models, and provides estimates of the likely ranges for equilibrium climate sensitivity and transient climate response based on the historical aerosol forcing.

How to cite: Smith, C., Harris, G., Palmer, M., Bellouin, N., Collins, W., Schulz, M., Myhre, G., Golaz, J.-C., Ringer, M., Storelvmo, T., and Forster, P.: Past, present and future aerosol forcing derived from CMIP6, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2863,, 2021.

Robert Allen, Larry Horowitz, Vaishali Naik, Naga Oshima, Fiona O'Connor, Steven Turnock, Sungbo Shim, Philippe Le Sager, Twan van Noije, Kostas Tsigaridis, Susanne Bauer, Lori Sentman, Jasmin John, Conor Broderick, Makoto Deushi, Gerd Folberth, Shinichiro Fujimori, and William Collins

Near-term climate forcers (NTCFs), including aerosols and chemically reactive gases such as tropospheric ozone and methane, offer a potential way to mitigate climate change and improve air quality--so called "win-win" mitigation policies.   Prior studies support improved air quality under NTCF mitigation, but with conflicting climate impacts that range from a significant reduction in the rate of global warming to only a modest impact.  Here, we use state-of-the-art chemistry-climate model simulations conducted as part of the Aerosol and Chemistry Model Intercomparison Project (AerChemMIP) to quantify the 21st-century impact of NTCF reductions, using a realistic future emission scenario with a consistent air quality policy.  Non-methane NTCF (NMNTCF; aerosols and ozone precursors) mitigation improves air quality, but leads to significant increases in global mean precipitation of 1.3% by mid-century and 1.4% by end-of-the-century, and corresponding surface warming of 0.23 and 0.21 K.  NTCF (all-NTCF; including methane) mitigation further improves air quality, with larger reductions of up to 45% for ozone pollution, while offsetting half of the wetting by mid-century (0.7% increase) and all the wetting by end-of-the-century (non-significant 0.1% increase) and leading to surface cooling of -0.15 K by mid-century and -0.50 K by end-of-the-century.  This suggests that methane mitigation offsets warming induced from reductions in NMNTCFs, while also leading to net improvements in air quality.

How to cite: Allen, R., Horowitz, L., Naik, V., Oshima, N., O'Connor, F., Turnock, S., Shim, S., Le Sager, P., van Noije, T., Tsigaridis, K., Bauer, S., Sentman, L., John, J., Broderick, C., Deushi, M., Folberth, G., Fujimori, S., and Collins, W.: Significant climate benefits from near-term climate forcer mitigation in spite of aerosol reductions, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-3162,, 2021.

Gillian Thornhill, William Collins, Ryan Kramer, Dirk Olivie, and Ragnhild Skeie

The effective radiative forcing (ERF) from aerosols and chemically reactive gases is calculated for several of the models that contributed to the CMIP6 project. The design of the experiments allowed for the calculation of the ERF due to each individual aerosol and gas, although where the aerosol and chemistry were modelled as interactive processes additional diagnostics were used to understand how these processes contributed to the overall ERF. The control used was the emission or concentration of the species in 1850 (considered the pre-industrial baseline for these experiments), with the perturbation using a specified emission or concentration of the species based on 2014 values as the present-day value.  The experiments were run over a period of 30 years, with sea-surface temperatures held constant, and the ERF was obtained as the net change in TOA radiative flux between the perturbed and control run.

The aerosols considered were black carbon (BC), organic carbon (OC), SO2 and NH3, and the combination of these constituents in was modelled in the ‘aer’ experiment.

The chemically-reactive gases included methane, nitrous oxide (N2O), and the ozone precursors nitrogen oxides (NOx) and volatile organic compounds (VOC) as well as ozone (O3).

For some models we were able to use radiative kernels to find the relative importance of rapid adjustments such as cloud changes, atmospheric temperature and water vapour in the overall value of the ERF. We also used double-calls, where the effect of the aerosol or gas was removed from the radiative transfer calculations in the model, to examine the relative contributions of aerosol-cloud interactions and direct radiative effects.

The spread of the results between models is also considered, and the differences attributed to how the models represent different processes, e.g. aerosol-cloud interactions, and the complexity of the atmospheric chemistry modelling. The multi-model means are given below.

Table 1 Multi-model means for ERF for aerosols

ERF Wm-2 aer BC OC SO2 NH3
Multimodel Mean -1.01 0.15 -0.25 -1.03 -0.07
S.D.  0.25 0.17 0.09 0.37 0.01


Table 2 Multi-model means for ERF for chemically reactive gases

Multi-model mean 0.67 0.12 0.26 -0.86 0.20 0.14 0.09
S.D. 0.17 0.21 0.07 0.18 0.07 0.13  0.14


We find the overall aerosol ERF (aer in Table 1) is consistent with previous work, although the results for black carbon (BC) show adjustments that are generally weaker than in previous studies. We also found that cloud effects  can have a large impact on the overall ERF, including for the chemically-reactive gases.

The impact of other processes, especially atmospheric chemistry on the overall results is also discussed.

How to cite: Thornhill, G., Collins, W., Kramer, R., Olivie, D., and Skeie, R.: Using the AerChemMIP experiments to calculate radiative forcing from aerosols and chemically reactive gases, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-7757,, 2021.

Steven Turnock, Mohit Davli, James Keeble, Eddy Robertson, and Fiona O'Connor

Short lived climate forcers (SLCFs) are important atmospheric components as they can influence climate, through interactions with the Earth’s radiative balance, but also impact regional air quality. Two important SLCFs are tropospheric O3 and fine particulate matter (with a diameter less than 2.5 microns – PM2.5). Future policy measures aim to provide co-benefits to both climate and air quality via mitigation of SLCFs. However, it is still uncertain how future reductions in SLCFs will impact both climate and air quality. Sensitivity experiments conducted as part of the 6th Coupled Model Intercomparison Project (CMIP6) provide an opportunity to assess the climate and air quality impacts of different mitigation scenarios.

Here we use results from UKESM1 (an Earth system model with interactive chemistry and aerosols) for the future climate and emission scenario ssp370SST, an atmosphere only simulation that assumes low mitigation of climate and air pollutants. We then compare results from this scenario to different sensitivity experiments to assess the impact on climate, through effective radiative forcing (ERF), and air quality, by changes in surface concentrations of O3 and PM2.5. The sensitivity experiments consider reductions to CH4 concentrations and emissions of O3 and aerosol precursors, individually and combined. Additional sensitivity experiments also consider the individual impact from land-use change, climate change and all emissions.

Compared to ssp370SST, scenarios that strongly mitigate both aerosol and O3 precursors, including CH4, produce the largest benefits in 2100 to global air quality, a 10-25% reduction in global annual mean O3 and PM2.5 concentrations, and climate, a change in ERF of up to -1.2 Wm-2. If CH4 concentrations are not reduced but other aerosols and O3 precursors are, then there are still benefits to air quality in 2100, relative to ssp370SST, but the change in the ERF becomes positive (up to +0.4 Wm-2), mainly due to aerosols reductions. The impact of solely reducing CH4 concentrations results in a large change in ERF (-1.4 Wm-2) and large reductions in surface O3 (-15%) in 2100 but has negligible impacts on surface PM2.5. Reducing only aerosol precursors decreases PM2.5 concentrations but results in a positive change in O3 concentrations and ERF in 2100. Reducing only tropospheric O3 precursors decreases surface O3 concentrations but has minimal impact on PM2.5 concentrations and the ERF in 2100. Implementing different land-use policies has a small impact on the ERF in 2100 but slightly increases both global surface O3 and PM2.5.

The results from this single model study show the importance of considering the different impacts of SLCFs on future air quality and climate metrics.

How to cite: Turnock, S., Davli, M., Keeble, J., Robertson, E., and O'Connor, F.: The Climate and Air Quality response under different future ssp370 pathways in UKESM1, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8216,, 2021.

Lei Lin, Zhili Wang, Yangyang Xu, Huizheng Che, Xiaoye Zhang, Hua Zhang, Wenjie Dong, Chense Wang, Ke Gui, and Bing Xie

Anthropogenic aerosol (AA) forcing has been shown as a critical driver of climate change over Asia since the mid-20th century. Here we show that almost all Coupled Model Intercomparison Project Phase 6 (CMIP6) models fail to capture the observed dipole pattern of aerosol optical depth (AOD) trends over Asia during 2006–2014, last decade of CMIP6 historical simulation, due to an opposite trend over eastern China compared with observations. The incorrect AOD trend over China is attributed to problematic AA emissions adopted by CMIP6. There are obvious differences in simulated regional aerosol radiative forcing and temperature responses over Asia when using two different emissions inventories (one adopted by CMIP6; the other from Peking university, a more trustworthy inventory) to driving a global aerosol-climate model separately. We further show that some widely adopted CMIP6 pathways (after 2015) also significantly underestimate the more recent decline in AA emissions over China. These flaws may bring about errors to the CMIP6-based regional climate attribution over Asia for the last two decades and projection for the next few decades, previously anticipated to inform a wide range of impact analysis.

How to cite: Lin, L., Wang, Z., Xu, Y., Che, H., Zhang, X., Zhang, H., Dong, W., Wang, C., Gui, K., and Xie, B.: Incorrect Asian aerosols affecting the attribution and projection of regional climate change in CMIP6 models , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8421,, 2021.

Catherine Hardacre, Jane P. Mulcahy, Richard Pope, Can Li, Steve Rumbold, and Colin Jones

UKESM1 is the latest generation Earth system model to be developed in the UK. It simulates the core physical and dynamical processes of land, atmosphere, ocean and sea ice systems which are extended to incorporate key marine and terrestrial biogeochemical cycles. These include the carbon and nitrogen cycles and interactive stratosphere-troposphere trace gas chemistry. Feedbacks between these components that have an important amplifying or dampening effect on the physical climate, and/or change themselves in response to changes in the physical climate are also included. One focus for the future development of UKESM1 is improved treatment of sulphur processes, including emission, chemical processing and deposition in the aerosol-chemistry scheme, UKCA-Mode. These processes span land-atmosphere and ocean-atmosphere boundaries and can therefore impact feedbacks between these systems. Emissions of SO2 can be oxidised to form sulphate aerosol, which plays a key role in both acid deposition, atmospheric aerosol loading and cloud properties, thereby directly contributing to the Earth’s radiative balance. Good representation of sulphur processes in UKESM1 is therefore essential for constraining uncertainties associated with the impacts of aerosols on the Earth system and thus understanding the global climate. Here we challenge UKESM1 with observations of SO2 and sulphate from ground-based measurement networks in Europe and the USA, and of SO2 from the Ozone Monitoring Instrument (OMI). We use these to evaluate temporal and spatial biases in the model’s simulation of SO2 and sulphate.  

We find that UKESM1 captures the historical trend for decreasing concentrations of atmospheric SO2 and sulphate in both Europe and the USA over the period 1987 to 2014. However, in the polluted regions of the Eastern USA and Europe, UKESM1 over-predicts surface SO2 concentrations by an average of 320-340%, while under-predicting surface sulphate concentrations by 25-35%. In the cleaner Western USA, the model over-predicts both surface SO2 and sulphate concentrations by 1200% and 150% respectively. The variability in the direction of UKESM1’s bias according to species and region suggests that the model bias may be driven differently depending on species and region. These drivers likely result from uncertainty in aspects of the sulphur cycle, including SO2 emission, loss processes (oxidation and deposition) or transport. To evaluate UKESM1 at the global scale we use a newly available data product for total column SO2 (TCSO2) from OMI. We find that UKESM1 over-predicts TCSO2 over much of the globe, particularly the large source regions of India, China, the USA and Europe as well as over background regions, including much of the ocean. 

In this study we also assess changes to UKESM1’s SO2 dry deposition parameterization. These changes increase SO2 dry deposition to land and ocean surfaces, thus reducing atmospheric SO2 and sulphate concentrations, and ultimately reducing cold bias in UKESM1's simulation of mid 20th C global mean surface temperatures. In comparison with the ground based and satellite observations, we find that the changes reduce UKESM1's over prediction of surface SO2 concentrations and TCSO2

How to cite: Hardacre, C., Mulcahy, J. P., Pope, R., Li, C., Rumbold, S., and Jones, C.: Challenging UKESM1 with SO2 and sulphate observational data to evaluate the aerosol sulphur cycle , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10736,, 2021.

Nick Schutgens and Qirui Zhong and the AEROCOM/AEROSAT teams

Absorbing aerosol has the capacity to warm the climate, but the amount of warming is highly uncertain. AAOD (Absorptive Aeorosl Optical Depth) is an optical measure of the abundance of this absorbing aerosol, comprising mineral dust, black and brown carbon and can be retrieved from satellite measurements providing an almost global view on absorbing aerosol.

In this study we evaluate AEROCOM models with satellite observations of AAOD and SSA (Single Scattering Albedo) and interpret the discrepancies. Over source regions, diversity in model AAOD is mostly due to emissions even though models employ different assumptions regarding the imaginary refractive index. On the one hand this suggests emissions to be a major error source, on the other hand it suggests that the AEROCOM ensemble as a whole may have a bias with regards to MAC (Mass Absorption Coefficient). We show that in the models AAOD scales almost linearly with emissions (either black carbon or dust) and this allows the use of observations as a constraint.  In contrast, model diversity in AOD is shown to depend in almost equal measure on emissions, lifetimes and MECs (Mass Extinction Coefficient). We also analyse mineral dust and black carbon lifetimes by considering the contrast in AAOD over source regions and over outflow regions, and again provide observations constraints.

While the older Phase II models generally underestimate AAOD, Phase III models tend to straddle the observations, with some models over-estimating and other models underestimating AAOD. Emissions seem to be the driving factor in this difference. The amount of diversity is larger in the Phase III than Phase II models.

This study was conducted using four satellite datasets of AAOD and SSA. These datasets were extensively evaluated with AERONET. Dearth of observations prevents global assesment of the satellite retrievals. However, we show that model evaluation is relatively independent of the chosen dataset, even though we identify significant biases between the datasets.

How to cite: Schutgens, N. and Zhong, Q. and the AEROCOM/AEROSAT teams: AEROCOM/AEROSAT: an intercomparison of AAOD & SSA in model and satellite data, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-11992,, 2021.

Matthew Kasoar, Douglas Hamilton, Daniela Dalmonech, Stijn Hantson, Gitta Lasslop, Apostolos Voulgarakis, and Christopher Wells

The CMIP6 Shared Socioeconomic Pathway (SSP) scenarios include projections of future changes in anthropogenic biomass-burning.  Globally, they assume a decrease in total fire emissions over the next century under all scenarios.  However, fire regimes and emissions are expected to additionally change with future climate, and the methodology used to project fire emissions in the SSP scenarios is opaque.

We aim to provide a more traceable estimate of future fire emissions under CMIP6 scenarios and evaluate the impacts for aerosol radiative forcing.  We utilise interactive wildfire emissions from four independent land-surface models (CLM5, JSBACH3.2, LPJ-GUESS, and ISBA-CTRIP) used within CMIP6 ESMs, and two different machine-learning methods (a random forest, and a generalised additive model) trained on historical data, to predict year 2100 biomass-burning aerosol emissions consistent with the CMIP6-modelled climate for three different scenarios: SSP126, SSP370, and SSP585.  This multi-method approach provides future fire emissions integrating information from observations, projections of climate, socioeconomic parameters and changes in vegetation distribution and fuel loads.

Our analysis shows a robust increase in fire emissions for large areas of the extra-tropics until the end of this century for all methods.  Although this pattern was present to an extent in the original SSP projections, both the interactive fire models and machine-learning methods predict substantially higher increases in extra-tropical emissions in 2100 than the corresponding SSP datasets.  Within the tropics the signal is mixed. Increases in emissions are largely driven by the temperature changes, while in some tropical areas reductions in fire emissions are driven by human factors and changes in precipitation, with the largest reductions in Africa. The machine-learning methods show a stronger reduction in the tropics than the interactive fire models, however overall there is strong agreement between both the models and the machine-learning methods.

We then use additional nudged atmospheric simulations with two state-of-the-art composition-climate models, UKESM1 and CESM2, to diagnose the impact of these updated fire emissions on aerosol burden and radiative forcing, compared with the original SSP prescribed emissions.  We provide estimates of future fire radiative forcing, compared to modern-day, under these CMIP6 scenarios which span both the severity of climate change in 2100, and the rate of reduction of other aerosol species.

How to cite: Kasoar, M., Hamilton, D., Dalmonech, D., Hantson, S., Lasslop, G., Voulgarakis, A., and Wells, C.: Improved estimates of future fire emissions under CMIP6 scenarios and implications for aerosol radiative forcing, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13822,, 2021.

Gunnar Myhre

Rapid adjustments play an important role for the climate effect of drivers of climate change. These adjustments to temperature, water vapour and clouds occur before changes to the surface temperature. We show results from CMIP6 models and results of various climate drivers in Precipitation Driver and Model Intercomparison Project (PDRMIP). Rapid adjustments are particularly important for the climate effect of black carbon. The magnitude and sign of the black carbon rapid adjustments depend strongly on the vertical profile of black carbon. Black carbon in the upper troposphere causes a strong reduction of clouds. On the other hand, CO2 increases upper-level clouds resulting in cloud rapid adjustment in some climate models is a substantial fraction of the cloud feedback

How to cite: Myhre, G.: Cloud rapid adjustments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14396,, 2021.

Chris Wells and Apostolos Voulgarakis

Aerosols are a major climate forcer, but their historical effect has the largest uncertainty of any forcing; their mechanisms and impacts are not well understood. Due to their short lifetime, aerosols have large impacts near their emission region, but they also have effects on the climate in remote locations. In recent years, studies have investigated the influences of regional aerosols on global and regional climate, and the mechanisms that lead to remote responses to their inhomogeneous forcing. Using the Shared Socioeconomic Pathway scenarios (SSPs), transient future experiments were performed in UKESM1, testing the effect of African emissions following the SSP3-RCP7.0 scenario as the rest of the world follows SSP1-RCP1.9, relative to a global SSP1-RCP1.9 control. SSP3 sees higher direct anthropogenic aerosol emissions, but lower biomass burning emissions, over Africa. Experiments were performed changing each of these sets of emissions, and both. A further set of experiments additionally accounted for changing future CO2 concentrations, to investigate the impact of CO2 on the responses to aerosol perturbations. Impacts on radiation fluxes, temperature, circulation and precipitation are investigated, both over the emission region (Africa), where microphysical effects dominate, and remotely, where dynamical influences become more relevant. 

How to cite: Wells, C. and Voulgarakis, A.: The local and remote atmospheric impacts of Africa’s 21st century aerosol emission trajectory, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14417,, 2021.

Qirui Zhong, Nick Schutgens, and Guido van der Werf

Biomass burning (BB) injects aerosols into the atmosphere and can thereby affect the earth climate and human health. Yet the modeling of BB aerosols exhibits significant bias. Here we present a comprehensive evaluation of AeroCom model simulations with satellite observations to understand such uncertainties. A total of 59 model runs using 17 models from three AeroCcom Phase III experiments (i.e., Biomass Burning emissions, CTRL2016, and CTRL2019 experiment) and 14 satellite products are involved. AOD (aerosol optical depth) at 550 nm wavelength during the fire season over three typical fire regions (Amazon, South Hemisphere Africa, and Boreal North America, or AMAZ, SHAF, and BONA) is the focus of our study, although we also consider AE and SSA from POLDER.

The 14 satellite products are shown to have quite substantial differences from AERONET observation. But we show that such differences have little impact on the model evaluation which is mainly affected by modeling bias. Through the comparison with POLDER observation, we found the modeled AOD are biased by -93% ~ 174% with most models showing significant underestimations even for the most recent modeling experiment (i.e., CTRL19). SHAF is among the three regions with the strongest underestimation in general. By scaling up the input emissions, such negative bias would be significantly reduced, which, however, has little impact on the day-to-day correlation between models and observations.

On top of the satellite-based model evaluation, we interpret the model diversity from the aspect of aerosol emissions, lifetime, and MEC (mass extinction coefficient), which are further linked with specific parameters in models. These three parameters cause similar levels of AOD diversity, which is quite different from the modeled aerosols during non-fire season when the contribution of lifetime is predominant. During the fire season, diversity caused by lifetime is strongly affected by local deposition; as a matter of fact, models tend to do quite poorly in simulating precipitation strength. Modeled MECs show significant correlations with aerosol wet-growth (which is known to be challenging to models) and AE (Angstrom Exponent) for some involved models. Comparisons with POLDER observed AE suggests some models tend to underestimate AE and thus MEC, which might be responsible for the overall AOD underestimation in certain models. Additionally, we show that model AOD biases correlate with satellite observed formaldehyde columns, suggesting SOA formation may be insufficiently captured by models.

How to cite: Zhong, Q., Schutgens, N., and van der Werf, G.: Evaluation and interpretation of modeling bias for biomass burning aerosols in AeroCom models, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14472,, 2021.

Max Coleman, William Collins, Keith Shine, Nicolas Bellouin, and Fiona O'Connor

We investigate a novel use of model nudging to interrogate radiative rapid adjustment mechanisms and their magnitudes in response to aerosol emission perturbations in an earth system model. The radiative effects of a forcing agent can be quantified using the effective radiative forcing (ERF). ERF is the sum of the instantaneous radiative forcing, and radiative adjustments – changes in the atmosphere’s state in response to the initial forcing agent that cause a further radiative forcing. Radiative adjustments are particularly important for aerosols, which affect clouds both via microphysical interactions and changes in circulation, stratification and convection. Understanding the different adjustment mechanisms and their contribution to the total ERF of different aerosol emissions is necessary to better understand how their ERF may change with future changes in anthropogenic aerosol emissions. In this work we investigate radiative adjustments resulting from changes in atmospheric temperature (and the resulting changes in stratification and convection) due to anthropogenic sulphate and black carbon aerosol forcing.

We have conducted multiple global atmosphere-only time-slice experiments using the UK Earth System Model (UKESM1). Each experiment has either control, black carbon perturbed, or sulphur dioxide perturbed emissions; and either no nudging, nudged horizontal winds (uv), or nudged horizontal winds and potential temperature (uvθ). The difference between nudged uvθ minus nudged uv simulations determines the atmospheric temperature related adjustments arising from the aerosol perturbation. We have also conducted repeats of each simulation, varying the nudging setup to test sensitivity to different nudging parameters.

We find that nudging horizontal winds affects the resulting ERF very little, whereas nudging potential temperature as well can cause a significant difference from the non-nudged experiments, primarily in the cloud radiative effect. However, this difference is sensitive to the strength of the nudging applied, for which we consider the most appropriate value.

How to cite: Coleman, M., Collins, W., Shine, K., Bellouin, N., and O'Connor, F.: Investigating model nudging as a novel technique to isolate aerosol radiative adjustment mechanisms, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15065,, 2021.

Zhenze Liu, Ruth M. Doherty, Oliver Wild, Fiona M. O’Connor, and Steven T. Turnock

A new version of the UKCA chemistry-climate model with highly reactive volatile organic compounds (VOCs) is used to investigate the ozone (O3) responses in historical (2004-2014) and future (2045-2055) shared socio-economic pathways (SSPs) scenarios of CMIP6 AerChemMIP experiments. Significant increases in surface O3 levels in South and East Asia are simulated in the new version compared with the standard UKCA model. The O3 production and the O3 burden averaged over the troposphere increase slightly by 6 % as a result of more highly reactive VOCs, but the O3 lifetime is quite similar. Comparing the different SSP scenarios using this new model version we find the averaged surface O3 concentrations are higher in the scenario with high emissions than for historical conditions. O3 concentrations are much lower than historical O3 concentrations when O3 precursor concentrations are low. However, regional O3 increases occur in East Asia in the future scenario with low emissions of short-lived climate forcers due to strong VOC limited regimes. Decreases in surface O3 concentrations occur globally in the future scenario that has lower methane (CH4) concentrations. We construct O3 and O3 production isopleths. These both suggest that the threshold of NOx/VOCs shifting from NOx limited to VOC limited regimes is approximately 0.8. More areas become VOC limited in South Asia in all future scenarios, but there is little change for East Asia. The hydroxyl radical (OH) concentrations generally increase in regions with high O3 precursor abundances in the future scenario, but the high OH levels are offset by lower CH4 concentrations in the future low CH4 scenario. We find that there are small changes in O3 production efficiency in continental regions in all future scenarios. Relative O3 burden changes between the future SSP and historical scenarios are larger in the troposphere than in the planetary boundary layer (PBL), illustrating that O3 burdens are less sensitive in the PBL under emission and climate change. The O3 lifetime in the troposphere decreases in all future scenarios as compared to the historical period. We find that the decreases in O3 precursors and CH4 concentrations play important roles in reducing O3 burdens in the future.

How to cite: Liu, Z., M. Doherty, R., Wild, O., M. O’Connor, F., and T. Turnock, S.: O3 responses in CMIP6 AerChemMIP experiments: different roles of O3 precursors, CH4 concentrations and climate change, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15685,, 2021.

Paul Griffiths, Zeng Guang, Sungbo Shim, Jane Mulcahy, Lee Murray, Fiona O'Connor, Alex Archibald, and John Pyle

A grand challenge in the field of chemistry-climate modelling is to understand the connection between anthropogenic emissions, atmospheric composition and the radiative forcing of trace gases and aerosols.   The AerChemMIP model intercomparison project, part of CMIP6, focuses on calculating the radiative forcing of gases and aerosol particles over the period 1850 to 2100.  We present an analysis of the trends in tropospheric ozone budget in the UKESM1 and other models from CMIP6 experiments. We discuss these trends in terms of chemical production and loss of ozone as well as physical processes such as transport and deposition.  Where possible, AerChemMIP attribution experiments such as histSST-piCH4, will be used to quantify the effect of individual emissions and forcing changes on the historical ozone burden and budget.  For future experiments, we focus on analogous experiments from the SSP3-70 scenario, a ‘regional rivalry’ shared socioeconomic pathway involving significant emissions changes.

How to cite: Griffiths, P., Guang, Z., Shim, S., Mulcahy, J., Murray, L., O'Connor, F., Archibald, A., and Pyle, J.: Analysis of ozone budget in CMIP6 experiments, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16097,, 2021.

Fiona O'Connor, Omar Jamil, Timothy Andrews, Ben Johnson, Jane Mulcahy, and James Manners

The pre-industrial (PI; Year 1850) to present-day (PD; Year 2014) increase in methane concentration leads to a global mean effective radiative forcing (ERF) of 0.97 ± 0.04 W m-2 in the UK’s Earth System Model, UKESM1. In comparison with the multi-model estimate of 0.75 ± 0.10 W m-2 from the Aerosol and Chemistry Model Intercomparison Project (AerChemMIP), UKESM1 has the highest methane ERF and lies outside the 1-sigma range. This is, in part, due to UKESM1 including interactive chemistry and positive indirect effects, such as methane-driven changes in tropospheric ozone. However, UKESM1 is the only model within AerChemMIP whose contribution to the methane ERF from tropospheric adjustments is positive – this is largely driven by the strong positive cloud adjustment in UKESM1, in contrast to other models. In this work, we apportion the total methane ERF between direct and indirect effects (including adjustments) and provide a process-based understanding of what is driving the positive cloud adjustment in UKESM1.

Using additional UKESM1 paired simulations, we apportion the total methane ERF between its direct methane contribution and indirect contributions from ozone, water vapour, and aerosols. This approach offers the advantage that linearity is not assumed and it distinguishes between cloud effects that are dynamically-driven via changes in temperature and those that are aerosol-mediated. By analysing the chemistry-aerosol budgets and the cloud responses, we find that the PI to PD increase in methane leads to an indirect positive aerosol ERF of up to 0.3 ± 0.06 W m-2, with a near-zero contribution from the instantaneous radiative forcing from aerosol-radiation interactions. Methane-driven changes in oxidants alter the relative contributions of the different sulphur dioxide oxidation pathways, causing a change in new particle formation rates and a shift in the aerosol size distribution towards fewer but larger particles. There is a resulting decrease in cloud droplet number concentration, an increase in cloud droplet effective radius, and a decrease in liquid water path in marine stratocumulus regions from aerosol-cloud interactions (mainly through the cloud lifetime effect). There is a subsequent change in the cloud radiative effect, with the positive change in the shortwave dominating over the negative change in the longwave. However, when aerosol-cloud interactions are disabled, the change in the cloud radiative effect is negative and is dominated by the reduction of cirrus clouds in the tropics, thus making UKESM1 more consistent with the other AerChemMIP models.

These results can explain some of the diversity in multi-model estimates of methane forcing and highlight the importance of chemistry-aerosol-cloud interactions when quantifying climate forcing by reactive greenhouse gases.

How to cite: O'Connor, F., Jamil, O., Andrews, T., Johnson, B., Mulcahy, J., and Manners, J.: Apportionment of the present-day forcing by methane using UKESM1: The role of chemistry-aerosol-cloud interactions , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16375,, 2021.

Manish Jangid and Amit Mishra

Atmospheric brown clouds (ABCs) are a dense and extensive pollution layer and have significant implications on air quality, agriculture, water cycle, and regional climate. The objective of the present study is to observe seasonal and spatial variations in the occurrence of ABCs and its radiative effects. The Indo-Gangetic plain (IGP) is the most populated region of India, which is an extended region in the foothills of the Himalayas. The IGP is one of the ABCs hotspots over the globe. The frequency of ABCs occurrences and radiative forcing were calculated using data from seven ground-based remote sensors situated across the IGP. We have used total ~ 5000 days of Level-2 aerosol measurements from seven AERosol Robotic NETwork (AERONET) stations (Karachi, Lahore, Jaipur, New Delhi, Kanpur, Gandhi college and Dhaka University) for three seasons (Pre-monsoon, Post-monsoon, and Winter) during 2000-2019. An algorithm based on the optical properties of aerosols is used to defined extreme pollution events (ABCs days) for each site. Our results show more frequent occurrences of ABCs over the region in the pre-monsoon out of all three seasons. However, spatial variation is found in all seasons, like maximum frequency of ABCs over western IGP region in post-monsoon and minimum is at eastern IGP region in the winter season. Further, we have used the Santa Barbara DISORT Atmospheric Radiative Transfer (SBDART) model to calculate radiative forcing during ABCs days on all sites of study. Aerosol optical depth (AOD) and absorption optical depth (AAOD) was used to calculate radiative forcing over the IGP region. Radiative forcing of ABCs is negative at both the surface (SRF) and top of the atmosphere (TOA), whereas it is positive in the atmosphere (ATM). In magnitude, it was found minimum in the pre-monsoon season at TOA. However, other seasons have specific features over specific locations, for example, in the winter season, radiative forcing is maximum over Kolkata at TOA, SRF, and ATM, which are -13.81 W/m2, -50.90 W/m2, and +37.09 W/m2 respectively. In the pre-monsoon season, radiative forcing is maximum at Delhi (-9.59 W/m2) at TOA. In post-monsoon season radiative forcing maximum at Gandhi-college (-11.30 W/m2) at TOA. This ground observation is also compared with Modern Era Retrospective analysis and Research and Applications-2 (MEERA 2) modal data. These results indicate the cooling effect of ABCs at the surface and TOA over the IGP region throughout the period.

How to cite: Jangid, M. and Mishra, A.: Radiative forcing of atmospheric brown clouds over the Indo-Gangetic Plain, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16481,, 2021.