Bias in CMIP6 models as compared to observed regional dimming and brightening

Abstract

Abstract. Anthropogenic aerosol emissions have increased considerably over the last century, but climate effects and quantification of the emissions are highly uncertain as one goes back in time. This uncertainty is partly due to a lack of observations in the pre-satellite era, making the observations we do have before 1990 additionally valuable. Aerosols suspended in the atmosphere scatter and absorb incoming solar radiation and thereby alter the Earth's surface energy balance. Previous studies show that Earth system models (ESMs) do not adequately represent surface energy fluxes over the historical era. We investigated global and regional aerosol effects over the time period 1961–2014 by looking at surface downwelling shortwave radiation (SDSR). We used observations from ground stations as well as multiple experiments from eight ESMs participating in the Coupled Model Intercomparison Project Version 6 (CMIP6). Our results show that this subset of models reproduces the observed transient SDSR well in Europe but poorly in China. We suggest that this may be attributed to missing emissions of sulfur dioxide in China, sulfur dioxide being a precursor to sulfate, which is a highly reflective aerosol and responsible for more reflective clouds. The emissions of sulfur dioxide used in the models do not show a temporal pattern that could explain observed SDSR evolution over China. The results from various aerosol emission perturbation experiments from DAMIP, RFMIP and AerChemMIP show that only simulations containing anthropogenic aerosol emissions show dimming, even if the dimming is underestimated. Simulated clear-sky and all-sky SDSR do not differ greatly, suggesting that cloud cover changes are not a dominant cause of the biased SDSR evolution in the simulations. Therefore we suggest that the discrepancy between modeled and observed SDSR evolution is partly caused by erroneous aerosol and aerosol precursor emission inventories. This is an important finding as it may help interpret whether ESMs reproduce the historical climate evolution for the right or wrong reason.

Opportunistic experiments to constrain aerosol effective radiative forcing

“…The co-incident upward trend in surface solar radiation ( Wild et al, 2005 ) caused by both ACI and clear-sky aerosol radiative interactions (ARIs) was found useful as an emergent constraint on simulated total aerosol effective radiative forcing (ERF) in the CMIP5 multi-model ensemble ( Cherian et al, 2014 ). In other regions, there are large discrepancies between surface radiation trends and model results ( Stjern et al, 2011 ; Moseid et al, 2020 ).…”

Energy Budget Constraints on the Time History of Aerosol Forcing and Climate Sensitivity

“…Nevertheless, CMIP6 models remain an important line of evidence in determining historical aerosol forcing. Unlike for greenhouse gases, proxy records for aerosol forcing are sparse before widespread surface radiation measurements became available in the 1950s (Bellouin, Quaas, et al., 2020; Moseid et al., 2020). No global observations of aerosols were available until the satellite era (late 1970s), whereas CMIP6 models provide aerosol forcing estimates from 1850.…”

Potential distribution of Artemisia annua L.under climate change

“…The future ones were derived from the projections of eight global climate models (GCMs): BCC-CSM2-MR, CNRM-CM6-1, CNRM-ESM2-1, CanESM5, IPSL-CM6A-LR, MIROC-ES2L, MIROC6, MRI-ESM2-0. These GCMs were selected according to their data availability and involvement in relevant model intercomparison projects within the CMIP6 (Moseid et al, 2020). The future climate data were 20-year averages for 2021-2040, 241-2060, 2061-2080 and 2081-2100 and for four Shared Socio-economic Pathways (SSPs): 126, 245, 370 and 585.…”