Energy Budget Constraints on the Time History of Aerosol Radiative Forcing
Abstract
Limitations in observation-based proxies for aerosol radiative forcing over the 20th century highlight the importance of climate models as a key line of evidence in determining historical aerosol forcing. However, as historical CMIP6 surface temperature simulations are cooler than the observations in the multi-model mean for much of the 20th Century, it is possible that some CMIP6 models may overestimate the strength of the historical aerosol forcing.
We use transient (1850-2014) aerosol forcing from 10 models participating in RFMIP, AerChemMIP and AMIP to derive simple relationships relating emissions to aerosol forcing. From these 10 models, a large ensemble of possible aerosol forcing time histories is produced by Monte Carlo sampling. This ensemble is run in a simple energy balance climate model with parameters tuned to CMIP6 models. Observational constraints of temperature (1850-2019) and ocean heat uptake (1971-2018) are then applied to the resulting energy-balance outputs to determine the most plausible historical aerosol forcing pathways. Our best estimate and 90% confidence range for aerosol forcing for 2005-2014 relative to 1750 is -1.12 (-1.75 to -0.53) W m-2. Forcing beyond 2014 depends on the emissions time series, and is -0.89 (-1.49 to -0.37) W m -2 in 2019 under SSP2-4.5 where a rapid reduction in global SO 2 emissions is projected to have occurred in the last few years. Our estimate shows that aerosol forcing is relatively constant from 1972 to 2014, in contrast to several CMIP6 models where a definitive peak in negative aerosol forcing occurs before 2014. Our results agree with reanalysis-derived estimates of aerosol forcing since 1980 which show no substantial trend in aerosol forcing over the last 40 years. Using our constrained energy balance model ensemble we derive estimates for equilibrium climate sensitivity of 3.1 (1.9-6.0)°C and transient climate response of 1.8 (1.3-2.7)°C. We also find that several CMIP6 models indeed overestimate global aerosol forcing in the 20th Century or show too positive a recent trend, or both.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2020
- Bibcode:
- 2020AGUFMA113.0005S
- Keywords:
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- 3310 Clouds and cloud feedbacks;
- ATMOSPHERIC PROCESSES;
- 3311 Clouds and aerosols;
- ATMOSPHERIC PROCESSES;
- 3337 Global climate models;
- ATMOSPHERIC PROCESSES;
- 3360 Remote sensing;
- ATMOSPHERIC PROCESSES