Origins of a Relatively Tight Lower Bound on Anthropogenic Aerosol Radiative Forcing from Bayesian Analysis of Historical Observations

Abstract

AbstractA variety of empirical estimates have been published for the lower bounds on aerosol radiative forcing, clustered around −1.0 or −2.0 W m−2. The reasons for obtaining such different constraints are not well understood. In this study, we explore bounds on aerosol radiative forcing using a Bayesian model of aerosol forcing and Earth’s multi-time-scale temperature response to radiative forcing. We first demonstrate the ability of a simple aerosol model to emulate aerosol radiative forcing simulated by 10 general circulation models. A joint inference of climate sensitivity and effective aerosol forcing from historical surface temperatures is then made over 1850–2019. We obtain a maximum likelihood estimate of aerosol radiative forcing of −0.85 W m−2 (5%–95% credible interval from −1.3 to −0.50 W m−2) for 2010–19 relative to 1750 and an equilibrium climate sensitivity of 3.4°C (5%–95% credible interval from 1.8° to 6.1°C). The wide range of climate sensitivity reflects difficulty in empirically constraining long-term responses using historical temperatures, as noted elsewhere. A relatively tight bound on aerosol forcing is nonetheless obtained from the structure of temperature and aerosol precursor emissions and, particularly, from the rapid growth in emissions between 1950 and 1980. Obtaining a 5th percentile lower bound on aerosol forcing around −2.0 W m−2 requires prescribing internal climate variance that is a factor of 5 larger than the CMIP6 mean and assuming large, correlated errors in global temperature observations. Ocean heat uptake observations may further constrain aerosol radiative forcing but require a better understanding of the relationship between time-variable radiative feedbacks and radiative forcing.