Abstract
AbstractAn 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−2relative 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).
Highlights
Aerosol effective radiative forcing (ERF) remains one of the most uncertain components of the present-day climate (Bellouin, Quaas, et al, 2020)
The weighted 5%–95% range from the CMIP6-constrained time series using both global-mean surface air temperature (GSAT) and energy uptake (EEU) as constraints is shown as a gray band with the weighted mean as a gray line
With the CMIP6-constrained time series, aerosol ERF exhibits a slight recovery between 1980 and 2014 of +0.025 W m−2 decade−1. This is a lower aerosol recovery than seven of the 11 CMIP6 models, the constrained 5%–95% range is wide (−0.074 to +0.111 W m−2 decade−1) and includes the means from all but the UKESM1-0-LL model (Figure 7). These results indicate that a rapid aerosol forcing recovery is unlikely and not consistent with the energy budget constraints, but whether aerosol forcing has been strengthening, weakening, or stable in recent decades is not conclusive
Summary
Aerosol effective radiative forcing (ERF) remains one of the most uncertain components of the present-day climate (Bellouin, Quaas, et al, 2020). Uncertainty in present-day forcing reduces our ability to confidently predict the future climate response to emissions (Forster et al, 2013) and the level of historical greenhouse gas warming masked by the cooling effect of aerosols (Samset et al, 2018). In most future socio-economic scenarios, aerosol forcing is projected to become less negative over the 21st century (Gidden et al, 2019; Huppmann et al, 2018; Rogelj et al, 2018), promoting an increase in the rate of warming unless there is a concurrent reduction in greenhouse gas emissions (Shindell & Smith, 2019). The time history of aerosol ERF is a necessary input to many reduced-complexity climate models (Nicholls et al, 2020), which SMITH ET AL.
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