Aggressive aerosol mitigation policies reduce chances of keeping global
warming to below 2C
Abstract
Aerosol increases over the 20th century delayed the rate at which Earth
warmed as a result of increases in greenhouse gases (GHGs). Aggressive
aerosol mitigation policies arrested aerosol radiative forcing from ~1980 to ~2010. Recent
evidence supports decreases in forcing magnitude since then. Using the
approximate partial radiative perturbation (APRP) method, future
shortwave aerosol effective radiative forcing changes are isolated from
other shortwave changes in an 18-member ensemble of ScenarioMIP
projections from phase 6 of the Coupled Model Intercomparison Project
(CMIP6). APRP-derived near-term (2020-2050) aerosol forcing trends are
correlated with published model emulation values but are
30-50% weaker. Differences are likely explained by
location shifts of aerosol-impacting emissions and their resultant
influences on susceptible clouds. Despite weaker changes, implementation
of aggressive aerosol cleanup policies will have a major impact on
global warming rates over 2020-2050. APRP-derived aerosol radiative
forcings are used together with a forcing and impulse response model to
estimate global temperature trends. Strong mitigation of GHGs, as in
SSP1-2.6, likely prevents warming exceeding 2C since preindustrial but
the strong aerosol cleanup in this scenario increases the probability of
exceeding 2C by 2050 from near zero without aerosol changes to
6% with cleanup. When the same aerosol forcing is
applied to a more likely GHG forcing scenario (i.e., SSP2-4.5),
aggressive aerosol cleanup more than doubles the probability of reaching
2C by 2050 from 30% to 80%. It is thus
critical to quantify and simulate the impacts of changes in aerosol
radiative forcing over the next few decades.