Near-term precipitation change highly sensitive to black carbon emissions

Changes in mean and extreme precipitation are arguably the most impactful aspects of climate change. Detailed and accurate projections are therefore crucial for climate risk assessments and adaptation strategies. Generally, global mean precipitation increases with surface warming, with a global mean hydrological sensitivty of around 1 to 2 %/K, and stronger increases in rates or extreme precipitation events. Local variations are however very large, and model projections are much more uncertain than for temperature. 

 

Absorbing aerosols, notably black carbon (BC), brown carbon (BrC) and mineral dust, are an exception to the rule. Their absorption of shortwave radiation inhibits precipitation formation, through rapid adjstments that overwhelm their influence on surface temperature. The hydrological sensitivty to black carbon emissions is therefore around -4 %/K, again with large regional variations, and with a very high spread between models ( -1 to -7 %/K)

 

In this talk, we discuss the near-term (2015-2045) dependence of precipitation change on the evolution of absorbing aerosols. We show the transient hydrological sensitivty in CMIP6, globally and regionally, and how it is affected by air quality policy (i.e. scenario choice) and model treatment of BC, BrC and dust. We confirm that, globally, while black carbon emissions have a modest impact on surface temperature, their influence on precipitation is outsized, causing a factor of 2 difference in hydrological sensitivty beween future scenarios with strict (SSP126) and weak (SSP370) air quality control measures. 

 

Further, for highly populated regions close to, or downwind from, major emission sources - notably India, China and northern Brazil - we find very high sensitivity of precipitation evolution to the levels of absorbing aerosol emissions. Several of these regions are therefore set for a "double whammy" of precipitation increase from global warming and a removal of short wave absorbing air pollution. We also discuss the rates of change of extreme precipitation events, and how they relate to absorbing aerosols in different regions. 

 

Our key message is that changes in absorbing aerosols over the coming decades is a key uncertainty in near term precipitation and extreme event evolution, and therefore a burning knowledge gap for the aerosol-climate community.