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Aerosols and climate

Reanalysis of the satellite observations with improved assumptions about the aerosol size distribution, the snow cover in high latitudes, etc. suggests that previous aerosol direct forcing estimates of approximately -1.0 W/m2 are biased and a value of around -0.65 W/m2. This observation-based estimate still may be biased low, due to choices for surface albedo, AOD, clear-sky to all-sky ratio and assumptions on missing AOD information over desert regions. Sensitivity studies demonstrate that direct cooling in access of -0.45 W/m2 is highly unlikely.

Model studies show that when more stringent air pollution abatements are implemented worldwide, utilizing the presently available most advanced control technologies, the present-day negative total aerosol top-of-the-atmosphere radiative forcing will be strongly reduced (by 50%) by 2030. As a consequence, climate change thereafter will be controlled to a larger extent by changes in greenhouse gas emissions. The temperature response of increasing GHG concentrations and reduced aerosol emissions leads to a global annual mean equilibrium temperature response of 2.18 K. This study thus highlights the huge potential impact of future air pollution mitigation strategies on climate and supports the need for urgent GHG emission reductions. As aerosols strongly impact surface forcings and have thus a high hydrological sensitivity, the consequences for precipitation increases associated with global warming are even stronger. GHG and aerosol forcings are not independent as they both affect and are influenced by changes in the hydrological cycle.

The future of aerosol forcings were also simulated using ECHAM5 climate model for time slices for 2000 and 2100 together with pre-industrial emissions simulations. Using all EUCAARI developed aerosol formation information, we found out that regardless of chosen RCP scenario, the total aerosol forcing will radically reduce in the future, especially if the EUCAARI determined nucleation formations are correct. Even very strong increases in global DMS or biogenic VOC productions cannot overcome the decrease in aerosol concentrations and thus related aerosol cooling (Fig. 7). This effect will strongly increase the potential of GHG gasses to warm the planet.