The turning point of the aerosol era

Climate change is the result of individual forcing agents changing their radiative balance at the top of the atmosphere over time, and as a result, if positive radiative forcings dominate over negative forcings, the troposphere warms. Over the historical period, based on the CMIP6 simulations ranging from 1850 until 2014, aerosol effects via their ability to absorb or scatter solar radiation and alter clouds, have provided the largest negative forcings compared to all other forcings and played an important role in counterbalancing some of the greenhouse gas (GHG) caused global warming. Trends in aerosol have been very diverse globally, depending on source and geographical region. While many regions in the Northern Hemisphere have been seeing decreasing emissions since decades, changes in Asia have been more recent, with some countries, such as China have recently reversed their trends and now have decreasing emissions, while other regions, such as India or parts of South Asia, e.g., are still on an increasing trajectory.

Here we study aerosol forcing trends in the CMIP6 simulations of the GISS ModelE2.1 coupled ocean climate model using a fully coupled atmosphere composition configuration, including interactive gas-phase chemistry, and either an aerosol microphysical (MATRIX) or a mass-based (OMA) aerosol module. The historical (1850-2014) CMIP6 as well as four Shared Socioeconomic Pathways (SSP) simulations (2015-2100) are analyzed, including the future scenarios, SSP1-2.5, SSP2-4.5, SSP3-7.0 and SSP 5-8.5.

The main conclusion of this study is that aerosol forcings have reached their turning point, switching from globally increasing to decreasing trends, in the first decade of the 21^st century. The turning point in aerosol direct forcing does depend on the individual SSP and model used, however forcings caused by aerosol cloud interactions fall under all studied scenarios into the historical period. The fact that aerosol-cloud forcings dominate in magnitude over direct forcings, leads to the conclusion that the turning point of total aerosol forcings has already been reached. As a consequence, it could be possible that the recently observed global warming which is primarily driven by greenhouse gases has been augmented by the effect of a decreasing aerosol cooling effect on the global scale.