Decreased Monsoon Precipitation in the Northern Hemisphere Due to Anthropogenic Aerosols

Abstract:

Observed precipitation in the monsoon regions underwent substantial changes during the second half of the 20th century, with drying from the 1950s to the mid-1980s and increasing precipitation in recent decades. Globally, anthropogenic forcing has been shown to have influenced changes in zonal mean and Northern Hemisphere precipitation. Modelling studies suggest human activity, in the form of anthropogenic aerosols, has been a key factor driving changes in tropical and monsoon precipitation. Here we use climate models from the Climate Model Intercomparison Project Phase 5 to investigate the influence of individual climate forcings (i.e. greenhouse gas, anthropogenic aerosol and natural) on monsoon precipitation in the Northern Hemisphere and apply a robust statistical analysis to determine whether observed changes are driven by these external forcings or whether they could be explained by internal climate variability alone. Four observational datasets were used to calculate the mean May-September (encompassing the whole monsoon season) precipitation anomalies in the Northern Hemisphere summer monsoon (NHSM) region. Multi-model ensembles of climate model simulations were used to derive response patterns to various forcings including all external forcings, which combines anthropogenic (greenhouse gases, aerosols, land use and ozone) and natural forcing (volcanic and solar), greenhouse gas forcing, anthropogenic aerosol forcing, natural forcing and all anthropogenic forcings. The results of a detection and attribution analysis show that the observed temporal changes can only be explained when including the influence of anthropogenic aerosols, even after accounting for internal climate variability. Anthropogenic aerosol, not greenhouse gas or natural forcing, has been the dominant influence on monsoon precipitation over the second half of the 20th century. Future monsoon precipitation will therefore depend on how aerosol emissions change in the coming decades.