Anthropogenic Aerosols Delay the Emergence of GHGs-Forced Wetting of South Asian Rainy Seasons Under a Fossil-Fuel Intensive Pathway

With continued fossil-fuel dependence, anthropogenic aerosols over South Asia are projected to increase until the mid-21st century along with greenhouse gases (GHGs). Using the Community Earth System Model (CESM1) Large Ensemble, we quantify the influence of aerosols and GHGs on South Asian seasonal precipitation patterns over the 21st century under a very high-emissions (RCP 8.5) trajectory. We find that increasing local aerosol concentrations could continue to suppress precipitation over South Asia in the near-term, delaying the emergence of precipitation increases in response to GHGs by several decades in the monsoon season and a decade in the post-monsoon season. Emergence of this wetting signal is expected in both seasons by the mid-21st century. Our results demonstrate that the trajectory of local aerosols together with GHGs will shape near-future precipitation patterns over South Asia. Therefore, constraining precipitation response to different trajectories of both forcers is critical for informing near-term adaptation efforts. Agricultural production, water availability, and the economy in South Asia depend closely on reliable rainfall. While much of this depends on monsoon season rainfall, the pre-monsoon season and the post-monsoon season are also important for these sectors. Understanding how and why South Asian rainfall patterns in these seasons are likely to change is, therefore, relevant for adaptation, planning, and infrastructure resilience. Multiple external climate forcers and natural climate variability influence South Asian rainfall. We examine how increasing greenhouse gases (GHGs) and changing anthropogenic aerosol distributions-two key forcers - shape seasonal rainfall patterns across South Asia using large-ensemble climate simulations for a very high emissions pathway. Our findings show that anthropogenic aerosols, which have a predominantly weakening influence on rainfall, could suppress the enhancement of monsoon and post-monsoon season rainfall projected in response to GHGs by several decades. Aerosols continue to be important influences on rainfall patterns in the region for at least the next few decades, after which the influence of GHGs will dominate. While aerosols from other regions have historically influenced regional rainfall, we find that local aerosols are primarily responsible for the projected changes in rainfall patterns. Aerosols could suppress greenhouse gases-forced intensification of rainfall in monsoon and post-monsoon seasons for several yearsAerosols will dominate monsoon and post-monsoon precipitation patterns until the mid-21st century under a very high emissions trajectoryLocal aerosols show a stronger influence on monsoon precipitation patterns throughout the 21st century relative to remote aerosols