Nonlinear Response of Asian Summer Monsoon Precipitation to Emission
Reductions in India and China
Abstract
Anthropogenic aerosols over South and East Asia currently have a
stronger impact on the Asian Summer Monsoon (ASM) than greenhouse gas
emissions, yet projected aerosol emission changes in these regions are
subject to considerable uncertainty in timescale, location, emission
type, and even the sign of the change, implying large uncertainties in
future ASM change. In addition, aerosol changes in either South or East
Asia cause circulation anomalies that affect both countries and
neighbouring regions. We use a circulation/climate model to demonstrate
that the sum of ASM responses to individual aerosol emission reductions
in each region is very different to the response to simultaneous
reductions in both regions, implying the ASM response to aerosol
emissions reductions is highly nonlinear. The phenomenon is independent
of whether aerosols are scattering or absorbing, and is driven by
large-scale teleconnections between the two regions. The nonlinearity
represents a new source of uncertainty in projections of ASM changes
over the next 30-40 years, and limits the utility of country-dependent
aerosol trajectories when considering their Asia-wide effects. To
understand likely changes in the ASM due to aerosol reductions,
countries will need to accurately take account of emissions reductions
from across the wider region, rather than approximating them using
simple scenarios and emulators. The nonlinearity in the response to
forcing therefore presents a regional public goods issue for countries
affected by the ASM, as the costs and benefits of aerosol emissions
reductions are not internalised; in fact, forcings from different
countries work jointly to determine outcomes across the region.