  The South and East Asian summer monsoons are globally significant
meteorological features, creating a strongly seasonal pattern of precipitation.
The stability of the monsoon is of extreme importance for a vast range of
ecosystems and for the livelihoods of a large share of the world's population.
Simulations are performed with an intermediate complexity climate model,
PLASIM, to assess the future response of the monsoons to changing
concentrations of aerosols and greenhouse gases. The aerosol loading consists
of a mid-tropospheric warming and a surface cooling, which is applied to India,
Southeast Asia and East China, both concurrently and independently. The primary
effect of increased aerosol loading is a decrease in summer precipitation in
the vicinity of the applied forcing, although the regional response varies
significantly. The decrease in precipitation is only partially ascribable to a
decrease in the precipitable water, and instead derives from a reduction of the
precipitation efficiency, due to changes in the stratification of the
atmosphere. When the aerosol loading is added in all regions simultaneously,
precipitation in East China is most strongly affected, with a quite distinct
transition to a low precipitation regime as the radiative forcing increases
beyond 60 W/m^2. The response is less abrupt as we move westward, with
precipitation in South India being least affected. This lower sensitivity in
South India is attributed to aerosol forcing over East China. Additionally, the
effect on precipitation is approximately linear with the forcing. The impact of
doubling carbon dioxide levels is to increase precipitation over the regions
and weaken the circulation. When the carbon dioxide and aerosol forcings are
applied at the same time, the carbon dioxide forcing partially offsets the
surface cooling and reduction in precipitation associated with the aerosol
response.
