Says Danielle, pollutants, particularly black carbon (BC), are affecting the monsoons by a process that starts in the winter when soot combines with dust blown from the west, creating huge clouds of haze that hug the southern slopes of the Himalayas.
Danielle explains, "The BC in these clouds absorb solar radiation, warming the air even faster than usual. This draws more moisture to the region sooner, causing the early monsoon to intensify. This theory is known as the Elevated Heat Pump, as the soot acts to pump heat up the Himalayan slopes. Observations show a widespread and sustained warming in the pre-monsoon season over the last three decades. In that same time period, early monsoon rainfall has increased by 20% ."
"Over the oceans, BC has a different, although equally damaging effect. BC combines with other anthropogenic aerosols, forming Atmospheric Brown Clouds (ABCs), large plumes of particles that can stretch over whole continents or ocean basins. These ABCs absorb solar radiation in the atmosphere, causing dimming below. This reduction of irradiance reduces evaporation and cools the surface, leading to a weakening of the later monsoon."
"The combined impact of these two phenomena, the Elevated Heat Pump and Solar Dimming, increases flooding during the early months of monsoon, and causes drought later on. Some studies suggest that over time the result will be an overall weakening of the monsoon and a reduction of rainfall over the region. Reduction in rainfall of great concern because in South Asia there is a strong positive correlation between the amount of precipitation and food production. The Indian summer monsoon is the biggest source of freshwater to the region: over 70% of the annual precipitation over India occurs during the summer monsoon."
