What about aerosols?

In the world of climate change aerosols are ultra-small particles which blanket vast areas in a haze that blocks and scatters sunlight reducing the solar energy that reaches the earth’s surface. Thus, evaporation is reduced and the water cycle (that governs where, when, and how much rain falls) is slowed. Currently, these effects are most pronounced in the Northern Hemisphere, which is the source of most of the world’s aerosols and thus suffers the most dimming from these pollutants. However, aerosols move around the world.

In the mid- to late-20th century global emissions of sulfur dioxide (which in the atmosphere becomes sulfate, a reflective aerosol) nearly doubled, reducing the amount of sunlight reaching the earth’s surface by about two percent, on average. As a direct result of this dimming, average rainfall in the Northern Hemisphere declined by between three and four percent over the same period. 

Now the problem is in East Asia and South Asia. These regions, which have rapidly industrialized over the past four decades, have seen a two- to fourfold increase in sulfur dioxide and black carbon emissions since the 1970s. As a result, in 2010, China and India received somewhere between ten and 15 percent less sunlight than they did in 1970. As the wind has carried sulfates and black carbon over thousands of miles, the dimming effect has extended to the atmos­phere over the Indian Ocean, reducing the evaporation of seawater and thus weakening the monsoons that bring much-needed water to East Asia and South Asia every year. From 1950 to 2002, the most recent period for which estimates are available, there was a seven percent decrease in average annual rainfall over the Indo-Gangetic Plain, the fertile belt of land crossing eastern Pakistan, northern India, and Bangladesh that is home to more than one billion people, many of them dependent on rain-fed agriculture. Over the same period, summer monsoon rainfall in parts of northern China decreased by more than ten percent.