Winds of Change: How Black Carbon Affects the South Asian Monsoon

In the previous post, I wrote about how the glaciers of the ‘Third Pole’ – the greater Himalayan-Tibetan Plateau region – are melting at a dramatic rate (just not as fast as mistakenly stated in the IPCC report). The key determinant of whether glaciers are retreating or advancing appears to be the South Asian summer monsoon (June – September). And the summer monsoon is changing.

The climate of South Asia and India is dominated by monsoons. Monsoons are large-scale wind patterns that predictably change direct with the seasons. Landmasses warm up – and cool down – faster than large bodies of water. In the summer, this means that the Tibetan Plateau warms faster than the Indian Ocean to the south. A large low pressure system builds over the land, drawing warm moist air from the ocean, and making the winds blow south to north. In the winter, the reverse is true: the landmass cools quickly, while the ocean holds onto its heat, causing the winds to reverse, blowing from north to south.

How the winds change: in the summer, the elevated Tibetan Plateau heats more rapidly than the ocean.

Pollutants, particularly black carbon (BC), are affecting this process. BC  – the black stuff in soot – is a byproduct of the incomplete combustion of fossil fuels and biofuels. In the winter, soot combines with dust blown from the west, creating huge clouds of haze that hug the southern slopes of the Himalayas. 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 (Lau et al., 2006a, b), 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% (Gautam et al., 2009a, b).

Mid-tropospheric temprrature trend for pre-monsoon season (March-May) from 1979 to 2007. Warming along the southern slopes of the Himalayas is noted in red (Gautam et. al., 2009b).

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 (Ramanathan 2005, 2008). 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.

Thankfully, mitigation of black carbon is much easier than CO2. Unlike CO2 and it’s greenhouse gas cousins, BC is a very short-lived pollutant. It only stays in the atmosphere for a week or so and doesn’t travel very far from its source. What that means is that reductions of BC will be felt immediately and locally. Reductions of BC have the added benefit of reducing air pollution, the 4th leading cause of death in the developing world. This is good news for India, the second largest producer of BC in the world after China. (The US is the largest producer of BC per capita).

This is NOT to say that global emissions reduction efforts should shift away from GHGs and the West’s responsibility to clean up its act. Reductions of CO2 and other long-lived GHGS are the ONLY way to stabilize global climate in the long run. BC reduction represents an opportunity for India to do something about climate change locally – stabilizing the monsoon, protecting local agriculture, and clearing the air – benefiting its own people in the near future.


Gautam et al., 2009a. Aerosol and rainfall variability over the Indian monsoon region: distributions, trends and coupling. Annales Geophysicae, 27:3691-3703

Gautam et al., 2009b. Enhanced pre-monsoon warming over the Himalayan-Gangetic region from 1979 to 2007. Geophysical Research Letters, Vol. 36, L07704, doi: 10.1029/2009GL037641

Lau et al.,2006a. Asian summer monsoon anomalies induced by aerosol direct forcing – the role of the Tibetan Plateau. Climate Dynamics, 26: 855-864.

Lau et al.,2006b. Observational relationships between aerosol and Asian monsoon rainfall, and circulation Geophysical Research Letters, Vol. 33, L21810, doi: 10.1029/2006GL027546

Ramanathan et al., 2005. Atmospheric brown clouds: Impacts on South Asian climate and hydrological cycle, Proceedings of the National Academy of Sciences, 102(15):5326-5333, doi: 10.1073/pnas.0500656102

Ramanathan and Carmichael, 2008. Global and regional climate changes due to black carbon. Nature Geoscience, 1:221-227, doi: 10.1038/ngeo156

….as well as numerous other papers by V. Ramanathan, a genius in this field and a genuinely nice and helpful guy.

Ritesh Gautam of NASA is well on his away to make an impact on this field too, and has been a great help, explaining the intricacies of the South Asian monsoon and the results of all the various models used to study it. Any errors in MY explanation above cannot be faulted to any of the above researchers, but are mine alone.


One thought on “Winds of Change: How Black Carbon Affects the South Asian Monsoon

  1. I liked the way you presented the scientific results in ordinary person’s language… Congrats. I am sharing this in my Facebook.

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