Black Carbon and Climate Change in South Asia

A changing climate greatly affects Nepal and the rest of South Asia.  Changing precipitation patterns affect drinking water supplies and agricultural production.  We have already seen our vulnerability to this during the past year’s unusual rainfall patterns.  Temperatures are, on average, increasing around the world; including in South Asia, but averages disguise the full picture.  Warming has been fastest at high elevations, resulting in melting of snowfields and glaciers, and changing river water availability during the dry seasons.  Even small increases in average temperatures can mean a large increase in heat waves or a large decrease in nights with temperatures below freezing.  Even a slight increase in temperature allows mosquitoes and other pests to reach elevations they wouldn’t otherwise, and can change which tree species dominate forests.

A large fraction of the climate change in South Asia is driven by the global increase in greenhouse gases such as carbon dioxide (CO₂), nine tenth of which are emitted outside of South Asia. Because CO₂ remains in the atmosphere for a century, and is well mixed, its impacts are not dependent upon where it is emitted — so a molecule emitted in the United States affects us as much as a molecule emitted in India.  Reducing the impacts of CO₂ requires global agreements and global attempts to reduce emissions.  It also takes a long time to have an impact, since the CO­₂ already in the atmosphere will not come down for a very long time.

The story is quite different for the substance that globally is the second biggest contributor to climate change: Black carbon. Consisting of fine soot particles emitted by diesel engines, brick kilns, industries, cooking fires and open fires, black carbon absorbs incoming sunlight, warming the air but cooling the ground underneath by depriving it of direct sunlight.  In South Asia it contributes to the warming of upper air layers and high mountain areas, and to the cooling of plains and valleys. It is a major ingredient of the haze that obscures our views of the mountains every spring. By depositing directly onto snow and ice surfaces and darkening them, black carbon accelerates the melting of the Himalayan cryosphere.

Compared to CO_­2, black carbon has a very short atmospheric life time of days to weeks. It deposits onto surfaces, or is washed out by rain, so it seldom travels more than a few thousand kilometers from its source.  Most of the impact of black carbon within South Asia are from black carbon emitted within South Asia.  Black carbon is classified as “short-lived climate pollutants” (SLCP), a class of air pollutants that impact climate, but that have a short atmospheric life time and go away quickly when their sources are turned off.  Reducing SLCPs has other co-benefits beyond climate. Black carbon is a carcinogen.  Switching from traditional wood stoves to cleaner cooking not only reduces black carbon’s effects on climate but also reduces mortality.  Black carbon is emitted during incomplete combustion, and essentially represents wasted fuel.  Many of the measures to reduce black carbon emissions, from cleaner cooking, to cleaner brick production, to cleaner diesel vehicles, are cost effective and save fuel.

Black carbon is usually not emitted alone, but in combination with other particles of different chemical composition that are also products of incomplete combustion.  Some of these may also be light absorbing, while others may reflect sunlight.  The direct local climate impacts vary depending on the exact composition of what is emitted, but they all affect climate and visibility.  Agricultural and forest fires tend to co-emit large amounts of white-ish organic carbon particles which attract water vapor and are quickly coated in a layer of water.  This was the source of the extreme haze experienced in Nepal in March-April 2016.

Meanwhile over the past two decades the plains of northern South Asia have been experiencing a four-fold increase in frequency of winter fog.  Instead of brief morning fog that burned off by late morning, there have been episodes of fog persisting for weeks and stretching from Lahore to Dhaka, impacting the lives of hundreds of millions in five countries. Scientists don’t yet have a consensus about why the fog has increased, but we do know that it is a result of changing water availability to the atmosphere (winter irrigation and different crops) and increased smoke.  We suspect that the smoke from garbage fires that the poorest people light to stay warm, along with the smoke from increased loads placed on thermal power plants by the richest people staying warm, maybe contributing to a change in fog microphysics that makes it more persistent over days.  While it may take another year or two of research to have full answers about why the fog has increased, and thus what can be done to decrease it, the fog clearly shows that regional climate change in South Asia is much more complex than just an average increase in temperature.