Carbon emissions and the Tibetans

The prominent scientific journal, Nature, has an article about climate change and Tibet in its July 2008 issue, entitled The Third Pole. The article highlights the significance of ‘black carbon’ as a major heat-absorbing contributor of atmospheric warming: black carbon contributes about 50% of the solar heating of earth’s atmosphere and is the second largest (carbon dioxide being the first) cause of atmospheric warming over Tibet.

At first I was struck by the author's definition of black carbon as “the soot that results when people cook with bio-fuels such as wood, crop waste or dung.” Even more surprisingly, the author identifies Himalayas as a global hotspot for black-carbon emissions. When I told this to my advisor, he was skeptical considering the sparse population and low economic activity in Tibet. So I did some research. I read the two key studies that the author had cited in the paper. I found that her description of black carbon and Himalayan region’s contribution of the soot are inaccurate.

The author had only referred to one of the two categories of black-carbon sources. The original study, “Global and regional climate changes due to black carbon” by Ramanathan and Carcimichael had defined black carbon as comprising of two categories of sources: 1) in door sources such as the use of “biofuels like wood, dung and crop residue; and 2) outdoor sources such as fossil fuel combustion (dieseland coal), open biomass burning (associated with deforestation and crop residue burning), and cooking with biofuels” (R&C, p. 21).

So there are more than just bio-fuels that make up black-carbon. Still, how could the Himalayas, a sparsely populated area, be one of the global hotspots for black-carbon emissions? The region, even in the Southern face of the Himalayas in India, is not that densely populated, nor is it economically developed. Shimla, the capital city of the state of the Indian state of Himachal Pradesh, may be one of the greatest contributors of black-carbon in the Indian Himalayas, but even Shimla cannot be that severe. As it turns out in Ramanathan and Carcimichael's study, the Himalayas is at the borderline between high and low black-carbon emitting regions. The Indo-Gangetic plains and the plains of “mainland China” are two of the major hotspots, not the Himalayas. The Tibetan plateau is clearly one of the lowest contributors of black-carbon, as shown in two simulated images of annual mean optical depth of black-carbon aerosols in the region (p. 225).

With these doubts clarified, I found two new (to me) noteworthy points:

1. Although CO2 and black carbon are the main contributors to atmospheric warming, water vapour actually has a even stronger greenhouse effect if measured per molecule. Water vapour is apparently not a big concern because it normally reaches no higher than 1–2 kilometres below the stratosphere, the layer of atmosphere which contains the most ozone. Research mentioned in the article says that climate warming (evaporation) and wind blowing over the plateau can transfer water vapour and pollutants (black carbon) into the stratosphere. If this is true, one can speculate that things could get pretty complicated and worrisome for the glaciers and snows on the the planet's Third Pole.

2. The article also touches on debates on the implications of the plateau's warming for the Monsoon. The obvious argument is that the increased (land) surface temperature would augment monsoon (higher temperature over land means more intense monsoon winds from the sea). However, some studies indicate that the plateau has had a weakening effect on the monsoon (see Fig. 4, R&C, p. 224). This is perhaps due to changes in land use patterns and aerosols that absorb solar radiation. So, it seems there is need for further studies before we can draw conclusions on this debate.