Glaciers on the Tibetan Plateau provide fresh water to over one billion people. However, the glaciers are melting, and they are melting so quickly that the glaciers may be gone within a few decades.
The glaciers are melting so quickly because the average temperature of the Tibetan Plateau is increasing twice as fast as the average warming on a global level. There's probably some unique environmental condition that is causing this rapid melting, but it's not known what it is.
The cause must be found and corrected quickly, or the water supplies of over one billion people will be threatened. James Hansen (NASA Goddard Institute for Space Studies, New York) and coworkers have shown that black soot (black carbon) from south and east Asia is driving the accelerated warming in the Tibetan Plateau, and the accelerated glacier melting.
Quantitating black soot.
The scientists began their long-term studies with ice cores from five geographically-dispersed glaciers on the Tibetan Plateau, over a period of time extending back to the 1950s. The outer ice was removed, the remainder was melted, and the amount of organic carbon (life-based) and black carbon (derived from combusted organic carbon) was quantitated.
They needed to distinguish between the two types of carbon. This is because black carbon absorbs visible light (unlike organic carbon), and when mixed with snow is expected to accelerate melting more than organic carbon.
To distinguish between the two types of carbon, the scientists first heated up their carbon sample. They then quantitated the difference in the amount of light absorbed by the sample before and after heating.
With increasing light absorbance after heating, there is more organic carbon (burnable) in the original sample. Therefore, when there is little change in light absorbance after heating, there is more black carbon (not burnable) in the original sample.
The scientists estimate 15% - 16% uncertainty in their data due to artifacts. This uncertainty does not negate their reported trends.
Black soot accelerates glacier melting.
The scientists found high concentrations of black soot (black carbon) in four of the five glaciers in the 1950s and 1960s, likely from European sources. Decreasing amounts are observed in the 1970s and 1980s, probably due to newly instituted environmental regulations.
However, since 1990, increasing levels of black soot was found in glaciers exposed to air currents from India. This is a reasonable observation, as total carbon emissions from India increased appproximately 30% between 1990 and 2003.
Implications.
It is clear that carbon dioxide is not the only large contributor to environmental changes. Black soot is also a contributor, at least with respect to glacier melting in the Tibetan Plateau.
The scientists note their previous findings that if unconventional fossil fuels are not developed, and if coal emissions are eliminated over the course of the next two decades, atmospheric carbon may be stabilized, and possibly even reduced through improved farming techniques and forest management.
In contrast, a "business as usual" attitude (heavily promoted by many politicians, industries, and common citizens in the United States) may be devastating to over one billion people in South Asia and China. Note that global warming will likely have many other detrimental effects worldwide, e.g. on coral reefs, the Amazon rainforest, and forests in the western United States.
Some scientists have predicted a centuries-long warming even if carbon dioxide emissions are halted. However, the devastating consequences of global warming should still be sufficient to motivate reductions in carbon emissions, even if in the end some long-term warming will still occur.
Hansen and coworkers' results are a strong incentive to adopt a cleaner economy, with the transitionary aid of biofuels, and the long-term solutions provided by hydrogen-, solar-, and wind-based power.
for more information:
Xu, B., Cao, J., Hansen, J., Yao, T., Joswia, D. R., Wang, N., Wu, G., Wang, M., Zhao, H., Yang, W., Liu, X., & He, J. (2009). Black soot and the survival of Tibetan glaciers Proceedings of the National Academy of Sciences, 106 (52), 22114-22118 DOI: 10.1073/pnas.0910444106