Survival of Tibetan Glaciers
Glaciers on the Tibetan Plateau, sometimes called Earth's "third pole", hold the largest ice mass outside the polar regions. These glaciers act as a water storage tower for South and East Asia, releasing melt water in warm months to the Indus, Ganges, Brahmaputra and other river systems, providing fresh water to more than a billion people. In the dry season glacial melt provides half or more of the water in many rivers.
Tibetan glaciers have been melting at an accelerating rate over the past decade. Glacier changes depend on local weather, especially snowfall, so glacier retreat or advance fluctuates with time and place. Thus it is inevitable that some Tibetan glaciers advance over short periods, as has been reported. But overall, Tibetan glaciers are retreating at an alarming rate.
Global warming must be the primary cause of glacier retreat, which is occurring on a global scale, but observed rapid melt rates suggest that other factors may be involved. To investigate the possible role of black soot in causing glacial melt, a team of scientists from Chinese research institutes extracted ice cores from five locations on the Tibetan Plateau (Figure 1).
Black soot, which includes black carbon (BC) and organic carbon (OC), absorbs sunlight and can speed glacial melting if BC reaches values of order 10 ng/g (nanograms per gram) or larger. The ice core data revealed that BC reached values of 20-50 ng/g in the 1950s and 1960s for the four stations that are downwind of European pollution sources. BC and OC amounts decreased strongly in the early 1970s, probably because of clean air regulations in Europe.
However, the ice cores also reveal that in the past decade BC and OC began to increase again, even on the Zuoqiupu glacier (Figure 2), which is mainly subject to Asian sources. The data suggest that increased black soot arises from Asian sources, especially the Indian subcontinent.
The measured concentrations of BC and OC refer to fresh snow. But as the snow melts in the spring and summer the black soot concentrations on the glacier surface increase, because the soot particles do not escape in the melt water as efficiently as the water itself. As a consequence, the soot noticeably darkens the glacier surface during the melt season, increases absorption of sunlight, and speeds glacier disintegration.
In a new paper by Xu et al., we concluded that black soot is contributing to the rapid melt of glaciers in the Himalayas. And continued, "business-as-usual" emissions of greenhouse gases and black soot will result in the loss of most Himalayan glaciers this century, with devastating effects on fresh water supplies in dry seasons.
But business-as-usual emissions are not inevitable. An alternative scenario, which stabilizes the glaciers and has other benefits for global climate and human health, requires a reduction of major human-made climate forcing agents that have a warming effect — that means greenhouses gases, especially carbon dioxide, as well as black soot.
Quantitative policy implications have been defined: coal emissions must be phased out over the next 20 years, and unconventional fossil fuels, such as tar sands and oil shale, must remain undeveloped. Combined with improved agricultural and forestry practices and reduction of methane and black soot emissions, these actions would avoid demise of the Tibetan glaciers.
Not coincidentally, these policy actions are the same as those required to stabilize Earth's energy balance and keep the climate near the Holocene climate range in which civilization developed. The question is whether the global community can exercise the free will to limit fossil fuel emissions and move to clean energies of the future — or is it inevitable that all fossil fuels will be burned?
The conclusion is that prospects for survival of Tibetan glaciers can be much improved by reducing black soot emissions. The black soot arises especially from diesel engines, coal use without effective scrubbers, and biomass burning, including cook stoves. Reduction of black soot via cleaner energies would have other benefits for human health and agricultural productivity. However, survival of the glaciers also requires halting global warming, which depends upon stabilizing and reducing greenhouse gases, especially carbon dioxide.
NASA Earth Observatory: Image of the Day: 2009-12-15
NASA News Release: Black Carbon Deposits on Himalayan Ice Threaten Earth's "Third Pole"
NASA News Release: New Study Turns Up the Heat on Soot's Role in Himalayan Warming
Xu, B, J. Cao, J. Hansen, T. Yao, D.J. Joswia, N. Wang, G. Wu, M. Wang, H. Zhao, W. Yang, X. Liu, and J. He, 2009: Black soot and the survival of Tibetan glaciers. Proc. Natl. Acad. Sci., 106, 22114-22118, doi:10.1073/pnas.0910444106.
Hansen, J., Mki. Sato, P. Kharecha, D. Beerling, R. Berner, V. Masson-Delmotte, M. Pagani, M. Raymo, D.L. Royer, and J.C. Zachos, 2008: Target atmospheric CO2: Where should humanity aim? Open Atmos. Sci. J., 2, 217-231, doi:10.2174/1874282300802010217.