Glaciers on the Tibetan Plateau, sometimes called Earth's "third pole," hold the largest ice mass outside the polar regions.
These glaciers act as water storage towers for South and East Asia, releasing melt water in warm months to the Indus, Ganges, Brahmaputra and other river systems that provide fresh water to more than a billion people. In the dry season, glacial melt provides half or more of the water in many rivers.
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, a team of scientists from Chinese research institutes extracted ice cores from five locations on the Tibetan Plateau.
Black Soot's Role in Global Warming
The team looked at the possible role of black soot, which arises from diesel engines, coal use without effective scrubbers, and biomass burning, including cook stoves, among other sources.
Black soot, which includes black carbon and organic carbon, absorbs sunlight and can speed glacial melting if black carbon reaches values of order 10 ng/g (nanograms per gram) or larger.
The ice core data revealed that black carbon had reached values of 20-50 ng/g in the 1950s and 1960s for the four stations that are downwind of European pollution sources. Black carbon and organic carbon amounts decreased strongly in the early 1970s, probably because of clean air regulations in Europe.
However, the ice cores also revealed that in the past decade, black carbon and organic carbon began to increase again, including on the Zuoqiupu glacier in southern Tibet. The data suggest that the increase in black soot was due to Asian sources, especially the Indian subcontinent.
The measured concentrations of black carbon and organic carbon 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.
The chart at right shows carbon concentrations in the Zuoqiupu ice core for the monsoon (June-September) and non-monsoon (October-May) seasons, and the annual mean.
In a new paper by Xu et al., we concluded that black soot is contributing to the rapid melt of glaciers in the Himalayas. 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 during the dry seasons.
But business-as-usual emissions are not inevitable.
Preserving the 'Third Pole'
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 the demise of the Tibetan glaciers and the consequences that would have on people.
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.
A 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.
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.
(Images: James Hansen)