Tibetan Glaciers Are Retreating
At An Alarming Rate
By James Hansen
20 December, 2009
Giss.nasa.gov
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.
Figure 1. Five ice cores were extracted from the indicated locationson the Tibetan plateau. The white dashed line is the northerly boundary of the Indian
monsoon.
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.
Figure 2. Black carbon (BC) and organic carbon (OC) concentrations in the Zuoqiupu ice core for the monsoon (June-Sept) and non-monsoon (Oct-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. 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.
Related Links
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
References
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., doi: 10.1073/pnas.0910444106, in press.
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.
