Resources for a sustainable future

14/12/2009 www.realclimate.org Black soot and the survival of Tibetan glaciers – an extract from the scientific paperby Baiqing Xua,b, Junji Caob, James Hansenc,1, Tandong Yaoa, Daniel R. Joswiaa, Ninglian Wangd, Guangjian Wua, Mo Wanga, Huabiao Zhaoa, Wei Yanga, Xianqin Liue, and Jianqiao Hed.

A Key Laboratory of Tibetan Environment Changes and Land Surface Processes, Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing
100085, China; bState Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710075, China;
cNASA Goddard Institute for Space Studies, 2880 Broadway, New York, NY 10025; dState Key Laboratory of Cryosphere Science, Chinese Academy of
Sciences, Lanzhou 730000, China; eInstitute of Atmospheric Physics, Chinese Academy of Sciences, Beijing 100029, China
Contributed by James Hansen, October 15, 2009 (sent for review July 6, 2009)

We find evidence that black soot aerosols deposited on Tibetan
glaciers have been a significant contributing factor to observed
rapid glacier retreat. Reduced black soot emissions, in addition to
reduced greenhouse gases, may be required to avoid demise of
Himalayan glaciers and retain the benefits of glaciers for seasonal
fresh water supplies.

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 to the Indus, Ganges,
Brahmaputra, and other river systems, providing fresh water to
more than 1 billion people (1, 2). Glacial melt provides up to
two-thirds of the summer flow in the Ganges and half or more
of the flow in other major rivers (3). One-quarter of the
population of China is in western regions where glacial melt
provides the main dry season water source (4).
Tibetan glaciers have been melting at an accelerating, alarming
rate over the past decade, raising the threat that many of the
glaciers could be gone by midcentury (5, 6). As glaciers recede
and release stored water, flow temporarily increases, but the
future fresh water supply is threatened (5–8). Once headwater
glaciers are gone, however, a dramatic decline in dry season
water availability may ensue. Total precipitation may increase
with global warming (1, 9), but a likely result of glacier loss will
be heavier spring floods and much reduced fresh water availability
during subsequent dry seasons.
Climate on the Tibetan Plateau is changing rapidly; over the
past three decades, the large area at altitude above 4,000 m has
warmed 0.3 °C per decade (Fig. S1), which is twice the rate of
observed global warming. Climate change on the plateau may
have large regional effects, for example, on the Asian monsoon,
and even global repercussions (3). Glacier retreat in the Tibetan
Plateau presumably is driven by warming due to increasing
greenhouse gases (1), but the rapidity of glacier retreat and the
up to 0.3 °C warming of per decade during the past 30 years
suggest additional mechanisms may be involved.
Black soot in aerosol pollution can warm the troposphere,
perhaps contributing to surface melt (10–12). Absorption is
caused primarily by the black carbon (BC), whereas organic
carbon (OC) absorbs mainly in the UV and slightly in the visible
region (13–15). Black soot incorporated in snowflakes darkens
snow and ice surfaces, increasing surface melt (16–18). Simulations
show that the added absorption by snow exceeds the
‘‘dimming’’ effect (reduced solar irradiance at the ground due to
atmospheric aerosols) and becomes significant when BC reaches
amounts on the order of 10 ng g
1 or more (15, 19).
The Tibetan Plateau is located close to regions in South and
East Asia that have been (20) and are predicted to continue to
be (21) the largest sources of black soot in the world. The
extensive black soot aerosols could be lofted to the high Tibetan
Plateau and incorporated in snowflakes that when falling on the
glaciers darken their surface, which has led to initial studies of
the amount of BC and OC in the snow and ice of Himalayan
glaciers (22, 23). Yet, black soot-induced reduction of snow
albedo and its contribution to glacier retreat have only begun to
receive attention, so there is a need for more extensive field data.
Here, we report measurements of the BC and OC content in ice
cores that sample snow deposited during the past half century on
five widely spaced glaciers on the Tibetan Plateau.