This is an aerial view of the Imja glacier and Lake Imja, Nepal, the
Himalayas. The lake appeared in the 1960s and has grown continuously
ever since. The sinking of the surface of the debris-covered glacier
tongue is also clearly discernible. (Credit: J. Kargel, University of
Arizona)
Several hundreds of millions of people in Southeast Asia depend, to
varying degrees, on the freshwater reservoirs of the Himalayan glaciers.
Consequently, it is important to detect the potential impact of climate
changes on the Himalayan glaciers at an early stage. Together with
international researchers, glaciologists from the University of Zurich
now reveal that the glaciers in the Himalayas are declining less rapidly
than was previously thought. However, the scientists see major hazard
potential from outbursts of glacial lakes.
Ever since the false prognoses of the Intergovernmental Panel on
Climate Change (IPCC), the Himalayan glaciers have been a focus of
public and scientific debate. The gaps in our knowledge of glaciers in
the Himalayan region have hindered accurate statements and prognoses. An
international team of researchers headed by glaciologists from the
University of Zurich and with the involvement of scientists from Geneva
now outlines the current state of knowledge of glaciers in the Himalayas
in a study published in Science. The scientists confirm that the
shrinkage scenarios for Himalayan glaciers published in the last IPCC
report were exaggerated.
Glacier area 20 percent smaller than assumed
The most up-to-date mappings so far based on satellite data revealed
that glaciers in the Himalayas and Karakoram cover a total area of
approximately 40,800 km². While this is around twenty times larger than
all glaciers of the European Alps put together, it is as much as twenty
percent smaller than was previously assumed. Lead scientist Tobias
Bolch, who researches at the University of Zurich and Dresden University
of Technology, mainly puts this down to erroneous mappings in earlier
studies.
Less shrinkage than predicted
The scientists took all the existing measurements of length, area and
volume changes and mass budgets into account for their study. While
some of the measurement series on length changes date back to 1840,
measurements of glacier mass budget that instantaneously reflect the
climate signal are rare. In addition, continuous measurement series do
not stretch back any further than ten years. The researchers recorded
average length decreases of 15 to 20 metres and area decreases of 0.1 to
0.6 percent per year in recent decades. Furthermore, the glacier
surfaces lowered by around 40 centimetres a year. "The detected length
changes and area and volume losses correspond to the global average,"
explains Bolch, summarizing the new results. "The majority of the
Himalayan glaciers are shrinking, but much less rapidly than predicted
earlier."
For the regions in the northwestern Himalayas and especially in the
Karakoram Range, the researchers noted very heterogeneous behaviour in
the glaciers. Many of them are dynamically unstable and prone to rapid
advances (so called "surges") that largely occur independently of the
climatic conditions. For the last decade on average, even a slight
volume increase was detected. Based on their analyses, the researchers
assume that glacier shrinkage will not have a major impact on the water
drainage of large rivers like the Indus, Ganges and Brahmaputra in the
coming decades.
Greater variability and menacing flooding of glacial lakes
Despite the partial all-clear for the Himalayan glaciers, however,
Bolch advises caution: "Due to the expected shrinkage of the glaciers,
in the medium term we can expect a greater variability in the seasonal
water drainage. Individual valleys could dry up seasonally."
Bolch and his colleagues also see a very serious threat to the local
population in newly formed or rapidly growing glacial lakes. The deluge
of water and debris from potential outbursts of these lakes could have
devastating consequences for low-lying regions. According to the
scientists, increased efforts are urgently needed to monitor the lakes
as well as changes in the glaciers and the climate in the Himalayas.
The study was conducted as part of the EU project High Noon and the European Space Agency project Glaciers_cci.
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