Glaciers are a sensitive indicator of climate change -- and one that can be
easily observed. Regardless of altitude or latitude, glaciers have been
melting at a high rate since the mid-20th century. Until now, however, the
full extent of ice loss has only been partially measured and understood. Now
an international research team led by ETH Zurich and the University of
Toulouse has authored a comprehensive study on global glacier retreat, which
was published online in Nature on 28 April. This is the first study to include
all the world's glaciers -- around 220,000 in total -- excluding the Greenland
and Antarctic ice sheets. The study's spatial and temporal resolution is
unprecedented -- and shows how rapidly glaciers have lost thickness and mass
over the past two decades.
Rising sea levels and water scarcity What was once permanent ice has
declined in volume almost everywhere around the globe. Between 2000 and
2019, the world's glaciers lost a total of 267 gigatonnes (billion tonnes)
of ice per year on average -- an amount that could have submerged the entire
surface area of Switzerland under six metres of water every year. The loss
of glacial mass also accelerated sharply during this period. Between 2000
and 2004, glaciers lost 227 gigatonnes of ice per year, but between 2015 and
2019, the lost mass amounted to 298 gigatonnes annually. Glacial melt caused
up to 21 percent of the observed rise in sea levels during this period --
some 0.74 millimetres a year. Nearly half of the rise in sea levels is
attributable to the thermal expansion of water as it heats up, with
meltwaters from the Greenland and Antarctic ice sheets and changes in
terrestrial water storage accounting for the remaining third.
Among the fastest melting glaciers are those in Alaska, Iceland and the
Alps. The situation is also having a profound effect on mountain glaciers in
the Pamir mountains, the Hindu Kush and the Himalayas. "The situation in the
Himalayas is particularly worrying," explains Romain Hugonnet, lead author
of the study and researcher at ETH Zurich and the University of Toulouse.
"During the dry season, glacial meltwater is an important source that feeds
major waterways such as the Ganges, Brahmaputra and Indus rivers. Right now,
this increased melting acts as a buffer for people living in the region, but
if Himalayan glacier shrinkage keeps accelerating, populous countries like
India and Bangladesh could face water or food shortages in a few decades."
The findings of this study can improve hydrological models and be used to
make more accurate predictions on a global and local scales -- for instance,
to estimate how much Himalayan glacier meltwater one can anticipate over the
next few decades.
To their surprise, the researchers also identified areas where melt rates
slowed between 2000 and 2019, such as on Greenland's east coast and in
Iceland and Scandinavia. They attribute this divergent pattern to a weather
anomaly in the North Atlantic that caused higher precipitation and lower
temperatures between 2010 and 2019, thereby slowing ice loss. The
researchers also discovered that the phenomenon known as the Karakoram
anomaly is disappearing. Prior to 2010, glaciers in the Karakoram mountain
range were stable -- and in some cases, even growing. However, the
researchers' analysis revealed that Karakoram glaciers are now losing mass
as well.
Study based on stereo satellite images As a basis for the study, the
research team used imagery captured on board NASA's Terra satellite, which
has been orbiting the Earth once every 100 minutes since 1999 at an altitude
of nearly 700 kilometres. Terra is home to ASTER, a multispectral imager
with two cameras that record pairs of stereo images, allowing researchers to
create high-resolution digital elevation models of all the world's glaciers.
The team used the full archive of ASTER images to reconstruct a time series
of glacial elevation, which enabled them to calculate changes in the
thickness and mass of the ice over time.
Reference:
Romain Hugonnet, Robert McNabb, Etienne Berthier, Brian Menounos,
Christopher Nuth, Luc Girod, Daniel Farinotti, Matthias Huss, Ines
Dussaillant, Fanny Brun, Andreas Kääb. Accelerated global glacier mass loss
in the early twenty-first century. Nature, 2021; 592 (7856): 726 DOI:
10.1038/s41586-021-03436-z
Tags:
Planet and Environment