The global cryosphere—all of the areas with frozen water on Earth—shrank by
about 87,000 square kilometers (about 33,000 square miles, an area about the
size of Lake Superior) per year on average between 1979 and 2016, as a
result of climate change, according to a new study. This research is the
first to make a global estimate of the surface area of the Earth covered by
sea ice, snow cover and frozen ground.
The extent of land covered by frozen water is just as important as its mass
because the bright white surface reflects sunlight so effectively, cooling
the planet. Changes in the size or location of ice and snow can alter air
temperatures, change the sea level and even affect ocean currents worldwide.
The new study is published in Earth's Future, AGU's journal for
interdisciplinary research on the past, present and future of our planet and
its inhabitants.
"The cryosphere is one of the most sensitive climate indicators and the
first one to demonstrate a changing world," said first author Xiaoqing Peng,
a physical geographer at Lanzhou University. "Its change in size represents
a major global change, rather than a regional or local issue."
The cryosphere holds almost three-quarters of Earth's fresh water, and in
some mountainous regions, dwindling glaciers threaten drinking water
supplies. Many scientists have documented shrinking ice sheets, dwindling
snow cover and loss of Arctic sea ice individually due to climate change.
But no previous study has considered the entire extent of the cryosphere
over Earth's surface and its response to warming temperatures.
Contraction in space and time
Peng and his co-authors from Lanzhou University calculated the daily extent
of the cryosphere and averaged those values to come up with yearly
estimates. While the extent of the cryosphere grows and shrinks with the
seasons, they found that the average area covered by Earth's cryosphere has
contracted overall since 1979, correlating with rising air temperatures.
The shrinkage primarily occurred in the Northern Hemisphere, with a loss of
about 102,000 square kilometers (about 39,300 square miles), or about half
the size of Kansas, each year. Those losses are offset slightly by growth in
the Southern Hemisphere, where the cryosphere expanded by about 14,000
square kilometers (5,400 square miles) annually. This growth mainly occurred
in the sea ice in the Ross Sea around Antarctica, likely due to patterns of
wind and ocean currents and the addition of cold meltwater from Antarctic
ice sheets.
The estimates showed that not only was the global cryosphere shrinking but
that many regions remained frozen for less time. The average first day of
freezing now occurs about 3.6 days later than in 1979, and the ice thaws
about 5.7 days earlier.
"This kind of analysis is a nice idea for a global index or indicator of
climate change," said Shawn Marshall, a glaciologist at the University of
Calgary, who was not involved in the study. He thinks that a natural next
step would be to use these data to examine when ice and snow cover give
Earth its peak brightness, to see how changes in albedo impact the climate
on a seasonal or monthly basis and how this is changing over time.
To compile their global estimate of the extent of the cryosphere, the
authors divided up the planet's surface into a grid system. They used
existing data sets of global sea ice extent, snow cover and frozen soil to
classify each cell in the grid as part of the cryosphere if it contained at
least one of the three components. Then they estimated the extent of the
cryosphere on a daily, monthly and yearly basis and examined how it changed
over the 37 years of their study.
The authors say that the global dataset can now be used to further probe the
impact of climate change on the cryosphere, and how these changes impact
ecosystems, carbon exchange and the timing of plant and animal life cycles.
Reference:
Xiaoqing Peng et al, A Holistic Assessment of 1979–2016 Global Cryospheric
Extent, Earth's Future (2021). DOI:
10.1029/2020EF001969