An interactive at NASA’s website shows how perennial sea ice has declined from 1980 to 2012. The bright white central mass shows the perennial sea ice while the larger light blue area shows the full extent of the winter sea ice including the average annual sea ice during the months of November, December and January. The data used for the interactive was compiled by NASA senior research scientist Josefino Comiso from NASA’s Nimbus-7 satellite and the U.S. Department of Defense’s Defense Meteorological Satellite Program. Credit: NASA/Goddard Scientific Visualization Studio.
new NASA study revealed that the oldest and thickest Arctic sea ice is
disappearing at a faster rate than the younger and thinner ice at the
edges of the Arctic Ocean’s floating ice cap.
thicker ice, known as multi-year ice, survives through the cyclical
summer melt season, when young ice that has formed over winter just as
quickly melts again. The rapid disappearance of older ice makes Arctic
sea ice even more vulnerable to further decline in the summer, said Joey
Comiso, senior scientist at NASA Goddard Space Flight Center,
Greenbelt, Md., and author of the study, which was recently published in
Journal of Climate.
new research takes a closer look at how multi-year ice, ice that has
made it through at least two summers, has diminished with each passing
winter over the last three decades. Multi-year ice “extent”—which
includes all areas of the Arctic Ocean where multi-year ice covers at
least 15% of the ocean surface—is diminishing at a rate of -15.1% per
decade, the study found.
another measurement that allows researchers to analyze how the ice cap
evolves: multi-year ice “area,” which discards areas of open water among
ice floes and focuses exclusively on the regions of the Arctic Ocean
that are completely covered by multi-year ice. Sea ice area is always
smaller than sea ice extent, and it gives scientists the information
needed to estimate the total volume of ice in the Arctic Ocean. Comiso
found that multi-year ice area is shrinking even faster than multi-year
ice extent, by -17.2% per decade.
average thickness of the Arctic sea ice cover is declining because it
is rapidly losing its thick component, the multi-year ice. At the same
time, the surface temperature in the Arctic is going up, which results
in a shorter ice-forming season,” Comiso said. “It would take a
persistent cold spell for most multi-year sea ice and other ice types to
grow thick enough in the winter to survive the summer melt season and
reverse the trend.”
differentiate multi-year ice from both seasonal ice, which comes and
goes each year, and “perennial” ice, defined as all ice that has
survived at least one summer. In other words: all multi-year ice is
perennial ice, but not all perennial ice is multi-year ice (it can also
be second-year ice).
found that perennial ice extent is shrinking at a rate of -12.2% per
decade, while its area is declining at a rate of -13.5% per decade.
These numbers indicate that the thickest ice, multiyear-ice, is
declining faster than the other perennial ice that surrounds it.
perennial ice retreated in the last three decades, it opened up new
areas of the Arctic Ocean that could then be covered by seasonal ice in
the winter. A larger volume of younger ice meant that a larger portion
of it made it through the summer and was available to form second-year
ice. This is likely the reason why the perennial ice cover, which
includes second year ice, is not declining as rapidly as the multiyear
ice cover, Comiso said.
sea ice hit its record minimum extent in the winter of 2008. That is
when it was reduced to about 55% of its average extent since the late
1970s, when satellite measurements of the ice cap began. Multi-year sea
ice then recovered slightly in the three following years, ultimately
reaching an extent 34% larger than in 2008, but it dipped again in
winter of 2012, to its second lowest extent ever.
this study, Comiso created a time series of multi-year ice using 32
years of passive microwave data from NASA’s Nimbus-7 satellite and the
U.S. Department of Defense’s Defense Meteorological Satellite Program,
taken during the winter months from 1978 to 2011. This is the most
robust and longest satellite dataset of Arctic sea ice extent data to
date, Comiso said.
ice, made from recently frozen ocean waters, is saltier than multi-year
ice, which has had more time to drain its salts. The salt content in
first- and second-year ice gives them different electrical properties
than multi-year ice: In winter, when the surface of the sea ice is cold
and dry, the microwave emissivity of multiyear ice is distinctly
different from that of first- and second-year ice. Microwave radiometers
on satellites pick up these differences in emissivity, which are
observed as variations in brightness temperature for the different types
of ice. The “brightness” data are used in an algorithm to discriminate
multiyear ice from other types of ice.
compared the evolution of the extent and area of multi-year ice over
time, and confirmed that its decline has accelerated during the last
decade, in part because of the dramatic decreases of 2008 and 2012. He
also detected a periodic nine-year cycle, where sea ice extent would
first grow for a few years, and then shrink until the cycle started
again. This cycle is reminiscent of one occurring on the opposite pole,
known as the Antarctic Circumpolar Wave, which has been related to the
El Niño-Southern Oscillation atmospheric pattern. If the nine-year
Arctic cycle were to be confirmed, it might explain the slight recovery
of the sea ice cover in the three years after it hit its historical
minimum in 2008, Comiso said.