Analysis complied from NASA’s Nimbus-7 satellite tv for pc and the U.S. Division of Protection’s Protection Meteorological Satellite tv for pc Program by senior analysis scientist Josefino Comiso reveals how multi-year ice, ice that has made it by at the least two summers, has diminished with every passing winter over the past three a long time.
GREENBELT, Maryland — A brand new NASA research revealed that the oldest and thickest Arctic sea ice is disappearing at a sooner fee than the youthful and thinner ice on the edges of the Arctic Ocean’s floating ice cap.
The thicker ice, referred to as multi-year ice, survives by the cyclical summer time soften season, when younger ice that has shaped over winter simply as shortly melts once more. The fast disappearance of older ice makes Arctic sea ice much more susceptible to additional decline in the summertime, mentioned Joey Comiso, senior scientist at NASA Goddard Area Flight Middle, Greenbelt, Maryland, and creator of the research, which was not too long ago revealed in Journal of Climate.
The brand new analysis takes a more in-depth take a look at how multi-year ice, ice that has made it by at the least two summers, has diminished with every passing winter over the past three a long time. Multi-year ice “extent” – which incorporates all areas of the Arctic Ocean the place multi-year ice covers at the least 15 p.c of the ocean floor – is diminishing at a fee of -15.1 p.c per decade, the research discovered.
There’s one other measurement that permits researchers to research how the ice cap evolves: multi-year ice “space,” which discards areas of open water amongst ice floes and focuses solely on the areas of the Arctic Ocean that are utterly lined by multi-year ice. Sea ice space is all the time smaller than sea ice extent, and it offers scientists the knowledge wanted to estimate the full quantity of ice within the Arctic Ocean. Comiso discovered that multi-year ice space is shrinking even sooner than multi-year ice extent, by -17.2 p.c per decade.
“The common thickness of the Arctic sea ice cowl is declining as a result of it’s quickly dropping its thick element, the multi-year ice. On the identical time, the floor temperature within the Arctic goes up, which ends up in a shorter ice-forming season,” Comiso mentioned. “It would take a persistent chilly spell for many multi-year sea ice and different ice sorts to develop thick sufficient within the winter to outlive the summer time soften season and reverse the pattern.”
Scientists differentiate multi-year ice from each seasonal ice, which comes and goes every year, and “perennial” ice, outlined as all ice that has survived at the least one summer time. In different phrases: all multi-year ice is perennial ice, however not all perennial ice is multi-year ice (it can be second-year ice).
Comiso discovered that perennial ice extent is shrinking at a fee of -12.2 p.c per decade, whereas its space is declining at a fee of -13.5 p.c per decade. These numbers point out that the thickest ice, multi-year ice, is declining sooner than the opposite perennial ice that surrounds it.
As perennial ice retreated within the final three a long time, it opened up new areas of the Arctic Ocean that might then be lined by seasonal ice within the winter. A bigger quantity of youthful ice meant that a bigger portion of it made it by the summer time and was accessible to type second-year ice. That is possible the explanation why the perennial ice cowl, which incorporates second 12 months ice, is just not declining as quickly because the multi-year ice cowl, Comiso mentioned.
Multi-year sea ice hit its file minimal extent within the winter of 2008. That’s when it was diminished to about 55 p.c of its common extent for the reason that late Nineteen Seventies, when satellite tv for pc measurements of the ice cap started. Multi-year sea ice then recovered barely within the three following years, finally reaching an extent 34 p.c bigger than in 2008, however it dipped once more in winter of 2012, to its second lowest extent ever.
For this research, Comiso created a time collection of multi-year ice utilizing 32 years of passive microwave knowledge from NASA’s Nimbus-7 satellite tv for pc and the U.S. Division of Protection’s Protection Meteorological Satellite tv for pc Program, taken in the course of the winter months from 1978 to 2011. That is essentially the most sturdy and longest satellite tv for pc dataset of Arctic sea ice extent knowledge so far, Comiso mentioned.
Youthful ice, produced from not too long ago frozen ocean waters, is saltier than multi-year ice, which has had extra time to empty its salts. The salt content material in first- and second-year ice offers them completely different electrical properties than multi-year ice: In winter, when the floor of the ocean ice is chilly and dry, the microwave emissivity of multiyear ice is distinctly completely different from that of first- and second-year ice. Microwave radiometers on satellites choose up these variations in emissivity, that are noticed as variations in brightness temperature for the several types of ice. The “brightness” knowledge are utilized in an algorithm to discriminate multi-year ice from different sorts of ice.
Comiso in contrast the evolution of the extent and space of multi-year ice over time, and confirmed that its decline has accelerated over the past decade, partly due to the dramatic decreases of 2008 and 2012. He additionally detected a periodic nine-year cycle, the place sea ice extent would first develop for a couple of years, after which shrink till the cycle began once more. This cycle is harking back to one occurring on the other pole, referred to as the Antarctic Circumpolar Wave, which has been associated to the El Niño-Southern Oscillation atmospheric sample. If the nine-year Arctic cycle have been to be confirmed, it’d clarify the slight restoration of the ocean ice cowl within the three years after it hit its historic minimal in 2008, Comiso mentioned.
Picture: NASA/Goddard Scientific Visualization Studio