Oppsummering: Smelter isen i Antaritika eller øker det med is? En forsknings-rapport hevdet nylig at det har vært en dramatisk reduksjon av is de siste årene. En ny studie av glasiolog Zwally ved NASA hevder nå at tap av ismasse i vestlige Antartika er kompensert av økt isvolum i østlige områder.
Is Antarctica melting or is it gaining ice? A recent paper claims Antarctica’s net ice loss has dramatically increased in recent years, but forthcoming research will challenge that claim.
NASA glaciologist Jay Zwally first challenged the “consensus” on Antarctica in 2015 when he published a showing ice sheet growth in eastern Antarctica outweighed the losses in the western ice sheet.
Zwally will again challenge the prevailing narrative of how global warming is affecting the South Pole. Zwally said his new study will show, once again, the eastern Antarctic ice sheet is gaining enough ice to offset losses in the west.
Much like in 2015, Zwally’s upcoming study will run up against the so-called “consensus,” including a paper published by a team of 80 scientists in the journal Nature on Wednesday. The estimates that Antarctic is losing, on net, more than 200 gigatons of ice a year, adding 0.02 inches to annual sea level rise.
“Basically, we agree about West Antarctica,” Zwally told The Daily Caller News Foundation. “East Antarctica is still gaining mass. That’s where we disagree.”
Reported ice melt mostly driven by instability in the western Antarctic ice sheet, which is being eaten away from below by warm ocean water. Scientists tend to agree ice loss has increased in western Antarctica and the Antarctic Peninsula has increased.
Measurements of the eastern ice sheet, however, are subject to high levels of uncertainty. That’s where disagreements are.
“In our study East Antarctic remains the least certain part of Antarctica for sure,” Andrew Shepherd, the study’s lead author and professor at the University of Leeds, told TheDCNF.
“Although there is relatively large variability over shorter periods, we don’t detect any significant long-term trend over 25 years,” Shepherd said.
However, Zwally’s working on a paper that will show the eastern ice sheet is expanding at a rate that’s enough to at least offset increased losses the west.
The ice sheets are “very close to balance right now,” Zwally said. He added that balance could change to net melting in the future with more warming.
So, why is there such a big difference between Zwally’s research and what 80 scientists recently published in the journal Nature?
There are several reasons for the disagreement, but the biggest is how researchers make what’s called a glacial isostatic adjustment (GIA), which takes into account the movement of the Earth under ice sheets.
Scientists use models to measure the movement of land mass in response to changes the ice sheet sitting on top. For example, Zwally said eastern Antarctica’s land mass has been going down in response to ice sheet mass gains.
That land movement effects ice sheet data, especially in Antarctica where small errors in GIA can yield big changes ice sheet mass balance — whether ice is growing or shrinking. There are also differences in how researchers model firn compaction and snowfall accumulation.
“It needs to be known accurately,” Zwally said. “It’s an error of being able to model. These are models that estimate the motions of the Earth under the ice.”
Zwally’s 2015 study said an isostatic adjustment of 1.6 millimeters was needed to bring satellite “gravimetry and altimetry” measurements into agreement with one another.
Shepherd’s paper cites Zwally’s 2015 study several times, but only estimates eastern Antarctic mass gains to be 5 gigatons a year — yet this estimate comes with a margin of error of 46 gigatons.
Zwally, on the other hand, claims ice sheet growth is anywhere from 50 gigatons to 200 gigatons a year.
Shepherd’s recently published paper found Antarctica lost 219 billion tons of ice from 2012 to 2017, about triple what annual ice mass loss was in the previous decade.
“There are several potential reasons for the remaining disagreement among the various satellite techniques, such as the models we use to account for snowfall and glacial isostatic adjustment,” Shepherd told TheDCNF.
“But the ice losses we detect in West Antarctica are highly accurate, and outstrip by far the signal or uncertainty in East Antarctica,” he said.
Zwally said the ice sheets are reacting to climate warming, the question is when receding started and how far it would go.
Kide: Global Warming Policy Foundation (GWPF) 15.06.2018
Michael Bastasch | Contributor