Antarktis smelter i økende tempo

i Klima av
Nye forskningsresultater konkluderer med at Antarktis smelter og det med økende hastighet. Riktignok er det områder hvor isen blir tykkere på grunn av økende nedbør som pakkes sammen til is, men dette oppveier ikke smeltingen andre steder på iskappen. Basert på disse forskningsresultatene blir det anslått at Antarktis vil bidra til at verdenshavene stiger med 15 centimeter fram til 2100. Det foreligger altså ikke noen umiddelbar fare for at verdens havnebyer skal forsvinne under vann fra Sydpollandet.  Smelteprosessen kan dessuten fortsatt snus dersom vi greier å redusere utslippet av klimagasser, men det økte tempoet er foruroligende.  I løpet av de siste 25 årene har Antarktis gitt fra seg så mye vann at det tilsvarer en høyde på åtte meter som dekker hele Norge.
Denne artikkelen som presenterer forskningsrapporten i tidsskriftet «Nature», er sakset fra en fritt tilgjengelig artikkel i New York Times.
Icebergs in the northern Weddell Sea of Antarctica. Credit John Sonntag/NASA

Between 60 and 90 percent of the world’s fresh water is frozen in the ice sheets of Antarctica, a continent roughly the size of the United States and Mexico combined. If all that ice melted, it would be enough to raise the world’s sea levels by roughly 200 feet.

While that won’t happen overnight, Antarctica is indeed melting, and a study published Wednesday in the journal Nature shows that the melting is speeding up.

The rate at which Antarctica is losing ice has tripled since 2007, according to the latest available data. The continent is now melting so fast, scientists say, that it will contribute six inches (15 centimeters) to sea-level rise by 2100. That is at the upper end of what the Intergovernmental Panel on Climate Change has estimated Antarctica alone could contribute to sea level rise this century.

 

 

Even under ordinary conditions, Antarctica’s landscape is perpetually changing as icebergs calve, snow falls and ice melts on the surface, forming glacial sinkholes known as moulins. But what concerns scientists is the balance of how much snow and ice accumulates in a given year versus the amount that is lost.

Between 1992 and 2017, Antarctica shed three trillion tons of ice. This has led to an increase in sea levels of roughly three-tenths of an inch, which doesn’t seem like much. But 40 percent of that increase came from the last five years of the study period, from 2012 to 2017.

Antarctica is not the only contributor to sea level rise. Greenland lost an estimated 1 trillion tons of ice between 2011 and 2014. And as oceans warm, their waters expand and occupy more space, also raising sea levels. The melting ice and warming waters have all been primarily driven by human emissions of greenhouse gases.

Sea ice and a small stretch of open water where the Venable Ice Shelf meets Farwell Island in Antarctica.CreditJohn Sonntag/NASA

The study also helps clear up some uncertainty that was linked to regional differences in Antarctica. West Antarctica and the Antarctic Peninsula, which reaches toward South America, have been known for some time to be losing ice. In East Antarctica the picture has been muddled as the ice sheet there gained mass in some years and lost mass in others.

 

East Antarctica, which makes up two-thirds of the continent, is a remote region of an already remote location, where data is scarcer because there are fewer measurement stations, Dr. Koppes said. Researchers must extrapolate a smaller amount of data over an area the size of the United States, which can make the analysis less precise.

To get around those problems in this study, more than 80 researchers from around the world collected data from about a dozen different satellite measurements dating to the early 1990s.

“We used different satellite missions and techniques because the various approaches we have at arriving at this number have different strengths and weaknesses,” Dr. Shepherd said. “And we find that by combining all of the available measurements we can iron out the problems that individual techniques have.”

The researchers concluded that the changes in East Antarctica were not nearly enough to make up for the rapid loss seen in West Antarctica and the Antarctic Peninsula. Antarctica is, on balance, losing its ice sheets and raising the world’s sea levels.