Glacial Meltdown

IT IS amazing to see because it feels so clearly out of place, but in some spots along Antarctica’s coast you can actually find hints of grass and other plant life — green evidence that in parts of the long-frozen continent the ice is thinning, and fast.

But just how much ice is being lost? A new, 10-year study conducted by Bristol University’s Glaciology Centre says that between Antarctica and Greenland, the answer is 300bn tonnes a year.

In scientific terms, a decade makes for a short study; this one can’t forecast whether this kind of loss will sustain into the future.

But if it continues at this pace, goodbye Miami, Hong Kong, New York City, etc, as well as the homes of hundreds of millions who live on estuaries, deltas, coral atolls and great city river basins around the globe.

The two huge ice sheets in Greenland and Antarctica — the ice in Antarctica is three miles thick at some places — contain 99.5% of the Earth’s glacier ice. If it were all to melt, sea levels would rise by 500 to 600ft (180m).

I’ve been to Antarctica on a couple dozen different trips, mostly along its long, skinny Peninsula, which because it is surrounded by warming ocean on both sides is the area most-impacted by the planet’s changing climate. It’s there, each austral summer, that more shorelines are exposed thanks to disappearing ice, and where you find hints of plant life. (How did the seeds get there? Most likely hitchhiking on fishing boats and visiting tourist ships, both growing in numbers.)

Another satellite study, conducted over the past 20 years by Nasa and the European Space Agency, confirms the Bristol study, reporting that the planet’s ice sheets have melted faster in the last 20 years than in the last 10,000.

According to this study, melting ice from both poles has been responsible for a fifth of the global rise in sea levels since 1992. The rest was caused by the thermal expansion of the warming ocean, the melting of mountain glaciers, small Arctic ice caps and groundwater mining. The share of the polar ice melt, however, is rising and most concerning.

When it comes to Antarctica, though, it can be tricky to generalise. It is a huge continent; the US could easily fit inside its borders. In the huge area of East Antarctica, where the ice is mostly above sea level, the air temperature is also much lower, and the experts do not expect the ice to melt on account of rising temperatures. In this part of Antarctica, the ice sheet is actually growing as a consequence of increased snowfall.

But today, using a variety of satellite reports and on-the-ice studies that have been going on for many decades, even conservative scientists say sea levels are rising 60% faster than what the Intergovernmental Panel on Climate Change predicted in 2007.

Another study has put forward a new reason for Greenland’s massive ice sheet’s accelerating slide toward the ocean is softening the interior of the ice like a stick of butter.

For more than a decade, scientists have reported rapid melting and shrinking at many of Greenland’s outlet glaciers, which snake into the ocean. Studies suggest warmer ocean water and rising atmospheric temperatures contribute to the melting ice. The new study finds the interior ice sheet also speeded up its flow to the sea in the past decade, but for a different reason. The ice sheet is changing from within.

“It’s not just melt at the edge that’s affecting how Greenland is changing,” said Thomas Phillips, lead study author and a glaciologist at the Cooperative Institute for Research in Environmental Sciences (CIRES) at the University of Colorado Boulder.

“It comes from within the ice, and that’s why we think this is such an important finding. This ice is changing its behaviour as a whole, not just at the edges.”

Surface meltwater ponds and lakes form on top of Greenland’s ice sheet and glaciers every summer. The water pressure can fracture the surface, creating a vertical drainpipe that carries the surface meltwater into the ice, even down to the rock at its base. The liquid lubricates the ice sheet, helping it slide more easily.

But the researchers also think the meltwater is softening the solid ice, making it deform like a stick of butter. The water transports heat into the chilly interior ice, according to computer modeling by the research team. The softer ice also flows more quickly, the modelling suggests.

The study focused on Sermeq Avannarleq Glacier in southwest Greenland. Located about 65km to 95km from the coast, the glacier is flowing about 1.5 times faster than it was 10 years ago. In 2000, the inland segment was flowing about 40m per year; in 2007-08, that speed was closer to 60m per year.

Additional work by the team hints that other regions of Greenland’s ice sheet are also accelerating because of surface melting, Phillips said.

The new findings also suggest Greenland’s future melt may be more extensive than recent studies conclude. Some scientists think the rapid shrinking of its outlet glaciers may slow in coming decades. But even if the outlet glaciers slow down, the interior ice sheet could continue to speed up, spurred on by the extensive surface melts of recent years, Phillips said.

In 2012, nearly the entire surface veneer of Greenland’s ice sheet melted, the biggest surface melt since record-keeping started 30 years ago.

“When we started studying this in 2005, we didn’t expect to see all of Greenland melt within our lifetime, and seven years later it happened,” said Phillips.

A European study has backed up the research, also concluding that while thermal expansion of the ocean and melting mountain glaciers are the most important factors causing sea-level change, the Greenland and Antarctic ice sheets will be the dominant contributors within the next two millennia.

Half of that rise might come from ice-loss in Antarctica which is currently contributing less than 10% to global sea-level rise.

“CO2, once emitted by burning fossil fuels, stays an awful long time in the atmosphere,” says Anders Levermann, lead author of the study and research domain co-chair at the Potsdam Institute for Climate Impact Research. “Consequently, the warming it causes also persists.”

The oceans and ice sheets are slow in responding, simply because of their enormous mass. “The problem is: once heated out of balance, they simply don’t stop,” said Levermann. “We’re confident our estimate is robust because of the combination of physics and data that we use.”

The study is the first to combine evidence from early Earth’s climate history with comprehensive computer simulations using physical models of all four major contributors to long-term global sea level rise. During the 20th century, sea level rose by about 0.2m, and it is projected to rise by significantly less than 2m by 2100 even for the strongest scenarios considered.

At the same time, past climate records, which average sea level and temperature changes over a long time, suggest much higher sea levels during periods of Earth history that were warmer than present.

For the study now published, the international team of scientists used data from sediments from the bottom of the sea and ancient raised shorelines found on various coastlines around the world.

“The Antarctic computer simulations were able to simulate the past 5m years of ice history, and the other two ice models were directly calibrated against observational data — which in combination makes the scientists confident that these models are correctly estimating the future evolution of long-term sea level rise,” says Peter Clark, a paleo-climatologist at Oregon State University and co-author on the study.

While it remains a challenge to simulate rapid ice loss from Greenland and Antarctica, the models are able to capture ice loss that occurs on long time scales where a lot of the small rapid motion averages out.

If global mean temperature rises by four degrees compared to pre-industrial times, which in a business-as-usual scenario is projected to happen within less than a century, the Antarctic ice sheet will contribute about 50% of sea level rise over the next two millennia. Greenland will add another 25% to the total sea level rise, while the thermal expansion of the oceans’ water, currently the largest component of sea level rise, will contribute about 20%, and the contribution from mountain glaciers will decline to less than 5%, mostly because many of them will shrink to a minimum.

“Continuous sea level rise is something we cannot avoid unless global temperatures go down again,” concludes Levermann. “Thus we can be absolutely certain that we need to adapt. Sea level rise might be slow on time scales on which we elect governments, but it is inevitable and therefore highly relevant for almost everything we build along our coastlines, for many generations to come.”


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