Friday, April 1, 2016

Sea level rise could be twice the level expected in this century--look to the big ice sheets.

We have recently seen not one but several papers suggesting that the Intergovernmental Panel on Climate Change has been far too conservative on sea level change.

That it’s likely to rise not just two or three but maybe twice as much in the coming decades.

Think about that in areas where waves are already eating at the foundations of home, are washing over roads, are eroding our beach parks.

(Image: Researchers service a PROMICE automatic weather station on the Greenland ice sheet. PROMICE stands for Programme for Monitoring of the Greenland Ice Sheet. Credit: William Colgan, York University.)

We’ve known for some time that the IPCC estimates left out big sources of ocean rise, simply because it wasn’t clear at the time how to make sense of what was likely to happen. Now new information is beginning to clear that up.

An article in Nature looks at the impacts of melting of the Antarctic ice sheet, both in the past and in the possible future. During the last interglacial period, 115,000 to 130,000 years ago, sea level was six to nine meters higher than now. Yes, in the range of 20 to 30 feet higher.

 “The Antarctic ice sheet has been implicated as the primary contributor, hinting at its future vulnerability,” wrote the authors, Robert De Conto of the University of Massachusetts and David Pollard of Pennsylvania State.

They estimate that Antarctic melting ice alone will provide three feet of rise, on top of the rise from other sites and from thermal expansion of warming oceans.

“Antarctica has the potential to contribute more than a metre of sea-level rise by 2100 and more than 15 metres by 2500, if emissions continue unabated,” they wrote.

Their work builds on numerous research projects, including one published in 2015 in the journal Science, which predicted that Antarctic and Greenland ice would be the dominant sources of sea level rise in the future—although they haven’t been the major sources until now.

“Although thermal expansion of seawater and melting of mountain glaciers have dominated global mean sea level (GMSL) rise over the last century, mass loss from the Greenland and Antarctic ice sheets is expected to exceed other contributions to GMSL rise under future warming,” these authors wrote.

Both groups looked at past sea level change data to recognize that we are at a turning point, at which the big ice sheets start contributing.

“Present climate is warming to a level associated with significant polar ice-sheet loss in the past,” wrote the authors of the Science article.

What’s been going on in the past is that mostly mountain glaciers and floating ice has been melting, but the melting of floating ice doesn’t cause sea level rise—any more than a melting ice cube causes your drink to overflow.

However, when the world's giant ice sheets begin melting, that adds significant amounts of water to the oceans. Researchers have been able to compare previous warming episodes with ice sheet actions and sea levels.

Still another study, this one in Geophysical Research Letters, looked into two exceptional melt episodes in 2012 in the Greenland ice sheet. It found that the ice melt incidents, which normally are caused directly by the sun, were significantly caused by warm, moist air.

“When we were analysing our weather station data, we were quite surprised, that the exceptional melt rates we observed were primarily caused by warm and moist air, because ice sheet wide melt is usually dominated by radiant energy from sunlight,” said Robert Fausto, of the Geological Survey of Denmark and Greenland. He was the lead author of the report.

“Exceptional melt episodes dominated by non-radiant energy are expected to occur more frequently in the future due to climate change. This makes it critical to better understand the influence of these episodes on ice sheet health,” Faust said.

© Jan TenBruggencate 2016

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