Sea level rise - No anomaly

Climate scientists had been concerned by an apparent slowing of sea-level rise over the last decade despite the increasing volume of run-off from glaciers. A recent paper in Nature Climate Change (DOI: 10.1038/nclimate2635) resolves the anomaly.

The authors showed that the rate of sea-level rise for 1993 to 2004 had been overestimated by about 20% due to inadequate calibration of the satellite-mounted altimeters. The new value, 2.6 to 2.9 mm/year, implies an acceleration in the rate of sea-level rise since 2004.

Antarctic Glacier in retreat

A new study by Gael Durand, at Grenoble Alps University, and others shows that the Pine Island Glacier, one of the largest in the Antarctic, is in “irreversible decline”.

The grounding line – the line along which the glacier lifts away from the sea bottom – has receded about six miles since 2003. But that’s just the start according to Durand; the glacier "has started a phase of self-sustained retreat and will irreversibly continue its decline." This conclusion is based on the use of three different models by the authors. The authors expect a five-fold increase in the volume of ice lost from the glacier for the period 2012 to 2031.

This is dramatic but unsurprising. The IPCC reports multiple measures of accelerating change:

·       The global mean sea level rose by an average of 1.7 mm per year from 1901-2010 but by 3.2 mm per year between 1993 and 2010.

·       Losses from Greenland's ice sheet have probably increased from 34 billion tons per year between 1992 and 2001 to 215 billion tons per year between 2001 and 2010.

·       In Antarctica, the rate of loss probably increased from 30 billion tons a year to 147 billion tons a year over the same timescale.

 
Forecasts of future warming, melting and sea level rise are highly uncertain precisely because the rates are increasing so sharply. It’s no secret that scientists are struggling to understand these changes. What’s not uncertain is that rising sea levels will threaten low-lying communities round the world, starting with the poorest. Without radical change in industry and agriculture the question is not whether they will threaten London and other cities in the developed world but when.

Greenland icecap melts

It's not only the Arctic sea ice that has melted this year. An almost unprecedented 97% of the Greenland ice also melted. This is not an actual catastrophe. It's only the surface that's melted (the ice sheet is up to two miles thick) and it has happened before.

But it is very rare. It happens about once in 150 years. The last time the melting reached Summit Station was 1889!

The reasons are perhaps more worrying than the simple fact. Thomas Mote of the University of Georgia blames the melting on the presence of a pocket of warm air over Greenland. In most years such pockets move away fairly quickly but this year has seen blocked weather systems so this year in stayed put.

Since these blocked weather systems are themselves attributed to climate change what we have here is another positive feedback effect: Global warming => blocking => local warming => melting => reduced albedo => warming.

If we get blocked weather systems next year - itself not unlikely - then we may have to throw away that 'once in 150 years' stuff.

The effects of increased surface melting may go beyond a simple loss of surface ice. When surface water enters crevasses it may lubricate the bottom of the icesheet and accelerate the movement of glaciers. More positive feedback.