Sea level will rise by more than a meter by 2100 according to a chapter in the new report from World Wide Fund for Nature on Arctic feedbacks. The two authors, Eric Rignot and Anny Cazenave are internationally recognized scientists in the field.

WWF has commissioned the report in order to publicize the most recent scientific findings regarding major arctic feedbacks of global significance. The third chapter is about sea-level rise. It draws together, authoritatively and coherently, the indications which point to sea-level rise as likely to be considerably more than predicted in the 2007 IPCC report. I have summarised some of the main points.

Sea level has been rising over the past 50 years, and its rate of rise has been accelerating. The rate in the past 15 years is about double that of the previous decades. Since 1993 sea level variations have been accurately measured by satellite altimetry.

Three major sources are currently roughly equally responsible. Ocean warming, and hence ocean thermal expansion, accounts for about 30% over the 1993-2008 period. Glacier melt accounts for another 30% for the period. The ice sheets of Greenland and Antarctica account for the rest. 

Observations show that the Greenland ice sheet is losing ice mass to the ocean. In 2008 the loss was about 280 gigatonnes. The loss has been increasing over the last 20 years by about 20 gigatonnes per year.

One third of this loss is due to increased surface melting or runoff, and the other two thirds to the acceleration of glaciers. It was thought that the acceleration was due to bedrock lubrication from meltwater, but this only accounts for about 20% of the acceleration.  The rest is due to the pressure change that occurs near the front of the glacier as it melts. 

More rapid melt due to warmer ocean and land temperatures causes the glacier to retreat inland, which reduces the backpressure (or resistance to flow) on the inland ice, meaning the glacier can flow more swiftly into the sea as a wave of acceleration is transmitted upstream over vast distances. 

These mechanisms of destabilization in a warmer climate were not sufficiently well understood to inform the forecasts of the IPCC 2007 report.  The ice sheets will continue to lose mass at an increasing rate in a warmer climate, though predicting those rates remains a serious scientific challenge at present.  Glaciers grounded below sea level are the most vulnerable because their frontal regions remain in contact with ocean water during their retreat. If Greenland continues to lose mass at the rate it has been it alone will contribute 31 centimeters to sea level rise this century.

In 2008 Antarctica, with a 220 gigaton net loss, lost nearly as much ice as Greenland, albeit at a much smaller fraction of its annual input of mass from snowfall. As in Greenland the mass loss is accelerating. Pine Island Glacier has been thinning more rapidly and its flow rate has been increasing every year for the past 35 years. When it becomes ungrounded from its ice plain, which could happen in only a few years, it will begin calving from a much deeper bed and speed up by a factor of 2 or 3. This sector of West Antarctica alone holds enough ice to raise sea level by an additional meter, a contribution to sea level not included in the IPCC 2007 predictions.

It is only recently that these complex ice sheet dynamics have begun to be understood.  The IPCC 2007 sea-level projections were not able to take them into account, which is why their projection of sea-level rise is likely to be exceeded.   Ice sheet losses are currently increasing faster than any other system contributing to sea-level rise which makes it likely that they will become the primary contributor during this century. 

Further progress is necessary to better understand and model the mechanisms of destabilization of glaciers and ice sheets and improve predictions.  In the meantime the report notes an alternative approach to predicting sea-level based on a simple 20^th century relationship between the observed average rate of global sea-level rise and the observed average global temperature of the Earth. On this basis global average temperature projections can be used to project future global average sea level.  The consequent estimate of a 60 to 120 centimeter rise this century the authors find plausible, while acknowledging uncertainties associated with it.

A big question is what sea-level rise will mean for the 25% of humans who live in low-lying coastal regions.  It will give rise to inundation (both temporary and permanent flooding), wetland loss, shoreline erosion, saltwater intrusion into surface water bodies and aquifers, and it will raise water tables. Sediment deposition in river deltas will decrease and there will be changes in coastal waves and currents. An additional factor is the combination of sea-level rise and vertical movement of the ground. Accelerated ground subsidence is reported in many regions because of local groundwater withdrawal or oil and gas extraction. Such sinking amplifies the effect of sea-level rise. It is very difficult to quantify future sea-level rise in specific regions where various factors interact in complex ways.

The chapter offers what struck me as a very restrained conclusion: “Despite the uncertainties, sea-level rise will almost surely cause significant impacts in coastal regions around the world.” That's as far as the scientists take us.  The rest is over to us and those we appoint to lead us. 

If you’d like to see glacier acceleration happening this recent video of a talk by photographer James Balog offers a dramatic and intelligently explained depiction.