More than a metre

By Bryan Walker 10/09/2009

Sea level will rise by more than a metre by 2100 according to the authors of the third chapter in the World Wide Fund for Nature’s new Arctic report, introduced by Gareth a few days ago. Eric Rignot, one of the two authors of the chapter, is principal scientist for the Radar Science and Engineering Section at NASA’s Jet Propulsion Laboratory, Pasadena, Calif.  The other author, Anny Cazenave, is an internationally renowned research scientist from France’s national centre for space studies. 

The value of the chapter is that it draws together, authoritatively and coherently, the evidence that points to considerably more sea-level rise over this century than projected in the 2007 IPCC Fourth Report (AR4). Happily politicians are taking IPCC reports much more seriously than in the past, but they should not rest on them.  Their responsibility is to be up to date with what the science is saying now. The WWF report assesses the most recent science, and finds that the impacts of warming will be more severe than indicated by the IPCC.

What follows is a summary of the main points made by the chapter.

Sea level has been rising over the past 50 years, and its rate of rise has been accelerating. The rate of rise 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. This 15-plus year data set shows that average global sea level is currently rising at a rate of about 3.3 millimetres per year (plus or minus 0.4 millimetres), roughly twice the average rate recorded by tide gauges over the previous decades.

Three major sources are currently contributing to sea-level rise. Ocean warming, and the resultant thermal expansion of the water, has significantly increased since 1950 and explains about 25% of the observed sea level rise of the last few decades and about 30% over the 1993-2008 period. Glacier melt accounts for another 30% for the 1993-2008 period, one third of that coming from Alaskan glaciers. 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 a glacier melts. The 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 destabilisation in a warmer climate were not sufficiently well understood to inform the forecasts in AR4. 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 centimetres to sea level rise this century.

The chapter then reports on Antarctica. In 2008 Antarctica lost nearly as much ice as Greenland, with a 220 gigatonne net loss, and as in Greenland the mass loss is accelerating. The 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 be 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 metre, a contribution to sea level not included in the IPCC’s 2007 predictions.

The AR4 sea-level projections did not take into account the complex ice sheet dynamics by which glaciers flow into the ocean, which have only recently begun to be understood.  This is why their projection 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 destabilisation 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 20th 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 centimetre rise this century the authors find plausible, while acknowledging that future sea level rates may not necessarily follow the past century’s dependence on the average global temperature, especially if ice sheet dynamics play a larger role in the future. They note that sea-level rise won’t stop in 2100. 

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.  What may happen in that arena is still very unclear.