Is the Greenland ice sheet collapsing?
The Greenland ice cap appears to be melting faster than previously thought. At least, that the gist of the round of recent news articles. Here's the U.P.I. version:
Report: Greenland ice change means troubleILULISSAT, Greenland, Sept. 8 (UPI) -- Mini earthquakes and glacier acceleration on the Greenland ice cap are signs climate change is speeding up, scientists said.
The quakes are caused by giant chunks of ice breaking off the rock they have been frozen to for hundreds of years, Robert Corell, chairman of the international Arctic Climate Impact Assessment, said in Greenland Friday.
Experts met at the Ilulissat glacier in Greenland to discuss the future of the arctic, The Guardian reported, while a group representing the world's major religions gathered to pray for Earth.
Though small in magnitude, the earthquakes bolster concerns that the entire ice shelf could collapse, causing a catastrophic change in sea levels worldwide.
The speed at which Greenland's glaciers flow into the sea has also accelerated. The Ilulissat glacier is dumping ice chunks into the ocean at a rate of 2 meters per hour -- more than three times faster than 10 years ago, the newspaper said.
Given the changes, the Intergovernmental Panel on Climate Change's current prediction sea levels will rise eight to 24 inches [18 - to 59 centimetres] this century may be too low, scientists at the meeting said. Some estimate the seas may rise by more than two meters.
The IPCC predictions are somewhat nuanced (see Table SPM-3 on page 13 of the IPCC report, Climate Change 2007: The Physical Science Basis): depending on which of six climate models are used (the models vary because they assume differing amounts of human-emitted greenhouse gases in coming decades), the expected sea level rise will be anywhere from 18 to 59 centimetres by the end of this century (the year 2099). But it's important to note the report's disclaimer for sea-level rise projections, found at the bottom of page 14 and the top of page 15:
Models used to date do not include uncertainites in climate-cycle feedback nor do they include the full effect of changes in ice sheet flow, because a basis in published literature is lacking. The projections include a contribution due to the increased flow from Greenland and Antarctica at the rates observed for the years 1993-2003, but these flow rates could increase or decrease in the future. For example, if this contribution were to grow linearly with global average temperature change, the upper ranges of sea level rise for SRES scenarios shown in Table SPM-3 would increase by 0.1 m to 0.2 m [10-20 centimetres]. Larger values cannot be excluded, but understanding of these effects is too limited to assess their liklihood or provide a best estimate or an upper bound for sea level rise.
That paragraph encapsulates the inherent conservatism of the IPCC process. Projections are made taking past, observable declines in alpine glaciers and in the thermal expansion of the oceans and projecting forward. So far as Greenland and Antarctic ice melt goes, only past rates of melt (from 1993 -2003) are considered, without any increased projections for warmer temperatures. As noted, including future warmer temperatures would increase the published projections by 10 to 20 centimetres (someone should tell the world's city and disaster planners, who now are building cities and preparing for storm surges with the 59-centimetre number in mind, not the far more deadly 79-centimetres). But even the additional 10-20 centimetres is based on a linear projection-- and does not consider the possibility of a non-linear collapse of the ice sheets.
This brings us back to Greenland, and even more important, Antarctica. What happens if these gigantic ice sheets don't simply melt at predictable rates, but instead collapse, more or less all at once?
How would that happen? In a paper published in May 2007 (Scientific reticence and sea level rise), NASA's Dr. James Hansen, arguably the world's most recognizable climatologist and the man who famously warned the US Senate of the dangers of global warming at a 1988 hearing, explains how the complete disintegration of the ice sheets could occur:
Ice sheet disintegration, unlike ice sheet growth, is a wet process that can proceed rapidly. Multiple positive feedbacks accelerate the process once it is underway. These feedbacks occur on and under the ice sheets and in the nearby oceans.A key feedback on the ice sheets is the `albedo flip' (Hansen et al 2007) that occurs when snow and ice begin to melt. Snow-covered ice reflects back to space most of the sunlight striking it. However, as warming causes melting on the surface, the darker wet ice absorbs much more solar energy. Most of the resulting melt water burrows through the ice sheet, lubricates its base, and thus speeds the discharge of icebergs to the ocean (Zwally et al 2002).
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Acceleration of ice sheet disintegration requires tapping into ocean heat, which occurs primarily in two ways (Hansen 2005): (1) increased velocity of outlet glaciers (flowing in rock-walled channels) and ice streams (bordered mainly by slower moving ice), and thus increased flux and subsequent melting of icebergs discharged to the open ocean, and (2) direct contact of ocean and ice sheet (underneath and against fringing ice shelves). Ice loss from the second process has a positive feedback on the first process: as buttressing ice shelves melt, the ice stream velocity increases.
Positive feedback from the loss of buttressing ice shelves is relevant to some Greenland ice streams, but the West Antarctic ice sheet, which rests on bedrock well below sea level (Thomas et al 2004), will be affected much more. The loss of ice shelves provides exit routes with reduced resistance for ice from further inland, as suggested by Mercer (1978) and earlier by Hughes (1972). Warming ocean waters are now thinning some West Antarctic ice shelves by several meters per year (Payne et al 2004, Shepherd et al 2004).
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The most compelling data for the net change of ice sheets is provided by the gravity satellite mission GRACE, which shows that both Greenland (Chen et al 2006) and Antarctica (Velicogna and Wahr 2006) are losing mass at substantial rates. The most recent analyses of the satellite data (Klosco) confirm that Greenland and Antarctica are each losing mass at a rate of about 150 cubic kilometers per year, with the Antarctic mass loss primarily in West Antarctica. These rates of mass loss are at least a doubling of rates of several years earlier, and only a decade earlier these ice sheets were much closer to mass balance (Cazenave 2006).
The Antarctic data are the most disconcerting. Warming there has been limited in recent decades, at least in part due to the effects of ozone depletion (Shindell and Schmidt 2004). The fact that West Antarctica is losing mass at a significant rate suggests that the thinning ice shelves are already beginning to have an effect on ice discharge rates. Warming of the ocean surface around Antarctica (Hansen et al 2006a) is small compared with the rest of world, consistent with climate model simulations (IPCC 2007), but that limited warming is expected to increase (Hansen et al 2006b). The detection of recent, increasing summer surface melt on West Antarctica (Nghiem et al 2007) raises the danger that feedbacks among these processes could lead to nonlinear growth of ice discharge from Antarctica.
Hansen goes on to note that climate change events of even less magnitude than those forecast for the next century have caused sea level rises of many metres per century many times in the earth's past (events he discussed in an earlier paper, Climate Change and Trace Gases), and argues that the IPCC projections are flawed because:
The linear approximation [of the IPCC report] fits the past sea level change well for the past century only because the two terms contributing significantly to sea level rise were (1) thermal expansion of ocean water and (2) melting of alpine glaciers.Under BAU [Business As Usual-- that is, we don't do anything about our increasing greenhouse gas emissions] forcing in the 21st century, the sea level rise surely will be dominated by a third term: (3) ice sheet disintegration. This third term was small until the past few years, but it is has at least doubled in the past decade and is now close to 1 mm/year, based on the gravity satellite measurements discussed above. As a quantitative example, let us say that the ice sheet contribution is 1 cm for the decade 2005–15 and that it doubles each decade until the West Antarctic ice sheet is largely depleted. That time constant yields a sea level rise of the order of 5 m this century. Of course I cannot prove that my choice of a ten-year doubling time for nonlinear response is accurate, but I am confident that it provides a far better estimate than a linear response for the ice sheet component of sea level rise under BAU forcing.
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The nonlinearity of the ice sheet problem makes it impossible to accurately predict the sea level change on a specific date. However, as a physicist, I find it almost inconceivable that BAU climate change would not yield a sea level change of the order of meters on the century timescale.
Yes, you read that right: Hansen is saying that if we consider the collapse of the Greenland and West Antarctic ice sheets, sea level rise this century may not be the IPCC's projected 59 centimetres but rather on the order of five metres.
A five-metre sea-level rise floods something like 90 percent of the world's largest cities, millions upon millions of square kilometres of agricultural land, the homes of billions of people.
Thankfully, there are a couple of caveats:
First, Hansen could be wrong. Of course, the news reports from Greenland might argue otherwise. But we should learn as much as we can about the potential problem, as soon as we can. Hansen is calling for an immediate world-wide scientific study to better understand what is happening in Greenland and Antarctica.
Second, even if Hansen is right, it doesn't follow that these ice sheet collapses are fated and there's nothing we can do about them. With political will and action we can prevent the worst of the potential harmful effects of global warming, including perhaps the loss of the ice sheets. I've discussed how to go about doing so in great detail, here, and there is a lot of information out there from many other authors about how to restructure our society to lessen the effects of global warming. A good place to start is the envioronmental blogs listed at left.
