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In an article published in Environmental Research Letters on May 24, 2007, James Hansen, NASA scientist, discusses the non-linearity of shelf ice melting and the reticence of scientists to announce their fears about this process. The whole article is worth a read. Below is just the section on sea-level rise risks:

"A field glaciologist, referring to a moulin on Greenland, said: `the whole damned ice sheet is going to go down that hole!' He was talking about his expectations, under the assumption of continued unchecked growth of global greenhouse gas emissions. Field glaciologists have been doing a good job of reporting current trends on the ice sheets. It is the translation of field data into conclusions needed by the public and policymakers that is at issue.

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).

The area with summer melt on Greenland increased from ~ 450 000 km2 when satellite observations began in 1979 to more than 600 000 km2 in 2002 (Steffen et al 2004). A linear fit to data for 1992–2005 yields an increase of melt area of 40 000 km2/year (Tedesco 2007), but this rate may be exaggerated by the effect of stratospheric aerosols from the 1991 volcanic eruption of Mount Pinatubo, which reduced the summer melt in 1992. Summer melt on West Antarctica has received less attention than on Greenland, but it is more important. Satellite QuickSCAT radiometer observations reveal increasing areas of summer melt on West Antarctica and an increasing melt season length during the period 1999–2005 (Nghiem et al 2007).

The key role of the ocean, in the matter of ice sheet stability, is as a conduit for excess global-scale heating that eventually leads to the melting of ice. The process begins with increasing human-made greenhouse gases, which cause the atmosphere to be more opaque at infrared wavelengths. The increased atmospheric opacity causes heat radiation to space to emerge from a higher level, where it is colder, thus decreasing the radiation of heat to space. As a result, the Earth is now out of energy balance by between 0.5 and 1 W m–2 (Hansen et al 2005).

This planetary energy imbalance is itself now sufficient to melt ice corresponding to one meter of sea level rise per decade, if the energy were used entirely for that purpose (Hansen et al 2005). However, so far most of the excess energy has been going into the ocean. 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).

The Antarctic peninsula recently provided a laboratory to study feedback interactions, albeit for ice volumes less than those in the major ice sheets. Combined actions of surface melt (Van den Broeke 2005) and ice shelf thinning from below (Shepherd et al 2003) led to the sudden collapse of the Larsen B ice shelf, which was followed by the acceleration of glacial tributaries far inland (Rignot et al 2004, Scambos et al 2004). The summer warming and melt that preceded the ice shelf collapse (Fahnestock et al 2002, Vaughan et al 2003) was no more than the global warming expected this century under BAU scenarios, and only a fraction of expected West Antarctic warming with realistic polar amplification of global warming.

Modeling studies yield increased ocean heat uptake around West Antarctica and Greenland due to increasing human-made greenhouse gases (Hansen et al 2006b). Observations show a warming ocean around West Antarctica (Shepherd et al 2004), ice shelves thinning several meters per year (Rignot and Jacobs 2002, Payne et al 2004), and increased iceberg discharge (Thomas et al 2004). As the discharge of ice increases from a disintegrating ice sheet, as occurs with all deglaciations, regional cooling by the icebergs is significant, providing a substantial but temporary negative feedback (Hansen 2005). However, this cooling effect is limited on a global scale as shown by comparison with the planetary energy imbalance, which is now sufficient to melt ice equivalent to about one meter of sea level per decade (table S1 of Hansen et al 2005). Yet the planetary energy imbalance should not be thought of as a limit on the rate of ice melt, as increasing iceberg discharge yields both positive and negative feedbacks on planetary energy imbalance via ocean surface cooling and resulting changes of sea ice and cloud cover.

Global warming should also increase snowfall accumulation rates in ice sheet interiors because of the higher moisture content of the warming atmosphere. Despite high variability on interannual and decadal timescales, and limited Antarctic warming to date, observations tend to support this expectation for both Greenland and Antarctica (Rignot and Thomas 2002, Johannessen et al 2005, Davis et al 2005, Monaghan et al 2006). Indeed, some models (Wild et al 2003) have ice sheets growing overall with global warming, but those models do not include realistic processes of ice sheet disintegration. Extensive paleoclimate data confirm the common sense expectation that the net effect is for ice sheets to shrink as the world warms.

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."

You can read the whole article here:
Scientific reticence and sea level rise