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Melting of Greenland and Antarctica

A concern for the future is that the ice sheets of Antarctica and Greenland may respond to climate change more rapidly than previously thought, with grave implications for the rise of sea level. The Greenland Ice Sheet (GIS) holds about 7 meters of equivalent sea level. Analysis of spaceborne microwave brightness temperatures indicate that 2007 was a record year with areas of the GIS higher than 2000 meters in elevation melting up to 25-30 days longer than the average number of days calculated for the previous 18 years. The Antarctic Ice Sheet holds about 57 meters of equivalent sea level. Measurements from satellites show that the mass of this ice sheet decreased at a rate equivalent to 0.4 mm global sea level per year for the period 2002-2005. Most of the loss came from the West Antarctic Ice Sheet, with rapid increases in near-coastal discharge during these years. The large glaciers flowing into Amundsen Sea are thinning, probably because of warm water eroding the ice shelves.

Our observations of these troubling signs in Greenland and Antarctica are relatively short, only a few decades. The less recent past provides critical information about possible future tipping points. The last major sea level rise above modern occurred during the Last Interglacial (LIG). Direct sea level measurements based on tropical coral sequences and coastal sedimentary deposits indicate that global sea levels were 4 to 6 meters higher than present from approximately 128 to 118 thousand years ago. Boreal forests expanding poleward to the Arctic Ocean coast in Russia and insects extending their range in eastern North America are interpreted as summer temperatures up to 5°C warmer than today. The summit of Greenland remained ice-covered but parts of southern Greenland became ice-free and the ice caps in the eastern Canadian Arctic disappeared.

For past interglacials, the dominant forcing is changes in latitudinal and seasonal insolation associated with changes in the Earth's orbit - changes in the tilt of the Earth's orbit, its eccentricity, and the time of year when the Earth is closest to the Sun. Scientists can accurately calculate the slow cyclic changes in the Earth's orbit and tilt by the gravitational attractions of the Earth with the Sun, Moon, and other planets. At 130 thousand years ago, at the beginning of the LIG, these orbital parameters conspired to lead to large increases in the amount of solar radiation received at polar latitudes during late boreal spring and summer. Atmospheric greenhouse gas concentrations were lower than today, similar to those before the start of industrialization.

Figure 1. The Earth's orbital cycles. Today the Earth is tilted at 23.5° off the vertical, with an orbit that is slightly elliptical and closest approach to the Sun on January 3. At the start of the Last Interglacial, the Earth was tilted at 24.2°, the orbit was more elliptical, and closest approach to the Sun occurred on July 1.

Past climates give us a gauge of the ability of climate models to simulate polar warmth under a variety of well-known forcings. Indeed, when the NCAR Community Climate System Model (CCSM) is forced by the seasonal and latitudinal changes in solar radiation of the LIG, the predicted summer warming of the Arctic shows very good agreement with the proxy data. Both show strong warming in the eastern Canadian Arctic and over Greenland. Large positive solar anomalies in spring allow sea ice to melt earlier, resulting in enhanced heating of the now ice-free ocean and nearby continents. Annual snow depths decrease significantly over the southern, western, and northern edges of Greenland with the warmer summer temperatures.

Figure 2. Summer surface temperature change relative to the present over the Arctic(left) and annual ice heights and extent for Greenland and eastern Canadian ice caps as simulated by models and recorded in proxy data for the Last Interglacial. Left panel shows summer temperature change contoured for the CCSM simulation as compared to paleotemperature proxies plotted as circles and diamonds. Right panel shows the predicted Greenland Ice Sheet configuration based on the modeling and ice core data.

When the CCSM results are combined with an ice sheet model, the results show that during the LIG, the GIS retreated to a steeply-sided dome in central and northern Greenland and the large ice caps in eastern Canada melted completely. Together, this melting likely contributed 2.2 to 3.5 meters of early LIG sea-level rise. The meltwater would have cooled the North Atlantic but not enough to negate the orbital warming. Since global sea level was 4 to 6 meters higher than modern at the LIG, Antarctica must have also contributed some sea level by partially melting.

The Last Interglacial, as well as other past interglacials and the mid-Pliocene, provide a paleo perspective on future outcomes of a warming Earth. Projections of future climate by the CCSM indicate that summer air temperatures in the Arctic and oceanic temperatures around Antarctica may be as warm or warmer by 2100 as during the Last Interglacial. This raises concern about the stability of the Greenland and Antarctic Ice Sheets, which may start contributing increasing amounts of meltwater to sea level rise.

For additional information, visit Paleoclimate Research in Climate Change Research (CCR).