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A recent study, published in the March 24 issue of the journal Science, revealed that the last time the mean summer temperatures in the Arctic were higher than today, was during the last interglacial period (Eemian) 125.000 years ago. The study modelled its findings based on evidence collected from existing ice-core records, marine sediments, corals and fossilized pollens and suggests that during this specific period of time the sea-level must have been 4-6 m above present levels. During that specific interglacial period, the melting Greenland ice sheet and other arctic sources only raised the sea-level by approximately 3.5m and implying that there must have been at least some Antarctic contribution to explain the total observed sea-level rise in the 4-6m range.
The scientists theorize that the rise in sea-level produced by the melting Arctic and Greenland along with a requisite circumpolar warming could have initialized a significant destabilisation of the ice shelves at the edge of the Antarctic ice sheet and might have caused their collapse back then with accelerating glacier leading to a sea-level rise of 4-6m.
By successfully recreating the last period of significant global warming by using palaeological records the scientists also projected a possible future scenario. While the global warming 125.000 years ago was likely gradual and driven by an orbital increase in solar radiation over the Arctic, the projected rise of temperature by the end of this century is thought to be human induced. Even more frightening the scientist suggest that the warmth needed to melt the ice sheets and to cause rising sea levels of up to 4-6 m even within this century isn’t that much above present conditions.

Fig. 1: Simulated climate for each of four time periods, from left to right: present day (Modern), 130,000 years ago (anomalies from present day, LIG), 2100 A.D. (the time atmosphere reaches three times preindustrial CO2 levels, climate anomalies from present day, D AD 2100), and 2130 A.D. (four times preindustrial CO2 levels, climate anomalies from present day, D AD 2130). (1)

The study suggests that by 2100 the eemian temperatures could even be outnumbered if the greenhouse gas emissions were increased by 1% per year resulting in a sea-level rise that might even exceed the observed one of 125.000 years ago. (1)

Eeamian conditions by the end of the 21st century?

Indeed a new analysis of data, published in the March and August 2006 issue of the journal Science, used satellite measurements taken with the Gravity Recovery and Climate Experiment (GRACE) and revealed quite alarming results about the latest melt processes in Greenland and the West-Antarctica. (2, 3)

The gathered data showed that the melting rate of Greenland’s ice sheet has accelerated dramatically over the past few years, with most of the ice loss occurring primarily along the south-eastern shoreline. Based on the results Greenland lost about 239 cubic kilometres of ice each year from 2002-2005.

Fig. 2: Greenland Melt Extent, 2005 (4)

Even more dramatic the measurements revealed that the acceleration rate really took of in the past two years suggesting that the whole process of glacial melting could be stepping up much more rapidly than previously thought and that the tipping-point (2.7 °C above pre-industrial times) of an irreversible collapse of the Greenland ice sheet could have been passed already. (2,4)

At the same time the two NASA satellites revealed that for the first time the Antarctic ice sheet lost a significant amount of mass since 2002. The scientist found the Antarctic ice sheet decreased by 152 (+/-80) cubic kilometres of ice annually between 2002 and 2005, contributing about 13 percent of the overall observed sea-level rise for the same period. (3)

Fig. 3: Ice mass loss in Antarctica in cubic kilometres as observed by Grace over the period 2002-2005. (3)

Only four years ago, it was scientific consensus that the West-Antarctic ice sheet was stable, but since the recently observed rapid acceleration ice loss from the West-Antarctica scientists start to re-think this assumption. In 2002 the Larsen B ice shelf collapsed in less than a month. Even though it didn’t contribute to the sea-level rise directly, the missing ice shelf reduced the backpressure on glaciers and enabled the land-based ice to speed up towards the ocean. Latest studies support this assumption by revealing that glaciers some especially around the missing Larsen B ice shelf have increased their speed towards the ocean with dramatically accelerated ice discharge from the Antarctic Peninsula. (5, 6)

Obviously the fast ice dynamics we are seeing now in Greenland and especially in the West-Antarctic have not been considered in the numerical IPCC models, and yet the dynamics seem to be much more important than melting and precipitation effect, which are included in the models.
With the ice sheets in Greenland and in the West-Antarctic already changing the scientists are urgently warning that the estimates of the numerical models used to project a possible sea-level rise in the future could be way too optimistic and therefore may be misleading policymakers.

1 Overpeck, J. T. et al.(2006): Paleoclimatic Evidence for Future Ice-Sheet Instability and Rapid Sea-Level Rise, In: Science, Vol. 311, Seite 1747.
2 Chen, J.L., Wilson, C. R. and B. D. Tapley (2006): Satellite Gravity Measurements Confirm Accelerated Melting of Greenland Ice Sheet. Science, 313:1958–1960, doi:10.1126/science.1129007.
3 Velicogna, I., and J. Wahr (2006): Measurements of Time-Variable Gravity Show Mass Loss in Antarctica, Science 311, 1754-1756.
4 Steffen, K. and R. Huff (2005): Greenland Melt Extent, 1992 - 2005. http://cires.colorado.edu/science/groups/steffen/greenland/melt2005/ (10.11.2006).
5 Rignot, E. et al. (2004): Accelerated ice discharge from the Antartic Peninsula following the collapse of Larsen B ice shelf. In: Geophysical Research Letters Vol. 31, L18401 doi:10.1029/2004GL020697.
6 Rignot, E. et al. (2002): Acceleration of Pine Island and Thwaites Glaciers, West Antartica, Annals of Of Glaciology 34, 189-194 .