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RealClimate has a current piece on the IPCC sea level numbers, which includes amongst the many comments a range of responses relating to the ice sheets and sea ice levels:

Rabett Run has a piece on Prof. Hansen’s current work on the same subjects, including an extremely useful link to the (discussion version of the)  paper in press:

You’ll notice that there are links to both these important and useful blogs in the sidebar.

The main item under discussion, I suppose, is whether the IPCC estimates of 21st Century sea level rise are too conservative, giving a maximum without ice sheet variables, of 59cm. Apparently, Stephan Rahmsdorf has already produced a paper in which a simple linear projection forward from the most recent data produces a sea level rise of ~1M by 2100. Hansen cites several authors and recent papers, in which some evidence for dynamic changes in the sheets is apparent. In addition to these, there are three new papers in GRL worthy of note:

The 1979–2005 Greenland ice sheet melt extent from passive microwave data using an improved version of the melt retrieval XPGR algorithm

Analysis of passive microwave satellite observations over the Greenland ice sheet reveals a significant increase in surface melt over the period 1979–2005. Since 1979, the total melt area was found to have increased by +1.22 × 107 km2. An improved version of the cross-polarized gradient ratio (XPGR) technique is used to identify the melt from the brightness temperatures. The improvements in the melt retrieval XPGR algorithm as well as the surface melt acceleration are discussed with results from a coupled atmosphere-snow regional climate model. From 1979 to 2005, the ablation period has been increasing everywhere over the melt zone except in the regions where the model simulates an increased summer snowfall. Indeed, more snowfall in summer decreases the liquid water content of the snowpack, raises the albedo and therefore reduces the melt. Finally, the observed melt acceleration over the Greenland ice sheet is highly correlated with both Greenland and global warming suggesting a continuing surface melt increase in the future.

Rapid volume loss from two East Greenland outlet glaciers quantified using repeat stereo satellite imagery

The coastal portions of Kangerdlugssuaq and Helheim glaciers in southeast Greenland lost at least 51 ± 8 km3 yr−1 of ice between 2001–2006 due to thinning and retreat, according to an analysis of sequential digital elevation models (DEMs) derived from stereo ASTER satellite imagery. The dominant contribution to this ice loss was dynamic thinning caused by the acceleration in flow of both glaciers. Peak rates of change, including thinning rates of ∼90 m yr−1, coincided with the rapid increases in flow speed. Extrapolation of the measured data to the ice divides yields an estimated combined catchment volume loss of ∼122 ± 30 km3 yr−1, which accounts for half the total mass loss from the ice sheet reported in recent studies. These catchment-wide volume losses contributed ∼0.31 ± 0.07 mm yr−1 to global sea level rise over the 5-year observation period with the coastal regions alone contributing at least 0.1 ± 0.02 mm yr−1.

Effects of ice melting on GRACE observations of ocean mass trends

The Gravity Recovery and Climate Experiment (GRACE) was designed to measure variations in the Earth’s gravity field from space at monthly intervals. Researchers have used these data to measure changes in water mass over various regions, including the global oceans and continental ice sheets covering Greenland and Antarctica. However, GRACE data must be smoothed in these analyses and the effects of geocenter motions are not included. In this study, we examine what effect each of these has in the computation of ocean mass trends using a simulation of ice melting on Greenland, Antarctica, and mountain glaciers. We find that the recovered sea level change is systematically lower when coefficients are smoothed and geocenter terms are not included. Assuming current estimates of ice melting, the combined error can be as large as 30–50% of the simulated sea level rise. This is a significant portion of the long-term sea level change signal, and needs to be considered in any application of GRACE data to estimating long-term trends in sea level due to gain of water mass from melting ice.

 This is already a long post, so I won’t add much comment at the moment, except to say that there does appear to be a growing body of evidence supporting the idea that there are already processes at work in both Polar regions which should cause concern to policy makers, in particular because these have not been considered at all in the IPCC SPM.


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March 2007