The Greenland Ice

In the earlier Science paper, Johanessen et al. found increased snow accumulation on the top of the interior Greenland ice sheet between 1992 and 2003. Above 1500m a.s.l in much of the interior Greenland they estimated an increase of 6.4 ± 0.2 cm/year and below 1500m they observed a decreasing trend of -2.0 ± 0.9 cm/year. Hence, growth in the interior parts and a thinning of the ice nearer the edges. However, Johanessen et al. were not able to measure all of the coastal ranges. Indeed, the thinning of the margins and growth in the interior Greenland is an expected response to increased temperatures and more precipitation in a warmer climate. These results present no contradiction to the accelerated sliding near the coasts, but both will affect the ice/snow (fresh water) mass estimate. Whereas the finding of Rignot and Kanagaratnam suggests a larger sink of the frozen Greenland fresh water budget (the ice is dumped into the sea), the snow deposition in Greenland interiors is a source term (increases the amount of frozen fresh water). It does not matter for the general sea level in which form the water exists (liguid or solid/frozen) when it is discharged into the sea: The same mass of liquid water and immersed ice affect the water level equally (Archimede’s principle).

A third relevant study is a recent paper in the Journal of Glaciology by Zwally et al. (2005) on the ice mass changes on Greenland and Antarctica. They use the same satellite obsevations (ERS 1 and 2) as Johanessen et al. and again find that the Greenland ice sheet is thinning at the margins (-42 ± 2 Gt/year = -46 ± 2 km3/year below the equilibrium-line altitude – ELA), but growing in the inland (+53 ± 2 Gt/year = 58 ± 2 km3/year). The mass estimates have been converted to volume estimates here, assuming the density of ice is 0.917 g/cm3 at 0°C, so that the mass of one Gt of ice is roughly equivalent to 1.1km3 ice*. This means that the Greenland ice has an overall mass gain by +11 ± 3 Gt/year (=10 ± 2.7 km3/year) which they estimated implied a -0.03 mm/year SLE over the period 1992-2002.

The critical point for Greenland is whether the increased rate of glacier motion more than compensates for the greater accumulation on the surface. While the broad picture of what is happening is consistent between these papers, the bottom-line value for Greenland’s mass balance is different in all three cases. Looking just at the dynamical changes observed by Rignot & Kanagaratnam, there is an increased discharge of about 0.28 mm/year SLE from 1996 to 2005, well outside the range of error bars. This is substantially more than the opposing changes in accumulation estimated by Johannessen et al and Zwally et al, and is unlikely to have been included in their assessments. Thus, the probability is that Greenland has been losing ice in the last decade. We should be careful to point out though that this is only for one decade, and doesn’t prove anything about the longer term. As many of the studies make clear, there is a significant degree of interannual variability (related to the North Atlantic Oscillation, or the response to the cooling associated with Mt. Pinatubo) such that discerning longer term trends is hard.

The largest contributions to sea level rise so far are estimated to have come from thermal expansion, with the melting of mountain glaciers and icecaps being of second order. Looking forward, the current (small) imbalance (whether positive or negative) of the Greenland ice sheet is not terribly important. What matters is if the melting were to increase significantly. Ongoing observations (most promisingly from the GRACE gravity measurements, Velicogna et al, 2005) will be useful in monitoring trends, but in order to have reasonable projections into the future, we would like to be able to rely on ice sheet models. Unfortunately, the physics of basal lubrication and the importance of ice dynamics highlighted in the Rignot & Kanagaratnam results are very poorly understood and not fully accounted for in current ice sheet models. Until those models include these effects, there is a danger that we may be under-appreciating the dynamic nature of the ice sheets.

References:

Hanna, E; Huybrechts, P; Janssens, I; Cappelen, J; Steffen, K; Stephens, A (2005) J. Geophys. Res.Vo. 110, D13108, doi:10.1029/2004JD005641

Johanessen, O.M; Khvorostovsky, K; Miles, M.W; Bobylev, L.P. (2005) ScienceVo. 310 no. 5750, pp 1013-1016

Ringnot, E; Kanagaratnam, P (2006) ScienceVo. 311 no. 5763, pp 986-990

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