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Friday, February 12, 2010

John Cook, Skeptical Science: Why is Greenland's ice loss accelerating?

Why is Greenland's ice loss accelerating? 

by John Cook, Skeptical Science, 17 November 2009

Recently, we looked at satellite gravity measurements of the Greenland ice sheet, finding that Greenland is losing ice at an accelerating rate. This has now been confirmed by independent calculations of surface mass balance and ice discharge rates (van den Broeke et al., 2009). More importantly, the extra data offers keen insights into the contributing factors of Greenland's accelerating ice loss.

Greenland's ice mass balance is governed by two factors: surface mass balance (SMB) and ice discharge (D). SMB is the result of ice gain from precipitation (rain and snow) minus ice loss from sublimation and runoff. In van den Broeke et al 2009, SMB is calculated by the Regional Atmospheric Climate Model which is then confirmed by on-site observations. Ice discharge is the loss of ice as glaciers calve into the ocean. Satellite measure the velocity of ice as it moves towards the coastline as well as the thickness of the glaciers. This data are combined to calculate the total amount of ice discharge (Rignot, 2008).

Greenland's total mass change is then calculated by taking the difference between SMB and D. This result SMB - D can be directly compared to Greenland's total mass change as determined by the GRACE satellites' gravity measurements. The two independent time series show strong agreement as seen in Figure 1.

Figure 1. Surface mass balance minus ice discharge (SMB - D) (red) compared with GRACE data (blue). GRACE data are offset vertically. Short horizontal lines indicate GRACE uncertainty. Dashed lines indicate linear trends. The scatter plot in the inset shows a direct linear regression between monthly GRACE values as a function of the cumulative SMB - D anomaly (
van den Broeke et al 2009).

The SMB reconstruction also offers deeper insight into the various components that cause Greenland's mass loss. Figure 2 shows Greenland's total mass balance broken into its two components: surface mass balance and ice discharge. Don't be put off by the D showing a positive upturn after 2000 -- this is actually meant to indicate that Greenland is losing more mass due to ice discharge. We see that not only is D increasing sharply due to faster moving glaciers, the SMB also fell sharply. Greenland's total mass loss over 2000-2008 is equally split between SMB changes and D.

Figure 2. Greenland mass balance and its components SMB and D. Before 1996, D and hence SMB - D, are poorly constrained and therefore not shown.

What caused SMB to fall? There are three components to SMB: precipitation (rain and snow), sublimation and runoff (primarily ice melting with the water running off into the ocean). Figure 3 shows the trends for each component along with the resultant SMB in blue. Again, note that while run-off (orange) is displayed as a strongly positive trend, this actually removes ice mass. On the other hand, precipitation adds ice mass through rain and snow. Between 1996 and 2004, runoff and precipitation anomalies both increase simultaneously, roughly cancelling each other out so that SMB remains relatively steady. After 2004, precipitation levels out while runoff remains high. This caused SMB to fall sharply.

Figure 3. SMB (blue) and its components precipitation (red), runoff (orange) and sublimation (green).

So SMB reconstructions tell us that the acceleration of Greenland ice mass loss is largely due to increased ice discharge and increased run-off. In other words, glaciers are moving faster into the ocean and more ice is melting. Currently, Greenland ice loss is contributing 0.74 mm of sea level rise per year. This is around 23% of the total sea level rise (3.1 mm per year).

Thanks to Ernst Schrama (co-author of the paper), John Cross, Chip Fletcher and Chris who all let me know about this paper within a few hours of each other. Obviously I was meant to write a post about this paper!

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