The latest issue of Science contains a paper by Eric Rignot and Pannir Kanagaratnam claiming that glaciers along the periphery of Greenland are melting at a rapidly increasing rate. Another paper on this subject was published by Science just last year. Ola Johannessen did not consider direct ice lost by glaciers into the ocean but instead only focused on elevations changes. Johannssen showed that increasing snowfall in Greenland was leading to greater ice accumulations than had previously been measured and this was acting to slow Greenland's contribution to sea level rise. It was conspicuously ignored in this new report.
The new paper uses radar measurements from satellites and concludes that there has been a widespread acceleration in the calving and loss of Greenland's glaciers during the past 10 years. The authors calculate that Greenland's glaciers were calving about 56 cubic kilometers of ice (km3) (give or take 30 km3) per year back in 1996, and that during the past 10 years, the volume of ice loss had risen to 167 km3 per year (give or take 40 km3). And this was just from glaciers dumping ice directly into the ocean (as opposed to melting ice).
Additionally, they speculated that snow and ice over other parts of Greenland are melting and the water is flowing into the ocean. Citing other work by Hanna et al. (2004), Rignot and Kanagaratnam figure another 35km3 in 1996 and 57 km3 in 2005 of ice loss occurred from surface melting bringing the total annual loss volume to 91 km3 in 1996 and 224 km3 in 2005.
This water and ice input leads to a sea level rise of 0.23 ± 0.08 mm/year in 1996 growing to 0.57±0.1mm/year by 2005. Not surprisingly, the reason that is given -- or at least presumed -- for the melting ice and the rising seas is that temperatures are going up because of global warming.
Why would Science publish this paper with no reference to Johannessen's earlier paper showing that Greenland is accumulating ice at a rate of about 5.4±0.2cm/year? Johannessen even used data from some of the same satellites. What's more, Johannessen used real data and Hanna et al., cited by Rignot, used a model of surface melt.
Consider what would have happened had the latest study included the ice and snow gains observed by Johannessen (and ignored the losses modeled by Hanna et. al.). Johannnessen's increase of 5.4cm/year averaged over Greenland converts to about 75km3/year. Rignot and Kanagaratnam could have subtracted Johannessen's gains. If they had done so, the total volume of ice loss from Greenland would only have become positive during the last 5 years, totaling 17km3 in 2000 and 92km3 in 2005. This translates to a sea level rise contribution of 0.04mm in 2000 and 0.23mm in 2005 -- values much less dramatic than those they published.
And what does all of this have to do with global warming? A look again at real data is instructive.
Temperatures fluctuations around Greenland are part of a phenomenon -- known as the Atlantic Multidecadal Oscillation (AMO) -- that connects with temperature changes further south, in the hurricane formation regions of the tropical Atlantic. The AMO goes through, as its name suggests, multidecadal swings from being cooler than normal both in the ocean around Greenland and in the tropical Atlantic, to being warmer than normal in both places. And modelers have suggested that the AMO has been part of the natural system for at least the past 10-15 centuries (see here).
When the AMO is in its positive (warm) phase, the Atlantic hurricane seasons become active with more and stronger storms; and, apparently, Greenland's glaciers flow faster and dump more ice into the ocean.
When the AMO is in its negative (cool) phase, hurricane activity in the Atlantic is suppressed and Greenland's glaciers flow slowly. The AMO changed from negative to positive in 1995 -- and since then hurricanes have become very active and glacier output has been accelerating. There is no need to invoke global warming for any of this.
Many of the world's foremost experts on hurricanes argue emphatically that it is the natural oscillations of the AMO -- not anthropogenic climate change -- that is primarily responsible for the enhanced Atlantic hurricane activity during the past decade or so (see here). And since the AMO ties together the sea surface temperatures in the tropical Atlantic with those around Greenland, the same argument applies to Greenland's glaciers.
Figure 1a shows the regions in the Atlantic Ocean that are most related to the AMO, and Figure 1b shows the AMO index for the past century.
Figure 1a. The relationship (correlation) between Atlantic sea surface temperatures and the AMO index; Figure 1b. The history of the AMO index since 1870 (source: Goldenberg, et al., 2001).
Figure 2 is the 20th century temperature history from the southern end of Greenland, from the United Nations' Intergovernmental Panel on Climate Change. Notice how similar it looks to the AMO history in Figure 1b. Also notice that both the AMO and Greenland temperatures were generally higher in 1930s and 1940s as they are today. We know that Atlantic hurricane activity has high back then, we can only assume that glacial flow rates were up there as well.
Figure 2. Temperature history from the southern portion of Greenland (source: http://www.ncdc.noaa.gov/gcag/gcag.html#HERE)
So what we have here are two stories making a lot of headlines -- Greenland is melting and hurricanes are strengthening. Both things are true. And, again, looking at real data it is apparent that at this time they are both part of a natural cycle that has been going on for thousands of years.
Goldenberg, S., et al., 2001. The recent increase in Atlantic hurricane activity: Causes and implications. Science, 293, 474-479.
Hanna, E., et al., 2005. Runoff and mass balance of the Greenland ice sheet, 1958-2003. Journal of Geophysical Research, 110, doi:10.1029/2004JD005641.
Johannessen, O.M., et al., 2005. Recent Ice-Sheet Growth in the Interior of Greenland. Science, 310, 1013-1016.
Knight, J.R., et al., 2005. A signature of persistent natural thermohaline circulation cycles in observed climate. Geophysical Research Letters, doi:10.1029/2005GL024233.
Rignot, E., Kanagaratnam, P., 2006. Changes in the velocity structure of the Greenland ice sheet. Science, 311, 986-990.