May 2, 2011

A 225-year reconstruction of Greenland ice melt

Last week, the most popular article from among those recently published in the American Geophysical Union’s (AGU) Journal of Geophysical Research-Atmospheres was one which presents a 225-yr reconstruction of the extent of ice melt across Greenland. We are happy to say that your obedient servants here at World Climate Report were part of the research team of this oft-downloaded paper.

The full citation (for those who may want to check it out) is:

Frauenfeld, O.W., P.C. Knappenberger, and P.J. Michaels, 2011. A reconstruction of annual Greenland ice melt extent, 1785-2009. Journal of Geophysical Research, 116, D08104, doi: 10.1029/2010JD014918.

Here is the abstract:

The total extent of ice melt on the Greenland ice sheet has been increasing during the last three decades. The melt extent observed in 2007 in particular was the greatest on record according to several satellite-derived records of total Greenland melt extent. Total annual observed melt extent across the Greenland ice sheet has been shown to be strongly related to summer temperature measurements from stations located along Greenland’s coast, as well as to variations in atmospheric circulation across the North Atlantic. We make use of these relationships along with historical temperature and circulation observations to develop a near-continuous 226 year reconstructed history of annual Greenland melt extent dating from 2009 back into the late eighteenth century. We find that the recent period of high-melt extent is similar in magnitude but, thus far, shorter in duration, than a period of high melt lasting from the early 1920s through the early 1960s. The greatest melt extent over the last 2 1/4 centuries occurred in 2007; however, this value is not statistically significantly different from the reconstructed melt extent during 20 other melt seasons, primarily during 1923–1961.

The importance of studying Greenland lies in the fact that about 7.2 meters (23.6 feet) of sea level rise is tied up in the massive ice sheet which lies atop Greenland’s bedrock. The longer that ice stays there (e.g., the slower it melts), the better.

Alarmist climate change scenarios call for Greenland to shed its ice cover extremely rapidly and send the sea level a meter or more higher by the end of this century. But a more rational look at the data makes such claims hard to justify (see here and here and here for a basis of a more rational interpretation).

It is not that we don’t expect Greenland to continue to lose ice and contribute to sea level rise as the climate around Greenland warms (or even stays similar to as it is now), but just that we don’t expect it to happen at such a pace that it requires us (or anyone else) to start packing up all our belongings that lie along the coast and plan our move inland. The slower the rate of sea level rise, the more options for dealing with it are on the table.

The vast amount of ice lying atop Greenland is a relict of the last ice age—it wouldn’t exist if it didn’t already exist. And as the current interglacial warm period drags on, human beings or no, Greenland would continue to lose its ice (after all, during the previous interglacial, Greenland seems to have lost a sizeable portion of its ice cover).

Climate conditions during the ice age led to a massive accumulations of snow (which was compressed into ice) and raised the surface elevation of much of Greenland by several kilometers. Since the temperature of the atmosphere drops by about 10°C for every kilometer of elevation, the surface temperature across the Greenland ice sheets is some 10°C to 30°C (~20°F to 50°F) lower than it would be if there were no ice there. Lacking its ice sheets, Greenland would resemble the northern portions of Canada or Russia—cold, windy, and barren, but not buried under 9,000 feet of ice. The ice atop Greenland creates conditions for ice to remain atop Greenland.

As the northern climate warms, it increases the opportunity for ice melt to occur on portions of Greenland’s ice sheet, which slowly erodes the surface elevation which can lead to more warming, etc.

But this process happens fairly slowly (to say the least).

For example, in 2009, University of Copenhagen’s B. M. Vinther and 13 coauthors published the definitive history of Greenland climate back to the last ice age, studying ice cores taken over the entire landmass. An exceedingly conservative interpretation of their results is that Greenland was 1.5°C (2.7°F) warmer than present for the period from 5,000-9,000 years ago. As the massive amount of ice currently overlying Greenland well attests, even 4,000 years of warm conditions there left plenty of ice around.

Another example can be found in the Fourth Assessment Report (AR4) of the IPCC. The AR4 includes a simulation of the evolution of the surface elevation across Greenland under conditions of quadrupled levels of atmospheric carbon dioxide (recall that since the Industrial Revolution, the atmospheric level of CO2 has increased by about 50%—in other words, we are a long way away from a quadrupling). Under this quadrupled CO2 scenario, it takes about 1000 years for Greenland to lose half of its ice volume and another 2,000 years to lose the rest (Figure 1).

Figure 1. Evolution of Greenland surface elevation and ice sheet volume versus time under a climate of constant quadrupled pre-industrial atmospheric CO2 (source, IPCC AR4).

And current observations also bear out the slow pace of ice loss across Greenland.

Averaged over the past several years, according to one recent estimate (Wu et al, 2010), Greenland is losing a bit more than 100 cubic kilometers (out of its total of 2,850,000 cubic km) of ice per year. This contributes to about 0.25 mm (0.01 in.) per year to the rate of global sea level rise (other recent estimates (e.g. Rignot et al., 2011) put this number at about twice that, or 0.02 in/yr).

In our study, we sought to place the observed ice melt that has taken place across Greenland (which has been only directly observable since the advent of satellite-borne instrumentation in the late 1970s) within the context of a longer-term history of ice melt there to get a sense of how the current situation stacks up against what has taken place in during past couple of centuries.

By using a relatively simple statistical model (multiple linear regression) we were able to define a pretty decent relationship between observations of total summer extent of ice melt across Greenland (available to us from 1979 through 2009) and a combination of summer temperatures in southern Greenland along with an index of wintertime atmospheric circulation in the region (specifically the North Atlantic Oscillation, or NAO). The winter NAO served probably as a proxy for snowfall accumulation during the previous winter, which impacted the degree of melt the following summer (probably acting through an albedo effect—that is, fresh winter snowfall whitened the surface which reflected away incoming solar radiation which retarded summer melting).

Since the temperature and NAO data extended back in time for a century or two, we were able to use those observations to “predict” the amount of ice melt that occurred each summer back to 1784 (Figure 2). While our predicted (i.e. “reconstructed”) summer ice melt extent is not a perfect measure of what actually took place, it at least serves to give us a decent idea of what was going on.

Figure 2. Reconstructed history of the total ice melt extent index over Greenland, 1784–2009. Observed values of the ice melt index (blue solid circles), reconstructed values of the ice melt index (gray open circles), the 10 year trailing moving average through the reconstructed and fitted values (thick red line), and the 95% upper and lower confidence bounds (thin gray lines) (from Frauenfeld et al., 2011).

We found that the extent of ice melt in recent individual years (notably 2007 and preliminarily 2010) was quite high, and probably higher than the melt which occurred during any other year in our record.

But, we also found that there were likely several decade-long periods in the early-to-mid 20th century during which the average annual melt extent exceeded that of the 10 years ending in 2009.

And, also of note, was that the average summer melt extent over Greenland during the late 1970s and early 1980s—the time of the start of satellite observations—was the lowest in over a decade and below the historical average. This is potentially important because it means that a decent portion of the recent increase in melt extent could be part of natural variability about the mean state and not directly caused by climate change from an anthropogenically-enhanced greenhouse effect.

Our findings give us some insight into the course of sea level rise for the next couple of decades.

Over the last decade, the rate of sea level rise was about 2.5mm/yr (0.1 in/yr)—this increase includes the effects of warming (expanding) oceans, and contributions from ice loss from Antarctica, Greenland, and other glaciers and ice fields scattered across the globe. Back in the period from about 1930 to 1960, an extended time during which our results show that the average melt across Greenland was similar to the average of the past decade (Figure 2), the rate of sea level rise was also similar to that of the last decade. In other words, the rate of sea level rise didn’t become particularly menacing even during a period of 30-40 years in which the average ice melt across Greenland was somewhat similar to what has taken place over the last decade.

This is not to say that if the current upward trend in Greenland melt extent continues into the future that it won’t contribute an increasing amount to sea level rise (e.g. Rignot et al., 2011), but just that this contribution seems likely to be relatively modest.

So, no need to run out and sell the beach house.

References:

Frauenfeld, O.W., P.C. Knappenberger, and P.J. Michaels, 2011. A reconstruction of annual Greenland ice melt extent, 1785-2009. Journal of Geophysical Research, 116, D08104, doi: 10.1029/2010JD014918.

Rignot, E., I. Velicogna, M.R. van den Broeke, A. Monaghan, and J.T.M. Lenaerts, 2011. Acceleration of the contribution of the Greenland and Antarctic ice sheets to sea level rise. Geophysical Research Letters, 38, L05503, doi: 10.1029/2011GL046583.

Vinther, B.M., et al., 2009. Holocene thinning of the Greenland ice sheet. Nature 461, 385-388.

Wu, X., et al., 2010. Simultaneous estimation of global present-day water treansport and glacial isostatic adjustment. Nature Geoscience, 3, 642-646, published on-line August 15, 2010, doi: 10.1038/NGE0938.




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