In a Recent Posting from NASA S Earth Observatory Lab Web News Letter (Aug 28, 2007) By

NASA AND GREENLAND ICE

By Peter McGurk, Joseph D’Aleo and George Taylor

We received this interesting brief posting from Peter McGurk, a senior meteorologist/climatologist and member of Icecap.

In a NASA posting by Kendall Haven, http://earthobservatory.nasa.gov/Study/Greenland/

There was a very interesting discussion entitled “Greenland’s Ice Island Alarm”. The article described how NASA is using advanced satellite technology to try and get an accurate estimate of the amount of ice loss currently occurring in the Greenland Ice Sheet. NASA noted that in terms of ice sheet elevation, most of the loss is occurring near the edges, but due to somewhat warmer temperatures snowfall has increased across interior portions of the ice sheet so that across many inland sections the ice sheet elevations are unchanged or increasing, especially across much of southern Greenland.

Using NASA’s satellite image and terminus locations of the Jakobshavn glacier in western Greenland as a rough proxy for estimating the amount of melting occurring along the edges of the ice sheet over the past 150 years or so we can get an idea of the relative melting rates per year. Certainly, as they [NASA] note in the article “without measurements of ice thickness, however, the picture of ice loss is incomplete”. That being said, if we assume that the middle of the glacier is of uniform thickness, we can estimate terminus retreat rates as a function of year.

We can see uniform retreat rates punctuated by three distinct increased rates of retreat. These increased rates of melting and glacial retreat all occur during the warm phase of the Atlantic Multidecadal Oscillation (AMO).

AMO Phase is in magenta with 1 denoting the cold phase years and 3 denoting the warm phase years. We can see increased spikes of glacial retreat during the first half of each of the past 3 AMO warm cycles with the rate of retreat growing markedly with each successive warm AMO cycle. It should also me noted that after each spike occurs the glacial retreat slows down to near or below pre-spike loss levels (already observed the last two years).

According to the best estimates from the NASA newsletter, the current rate of ice loss from the Greenland Ice sheet is somewhere around 134 Gigatons of ice per year. (This number is the result of averaging all of their 4 estimates from several different sources together). That certainly is a lot of ice, but we need to keep that total in perspective with how much ice is currently sitting atop Greenland.

In a USGS paper from 1999 (Williams & Ferrigno) the Greenland Ice Sheet was estimated to include a total of 2,620,000 cubic kilometers of ice. That translates to 2.62 x 10 21 cubic cm which when multiplied by the average density of ice at 0 deg C of 0.917 comes to 2.40254 x 10 21 grams of ice. If my math is correct this converts to2.649347 x 1015 tons (2.648347 Petatons). Even if you assume that the current loss rate is going to remain the same for a very long time and not settle down, it will take approximately 19,700 years for the entire Greenland ice sheet to melt completely.

Presently we are in an interglacial period (Holocene) which has been occurring for the past 12,000 to 15,000 years or so. Past history tells us that these interglacial periods don’t last forever and eventually there will be another ice age. If we take a minute to look at how long these warm interglacial periods and ice ages last we notice an interesting trend, maybe its just chance, but maybe not. Since about 650,000 years ago interglacial periods are growing shorter and shorter while the ice ages are lasting longer and longer.

Perhaps these trends whether real or not will reverse themselves, but chances are that the current Holocene interglacial period will last around 21,000 years followed by the next ice age which might last up to 80,000 to 90,000 years! Clearly none of us are going to be around to verify this forecast, but based on the above numbers we’ve got around 6,000 to 9,000 years left of our relatively mild climate before the deep freeze returns. At that stage of the game, the Greenland Ice sheet would only be 1/3 to ½ the size it is today assuming the recent rapid ice loss rates of the late 1990s and early part of this century remain the same for the next 6 to 9 millennia.

It seems to me that climate cycles have been occurring on this planet for millions of years and will continue to do so. Latching on to little pieces of these cycles and projecting them out linearly forever is not very realistic. We are still trying to unravel the complexities of our climate system with all its wonderful checks and balances. Maybe someday we might even figure it all out, but as my late professor Dr. Theodore Fujita used to say “nothing integrates as fast as Mother Nature”. In the long run, the Earth will take care of itself and the Greenland ice sheet will be just fine.

Thanks Peter for an interesting perspective on the latest Greenland story.

JUST ANOTHER CYCLE?

As we (D’Aleo and Taylor) noted here on Icecap, there have been many recent studies have addressed Greenland mass balance. They yield a broad picture of slight inland thickening and strong near-coastal thinning, primarily in the south along fast-moving outlet glaciers. AR4 assessment of the data and techniques suggests overall mass balance of the Greenland Ice Sheet ranging between growth by 25 Gigatonnes per year (Gt/year) and shrinkage by 60 Gt/year for 1961-2003. This range changes to shrinkage by 50 to 100 Gt/year for 1993-2003 and by even higher rates between 2003 and 2005.

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However, interannual variability is very large, driven mainly by variability in summer melting and sudden glacier accelerations. Consequently, the short time interval covered by instrumental data is of concern in separating fluctuations from trends. But in a paper published in Science in February 2007, Dr Ian Howat of the University of Washington reports that two of the largest glaciers have suddenly slowed, bringing the rate of melting last year down to near the previous rate. At one glacier, Kangerdlugssuaq, "average thinning over the glacier during the summer of 2006 declined to near zero, with some apparent thickening in areas on the main trunk." Dr. Howat went on to add

"Greenland was about as warm or warmer in the 1930's and 40's, and many of the glaciers were smaller than they are now. This was a period of rapid glacier shrinkage world-wide, followed by at least partial re-expansion during a colder period from the 1950's to the 1980's. Of course, we don't know very much about how the glacier dynamics changed then because we didn't have satellites to observe it. However, it does suggest that large variations in ice sheet dynamics can occur from natural climate variability."

Thomas, et al. (2000) showed great variance in mass balance of the Greenland ice sheet with highly variable thickening and thinning depending on location.

Temperatures indeed were warmer in the 1930s and 1940s in Greenland. They cooled back to the levels of the 1880s by the 1980s and 1990s. In a GRL paper in 2003, Hanna and Cappelen showed a significant cooling trend for eight stations in coastal southern Greenland from 1958 to 2001 (-1.29ºC for the 44 years). The temperature trend represented a strong negative correlation with increasing CO2 levels. Shown below, the temperature plot for Godthab Nuuk in southwest Greenland. Note how closely the temperatures track with the AMO (which is a measure of the Atlantic temperatures 0 to 70N). It shows that cooling from the late 1950s to the late 1990s even as Greenhouse gases rose steadily, a negative correlation over almost 5 decades. The rise after the middle 1990s was due to the flip of the AMO into its warm phase. They have not yet reached the level of the 1930s and 1940s.