June 4, 2012

Historical Imagery of Greenland Glaciers Lessens Sea Level Rise Alarm

Filed under: Arctic, Polar

A new study using historical images of glaciers in southeast Greenland to investigate glacier response to climate changes suggests that the recently observed acceleration of ice loss from Greenland may not be a long-term phenomenon. Instead, as marine terminating glaciers reach their grounding line and as the termini of land-terminating glaciers migrate upwards in elevation, ice loss rates from glacial discharge may slacken. According to Anders Bjørk and co-researchers:

[T]he recent high rate of retreat may come to a slowdown when retreating marine-terminating glaciers reach their grounding line and become less sensitive to the influence of ocean temperature, or through positive or negative feedback mechanisms relating to the cold East Greenland Coastal Current.

and,

Our results have implications for future estimations of sea-level rise as retreat rates for marine-terminating glaciers are likely to increase as temperature rises until glacier fronts reach the grounding line, or when cold ocean currents re-establish, whereas retreat rates for land-terminating glaciers are not likely to rise in the same order of magnitude.

Such results throw a bit of cold water on alarmist ideas that rising temperatures will lead to ever-accelerating ice loss from Greenland and accelerating sea level rise.

For example, Eric Rignot and colleagues (2011) last year documented an acceleration in the rate of ice loss from Greenland (and Antarctica) over the past two decades. They extrapolated this acceleration into the future, to arrive at potential future sea level contributions from the loss of ice sheets:

At the current rate of acceleration in ice sheet loss, starting at 500 Gt/yr in 2008 and increasing at 36.5 Gt/yr2, the contribution of ice sheets alone scales up to 56 cm [22 in.] by 2100.

Rignot et al. (2011) were quick to note that this rather naïve extrapolation was probably less than reliable:

While this value may not be used as a projection given the considerable uncertainty in future acceleration of ice sheet mass loss, it provides one indication of the potential contribution of ice sheets to sea level in the coming century if the present trends continue.

The new Bjørk et al. (2012) study (as well as a slew of other recent studies which we have discussed: see here, here, here, here, here , and here for example) strongly suggests that “present trends” will not continue—and thus the Rignot et al. extrapolation is grossly inaccurate (i.e., far too large).

Bjørk and colleagues came to their conclusion by studying some recently unearthed historical photographs of the coast of southeastern Greenland that had been taken by early 20th century Danish expeditions to the region. The earliest photos were from the early 1930s. Combining those photos with a set of U.S. WWII military photos and modern satellite imagery, Bjørk and fellow researchers were able to build a photographic database of the behavior of the glaciers in the region for much of the 20th and early 21st centuries, and related that behavior to climate changes observed across southeast Greenland.

They found that the glaciers respond rather quickly to climate changes. The well-known warmth of the region during the early 20th century was accompanied by relatively rapid retreat of both marine-terminating and land-terminating glaciers. As temperatures cooled from the 1950s through the 1980s, many glaciers advanced, and the warming of the past two decades has again been marked by rapid glacier retreat (Figure 1).


Figure 1. Behavior of glaciers along Greenland’s southeastern coast during different periods during the past 80 years. Glaciers generally retreated during the relative warmth of the 1930s-1940s and again in the early 21st century. Some glacier advancement was observed during the relatively cool conditions of the 1960s-1970s. (source: Bjørk et al., 2012)

This pattern of behavior of glacial dynamics is exactly like that of surface ice melt in Greenland that we documented in our paper (Fraunfeld et al., 2011) from last year and discussed here. In that paper, we concluded the following:

The forces acting in concert with ice melt across Greenland to produce higher global sea levels currently, should also have been acting during the extended high‐melt conditions from the mid‐1920s to the early 1960s. There is some qualitative indication of an observable influence of the variations in input from Greenland in the decadal rates of sea level change over the course of the past century… However, there is no indication that the increased contribution from the Greenland melt in the early to mid 20th century, a roughly 40 year interval when average annual melt was more or less equivalent to the average of the most recent 10 years (2000–2009), resulted in a rate of total global sea level rise that exceeded ∼3 mm/yr. This suggests that Greenland’s contribution to global sea level rise, even during multidecadal conditions as warm as during the past several years, is relatively modest.

The implications of the new results reported by Bjørk at al. are in accordance.

Bjørk at al. note that in the early 20th century warmth, both marine- and land-terminating glaciers increased their rates of retreat, but during the relative warmth of the early 21st century, the marine-terminating glaciers retreated at a greater rate than the land-terminating glaciers. They speculated that the reason the land-terminating glaciers did not respond as much to the recent warmth was that they had retreated to higher elevations during the warmth of the early 20th century and thus were less sensitive to further warming. Bjørk et al. further speculated that even though the marine-terminating glaciers may continue their increasing rate of retreat with further ocean temperature increases, that this increase will not continue indefinitely, but will begin to slow as the marine-terminating glaciers reach their grounding line. Together, their results indicate the potential for only modest increases in the annual amount of ice loss as condition warm (similar to our findings reported above).

Currently, ice loss from Greenland contributes about one-to-two hundredths of an inch per year to the global average sea level—that’s a total of 1-2 inches by the year 2100. So, even if the rate of ice loss doubles or triples, the resulting sea level rise falls far short of being catastrophic. And the more evidence that comes in, the more confidence we have that as temperatures rise, Greenland will shed ice only gradually and through somewhat self-limiting processes—rather than catastrophically through rapid acceleration and “slipping into the sea” as Al Gore so indelicately described his (incorrect) vision of the future.

References:

Bjørk, A. A., et al., 2012. An aerial view of 80 years of climate-related glacier fluctuations in southeast Greenland. Nature Geoscience, 5, 427-432, doi:10.1038/NGEO1481.

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., et al., 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.

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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