Melting Ice Sheets: Much Ado About Nothing; Tree Rings: What Do They It Tell Us?

Melting Ice Sheets: Much Ado About Nothing

A recent study in Science (July 21, 2000) has prompted the press to run wild with lurid stories of melting glaciers, rising sea levels and looming disaster. If any of the reporters had taken the time to read the actual study, they would have learned that it is a non-story.

The study used aircraft laser altimeter surveys to determine the mass balance of the Greenland Ice Sheet. It found that above 2000 meters there is both thickening and thinning of the ice sheet. In the north the ice sheet is thickening at a rate of 14 7 mm/year and a thinning in the south of 11 7 mm/year with an overall thickening of 5 5 mm/year. The authors estimate a bedrock uplift of 4 to 5 mm/year, meaning that the balance is essentially zero.

Below 2000 m, the coastal regions of the ice sheet, the authors estimate that "thinning predominates along approximately 70 percent of the coast." The reason this is an estimate is because the coasts were "sparsely" measured. The authors explain that their estimate was obtained by calculating "a hypothetical thinning rate at the coast on the basis of the coastal positive degree day anomalies." They then "interpolated between this calculated coastal thinning rate and nearest observed elevation changes to yield thinning rates within the ice-covered coastal regions."

This interpolation from a calculation of a hypothetical thinning rates shows a total net reduction in ice volume of "51 km³/year, which is equivalent to 0.13 mm/year sea-level rise or, about 7 percent of the observed rate of sea-level increase." The authors concede, "We do not have a satisfactory explanation for the observed, widespread thinning at elevations below 2000 m." Perhaps the answer has to do with "hypothetical" rather than observed thinning.

To further complicate matters, the authors also note that "The 1980s and early 1990s were about half a degree cooler than the 96-year mean. Consequently, if present day thinning is attributable to warmer temperatures, thinning must have been even higher earlier this century." But thinning rates on many glaciers are too large to be explained by warming, "leaving a change in ice dynamics as the most likely cause," they argue. "We have no evidence for such changes, and we cannot explain whey they should apply to many glaciers in different parts of Greenland."

Tree Rings: What Do They It Tell Us?

A study in Science (July 14, 2000) concludes that "A 21^st-century global warming projection far exceeds the natural variability of the past 1000 years and is greater than the best estimate of global temperature change for the last interglacial." The researcher, Thomas J. Crowley, at Texas A&Ms Oceanography Department, comes to this conclusion using computer models that test various forcings for a 1000 year temperature time series.

Using an energy balance model, Crowley determined that solar irradiance and volcanism account for a large part of temperature variations prior to 1850, before the advent of man-made greenhouse gases. By removing solar and volcanic forcings from the temperature data (reconstructed from proxies such as tree ring data) Crowley determined that the resulting natural variability was similar to the control run of the climate model. A model run with only greenhouse gas forcing resulted in a warming similar to the "very large late-20^th-century warming that closely agrees with the response predicted from greenhouse gas forcing."

The temperature data used by Crowley was a combination of proxy data up to 1860 and data from the instrumental surface record beyond 1860. There are several problems with combining the two data sets, which result in an apparent pronounced warming in the 20^th century. According to the study in the Quaternary Science Reviews (January 2000), there are several puzzles within tree ring data that complicate its use in climate science.

Keith R. Briffa, with the Climate Research Unit, University of East Anglia, notes that "The evidence from dendroclimatology in general, supports the notion that the last 100 years have been unusually warm, at least within the context of the last two millenia." He cautions, however, that "This evidences should not be considered equivocal. The activities of humans may well be impacting on the natural growth of trees in different ways, making the task of isolating a clear climate message subtly difficult."

In the section titled "Reconstructing large-scale patterns of climate change", Briffa discusses a tree ring data set of the northern boreal forest which he says "provides the best overall indicator to date of long-term temperature changes over the higher northern land areas," the area where climate models predict the most warming. There is a divergence in temperature trends between proxy records and the instrumental record after 1950, notes Briffa. "Average [tree ring] density levels have continuously fallen while temperatures in recent decades have risen." The reason for this "is not known" says Briffa, but he gives several possible explanations. One explanation that he doesnt entertain is the possibility that the instrumental record is wrong.

On the whole, however, tree-ring chronologies show an increase in density that has been widely interpreted as evidence for "anomalous global warming." Biffra argues that "Some of this accelerated growth is no doubt temperature driven but fertilization by increased nitrogen and/or CO2 levels may also be involved."

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