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To: cogitator
Hansen says his biggest concern is the potential for a large rise in sea-level. Yet empirical evidence shows that the rate of sea-level rise over the course of the 20th century (during which there was about 0.75ºC of warming) was about 1.8 mm per year

I am wondering what is the source of Hansen's information. The most detailed report I've read on sea level change have concluded that a significant measurement of sea level change will not be obtained until 2030. Previous reports of sea level changes did not account for changes in land elevation due to tectonics, and were complicated by sea level changes due to such effects as El Nino.

37 posted on 05/11/2004 11:23:37 AM PDT by kidd
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To: kidd
Hansen's SciAM article does not have those numbers. Michaels quoted them from somewhere else. Hansen expresses a concern about potential destabilization of the large ice sheets and relates this to the current amount of radiative forcing in the Earth climate system. Michaels argues around what Hansen is saying in the article; the current rate of sea level rise is minimal (and confirmed by satellite altimetry), but increasing temperatures could initiate a major change whose outcome could still be centuries in the future. Quoting:

"The main issue is: How fast will ice sheets respond to global warming? The IPCC calculates only a slight change in the ice sheets in 100 years; however, the IPCC calculations include only the gradual effects of changes in snowfall, evaporation and melting. In the real world, ice-sheet disintegration is driven by highly nonlinear processes and feedbacks. The peak rate of deglaciation following the last ice age was a sustained rate of melting of more than 14,000 cubic kilometers a year—about one meter of sea-level rise every 20 years, which was maintained for several centuries. This period of most rapid melt coincided, as well as can be measured, with the time of most rapid warming. Given the present unusual global warming rate on an already warm planet, we can anticipate that areas with summer melt and rain will expand over larger areas of Greenland and fringes of Antarctica. Rising sea level itself tends to lift marine ice shelves that buttress land ice, unhinging them from anchor points. As ice shelves break up, this accelerates movement of land ice to the ocean. Although building of glaciers is slow, once an ice sheet begins to collapse, its demise can be spectacularly rapid. . . . These considerations do not mean that we should expect large sea-level change in the next few years. Preconditioning of ice sheets for accelerated breakup may require a long time, perhaps many centuries."

40 posted on 05/11/2004 11:34:26 AM PDT by cogitator
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