Thread: Satellite Survey: Thinning glaciers driving polar ice loss

Thinning glaciers driving polar ice loss, satellite survey finds


by Ian Sample
The Guardian
2009-09-23


Satellite survey of Greenland and Antarctic ice sheets reveals extensive network of rapidly thinning glaciers that is driving ice loss in the regions.

A comprehensive satellite survey of the Greenland and Antarctic ice sheets has revealed an extensive network of rapidly thinning glaciers that is driving ice loss in the regions.

The most profound loss of ice was seen along the continental coastlines, where glaciers speed up as they slip into the sea. In some regions, glaciers flowing into surrounding waters were thinning by nearly 10m a year.

Scientists used data from Nasa's ICESat (Ice, Cloud and and land Elevation Satellite) to piece together a picture of the changing fortunes of glaciers on the ice sheets. The satellite bounces laser light off the ground, allowing researchers to measure the terrain with extraordinary precision.

The survey, compiled from 50m satellite measurements taken between February 2003 and November 2007, shows glaciers thinning at all latitudes in Greenland and along key Antarctic coastlines. Thinning penetrated deep into the interior of the ice sheets and continues to spread as ice shelves melt into the sea.

"We were surprised to see such a strong pattern of thinning glaciers across such large areas of coastline. It's widespread and in some cases, thinning extends hundreds of kilometres inland," said Hamish Pritchard who led the study at the British Antarctic Survey.

In Greenland, glaciers in the south-east were found to be flowing at speeds of more than 100m per year, during which they thinned by 84cm. More slow-going glaciers lost around 12cm a year.

In a vast region of western Antarctica that drains into the Amundsen Sea, the Pine Island glacier and neighbouring Smith and Thwaites glaciers are thinning by 9m a year, the satellite measurements show. The study is published in the journal Nature.

Previous satellite surveys of polar regions have relied upon radar measurements that cannot map the Earth's surface with the same precision as the ICESat laser rangefinder. The satellite allows scientists to take 65m-wide snapshots of the ground, giving an unprecedented view of glaciers on the steep terrain where ice meets ocean.

This satellite survey helps scientists explore how different aspects of climate change are driving ice loss in polar regions. Higher air temperatures can increase surface melting, but warm ocean currents accelerate ice loss more when glaciers flow into the sea.

"The majority of the thinning we see is not due to increased melting from higher atmospheric temperatures, but because the glaciers are flowing faster thanks to their interaction with the oceans," said Prof David Vaughan, a co-author on the study.

http://www.globalresearch.ca/index.php? ... &aid=15369

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The Truth About Arctic and Greenland Ice
Leonard Weinstein, ScD
May 21, 2009

There is an interesting story going around that Anthropogenic Global Warming (AGW) is resulting in unusual melting of the polar and near polar ice caps, especially the Arctic sea ice and Greenland ice cap, and this is going to contribute to the Earth’s problems in a big way (flooding, feedback temperature increase, etc.). In order to understand the issue, a few simple facts and recent observations need to be shown. The following discussion is restricted to the Arctic and Greenland, since these are the main regions of contention. It should be noted that the Antarctic is presently cooling (and has been for several years) and sea ice extent is expanding.

We first need to examine the external energy balance for the Arctic. Due to the axial tilt of the Earth, the Arctic is in full dark about half of the year, and in partial to full Sunlight about half of the year. The maximum Sun angle at the North Pole is about 24 degrees above the horizon. This angle only occurs for a relatively short time, and smaller angles occur most of the daylight time. The maximum Solar insolation from this (due to the angle and atmospheric absorption) is about 400 W/m2. The high reflectivity of clean snow (always present on ice) results in only about 40 W/m2 being absorbed into the ice. The maximum air temperature near the surface is just a small amount above freezing, so the maximum thermal heat transfer from air to ice is generally just a few 10’s of W/m2. The ice (or snow) gray-body emissivity (about 0.9) is much higher than the low absorption (about 0.1) at the short wavelengths of Sunlight, so the reemitted long wavelength radiation is about 280 W/m2. The result is that solid ice cannot normally melt from Solar radiation plus air warming from the top, even at peak summer. The rest of the year has even less energy input, so ice is strongly cooled all year long.

The actual cause of the significant melting of summer ice has been shown to not be possible from the top. However, there is considerable summer melting. It is clear from the physics that the summer melting has to have been caused by water flowing under the ice. The water flows from lower latitudes, so has a higher temperature than the ice. If the water is warm enough it melts the ice from below. However, if the ice is cold enough and the water cool enough, this flow will freeze and add thickness of solid ice to the underside. This is the source of new solid ice. Snow on the top also adds to the thickness, but is not solid ice.

The summer melting can result in some Arctic open water. Open water can absorb much more of the Solar insolation than ice or snow, but even at the maximum summer level, absorbs only about 350 W/m2 near the pole. The water emissivity is slightly higher than ice, and radiates about 300 W/m2. These result in a maximum excess of absorption in open water of only about 50 W/m2 for a very short part of the peak of summer, and a large net cooling over the full year. Thus to claim that a temporary full melting of Arctic ice (from underneath) will keep the ice from reforming is incorrect based on Solar insolation and reasonable air temperature variations.


The following figure is show as an example of the very rapid temperature rise experienced at the Artic in recent years. This is supposed to prove the model predictions of unusual temperature rise rates are valid, and that we are heading for big problems:
The figure shows a rise rate about 0.25C per decade. This looks scary. However if we look at records back to 1880, we get a different story altogether:

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