Friday, February 10, 2012

Himalayas not losing much ice, but sea levels still rising

Michael Marshall, environment reporter
(Image: Gerard Lacz/Rex Features )

What on Earth is going on with the world's glaciers? Reports today suggest that the Himalayan glaciers have not lost any mass in the last decade. But while that comes as a real surprise, the global pattern remains basically the same. Overall, the world's ice is melting away and causing sea levels to rise.
John Wahr of the University of Colorado at Boulder and colleagues used data from the twin GRACE satellites to measure the total amount of water lost from the world's glaciers and ice sheets - excluding Greenland and Antarctica - between 2003 and 2010. They write:
[The world's glaciers and ice sheets] lost mass at a rate of 148 [gigatonnes per year] from January 2003 to December 2010, contributing 0.41 [millimetres per year] to sea level rise... The rate for 2003-2010 is about 30 per cent smaller than the previous mass balance estimate.
What's more, they find that hardly anything was lost from high mountains of South Asia, including the Himalayas. Glaciers in this region are particularly important, because so many people depend on the runoff for their livelihoods. Wahr writes:
The high mountains of Asia, in particular, show a mass loss of only 4 ± 20 gigatonnes per year for 2003-2010, compared with 47-55 gigatonnes per year in previously published estimates.
What went wrong with the previous estimates, both worldwide and in the Himalayas?
For one thing, most previous estimates were based on ground-level measurements of individual glaciers, which were then extrapolated to the world as a whole. Many of the glaciers measured were low-lying, which meant they were easier to reach but also more vulnerable to melting than their higher, colder counterparts.
Earlier studies using satellites also suggested that, as a whole, the world's glaciers are melting, but their measurements were not as comprehensive and appear to have overestimated the melt-rate.
2nd-pic-622074main_grace20120208-946.jpgChanges in ice thickness (in centimetres per year) during 2003-2010 as measured by NASA's GRACE satellites, averaged over each of the world's ice caps and glacier systems outside of Greenland and Antarctica. Blue represents ice mass loss, while red represents ice mass gain. (Image: NASA/JPL-Caltech/University of Colorado)

Glaciergate legacy
It's the lower Himalayan figures that are getting the most attention, because of a much-publicised mistake in the last report of the Intergovernmental Panel on Climate Change. The IPCC stated that the Himalayan glaciers could disappear by 2035, far sooner than glaciologists believed. That statement now appears to be even more wrong than it already was.
Regardless, Wahr emphasises that the new, smaller estimate of ice loss does not mean everything is ok. Reuters quotes him saying that his estimate of the total mass lost is
a large number, and represents a lot of melting ice.
What does this mean for sea-level rise? The mountain glaciers in Wahr's study contributed 0.41 millimetres per year of sea level rise between 2003 and 2010. Overall, sea levels rose by 3.3 millimetres per year between 1993 and 2009. Much of that came from ice lost from Greenland, and the fact that water expands as it warms up, so global estimates are largely unaffected by this latest study.

Looking ahead, The Guardian quotes glaciologist Jonathan Bamber:

The projections for sea level rise by 2100 will not change by much, say 5 centimetres or so, so we are talking about a very small modification.
Previous studies suggest that sea level could rise as much as 2 metres by 2100.
Writing in Nature News, Bamber also notes that 8 years is a relatively short time so the study may have captured the Himalayan glaciers in a temporary stable period. It's not at all clear that they will remain that way.

Journal reference: Nature, DOI: 10.1038/nature10847

No comments:

Post a Comment