April 9, 2011 — andyextance

South American glaciers have melted unusually quickly in recent years, a trend that could threaten people’s water supplies if found to be happening elsewhere in the world. Neil Glasser and his colleagues have shown that current melting is around ten times faster than the average since the glaciers reached their most recent maximum extent. “We’ve established that the rates of volume loss have increased dramatically in the last the 30 years,” the University of Aberystwyth, UK, scientist told Simple Climate. Glasser’s team has reached this conclusion after two years of painstaking research into glaciers in Patagonia, the geographic region containing the southernmost portion of South America. “To do any form of calculations of ice volume losses you need to know the height of the ice now and you need to know what it was in the past,” Glasser explained.

With the appearance of satellites in the sky over our heads, calculating the volumes of glaciers has become much more straightforward. Comparing images recorded by satellites and aerial photography shows that melting glaciers across the world contributed an average of half a millimetre per year to global sea-level rise from 1961-2003. That has increased to around three-quarters of a millimetre per year from 1993 to 2003, with Patagonia contributing around one-tenth of that rise. While in the region for other research, Glasser and his colleagues had the idea to use the landscape as a window through which they could scrutinise melt rates much further in the past. “We realised that we could make those calculations back several hundreds of years,” the scientist said.

Valleys bathed in ice

Writing in scientific journal Nature Geoscience on Sunday, the scientists describe how they measured when the glaciers last stopped growing, and how large they were when they did. To do this they studied two key features that are left behind as the glaciers melt and retreat: moraines and trim lines. “Moraines are the big piles of material that they push out in front of them,” Glasser explained. “The trim lines are where the ice has scoured off the vegetation, very sharp boundaries between the mature vegetation above, and no vegetation or younger underneath. It’s like a dirty line in the bath showing you where the water was.”

The researchers studied these features for the 270 largest glaciers extending into valleys across two separate icefields – the North Patagonian Icefield (NPI) and South Patagonian Icefield (SPI). That enabled them to locate both where the glaciers ended and, thanks to the trim lines, how high up the valley sides they reached at their largest. But to work out how rapidly the ice has been lost, they had to establish when those limits were reached, and for that they studied the nearby long-lived vegetation. “Lichens grow on rocks, especially in damp areas,” Glasser said. “If you measure the size of the largest lichens, you get an idea of when they could first grow and work out at what time period the boulders on the moraines were first exposed.” The team also got date information by boring narrow sections from trees growing on or near the moraines, finding their age by counting their rings.

A power of ten

Glasser’s team found that glaciers covered the greatest area in the NPI in 1870, and during two peaks in 1650 and 1750 for the SPI. They then determined how much of the glaciers had melted since by reconstructing the ice mass from the outlines provided by trimlines and moraines and comparing them against current satellite images. That equated to sea level rise around two thousandths of a millimetre per year on average for the past 140 years from the NPI. For the SPI the figure was either five or three thousandths of a millimetre per year, depending upon whether you calculate the average over 260 or 360 years, respectively. This is around one-tenth of the speed of melting seen in recent years, Glasser noted. “This will be used to confirm that the rate is increasing,” he said.

Glasser added that beyond sea-level rise the effect of melting the Patagonian glaciers is not especially grave. “The population’s reasonably sparse, the glacier melt is used for hydroelectric power here and there, but generally speaking it’s not utilised for any great human requirement,” he explained. However taking the method to the Himalayas, where hundreds of millions of people depend on glaciers, could reveal one way in which climate change is set to affect lives. “The glaciers are basically a store of fresh water, so you’d be looking at how fast those stores are being depleted,” Glasser explained. “If you live in the Himalayas, increased runoff is great in the short term, but in the long term it’s a disaster.” Glasser now hopes that the date of the last glacier advance in the Himalayas can be found. It can then be used to establish whether and how much melt rates have quickened, and to prepare for the consequences. “People downstream of the Himalayas rely on glacier melt for drinking and sanitation,” he underlined.