Cogley says there are two reasons for the increase. "We now have measurements for the year 2005 which were not available in 2006," he said. "2005 turned out to be a very bad year for glaciers around the world and it boosted the five-year average from 1.0 to 1.1 mm per year of sea-level rise."
Secondly, Cogley has worked out how to incorporate measurements based on advanced surveying methods in the estimate. Traditional techniques for glacier measurement involve drilling a hole in the glacier, inserting a stake, and monitoring how the surface level of the glacier changes. This is expensive, time-consuming, potentially dangerous and manpower-intensive. Direct measurements of this type are available for around 350 glaciers worldwide. Advanced surveying methods, on the other hand, use laser altimeters from planes or satellites to measure glacier elevation. The technique can measure hundreds or even thousands of glaciers relatively easily. Cogley figured out a statistical method for handling the glacier data and for including areas with limited data availability.
"This gives us a valuable payoff because the surveying dataset, which we haven’t been able to use until now, is large and includes many more tidewater glaciers," he explained. "We have long suspected that these glaciers, which calve icebergs directly into the ocean, are losing mass faster than the ordinary ones. It turns out that we were right, and they further increase the estimate for 2001 to 2005 to 1.4 mm per year of equivalent sea-level rise."
According to Cogley, 2006 was an even worse year for glaciers than 2005. "Knowing what we know about the greenhouse effect, there doesn’t seem to be any reason why the rate of glacier loss shouldn’t keep on increasing until we begin to run out of ice, starting in maybe half a century or so," he said. "The tidewater glaciers are an added puzzle because their behaviour is governed only indirectly by the climate. But, whatever the reason, they are suffering more than the glaciers which terminate on land."
Georg Kaser of the University of Innsbruck led the October 2006 study, with contributions from Mark Dyurgerov and Mark Meier of the University of Colorado and Atsumu Ohmura of the Federal Technical Highschool in Zurich.
Cogley reported on his update to the team’s work at the European Geosciences Union General Assembly in Vienna, Austria.
• Mark Dyurgerov of the University of Colorado and Stockholm University has found that glacier mass balances have been moderately negative since the 1960s with increased loss since the 1980s and a further acceleration of loss since the mid-1990s. For 1993 to 2006 he estimated the trend to faster loss as 11 mm of water per year when averaged over the glacier cover. This translates to an additional sea-level rise of 0.02 mm per year with every passing year.