A new study on how quickly Earth’s glaciers and ice caps are melting - a crucial piece of the sea level rise puzzle - has received quite a bit of press attention during the past week, some of it rather misleading. While the paper (technically a letter) published in the journal Nature, concluded that glaciers and ice caps worldwide lost about 4.3 trillion tons of mass between 2003-2010 - enough to cover the entire United States with water 1.5 feet deep, according to NASA - there were some regions where glaciers and ice caps did not lose as much ice as previously thought.
Naturally some media outlets and climate skeptics chose to focus on the exceptions to the rule, losing track of the main conclusions in the process.
The study, by scientists at the University of Colorado at Boulder and the National Center for Atmospheric Research, constitutes the first global observational measurements of ice mass loss from glaciers and ice caps, gleaned from a pair of satellites known as the Gravity Recovery and Climate Experiment, or GRACE.
The study found that the contribution to sea level rise from land ice other than the Greenland and Antarctic ice sheets is about a third less than previously thought, at 0.4 mm per year between 2003 and 2010. However, these areas only comprised about 25 to 30 percent of the total global mass loss during that period, since most mass was lost from the polar ice caps. The study found that outside of Greenland and Antarctica, the glaciers of Alaska lost ice at the fastest rate between 2003 and 2010.
An animation shows the location of mountain glaciers and ice caps around the world with data from the GRACE mission to show recent trends in ice mass loss or gain. (NASA / Goddard Space Flight Center)
The GRACE satellites allow scientists to track changes in Earth’s gravity field by detecting regional variations in the planet’s mass. The satellites work as a team, discerning changes in the distance between them due to fluctuations in the planet’s gravitational field. According to NASA, they can detect changes in distance down to one-hundredth of the width of a human hair.
Study coauthor John Wahr of the University of Colorado said GRACE allows scientists to take stock of glacier systems and ice caps, but the space-based approach doesn’t allow for studies of individual glaciers.
“It’s sort of like looking down on a forest from an airplane,” he said via email. “You can easily see the extent of the forest without too much trouble, but you have a hard time identifying individual trees.”
As a commentary accompanying the Nature study noted: scientists have calculated the mass balances of very few glaciers and ice caps, and global mass loss rates come with a large amount of uncertainty.
“There are more than 160,000 glaciers and ice caps worldwide,” wrote Jonathan Bamber of the Bristol Glaciology Center in the U.K. “Fewer than 120 (0.075%) have had their mass balance (the sum of the annual mass gains and losses of the glacier or ice cap) directly measured, and for only 37 of these are there records extending beyond 30 years. Extrapolating this tiny sample of observations to all glaciers and ice caps is a challenging task that inevitably leads to large uncertainties.”
According to Wahr, researchers relying on sampling a limited number of glaciers and extrapolating the results from there have actually done a very good job. “The only place where things seem to break down, is high mountain Asia,” Wahr said.
Ah yes, the exception to the rule I noted earlier.
Contrary to previous studies, the new research found the glaciers and ice caps stretching across the high mountains of Asia (sometimes referred to as Earth’s “Third Pole”) have shown a high degree of variability from one year to the next, and did not lose much mass during the eight-year study period. In fact, they might have added some mass during that time.
The researchers found that glaciers and ice caps in the high mountains of Asia, including the Himalayas, Pamir, and Tien Shan, lost about 4 gigatonnes per year during the 2003-2010 period, with an uncertainty of about plus or minus 20 gigatonnes per year.
The fate of Himalayan glaciers has been a hot button topic in the media, especially since 2010, when the U.N. Intergovernmental Panel on Climate Change admitted that its 2007 Fourth Assessment Report had misstated when these glaciers might melt - the report stated 2035, when it should have said 2350.
Previous estimates showed more ice loss occurring in this region, which is a major concern given the large amount of ice in the area and its importance for supporting rivers that help nourish booming populations in India, Bangladesh, Pakistan and China.
Wahr said high mountain Asia is very challenging terrain for ground-based monitoring of glaciers, given the many microclimates, forbidding topography, and other factors. “There are tens of thousands of glaciers in that region, spread over a wide range of elevations and subject to many different kinds of weather patterns,” he said. “Those things strongly influence changes in temperature and snowfall, which are two of the primary factors in determining the rate of glacial mass loss.”
The British press, along with climate skeptics in the U.S., played up the Himalayan finding, and in the process implied that it undermines the basic conclusion that glaciers and the polar ice caps are melting and sea level is rising as a result. Even the Guardian, one of the bright spots in media coverage of global warming, got into the act with the headline: “The Himalayas and nearby peaks have lost no ice in past 10 years, study show]s]”.
As Charlie Petit of the Knight Science Journalism Tracker noted in response to these news reports, “It’s newsy enough to learn that Himalaya will keep its glaciers a good long while, but to package it as evidence that the same is true of, oh, Glacier National Park in Montana, is a stretch. So is to frame the letter generally as a blow against standard climate science’s ice-melt division. It makes climate science more robust and not precarious.”
Thankfully, a blog post by the Guardian’s “EcoAudit” man Leo Hickman provided crucial context by laying out other recent research findings on melting glaciers, and soliciting the views of other experts in the field to determine the state of the science on this issue. After exploring the topic in depth, Hickman concluded:
The surprising finding, reported in this new study, that satellite evidence shows that there wasn’t any loss in ice mass between 2003 and 2010 in the wider Himalayan region has, again, been welcomed with much delight by climate skeptics. However, the headline finding distracts somewhat from the rest of the data presented in the paper. It shows clear evidence that other regions, most notably Greenland and Antarctica, recorded a significant loss in ice mass over this same period. But, because this was largely expected, it didn’t become the headline.
There’s little comfort to be found, though, in the news that, in total, 536 gigatonnes (+/- 93Gt) of ice was “lost” globally between 2003 and 2010. What this study shows is that our understanding of how glaciers are affected by climate change can, as you would expect, be improved. For example, there are huge regional variations for reasons that scientists are still trying to fully understand. But to pin our hopes that climate change might be more benign than first feared on an unexpected finding in one region over a period of just eight years seems unwise when the wider global trend is clear, as the study clearly shows.
Wahr also cautioned against making predictions based on just eight years of data, calling such attempts “dangerous.” Instead, he described the study as a “comprehensive picture of present-day ice loss.”
In an email conversation, study coauthor Tad Pfeffer of the University of Colorado at Boulder also rejected the view that the study demonstrates that mainstream scientists had erred in previous estimates of ice mass loss. “... Our report shows that we (the mainstream) are actually trying to discover what is really happening out there in the world. If improved research tells us that High Mountain Asia is losing mass at a slower rate than we thought earlier, we say so.” Pfeffer noted that the study’s ice mass loss rates “match other reported estimates pretty closely in other parts of the world.