Answer: Ice sheets on Greenland and Antarctica, as well as most mountain glaciers worldwide, have been rapidly losing mass in recent decades.

The Facts: As with the rapid decline of Arctic sea ice (see question 1 ), melting glaciers and ice sheets are a powerful image of a warming planet. Page 11  |  Top of ArticleThe Extreme Ice Survey, led by photographer James Balog, has captured the disintegration of 23 glaciers around the world, including Alaska, Greenland, Iceland, and Antarctica, using time-lapse video sequences, viewable at various locations online and in the documentary film Chasing Ice (2012) . Repeat photography—replicating an old photograph to see how conditions have changed in the intervening years—provides a less detailed but longer-term view of the pattern of melting glaciers.

Careful measurements of the mass of glaciers and ice sheets over time, both on the ground and from satellites, support the anecdotal, but visually powerful, photographic evidence. The conclusion is clear: glaciers and ice sheets around the world are rapidly losing mass. In fact, the evidence for melting glaciers and ice sheets is so clear and strong that it is difficult to see just how a myth to the contrary could gain any traction. To understand the myth, it is necessary first to understand more about the details of glaciers and how they are measured.

Glacier ice forms as snow accumulates year after year, so that the weight of the snow on top gradually compresses the snow at the bottom. This squeezes out the air, making it denser over time, so that the snow is eventually transformed into ice. Consequently, glaciers will only form where more snow falls during the winter than melts away in the summer, because only then will there be a surplus of snow to build up year after year. This situation, where more snow falls than melts, is known as a positive net mass balance—the net result of adding mass by snowfall and subtracting mass by snowmelt is positive—and this allows a glacier to grow. Positive net mass balances are found where snowfall is heavy, or the snowmelt is small, that is, places that are generally cold, by virtue of either high altitude or high latitude or both. This explains why glaciers are found in high mountains or near the poles.

Conversely, if more snow and ice melt in summer than snowfall adds in winter, a glacier will have a negative net mass balance. If this persists for long enough, the glacier will shrink. Net mass balance, and its translation into visible changes in the size of a glacier, means that glaciers can act as indicators of climate change. A prolonged cold or snowy spell should see glaciers advancing, while a warm or dry spell should see the opposite. The available evidence tells us that most of the world’s glaciers and ice sheets are shrinking, or losing mass.

How is this known? After all, there are an estimated 160,000 glaciers in the world, not counting those on the edges of the big ice sheets of Greenland and Antarctica. Only a handful of these have been surveyed. Climate scientists, however, have devised several ways to address this challenge. First, the World Glacier Monitoring Service (WGMS) coordinates an Page 12  |  Top of Articleinternational program to monitor 37 “reference” glaciers located in 10 different mountain ranges around the world. These reference glaciers have continuous on-the-ground measurements of mass balance stretching back to 1980 or earlier.

Many other glaciers have been monitored for much shorter periods of time, but because there are so many more of them, they cover a larger area than just the reference glaciers, though the total number of measured glaciers is still small compared with the estimated total number of glaciers in the world. WGMS regularly compares the mass balance for the reference glaciers with the mass balance for all glaciers in the archive. The two sets of mass balance data agree closely. For surveyed glaciers, there has been a striking loss of ice since 1980, with an overwhelming majority of monitored glaciers retreating since that time. Given that almost all the glaciers surveyed around the world are losing mass, this suggests that, even though there are large gaps between glaciers, they are in fact capturing an accurate picture of global glacier behavior.

Credence is added to these on-the-ground measurements by recent satellite observations. Measurements of the Earth’s gravitational field have been made since 2003, using NASA’s twin satellites called GRACE. Since gravity is a function of mass, changes in the gravitational field can indicate changes in mass.

Recent measurements of the mass of mountain ranges in Alaska and South America show striking mass losses, indicating melting glaciers. GRACE provides similarly clear evidence of accelerating mass loss for the great ice sheets of Greenland and Antarctica.

Measuring the changing gravitational field of the Earth from space is a serious technical challenge, and the difficulties of accurately measuring changes in glacier and ice sheet mass with GRACE should not be underestimated. However, other satellite measurements provide a cross-check for the GRACE data. Instruments measuring the extent of the summer melt area on the Greenland ice sheet since 1979 show a progressive increase in melt area over time. Although the natural variability of Earth’s climate means some summers are colder and others warmer, the long-term trend is evident: more and more of Greenland’s surface is melting in summer.

For Antarctica and Greenland, satellite-mounted laser altimeters have measured the height of the ice sheets and recorded changes over time. These measurements also largely agree with the GRACE data: Greenland and Antarctica are losing mass. Currently, Greenland alone is losing more than 300 billion tons of ice every year. That’s more than the entire weight of Mount Everest.

Finally, a long-running scientific project to monitor changes in mountain glaciers using satellites is now coming to fruition. The GLIMS project (Global Land Ice Measurements from Space) is using space-based visual observations—essentially, pictures of glaciers—to map modern-day glacier locations, lengths, and shapes, and compare them to historical records. In one application of these techniques, to the Cordillera Blanca mountain range in Peru, scientists found the glacier-covered area shrank from 665.1 km² in 1970 to 516.1 km² in 2003, a reduction of 23% ( Racoviteanu et al., 2008 ). In another recent study, of the eastern Himalayas, every single one of over 500 glaciers in the study was found to have retreated since the 1960s, giving a total reduction in the area covered by glaciers of around 12%—and the rate of retreat is accelerating ( Racoviteanu et al., 2014 ). GLIMS is significantly increasing the number of glaciers being monitored, and finding results consistent with the surface observations: glaciers are melting.

Given the overwhelming evidence to the contrary, how can anyone say that glaciers and ice sheets are not melting? One way is simply to make inaccurate statements. The Heartland Institute’s 2013 report Climate Change Reconsidered II and Steve Goreham’s 2013 book The Mad, Mad, Mad World of Climatism state that the Greenland ice sheet’s mass balance is stable, in flat contradiction of the available data.

Another approach is to focus on a small number of glaciers around the world that are not shrinking and ignore the rest. A third approach is to emphasize how small the number of measured glaciers is compared with the estimated total number of glaciers worldwide. This approach is evident in the Climate Change Reconsidered II report, as well as in the work of the late science fiction writer Michael Crichton, who presented this myth (along with many others related to global warming) in his 2004 novel State of Fear.

It is true that the number of directly measured glaciers is very small compared with the global total—at most a few hundred ever surveyed, out of 160,000, with long-term detailed measurements for only 37, the WGMS reference glaciers. However, to emphasize the uncertainty is misleading, because so many of the glaciers that have been surveyed, which are widely separated in space and scattered all across the globe, show similar patterns of mass loss. The GRACE and GLIMS measurements also confirm this view. Multiple lines of evidence all point to the same conclusion: glaciers and ice sheets worldwide are losing mass, in some cases rapidly.

A final variant on this myth comes in the form of exaggerations of the rate of glacier melt. In an unfortunate case of bad proofreading and editing, the Intergovernmental Panel on Climate Change (IPCC)’s 2007 Page 14  |  Top of Articlereport included a statement that the Himalayas would lose 80% of its glacier area by 2035. The original source for this claim appears to be a 2005 World Wildlife Fund report, which cited an interview in New Scientist with Indian scientist Syed Hasnain. In this interview, Hasnain speculated that the glaciers might disappear by 2035, although this speculation was not supported by peer-reviewed research. Unfortunately, the rigorous IPCC processes were not followed in this case, which underscores the importance of adhering to their stringent standards and reliance on peer-reviewed scientific research (see Cook, 2010 , for a complete account). The IPCC is the world’s most authoritative source on climate change and has an exhaustive process of peer review, so letting such a mistake slip into its final report was a serious error. However, the IPCC eventually recognized the mistake and issued a correction. The facts about the melting of glaciers and ice sheets worldwide are serious enough in their own right. They do not need to be exaggerated.