Rate of glacial retreat doubled

The study’s lead author, Joshua Maurer, a doctoral student in earth sciences at Columbia University, used recently declassified spy-satellite images from the nineteen-seventies to observe how the volume of the region’s largest glaciers has changed. Scientists had already documented the rate at which the Himalayas had lost ice mass in the course of the twenty-first century using more sophisticated satellite imagery.

Maurer developed computer software that created three-dimensional images from the overlapping pictures, allowing him and his colleagues to digitally “walk” across the glaciers’ surfaces as they appeared in 1975. In the end, they found that the six hundred and fifty largest glaciers across India, China, Nepal, and Bhutan, which together represent fifty-five per cent of the region’s total ice volume, have lost the equivalent of a vertical foot and a half of ice each year this century.

That’s roughly twice as much as what they lost from 1975 to 2000, when temperatures were, on average, 1.8 degrees Fahrenheit cooler. (Previous studies that looked at melt since the seventies, like Maurer’s, found similar results, but with much less precision and over a smaller area.) “The correlation we observed between rising air temperatures and acceleration of glacial melt really highlights how vulnerable these glaciers are to climate change,” Maurer told me by phone on Monday. “As temperatures continue to rise, ice loss is going to continue to accelerate.”

Other factors can affect the rate of glaciers’ retreat, such as black carbon, from nearby power plants, or changes in precipitation. But Maurer’s analysis found that warming temperatures seemed to be the factor that overwhelmed others. “With such a homogeneous pattern of ice loss over such a large region that’s also climatically complex, really the only climate driver that makes any sense is atmospheric warming,” he told me.

To further test that interpretation, they compiled temperature data for the same timespan from weather stations all over the Himalayas, and ran the data through computer simulations. “We show that the amount of ice loss that we see falls right in the range of what we would expect if these measured air temperatures were the primary drivers of the ice loss. It’s just very good agreement.”

If the emission of black carbon, or a decrease in precipitation, were the primary drivers of ice loss, he said, there would be a much more variable spatial pattern of ice loss, because those factors are much more localized.

Maurer’s study builds on an extensive body of research on Himalayan glacierssometimes called the Third Pole, for the amount of ice the mountains hold, a quarter of which has already disappeared as temperatures have become hotter in the last century. Scientists who study glaciers and climate change have generally approached the subject at much longer timescales, relying on excavated ice cores from ancient glaciers, which, in their frozen layers of ice and air bubbles, contain evidence of earth’s climate going back tens of thousands, even hundreds of thousands, of years.

“Usually we’re using the geologic record globally to see how the cryosphere responded to the changing climate in the past,” Joerg Schaefer, a research professor at Columbia University’s Lamont-Doherty Earth Observatory and one of Maurer’s co-authors, said.

“It always bothered me that, if you look at the geological record, glaciers look like they respond very coherently, as an army, to climate change. But then, if you look into the field of glaciology, where people look at daily, weekly, monthly changes, it really gets complicated, everything seems to be out of phase.” Based on many other studies, the suspicion already had been that temperature is the main climate driver melting glaciers. “Of course, if it gets warmer, ice melts, we knew that,” Schaefer said.

But he would have been happy if the study showed that melting is much more variable, and more strongly impacted by other factors. “We were obviously hoping that for the environment, and the livelihood of society, that it would be a more local pattern,” he said. “Instead, this means that just everywhere these glaciers will follow the temperature curve.” Schaefer added, “Of all the possibilities, that’s the worst result.”

A major report on the state of the Himalayas“Hindu Kush Himalaya Assessment: Climate Change, Sustainability and People”was published last February and projected that, even in the best-case scenario, if the world rapidly decarbonized and was carbon neutral by 2050, limiting global warming to 1.5 degrees Celsius, the Himalayas are very likely to lose a third of their total ice, partly because the peaks are warming at a faster rate than the global average.

If things continue as they are now, they are likely to lose two-thirds of their total ice. For a region already plagued by poverty, inequality, and discrimination, that eventuality is catastrophic, and much of the report was an attempt to thoroughly examine all the effects that the glaciers’ retreat will have on agriculture, ecology, and energy, especially hydro-power.

Densely populated and quickly developing downstream areas in China, India, Pakistan, and Bangladesh are likely to face even more severe water crises than what they’ve already seen. The report also provides some solutions, including necessary cooperation between China, Nepal, and India on early-warning disaster systems for expected glacial-lake-outburst floods a type of risk to which Maurer’s new study provides helpful clarifications.

Scientists have long known about the trouble that rapidly increasing atmospheric greenhouse gases would bring. But they have, in the past, stumbled to properly communicate that trouble to the public especially in complex climates like the world’s highest mountain range. In the 2007 Intergovernmental Panel on Climate Change’s fourth climate assessment, some scientists made a bad error, predicting that all Himalayan glaciers would be gone by 2035. That has haunted the field ever since.

Tobias Bolch, a glaciologist at the University of St. Andrews and one of the lead authors of February’s Hindu Kush Himalayan Assessment, was slightly unnerved by the new paper’s bold message. “I would have liked that the authors would have been more cautious with the main message’ Glacier ice loss rates across the Himalayas have doubled over the past four decades’ as we do not want other false messages,” he said, recalling the I.P.C.C. error. Bolch noted that, although the evidence is clear that, over all, ice loss has accelerated significantly, there is some degree of uncertainty in the data as to whether this acceleration has fully doubled in speed.

Schaefer told me that people often ask him when the Himalayas are going to be ice-free. “It’s a little bit like asking, When are the Antarctic or Greenland ice sheets gonna be gone?” he said. (Greenland’s summer heat is already weeks ahead of average, breaking the record for such extensive melting of its ice sheet at an early date.) “They are interesting questions, but they are not that relevant for us. If the ice sheets are gone, we are toast anyway. We will be gone way before.”

The relevant question, he continued, “is what is the societal impact of the doubling in speed of these Himalayan glaciers’ retreats. We know, of course, that it’s really bad, but at least now we have a study that gives us a decently robust point of view of what’s actually going to happen as the temperature rapidly increases.”

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