A rise of 1.5 degrees Celsius beyond preindustrial temperatures is now extraordinarily difficult to avoid, suggesting that a change of this magnitude may be nearly unstoppable. With each additional increment of temperature increase, the study finds, the outlook becomes worse.
Three degrees C (5.4 degrees F) of warming, the research finds, would translate into a loss of over 70 percent of global glaciers and result in about 5 inches of global sea-level rise. So, even if many losses are baked in, the authors say, it is still worth trying to avoid whatever warming we can.
“Any reduction in the temperature increase will have a substantial impact on sea-level rise and the loss of glaciers globally,” said David Rounce, the study’s lead author and a researcher at Carnegie Mellon University and the University of Alaska at Fairbanks.
Rounce conducted the research with an international group of glaciologists affiliated with research institutions in Austria, Canada, France, Norway, Switzerland, Britain and the United States.
The planet has been gradually losing glacial ice since the peak of the last major ice age, some 20,000 years ago. But there is still a lot to give. The largest amount of remaining ice is concentrated in the ice sheets of Greenland and Antarctica, which, accordingly, pose the greatest threat of major sea-level rise.
But across the Arctic and Antarctic, as well as in the planet’s more temperate latitudes, many high mountain regions also feature numerous glaciers, where thick ice has accumulated because of centuries or even millennia of snowfall. These glaciers then accumulate more ice in winters and often lose some of it in spring and summer, feeding rivers downstream.
Human societies rely on these ice masses for water supplies, often heavily, as in the case of the thick glaciers of the high mountains of the Hindu Kush-Himalayan region, sometimes called the planet’s “third pole.” Glaciers in this region feed water into massive river systems including the Indus and Ganges. An estimated 1.9 billion people worldwide depend on glaciers for waterthe research notes.
In many cases, mountain glaciers also have great cultural and tourism significance — an example being Glacier National Park in the United States. The glaciers in this Montana region are in severe trouble, their size and number having plummeted since 1850, according to surveys and mapping by the US Geological Survey. A 2017 study of the park by the agency showed that its remaining 37 named glaciers had shrunk by 68 percent.
The study released Thursday finds that worldwide, this process of shrinkage, up to and including total loss, will especially affect many of the world’s smaller glaciers, those less than 1 square kilometer (0.39 square miles) in area.
Larger glaciers and more densely glaciated regions, in the Arctic and Antarctic, will be more resilient as the century wears on and temperatures continue to rise, the research finds, melting and contributing to sea level rise but not necessarily disappearing.
The glaciers of Alaska, for instance, are a leading contributor to sea-level rise. This will continue, but some of the ice deposits are vast enough, and in cold enough places, that they are projected to withstand even 4 degrees C (7.2 degrees F) of warming. But that’s not always true of many smaller glaciers in the middle latitudes — in regions including the Alps in Europethe Peruvian Andes and the peaks of New Zealand’s South Island.
“Roughly 80 percent of the world’s glaciers are less than 1 square kilometer,” Rounce said. “They are very small in terms of area, so when you think about future changes in a warming climate, they are very challenged in order to survive.”
In essence, the new research finds that while glacial ice will persist at the poles as the planet warms, in temperate and tropical mountain ranges, it will be far more difficult. In such places, it is mainly altitude, rather than long and dark winters, that protects and preserves the glaciers. And this ice, accordingly, becomes scarcer as warming progresses.
The new study goes beyond prior research in seeking to project the individual fates of all 215,000 or more of the world’s cataloged glaciers, and added techniques to account for some of their special attributes. For instance, especially in the poles, many glaciers flow far outward into the sea and even partially float on its surface. This means they can be melted not only by warm air but also by warming ocean water.
In some mountain areas, meanwhile, some glaciers are covered with dirt and rocky debris — which, if thick enough, can insulate and protect glaciers. But such debris is actually a vulnerability if the layer is thin, because the sun heats darker surfaces faster than would happen with bare, reflective ice.
“The glacier areas around the globe are quite varied and have unique responses to the changing climate,” said Christopher Harig, a glaciologist at the University of Arizona who was not involved in the latest research. “So there has been a rapid advance in glacier modeling to really capture what is happening in the different settings.”
The study suggests a greater overall glacier vulnerability than previous work has posited — especially at lower emissions scenarios consistent with a warming goal of 1.5 to 2 degrees Celsius (2.7 to 3.6 degrees Fahrenheit). At these temperatures, the research suggests, glaciers could have a 14 to 23 percent larger overall contribution to sea level rise than earlier studies had found.
It’s part of a pattern of newer research findings ever more intense impacts at ever lower levels of warming — levels quite close to where we are at present.
Paradoxically, however, some experts see hope despite the bad news about the world’s glaciers.
“The good news is that our decisions and actions today will have an impact on ice loss and on sea-level rise,” said Mathieu Morlighem, a Dartmouth College glaciologist who was not involved in the study. “Reducing carbon emissions to achieve the Paris agreement [1.5 degrees C goal] is particularly important, as the contribution from glaciers to sea level increases sharply beyond this point. It would also give people and ecosystems more room to adapt.”