Glaciers are melting. That’s nothing new. But confidence about the role of climate change in their retreat has grown substantially. A new study concludes that the loss of glaciers is, in fact, “categorical evidence of climate change.”
For this study, researchers studied 37 glaciers from around the world. The team calculated how likely it was that each glacier was melting due to a warming climate. Scientists call this kind of calculation a probability. It’s essentially the chance that one thing is due to something else. For 36 of the 37 glaciers studied, there is at least a 90 percent probability that their melting — or retreat — was due to climate change. That means the melting's link to Earth’s growing fever is now “very likely.”
That’s a bit more worrisome than previous estimates had indicated. Before, the link between global warming and glacier health was rated as only “likely.” That meant there was at least a 66 percent probability that a warming climate was behind the glacial melting.
The ratings come from the Intergovernmental Panel on Climate Change. This group of scientists keeps tabs on all research related to climate. Its new rating of “very likely” links glacier retreats much more directly to climate change.
Making that link hasn’t been easy. A glacier’s ice pack can change as a result of natural shifts in climate. Scientists call this natural variability. El Niño is one driver of natural variability. It’s a semi-regular shift in climate that occurs when the surface water around the equator in the eastern and central Pacific Ocean warms for a long bit of time.
An El Niño can trigger heavy rains on the U.S. West Coast and in South America. Meanwhile, the same event can lead to drought and possibly wildfires in Australia and Southeast Asia. Other factors also can affect a glacier’s ice cover and depth. So natural variability and these other factors make it hard for scientists to assess the role of any human influence on climate.
Gerard Roe is an Earth system scientist. He works at the University of Washington in Seattle. He and his colleagues have just calculated the natural ups and downs of glacier health across the planet. They measured how the ice in each glacier has melted or accumulated over time. They also noted how much the sizes of these glaciers have drifted from their natural range of variation. Finally, the team looked at what was happening in regions close to the glaciers.
These analyses revealed something startling. All but one glacier earned the “very likely” label that it was retreating in size because of climate change. The rating for 21 out of the 37 glaciers was even more alarming. For them, it is “virtually certain” that climate change caused them to melt, the scientists conclude. They shared their findings December 12 in Nature Geoscience.
Glaciers hold about 75 percent of Earth’s freshwater. Their decline is troublesome because it serves as a ‘canary in the coal mine’ for climate change. When someone says something is a canary in a coal mine, it means it is an advanced warning of danger. Glacier melt, the scientists say, is sending humanity such a message about climate change.
canary in the coal mine A phrase that points to some sign of impending doom. The term comes from the fact that in the 1800s (and at some workplaces into the early 1900s), coal miners often kept a caged canary in their tunnels. If dangerous levels of methane or carbon monoxide began building up, the bird would die before the men would be affected. That early warning of danger alerted the workers it was time to evacuate.
climate The weather conditions prevailing in an area in general or over a long period.
climate change Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.
colleague Someone who works with another; a co-worker or team member.
drought An extended period of abnormally low rainfall; a shortage of water resulting from this.
El Niño Extended periods when the surface water around the equator in the eastern and central Pacific warms. Scientists declare the arrival of an El Niño when that water warms by at least 0.4 degree Celsius (0.72 degree Fahrenheit) above average for five or more months in a row. El Niños can bring heavy rainfall and flooding to the West Coast of South America. Meanwhile, Australia and Southeast Asia may face a drought and high risk of wildfires. In North America, scientists have linked the arrival of El Niños to unusual weather events — including ice storms, droughts and mudslides.
factor Something that plays a role in a particular condition or event; a contributor.
freshwater A noun or adjective that describes bodies of water with very low concentrations of salt. It’s the type of water used for drinking and making up most inland lakes, ponds, rivers and streams, as well as groundwater.
geoscience Any of a number of sciences, like geology or atmospheric science, concerned with better understanding the planet. People who work in this field are known as geoscientists.
glacier A slow-moving river of ice hundreds or thousands of meters deep. Glaciers are found in mountain valleys and also form parts of ice sheets.
global warming The gradual increase in the overall temperature of Earth’s atmosphere due to the greenhouse effect. This effect is caused by increased levels of carbon dioxide, chlorofluorocarbons and other gases in the air, many of them released by human activity.
Intergovernmental Panel on Climate Change, or IPCC This international group keeps tabs on the newest published research on climate and on how ecosystems are responding to it. The United Nations Environment Programme and the World Meteorological Organization jointly created the IPCC in 1988. Their aim was to provide the world with a clear scientific view on the current state of knowledge in climate change and its potential environmental and social impacts.
Pacific The largest of the world’s five oceans. It separates Asia and Australia to the west from North and South America to the east.
probability A mathematical calculation or assessment (essentially the chance) of how likely something is to occur.
semi An adjective meaning “somewhat.”
Meeting: G. Roe, M.B. Baker and F. Herla. A formal attribution of glacier retreat to climate change. American Geophysical Union annual fall meeting, San Francisco, December 16, 2016.
Journal: G.H. Roe, M.B. Baker and F. Herla. Centennial glacier retreat as categorical evidence of regional climate change. Nature Geoscience. Published online December 12, 2016. doi: 10.1038/ngeo2863.