Shrinking glaciers are ‘categorical evidence’ of climate change, study says

§ Attribution

The study, just published in Nature Geoscience, is the latest in the relatively new field of attribution, where scientists identify the fingerprints of human influence on observed changes in temperature, rainfall, and other climate parameters.

Attribution studies typically focus on specific extreme events. Recent research has found, for example, that climate change boosted the odds of the UK’s very wet winter in 2013-14 by 43%. Another paper identified how many extra deaths there were in London and Paris during the 2003 summer heatwave because of our warming climate.

Rather than an event of a few hours, weeks or months, the new study looks at how glaciers have changed over the past century.

The near-global retreat of these rivers of ice “seems improbable” without the influence of climate change, the paper says, but most studies to date have only been able to test this theory on a few glaciers at most.

So the researchers collected together records of glacier length for 37 glaciers in North America, South America, Europe, Asia and Australasia. The records were selected for their length and completeness – the longest goes back to 1534.

You can see the glacier locations in our interactive map below.

Whether a glacier retreats or advances each year largely depends on its mass balance – the difference between how much snow it receives and the amount of its ice that melts away. Glaciers that lose more mass through melting than they gain from snow will shrink and retreat back up the valley.

The chart below shows how the length of each glacier has changed over their observed records.

Using the long-term records, the researchers estimated how glacier lengths have varied in response to the yearly ups and downs of temperature and snowfall. They then compared these natural fluctuations with the large-scale retreat seen in the glaciers in the 20th and 21st centuries.

Their findings show there’s only a very small chance that the retreat of these glaciers could have happened without climate change, says lead author Prof Gerard Roe from the Earth and Space Sciences department at the University of Washington. He tells Carbon Brief:

“Our paper demonstrates that the worldwide retreat of glaciers necessarily requires a climate change that is centennial in duration and global in extent.”

The findings constitute “categorical evidence” that the climate is warming, the paper says.

§ ‘Virtually certain’

The results suggest for all but one of the 37 glaciers, there’s a more than 90% probability that their retreat is a result of a change in climate. In the vernacular of the IPCC, this level of likelihood means it is “very likely” that the changes are attributable to human-caused climate change.

For 21 of the 37, the findings are even more emphatic. They find that it is over 99% likely – or “virtually certain” – that glacier retreat is being driven by climate change.

These likelihoods are stronger than the findings on glaciers in the most recent IPCC assessment report (“AR5”), published in 2013, the paper notes.

According to AR5, there is “high confidence that a substantial part of the mass loss of glaciers is likely due to human influence.” Here, “likely” means a greater than 66% probability. This lower level of confidence reflected the lack of published literature on attributing glacier change, says Roe:

“There were very few studies [available in 2013] that tried to do what we have done. In the absence of such studies, they [the IPCC] were right to be cautious.”

The new study convincingly shows that the retreat of these glaciers is a response to a changing climate, says Dr Alex Gardner, a solid earth research scientist at the Jet Propulsion Laboratory at Caltech, who wasn’t involved in the study. He tells Carbon Brief:

“It is now almost certain that we will continue to see the near global demise of low latitude glaciers in the coming centuries as our planet warms in response to increased concentration of greenhouse gases in the atmosphere.”

The question to answer next, he adds, is how quickly they’ll retreat and by how much.

§ ‘Ingenious approach’

The researchers hope their findings will feed into future IPCC reports, strengthening the panel’s conclusions on how climate change is affecting glaciers around the world.

This would be helped by other studies building on their “ingenious approach,” says Prof Peter Stott, head of the climate monitoring and attribution at the Met Office Hadley Centre and professor of detection and attribution at the University of Exeter, who also wasn’t involved in the study. He tells Carbon Brief:

“[Their conclusions] depend crucially on the scientific robustness of their techniques so it will be interesting to see if investigations by independent teams support these new results.”

The influence of climate change on glacier retreat could also be used to help communicate the consequences of human activity, says Roe:

“Alongside the dry, technical information of the data and the numbers, I think it is important for students, the public, policymakers, and scientists to have something more definite and easy to grasp, that they can understand and reflect on. Hopefully, in strengthening the link between glaciers and climate, our study facilitates that happening.”