How we sped up the glacial melt
A new model of glacial melting from 1850 onwards shows a dramatic increase as human-induced climate change begins to bite. James Mitchell Crow reports.
A grainy alpine image from the early days of photography beside a modern picture of the same view – "before and after" photos of retreating glaciers provide some of the most arresting images of climate change. But do we really know how much of the ice loss is due to human-produced greenhouse gases? We do now, with new modelling pointing firmly to this cause for most glacier melt since the late 1970s.
Glaciers are good recorders of climate variation because they filter out the noise of daily weather changes to reveal long-term underlying trends. But this slow and stately response also adds a complication. Like an ice cube in a drink, the glaciers take a while to melt after their environment warms.
For Ben Marzeion, a climate scientist at the University of Innsbruck in Austria, and his colleagues, that raised a question. How much of the glacial melting observed since 1851 is due to man-made global warming and how much to the glaciers’ slow adjustments as the world emerged out of the Little Ice Age, a series of natural icy episodes between 1500 and 1850?
“Those repeat photographs are really powerful images for transmitting the message of what’s going on under climate change,” says Marzeion. “The problem I saw was that nobody had tested whether it is right to use those images.”
How to test it? In the lead-up to the latest IPCC report released late last year, glacier scientists got together to use the latest satellite data to accurately map virtually every glacier on the planet – all 198,000 of them. The reason for the work was to improve predictions of future glacial melt and its contribution to sea level rise. It also provided Marzeion with the perfect data set to answer his question.
Marzeion and his team took the modern maps and began to play with time. Using computer reconstructions of the world climate back to 1851 they first hit rewind to reconstruct the area and volume of all 198,000 glaciers at that time. Then they hit play and watched how the glaciers melted – first using models that include all known climate impacts, including human factors, and then using models that only included natural effects such as solar variability and volcanic eruptions.
The results proved that Marzeion’s suspicions about old glacier photographs were correct. It is only when the two simulations reach the late 1970s that they begin to diverge – the simulation that includes human impacts continues to track field research data of actual melting quite closely, whereas the simulation that does not include human influence predicts significantly less melting.
In other words, for most of the period since 1851 natural processes have dominated glacial melting. Only around a quarter of the melting in that time has been caused by human activity.
However, by narrowing the range to more recent decades, human influence becomes clear. Between 1991 and 2010 about 70% of melting was human-caused – and that number will keep rising. “Looking at the glaciers now, most of the change is due to human activities,” Marzeion says.
“Impressive,” is how Ian Willis, a glaciologist at Scott Polar Research Institute in Cambridge, UK, sums up the work. “I'm convinced by the numbers,” he says. For example, the areas that experience the greatest human-induced impact in Marzeion’s models match those where glacier melt is known to be accelerating – regions including Alaska, west Canada and USA, north Asia and New Zealand.
The inherent lag time in glaciers’ response to climate warming means that, even under the IPCC’s best-case scenarios for cutting greenhouse gas emissions, glaciers will keep on melting for the next few decades and end up at around half their current sizes. In their worst-case, business-as-usual scenario, glaciers will ultimately disappear almost completely. That would add up to half a metre to global sea levels.