Time lapse photography of shrinking glaciers makes a powerful visual case for the impact of human-caused global warming. That's why, for example, it's used in former Vice President Al Gore's slideshow, An Inconvenient Truth, and by GlacierWorks, a nonprofit started by famed mountaineer and filmmaker David Breashears to document climate change in the Himalaya.
But is human activity really causing the world's glaciers to melt? After all, these rivers of ice have been melting ever since the Little Ice Age came to an end nearly two centuries ago, long before humans pumped out enough greenhouse gases to change the global climate. Perhaps the glaciers would be retreating even if humans never once burned a fossil fuel or cleared a forest.
"It seems to be so obvious that when it is getting warmer — and it is getting warmer because of human activity — the glaciers are melting because of human activity, but that actually hasn't been shown before," Ben Marzeion, a climate scientist at the University of Innsbruck in Austria, told NBC News.
Now it has.
He and colleagues looked at all the world's glaciers outside Antarctica and used climate models to study their evolution with and without greenhouse gas emissions from human activity. Their findings were published Thursday by the journal Science. "The big news behind this paper is that yes, it is actually possible to say really robustly it is humans which are responsible for the melting glaciers," Marzeion said.
Let our news meet your inbox. The news and stories that matters, delivered weekday mornings.
This has been especially true since 1991, according to the study. Data show that since then about two-thirds of the observed glacier melt is attributable to human activity. Prior to then, recovery from the Little Ice Age was the dominant signal. "We can see how the impact of humans is changing," Marzeion added. "It is getting bigger and bigger all the time."
The study is solid and for the first time quantifies prevailing arguments within the scientific community – humans are causing the climate to warm, but since glaciers are slow to respond to warming, their response lags behind, according to Richard Alley, a glaciologist at Pennsylvania State University, who was not involved in the paper.
"At some level, this is the pound on the table 'no there are no holes in this (argument).’ We've done the work. We've done the work. We've done the work," he told NBC News.
While future studies are unlikely to overturn these findings, Alley said, the results will guide researchers to places such as southern Andes to understand why the models underestimate the rate of glacier loss there and the Russian Arctic to figure out why the loss there is overestimated.
"And then also by doing the global picture all of the sudden they say, look there is a funny one, there is a funny one, and there is a funny one,” he said. “Let's go see what is going on in those places that are doing something slightly different."
Glaciers’ delayed response to climate warming means that they will keep on melting no matter what actions humans take today to curb global climate change. "Over the next 50 years or so there is hardly anything we can do about it," Marzeion said. "Whether we really limit the greenhouse gas emissions or not, the glaciers are going to continue to melt."
Longer term, the glaciers’ fate is firmly within humanity's control, he added. If strong actions are taken to curb greenhouse gas emissions now, the glaciers still may recede another 40 percent by the end of this century. If, however, little action is taken, up to 70 percent of the glacier mass could be gone by 2100 and the shrinkage would continue from there, he said.
Either way, noted Alley, people can expect more of the type of climate impacts they've experienced in recent decades. Coastal dwellers will have to contend with swelling seas, skiers will watch glaciers waste away, supplies of water for drinking and irrigation will dry up in some regions, and the rivers that drain glacial waters out to sea will be prone to sudden flooding as newly created glacial lakes burst through moraine dams.
"There is a memory in the glacier system," he said. "That means what you've seen, you are not done … the past is a guide to the future."