The debate over whether global warming is natural or manmade is an artificial one: scientists know that both factors can affect the planet’s temperature. The real question is which factor is doing the heavy lifting — and a new report in Nature released Wednesday says that on the Antarctic Peninsula, at least, human-generated greenhouse gases have almost certainly been by far the most important driver of warming over the past half-century.
Scientists are intrigued with this corner of the world because it’s warming faster than anyplace else on Earth. The planet as a whole has heated up by about 1.3°F since 1900, but on the peninsula, it has shot up by a whopping 5° in just 50 years, forcing massive ice shelves to disintegrate and penguin colonies to collapse.
Heat trapping greenhouse-gas emissions are the obvious culprit, since they’ve increased dramatically over that same 50 years, but scientists prefer hard evidence to presumption, so a team from the British Antarctic Survey has been drilling into ancient ice to see how the current warming stacks up against what happened in the ancient past. If the kind of warming happening now also happened before we started burning fossil fuels, it would cast doubt on the human contribution.
What the scientists discovered, however, removed any doubt. “We found that the peninsula has been warming for the past 600 years,” said lead author Robert Mulvaney, of the British Antarctic Survey, in an interview. “But the rate of warming has been much faster during the past century, and fastest over the past 50 years.”
The currently skyrocketing temperatures, in short, are very likely the result of human-caused warming, which is superimposed on a natural climate variation. “Asking whether it’s natural or caused by humans is silly,” said Eric Steig, of the University of Washington, who wrote a Nature commentary on the new research. “We’ve changed the atmosphere so dramatically that it has to be mostly human. The only question,” he said in an interview, “is how large the human influence is relative to other things.”
Given the rapid rise in recent decades, the answer seems to be “pretty large,” but emissions from the burning of fossil fuels may only be part of the human contribution. Another is almost certainly the infamous ozone hole that opened up over Antarctica in the 1980s, and which is still there. The loss of stratospheric ozone has changed the local energy balance in the atmosphere, which in turn has changed wind patterns, bringing warmer air to the Peninsula. “The ozone hole alone would have caused some warming,” Steig said.
The disintegration of ice shelves is clearly caused by warmer air, according to Steig. “It’s clear from satellite images that the breakups are preceded by the appearances of large ponds on the ice surface,” he said. “The meltwater then drills down, turns the ice into Swiss cheese.”
Warm water, by contrast, is the problem in other parts of Antarctica — at the Ronne-Fitchner Ice Shelf, for example, to the east of the Peninsula, or along the Antarctic coast to the west. In both cases, however, the melting of ice that reaches the sea allows ice further inland to flow faster, boosting the overall transfer of ice from the continent to the ocean and adding to the rise in sea level. (The same thing is happening in Greenland).
Unlike some other scientists, Mulvaney declined to comment on whether the warming will continue. “I cannot say for sure,” he said. But he did say that when he started doing Antarctic research several decades ago, he was a climate skeptic. “Not any more,” Mulvaney said. “You can’t keep doing this year to year and decade to decade and not become a very strong believer.”
“You can look at one particular location and say ‘oh, it’s happened before,’ “ Steig said. “But this is one of the very few times you can say it’s warming all over the planet.”