One of the criticisms most frequently lobbed at Al Gore's An Inconvenient Truth is that he has his facts wrong about the snows of Kilimanjaro. Yes, those immortal snows are vanishing (actually, they're glaciers, but we can blame Ernest Hemingway for that bit of poetic license), as Gore's global-warming documentary contends, but they've been receding since the early 1900s at least long before the planet began to warm.
Now that bit of fact-checking is looking a lot less convincing with the publication of a study on Tuesday in the Proceedings of the National Academy of Sciences. Lead author Lonnie Thompson, a glaciologist at Ohio State University who has been to the summit of Africa's tallest mountain repeatedly over more than a decade, says that while the glaciers did start melting a century ago, their retreat has sped up dramatically in recent years. "We've lost 26% of the ice since 2000 alone. And that, unfortunately, is just what we predicted would happen." Within a few decades, he says, most if not all of Kilimanjaro's glaciers will be gone.
That's not to say that Thompson's research is the final word on the debate. Indeed, glacier experts have been waging an intellectual war for years over what's really causing the ice loss atop Kilimanjaro. The simplest explanation would be that warming temperatures are making the ice melt and indeed, Thompson believes this is a big part of what's going on.
But other scientists insist that melting, if it's occurring at all, has a relatively minor effect. "The fact that you have melting may mean air temperatures have increased, but it doesn't necessarily," says Philip Mote, who heads the Oregon Climate Change Research Institute at Oregon State University. "And in fact, the temperature on the summit of Kilimanjaro is essentially always below freezing, which makes it hard to accept warming as the reason [for glacier loss]."
The likelier explanation, he and others say, is a combination of factors, including changes in weather patterns. Fewer clouds and more sunlight would create a layer of warm air right at the glaciers' surface, which would cause some melting. But most of the ice loss, he suggests, is due to sublimation that is, ice turning directly into water vapor with no intermediate step. That tends to happen when temperatures are cold and the air is extremely dry, which is the case at Kilimanjaro's higher-than-19,000-ft. summit (it's the same reason ice cubes slowly wilt away in a frost-free freezer). That happens all the time, but if there's less precipitation to build the glaciers back up, which may be the case here, the result is a net loss of ice.
Thompson agrees that this is a factor. "But the idea that at the end of the day, they're sitting in the mid-troposphere sublimating away is false. There are lakes on the surface of the glaciers," he says, noting that summit temperatures aren't so cold as to preclude the existence of water, "and when you drill down, the ice is saturated with water."
All of this back-and-forth might suggest on first blush that the scientists are arguing over whether global warming is even happening. That would be a mistake. While he doesn't think Thompson's case is proven, Mote has no doubt that the climate is changing and that humans are largely responsible; he just doesn't think Kilimanjaro's glaciers are being melted out of existence.
Still, even if Mote is right, that doesn't rule out global warming as a root cause of glacier retreat. Climate scientists have long maintained and evidence from the real world is already confirming that warming doesn't just result in higher temperatures. It also leads to changes in weather patterns, including more intense precipitation in some areas, more severe droughts in others (and sometimes, as in the case of the American Southeast, a little of both). And that may well be what's happening at Kilimanjaro. While strongly disputing Thompson's explanation, Georg Kaser of the Institut für Geographie in Innsbruck, Austria, writes that "we are confident that global warming is the ultimate driver of glacier shrinkage on Kilimanjaro due to the strong relation between Indian Ocean dynamics and East African precipitation."
In short, the answer to the question "Is it global warming or changes in weather patterns?" may be, simply, yes.
Michael D. Lemonick is the senior science writer at Climate Central.