Just ain’t natural

Epic data review blames climate change for changes

It’s not Colonel Mustard in the kitchen with the candlestick. It’s human-driven climate change that is causing myriad disruptions — dwindling snowpacks and earlier salmon migrations and so on — in the Northern Hemisphere, says an international research team.

MELTDOWN A picture from 1940 (top) shows the Chacaltaya glacier and ski area in Bolivia, which looked rather different in 2005 (bottom). Records of glaciers melting join thousands of other data sets in a new analysis of the impacts of climate change. IPCC

That link has been predicted but is hard to demonstrate scientifically, NASA’s Cynthia Rosenzweig says. Now she and 13 other environmental scientists say they have reviewed so much data on change and analyzed possible causes in such a way that the culprit’s fingerprints show up.

The team’s work demonstrates that there’s “a huge suite of impacts of warming now discernible,” says Chris Field of the Carnegie Institution of Washington’s Department of Global Ecology at StanfordUniversity. And globally these wide-ranging impacts can be “attributed with confidence” to the warming caused by humans. “In the body of evidence about climate change, this is a very important layer,” he says.

Rosenzweig, based in New York, and colleagues around the world analyzed some 80 studies that document changing aspects of natural processes: when gingko trees bloom in Japan, how much glaciers melt in the Alps, when frogs start their springtime calls in New York and so on. Each study offered at least 20 years of data collected between 1970 and 2004. In all, the mountain of data comprised around 29,500 data series that each revealed a significant change. (Twenty years of melt measurements from two separate glaciers counted as two data series.)

Of all the changes revealed in those data series, 90 percent matched the direction predicted for a climate-change response. The ginkgos bloomed earlier, not later. The glaciers shrank rather than expanded.

The team overlaid data documenting varying temperature changes around the globe with the tally of local impacts. The overlap of the trends was very close.

During those years, the temperature rose 0.6 degrees Celsius on average around the globe, says Rosenzweig. “If we’ve had this many impacts with just 0.6 degrees, think what will happen with more.” By the end of the century, the global temperature could increase an additional 2 to 6 degrees Celsius, according to the Intergovernmental Panel on Climate Change.

Three continents had enough data by themselves for their own analyses. In North America, Europe and Asia, at least 88 percent of the changes match the direction expected for climate change as a cause, the researchers report in the May 15 Nature.

“It’s the first study to link human-caused climate change with biological and physical impacts at the continental level,” Rosenzweig says.

“I think they’ve been quite strict in what they’ve included,” says environmental scientist Tim Sparks, of the Centre for Ecology and Hydrology in Monks Wood, England.

With so much change documented now, he says, it’s time for scientists “to look at the ‘so what’ question.” Insects may expand their ranges, lakes may warm, birds may nest earlier, but he wants to know which changes will spread diseases or wipe out species.

It’s also time for researchers to get to work on filling information gaps. For massive as the study is, it has holes, says Karen Lips of Southern Illinois University in Carbondale. Rosenzweig’s team included one of Lips’ studies of amphibian populations dwindling in Mexico, a loss she attributes not to any climate effect but to the deadly chytrid fungus spreading north and south. Her work is among the few data sets from low latitudes, she says. The Southern Hemisphere likewise needs attention. The new study worked 64 data sets for South America, 30 for Australia and 14 for Africa. North America had 984 and Europe 28,163.

The analytic approach may be novel, but climate scientist Camille Parmesan of University of Texas, Austin says several of her own analyses of existing studies likewise reported fingerprints of climate change. “Still,” she says, “we need to keep publishing the conclusions in hopes that governments and the public will someday take it in and do something.”

Susan Milius is the life sciences writer, covering organismal biology and evolution, and has a special passion for plants, fungi and invertebrates. She studied biology and English literature.

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