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The golden toad was last seen in 1989 in the Costa Rican cloud forest of Monteverde—and 5 years later, its disappearance was the first extinction to be blamed on humanmade global warming. New evidence, however, suggests that humans may not have been at fault after all.

Here’s the current line on what drove the golden toad extinct. As humans pumped carbon dioxide and other greenhouse gases into the atmosphere, Costa Rican rainforests became hotter and dryer in the mid-1980s. These conditions made the toad vulnerable to the chytrid fungus, which is thought to thrive in warmer, drier climes. Chytridiomycosis is a fatal skin disease that eventually causes convulsions, skin loss, and death in amphibians. Although amphibian populations all over the region declined, the golden toad’s limited habitat and small population made it especially vulnerable.

But it’s hard to tell if the unusually dry conditions that contributed to the extinction were part of a natural cycle or connected to global climate change patterns. That’s because reliable temperature and moisture data for the Monteverde forests go back only to the 1970s. And due to the tropical climate, trees in Monteverde don’t form the rings usually used to study temperature and moisture patterns over time.

Instead, Columbia University climate scientist Kevin Anchukaitis and paleoclimatologist Michael Evans of the University of Maryland, College Park, took samples the diameter of a pencil from two trees in the region. The duo then sliced the samples just 200 microns thick—about the width of a human hair—and analyzed them in a mass spectrometer to see what kind of oxygen isotopes they contained. Isotope ratios associated with dry conditions alternated with wetter conditions, allowing the researchers to establish the annual moisture cycle. The time-consuming analysis—there were 2000 pieces of wood to look at—created a climate record for the forest going back a century, which they describe online today in the Proceedings of the National Academy of Sciences.

“We expected to see some kind of trend related to global warming, yet when we developed the record it turned out the strongest trends were El Niño-related,” says Anchukaitis, referring to the cyclical weather pattern that affects rainfall and temperatures on the Pacific coast of North America. In Monteverde, El Niño caused an unusually severe dry season at about the time the golden toad was wiped out.

“The paper provides a much-needed long-term climate and hydroclimate context for this region,” says Christopher Still, a climatologist at the University of California, Santa Barbara. However, he urges caution, because the team sampled only two trees, and because interpreting oxygen isotopes in the cellulose of tropical trees is still a relatively new field.

J. Alan Pounds, a biologist at the Monteverde Cloud Forest Preserve and one of the researchers who originally put forward the argument that global warming played a role in the extinction of the golden toad, disagrees with the paper’s conclusions. Pounds writes in an e-mail that climate change has caused greater day-to-day variations in precipitation at Monteverde, something the isotope method isn’t precise enough to measure.

Even so, Anchukaitis says the impact of a single El Niño cycle on the amphibian indicates that climate change could have devastating consequences. “The fact that our research suggests it was El Niño and not anthropogenic climate change shouldn’t be any comfort when considering the future impact of climate change,” he says.

Science Now, 2 March 2010