A Complicated Death

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Last year was the hottest on record, or the second hottest, depending on the records climatologists look at. The planet has warmed .8 degrees C over the past 150 years, and scientists are generally agreed that greenhouse gases have played a major part in that warming. They also agree that the warming will continue in the decades to come. Many experts are concerned that warming may make two unpleasant things more common: extinctions and diseases. In tomorrow’s issue of Nature (link to come here), a team of scientists report on a case that ties these two dangers together: frogs have become extinct as climate change spreads a deadly fungus. It’s an important study, but it can’t be boiled down to simple slogans. It highlights the dangers of global warming, but it shows that global warming’s effects can be counterintuitive and unpredictable.

Since the 1980s, scientists have observed that frogs and toads have been disappearing. Species that live in mountain cloud forests in the tropics have been particularly hard hit. Take Harlequin frogs (Atelopus). Scientists have described 110 species from Central and South America. But they can no longer find a single individual from 67% of those species. They’ve been identifying potential agents of the extinctions. It’s been much harder to pinpoint the actual culprit (or culprits).

Climate change was one suspect. Species that live in mountains may be particularly vulnerable to warming temperatures because they live in small ranges. If it’s too hot for an animal at 5,000 feet, it may respond by moving uphill. But it can’t go uphill forever, and before long its range may simply vanish. Another leading suspect was a fatal fungus, which has been sweeping through frog populations in recent years.

There was some reason to think that the two suspects might be working together. Scientists have found some evidence that warmer temperatures encourages the spread of diseases. Pathogens that might be killed off by cold weather can thrive if the climate changes. It was also possible that warmer temperatures were putting stress on the frogs, making them more vulnerable to attack.

This might all sound quite logical, but some evidence didn’t seem to fit in. In one Australian study, for example, the fungus proved deadlier at cooler temperatures. When scientists exposed 16 frogs to the fungus at 17 degrees C all died. But only 4 out of 8 frogs died at 27 degrees C.

A network of 75 scientists came together to sort this mystery out. They gathered data on almost all the harlequin frog species, including weather records for their ranges. If a major force has been driving a lot of species to extinction, it should be easier to pinpoint than the cause of a single species’s disappearance.

The results indicate that global warming has had a hand in the extinctions. The warmer the average temperature is in the tropics in a given year, the more likely that frogs are going to disappear in the following year.

But the results also clash with simple notions of how global warming can drive extinctions. The most vulnerable harlequin frog species live between 1,000 and 2400 meters. Harlequin frogs living at higher elevations have actually suffered fewer extinctions. So vanishing real estate is not to blame (at least in this case).

The study is also a vivid illustration of the fact that global warming can lead to lots of strange local climate change. At several research stations in the study, scientists have found that the maximum daytime temperature has actually gone down. At night, on the other hand, the minimum temperature has been going up. Clouds may be causing this pattern. Global warming causes more water to evaporate, creating more clouds in mountain forests. At night these clouds may trap heat, keeping the forests warm. But in the daytime, incoming sunlight may bounce off the clouds, leading to cooler days.

It’s these local peculiarities of climate change, the scientists argue, that may be helping the fungus kill harlequin frogs. The fungus doesn’t like temperatures over 28 degrees C and dies at 30 degrees C. It can’t survive in lowland forests, and even a harlequin frog living on a mountain could cure itself with a good bake in the sun. But these days that frog is less likely to find a spot of sun, thanks to the increasing cloud cover. On the other hand, very cold temperatures keep the fungus from growing. The highest elevations are still cold enough to block its spread, the scientists argue, which is why harlequin frogs have suffered fewer extinctions there. But as nights get warmer, the mid-elevation forests are becoming the perfect breeding ground for the fungus. And harlequin frogs there have paid the price.

I’m writing this post just before this paper goes public, and I’m cringing at the thought of how it will be spun. I’ve seen how pseudo-skeptics try to claim that we can’t learn anything about extinctions or how they might be accelerated by future climate change (see my posts here, here, and here). On the other hand, it would be wrong to make a blanket statement that climate change triggers outbreaks because it makes the planet warmer. The equation is far from simple. If not for some cooling, fungi would not be such a threat to harlequin frogs. This interplay is not just complex but hard to forecast. Scientists have known about global warming and fungus outbreaks and frog extinctions for twenty years. But as far as I know, no one predicted that it would be nighttime warming and daytime cooling that would make the fungus so deadly. A commentary that accompanies the new paper in Nature points out that few computer models used to forecast climate-driven extinctions take parasites into account. And so we have no idea just what sort of future the Harlequin frogs are pointing us towards.