Over the past 540 million years, life on Earth has passed through five great mass extinctions. In each of those catastrophes, an estimated 75 percent or more of all species disappeared in a few million years or less.
For decades, scientists have warned that humans may be ushering in a sixth mass extinction, and recently a group of scientists at the University of California, Berkeley, tested the hypothesis. They applied new statistical methods to a new generation of fossil databases. As they reported last month in the journal Nature, the current rate of extinctions is far above normal. If endangered species continue to disappear, we will indeed experience a sixth extinction, over just the next few centuries or millennia.
The Berkeley scientists warn that their new study may actually grossly underestimate how many species could disappear. So far, humans have pushed species toward extinctions through means like hunting, overfishing and deforestation. Global warming, on the other hand, is only starting to make itself felt in the natural world. Many scientists expect that as the planet’s temperature rises,global warming could add even more devastation. “The current rate and magnitude of climate change are faster and more severe than many species have experienced in their evolutionary history,” said Anthony Barnosky, the lead author of the Nature study.
But equally as strong as the conclusion that global warming can push extinctions is the difficulty in linking the fate of any single species to climate. Policy makers would like to get a better idea of exactly what to expect — how many species will risk extinction, and which ones are most likely to wink out of existence. But scientists who study the impact of global warming on biodiversity are pushing back against the pressure for detailed forecasts. While it’s clear that global warming’s impact could potentially be huge, scientists are warning that it’s still impossible to provide fine-grained predictions.
“We need to stand firm about the real complexity of biological systems and not let policy makers push us into simplistic answers,” said Camille Parmesan, a biologist at theUniversity of Texas. She and others studying climate’s effects on biodiversity are calling for conservation measures that don’t rely on impossible precision.
Dr. Parmesan herself has gathered some of the most compelling evidence that global warming is already leaving its mark on nature. In 2003, she and Gary Yohe, an economist at Wesleyan University, analyzed records of the geographical ranges of more than 1,700 species of plants and animals. They found that their ranges were moving, on average, 3.8 miles per decade toward the poles. Animals and plants were also moving up mountain slopes.
These were the sorts of changes you’d expect from global warming. The warmer edges of a range might become too hot for a species to survive, while the cooler edge becomes more suitable. What’s more, only worldwide climate change could explain the entire pattern. “Because it’s happening consistently on a global scale, we can link it to greenhouse gases changes,” Dr. Parmesan said.
Dr. Parmesan and her colleagues have continued to expand their database since then. But other researchers have been moving in the opposite direction, seeking to attribute changes in individual species to climate change. Last year, for example, Michael Kearney of the University of Melbourne and his colleagues published a study on the common brown butterfly of Australia. From 1941 to 2005, adult butterflies had been emerging from their pupae 1.5 days earlier per decade around Melbourne.
To see if the brown butterfly is actually responding to climate change, Dr. Kearney and his colleagues first analyzed historical temperature records in Melbourne. Temperatures have gradually risen over the past 60 years. Computer models indicate that natural climate cycles can explain only a small part of the change.
The scientists then observed how temperature affects how brown butterflies develop. The warmer the temperature, the faster the butterflies emerged from their pupae. Dr. Kearney and his colleagues used those results to build a mathematical model to predict how long the butterflies would develop at any given temperature. They determined that Melbourne’s local warming should have led to the butterflies emerging 1.5 days earlier per decade — exactly what the butterflies are, in fact, doing.
In the journal Nature Climate Change, Dr. Parmesan and her colleagues argue that trying to attribute specific biological changes to global warming is the wrong way to go. While the global fingerprint of climate change may be clear, the picture can get blurry in individual species. “When you go to the local level, the outcome of climate change on one particular species is not dependent just on what climate change is doing,” said Dr. Parmesan.
In Europe, for example, the map butterfly has expanded its range at both its northern and its southern edge. Global warming probably has something to do with its northern expansion. But the butterflies are also benefiting from the mowing of roadsides, which allows more nettle plants to grow. Because map butterflies feed on nettles, they’re able to survive across a broader range of Europe.
A number of experts applaud the commentary from Dr. Parmesan and her colleagues. “I think they really hit the nail on the head,” said Richard Pearson, the director of biodiversity informatics research at the American Museum of Natural History. “Biologists shouldn’t get drawn heavily into the attribution debate.”