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Consensus on Ecological Impacts Remains Elusive

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Science  03 Jan 2003:
Vol. 299, Issue 5603, pp. 38
DOI: 10.1126/science.299.5603.38

Two big new studies strengthen the case that global warming is causing biological effects, but critics say even the additional data fall short of proof

Working Group II had a problem. The group, part of the Intergovernmental Panel on Climate Change (IPCC), reviewed 44 studies showing that more than 400 species of plants and animals across the globe had shifted their ranges or changed behaviors such as the timing of egg laying. To the biologists on the committee, this was a strong signal of climate-induced effects on a variety of biota. They wanted to give the finding a very high level of confidence, 95%. But the nonbiologists, mostly economists, advocated a confidence level of 33% to 67%, and no more.

Ultimately, IPCC's consensus document listed a high confidence level, 67% to 95%. But despite that paper compromise, the group remained split on how certain it was that global warming caused the observed biological changes. The issue: which data should be considered in such an analysis.

“We all think we know how to analyze data, so you'd think there wouldn't be any disagreements,” says one of the IPCC authors, Camille Parmesan, a population biologist at the University of Texas, Austin. “But we would look at the same data, … and one person says, ‘So what?’ and the other person says, ‘Wow, look at that!’”

Richard Tol, an environmental economist at Hamburg University in Germany, for instance, has questioned whether the data set represented a “fair sample.” He points out that biologists tend to do studies in regions where impacts of climate change are expected; in addition, he said, studies showing no effect are unlikely to be published. Others, including economist Gary Yohe of Wesleyan University in Middletown, Connecticut, said that high confidence was unwarranted because the analysis simply showed a correlation, not cause and effect.

In an effort to persuade the skeptics, Parmesan teamed up with Yohe, also an IPCC author, to reanalyze these and other data. Another IPCC author, Stanford University ecologist Terry Root, independently embarked on a similar study. The two papers, published in the 2 January issue of Nature, are being touted as the most comprehensive meta-analyses to date of the biotic effects of global warming. To many, they clinch the case. But Tol and other working group members maintain that the two studies don't provide any greater level of confidence than before that global warming is causing the observed biological changes.

Northward migration.

As temperatures have warmed in Europe, the Sooty copper butterfly has gone extinct in large parts of Spain and has expanded north into Estonia.


As part of her joint project with Yohe, Parmesan sorted species into four categories: those that changed their ranges or behaviors in accord with global warming predictions, those that did the opposite, species that did not change, and species showing changes that couldn't be ascribed to global warming. She found that 87% of 484 species analyzed changed their timing as predicted by models of global warming. Distributional shifts were consistent with predictions for 81% of 460 species. Such changes would occur by chance less than one time in 10 trillion—an airtight case, argued Parmesan.

Not so, said Yohe, pointing out that the result was still based on correlations. He then developed a probabilistic model that could use Parmesan's data. A key variable controlling the confidence level was the likelihood that a species' observed change was properly attributed to climate change. The model's result: an estimate of medium confidence, or 33% to 67%.

Parmesan then focused on effects she calls “sign switching”; these can only be explained by a temperature increase or decrease—for example, spring signals such as flowering that happen earlier during warmer decades and later during cooler decades. Between 80% and 100% of the 294 species examined switched as predicted by temperature flip-flops.

Yohe found these tests to be a compelling demonstration of causation, even though the data sets were smaller. “To the degree to which sign switching occurs, it's very convincing,” he says.

In a separate study, Root attempted to strengthen the literature review started at IPCC by gathering a larger data set. Her team analyzed studies in which either positive or negative changes (such as range shifts or earlier flowering) were observed and linked to temperature, but, unlike Parmesan, it did not include studies that showed no effects. In all, the team analyzed 143 studies covering 1468 species of plants and animals around the globe; of those, 81% exhibited changes consistent with those predicted for global warming. When Root's team focused on timing specifically, it found that spring events shifted an average of 5.1 days earlier per decade.

“What we're finding is a correlation,” says Root. “But it's such a robust correlation that the probability it's by random chance is minuscule.”

Harvard University biological oceanographer James McCarthy, co-chair of Working Group II, says, “Before these analyses, [the idea of] consistent response of organisms to temperature was just a hypothesis. Now we know that organisms are responding globally—on all continents, across a large range of organisms.”

But economists say that the biologists have not yet proved their case. Tol says Parmesan and Yohe try to deal with problems of study selection and bias, but they fail because the bias is inherent in the nature of published literature. “The Root et al. paper does not try,” he adds. Even so, he says the papers will stimulate other groups to work on the problem, and therefore he finds the papers “a useful first step.”

Yohe still won't assert with 95% confidence that global warming is causing biotic changes. However, as more evidence accumulates that he can apply in his probabilistic model, he expects to come around.

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