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N E W S F O C U S Consensus on Ecological Impacts Remains Elusive 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, versity ecologist Terry Root, independently part of the Intergovernmental Panel on embarked on a similar study. The two Climate Change (IPCC), reviewed 44 studies papers, published in the 2 January issue of showing that more than 400 species of plants Nature, are being touted as the most comand animals across the globe had shifted their prehensive meta-analyses to date of the ranges or changed behaviors such as the biotic effects of global warming. To many, timing of egg laying. To the biologists on they clinch the case. But Tol and other the committee, this was a strong signal of working group members maintain that the 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 Northward migration. As temperatures have warmed in Europe, the in such an analysis. Sooty copper butterfly has gone extinct in large parts of Spain and “We all think we know has expanded north into Estonia. how to analyze data, so you’d think there wouldn’t be any disagree- two studies don’t provide any greater level ments,” says one of the IPCC authors, of confidence than before that global Camille Parmesan, a population biologist at warming is causing the observed biologithe University of Texas, Austin. “But we cal changes. would look at the same data, … and one perAs part of her joint project with Yohe, son says, ‘So what?’ and the other person Parmesan sorted species into four catesays, ‘Wow, look at that!’ ” gories: those that changed their ranges or Richard Tol, an environmental economist behaviors in accord with global warming at Hamburg University in Germany, for predictions, those that did the opposite, instance, has questioned whether the data set species that did not change, and species represented a “fair sample.” He points out showing changes that couldn’t be ascribed that biologists tend to do studies in regions to global warming. She found that 87% of where impacts of climate change are expect- 484 species analyzed changed their timing ed; in addition, he said, studies showing no as predicted by models of global warming. effect are unlikely to be published. Others, Distributional shifts were consistent with including economist Gary Yohe of Wesleyan predictions for 81% of 460 species. Such University in Middletown, Connecticut, said changes would occur by chance less than that high confidence was unwarranted one time in 10 trillion—an airtight case, because the analysis simply showed a corre- argued Parmesan. lation, not cause and effect. Not so, said Yohe, pointing out that the In an effort to persuade the skeptics, result was still based on correlations. He then Parmesan teamed up with Yohe, also an developed a probabilistic model that could IPCC author, to reanalyze these and other use Parmesan’s data. A key variable controldata. Another IPCC author, Stanford Uni- ling the confidence level was the likelihood 38 3 JANUARY 2003 VOL 299 SCIENCE 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. –MARI N. JENSEN Mari N. Jensen is a science writer in Tucson, Arizona. www.sciencemag.org CREDIT: CAMILLE PARMESAN/UNIVERSITY OF TEXAS, AUSTIN Climate Change