THE evidence is in. Global warming is already having a widespread impact on the world’s plants and animals, driving them closer to the poles and to higher altitudes, and altering the times of year they migrate and reproduce.
Two analyses published this week, the most comprehensive so far, strengthen similar but tentative conclusions made in 2001 in a landmark report by the Intergovernmental Panel on Climate Change (IPCC). Both studies scoured scientific literature for data on thousands of plant and animal species, with strikingly similar conclusions (Nature vol 421, p 42 and p 57).
Of the species showing recent changes in range or seasonal behaviour, four out of five shifted their ranges towards the poles or higher altitudes, and began mating and migrating earlier in the spring. Frogs are breeding, flowers are blooming and birds are migrating 2.3 days earlier on average each decade, and butterflies, birds and plants are moving toward the poles by 6.1 kilometres a decade.
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Such relatively small changes can sometimes mean extinction, say ecologists. Organisms can’t move higher to escape the heat if they’re already living on a mountaintop, for instance. Costa Rica’s golden toad has been driven to extinction by climate change, as the cloud forests in which it lived have warmed and dried.
“We’ve tended to assume all species are fixed in place and we can build parks around them to protect them,” says Lee Hannah of Conservation International. “[But] with the level of habitat loss, dynamic species ranges mixed with static conservation strategies is a recipe for mass extinction.”
For example, in Europe and North America, some birds have been driven away from the plant-bound insects they eat. Species that live at higher latitudes, where warming is greatest, have felt the effects the most, says Terry Root of Stanford University, a former IPCC scientist who led one of the studies. And strikingly, many effects have switched direction over time: butterflies that moved north in the warm 1930s to 40s moved south in the cooler 1950s to 60s, then north again with more recent warming.
Both biologists and economists worked on the IPCC report, but they argued over the strength of evidence for warming’s effects on wildlife. Economists tend to view only major present-day changes as important; biologists focus on the cumulative long-term consequences of minor impacts. Biologists at the IPCCdemanded that the 2001 report assign the ecological results “very high confidence” – meaning “very serious” – but the economists pushed for “medium confidence”. The final report compromised by stating that there was “high confidence” for a climate impact.
So biologist Camille Parmesan of the University of Texas in Austin teamed up with economist Gary Yohe of Wesleyan University in Middletown, Connecticut, to blend the two approaches. In one of this week’s papers they end up endorsing confidence levels as high or higher than the IPCC’s.
Some economists are not convinced. The studies examine “only a tiny fraction of all species out there”, says Richard Tol of the University of Hamburg in Germany. “Besides, scientists prefer to investigate species that are likely to be affected by climate change, and journals prefer to publish papers with strong results. The sample is therefore not representative.”
But Thomas Lovejoy, president of the Heinz Center for Science, Economics and the Environment in Washington DC, comes to the authors’ defence. “You simply can’t apply that economics stuff to this,” he says. “Biologists are trying to answer the question, ‘Are you detecting responses in nature?’ Of course the first responses are smaller than they will be later. And that’s precisely why you want to find them.”
