THE evidence is in. Global warming is already having a widespread impact on the world鈥檚 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鈥檛 move higher to escape the heat if they鈥檙e already living on a mountaintop, for instance. Costa Rica鈥檚 golden toad has been driven to extinction by climate change, as the cloud forests in which it lived have warmed and dried.
鈥淲e鈥檝e tended to assume all species are fixed in place and we can build parks around them to protect them,鈥� says Lee Hannah of Conservation International. 鈥淸But] with the level of habitat loss, dynamic species ranges mixed with static conservation strategies is a recipe for mass extinction.鈥�
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鈥檚 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 鈥渧ery high confidence鈥� 鈥� meaning 鈥渧ery serious鈥� 鈥� but the economists pushed for 鈥渕edium confidence鈥�. The final report compromised by stating that there was 鈥渉igh confidence鈥� 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鈥檚 papers they end up endorsing confidence levels as high or higher than the IPCC鈥檚.
Some economists are not convinced. The studies examine 鈥渙nly a tiny fraction of all species out there鈥�, says Richard Tol of the University of Hamburg in Germany. 鈥淏esides, 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.鈥�
But Thomas Lovejoy, president of the Heinz Center for Science, Economics and the Environment in Washington DC, comes to the authors鈥� defence. 鈥淵ou simply can鈥檛 apply that economics stuff to this,鈥� he says. 鈥淏iologists are trying to answer the question, 鈥楢re you detecting responses in nature?鈥� Of course the first responses are smaller than they will be later. And that鈥檚 precisely why you want to find them.鈥�
