"Birds are laying eggs earlier than usual, plants are flowering earlier and mammals are breaking hibernation sooner," said Terry L. Root, a senior fellow with Stanford University's Institute for International Studies (IIS) and lead author of the Jan. 2 Nature study.
"Clearly, if such ecological changes are now being detected when the globe has warmed by an estimated average of only 1 degree F (0.6 C) over the past 100 years, then many more far-reaching effects on species and ecosystems will probably occur by 2100, when temperatures could increase as much as 11 F (6 C)," Root concluded.
Climatic and biological changes
In their Nature paper, Root and her colleagues analyzed 143 scientific studies involving a total of 1,473 species of animals and plants. Each study found a direct correlation between global warming and biological change somewhere in the world. For example, several studies revealed that, as temperatures increased in recent decades, certain species began breeding and migrating earlier than expected. Other studies found that the geographical range of numerous species had shifted poleward or moved to a higher elevation -- indicating that some plants and animals are occupying areas that were previously too cold for survival.
Were these biological and behavioral changes isolated events, or did they reflect a worldwide pattern consistent with global warming? After exhaustive statistical analyses of all 143 studies, Root and her co-authors concluded that global warming is, in fact, having a significant impact on animal and plant populations around the world.
"Our study shows that recent temperature change has apparently already had a marked influence on many species," they wrote, noting that a rapid temperature rise in combination with other environmental pressures "could easily disrupt the connectedness among species" and possibly lead to numerous extinctions.
Swallows, geraniums and spruce
In their analysis, Root and her co-workers revealed that nearly 1,200 species - roughly 81 percent of the total number analyzed - have undergone biological changes that were "consistent with our understanding of how temperature change influences various traits of a variety of species and populations from around the globe."
Their overall analysis of studies involving temperate-zone species revealed that springtime events - such as blooming, egg laying and the end of hibernation - now occur about 5.1 days earlier per decade on average.
The North American tree swallow offers a good example. Field biologists, who kept track of some 21,000 tree swallow nests in the United States and Canada over the last 40 years, concluded that the average egg-laying date for female swallows has advanced by nine days - a phenomenon that mirrors other North American studies confirming higher temperatures and the earlier arrival of spring.
Similar long-term observations of flowering plants in Wisconsin revealed that wild geraniums, columbine and other species are blooming earlier than before. Studies in Colorado also showed that marmots are ending their hibernations about three weeks sooner than they were in the late 1970s.
Other studies confirmed that a variety of species - including butterflies and marine invertebrates - have shifted their ranges northward as temperatures increased. Measurements taken in Alaska revealed that growth in white spruce trees has been significantly stunted in recent years - another expected consequence of a rapidly warming climate, Root said.
"Climate change models predict that the poles will warm more quickly than the equator, so it's not surprising that we're getting the strongest signals of biological change from Alaska and other northern regions," she added.
The authors pointed out that, although plants and animals have responded to climatic changes throughout their evolutionary history, a primary concern for wild species and their ecosystems is the rapid rate of change predicted during the next century.
"The problem will be the differential response of species," Root explained. "I call it the tearing apart of communities. For example, four types of warblers feed on spruce budworm caterpillars. But the birds are shifting north. What happens when the birds no longer are present in the southern portion of their ranges, and the caterpillar population is no longer kept in check?"
She predicted that rapid climate change, coupled with the loss of habitat and other ecological stressors, could lead to the disappearance of species - a consequence that might be avoided by taking proactive instead of reactive conservation measures.
"For example, there's a very high probability that global warming could contribute to a 50 percent decline in breeding waterfowl populations," Root noted. "One thing we might do now is to consider adjusting the bag limits for hunters so we don't add insult to injury in the coming years. Because anticipation of changes improves our capacity to manage, it behooves us to increase our understanding about the responses of plants and animals to a changing climate."
Other co-authors of the Nature study are Jeff T. Price of the American Bird Conservancy in Colorado; Kimberly R. Hall of Michigan State University; Stephen H. Schneider, a professor of biological sciences at Stanford and an IIS senior fellow; Cynthia Rosenzweig of the NASA Goddard Institute for Space Studies; and Alan Pounds of the Golden Toad Laboratory for Conservation in Coast Rica. The study was supported by the U.S. Environmental Protection Agency, the Winslow Foundation and the University of Michigan.
By Mark Shwartz
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EDITORS: The study, "'Fingerprints' of global warming on wild animals and plants," will be published in the Jan. 2 issue of Nature.