A team of international scientists says global warming could drive to extinction more than a third of the wildlife in the world's most ecologically sensitive areas by 2050 - and have similar, if less devastating effects on plants and animals worldwide.
The researchers' study says rising temperatures will make it impossible for many plants and animals to fight for shrinking habitats in the Amazon, Australia, Africa and Mexico.
"It's a wakeup call for conservationists and biologists that climate change is potentially having a dramatic effect on wildlife, especially when you consider the loss of habitat worldwide," said Lee Hannah, a co-author of the study and researcher with Conservation International in Washington.
The research, published in today's issue of Nature, is expected to rekindle scientific and political debates about the costs of global warming.
"I think the point is to get people talking about the fact that we're standing at the brink of a massive extinction, and we have to start thinking about what that means," said Terry Root, an ecologist at Stanford University's Center for Environmental Science and Policy. "I think it's a fantastic study."
But some experts said there are too many unknown factors to predict results 50 years from now.
"You have to take the numbers they give with a grain of salt," said Lewis H. Ziska, a weed expert with the U.S. Agricultural Research Service in Beltsville who studies the effects of climate change on invasive plants that damage crops. The study warns that projected extinction rates should not be "taken as precise predictions."
The authors also acknowledged that their findings are based on assumptions about population, land use and energy consumption that are likely to change over the next 50 years. Nor do the results take into account future policies or technologies that could reduce greenhouse gas emissions and curb global warming.
"It's true. We don't know how fast China is going to grow. We don't know what's going to happen with the U.S. economy, whether there'll be new policies or an increased use of things like hybrid cars," Hannah said.
But the study is the first to predict extinctions due to global warming on such a huge scale. "None of the other work quantified what's going on to the extent that this does," Root said.
The researchers used computer projections to estimate future populations of 1,103 types of native plants, birds, butterflies, frogs and other animals in hotspots from the Amazon rainforests of Brazil to the hills of Scotland to the Australian outback. Together, the areas they studied account for about 20 percent of the earth's surface, the researchers said.
The team - made up of researchers from the United States, England, Mexico, Australia, Brazil and South Africa - stopped short of predicting the extinction of any particular plant or animal. But they said the most threatened creatures depend on shrinking habitats. They include Australian reptiles, European songbirds, South African wildflowers and Brazilian trees.
Global warming is caused by a buildup of carbon dioxide and greenhouse gas emissions from factories, cars and other man-made sources.
The Intergovernmental Panel on Climate Change, a United Nations scientific advisory group, says average global temperatures rose by 1.8 degrees Fahrenheit in the 20th century. The panel predicted last year that temperatures will rise another 2 to 6 degrees by 2100.
The study projected three climate change scenarios - one with minimal increases in temperature and carbon dioxide levels, and others with moderate and severe increases. Using IPCC data, they predicted that even minimal increases will threaten 18 percent of the wildlife studied with extinction. Moderate changes posed a risk to 24 percent, while maximum changes created a 37 percent at-risk population.
Hannah said researchers focused on areas that make up ecological hotspots because they have been extensively studied and data were available. Even so, he said, scientists were surprised by the impact of global warming on such a wide range of geography.
"I think we were shocked by the overall magnitude of the changes we were seeing," Hannah said.
Researchers established three years ago that habitat losses worldwide are exposing plants and animals to an increased risk of extinction.
Researchers also have implicated global warming in reducing the population of various species. Studies have shown that higher temperatures are killing toads in Costa Rica, prompting birds to nest in more northern U.S. climates, and reducing waterfowl populations in the Midwest by drying up watering holes.
"I don't think anyone would argue that climate change is having an impact - the question is how soon and how much," said Jerry Michael Melillo, co-director of the Ecosystems Center at the Marine Biological Laboratory in Woods Hole, Mass.
But experts say the Nature study is one of the first to consider the effects of a number of factors, ranging from climate change and habitat loss to the introduction of invasive species into many habitats.
"It's the synergistic effects of these things that is really quite scary," Root said.