WASHINGTON — What's likely to happen if the world does nothing to combat global warming? The answer from the Nobel Prize-winning Intergovernmental Panel on Climate Change was jaw-dropping: more than 40 percent of known plant and animal species could become extinct by the end of this century.
Many scientists who've been studying climate change say extinctions aren't inevitable if the world greatly reduces its dependence on oil, coal and natural gas. As daunting as the warning signs and projections are, there's still time to fend off the worst, they say. But they also warn that "business as usual" would bring devastating changes in the decades ahead.
"We're locked into a different planet, but we can still make it a planet similar to what we have known," said Lara Hansen, an ecologist who's the chief climate-change scientist at the World Wildlife Fund. The Arctic Ocean will be ice free in the summer in a few years, "but we're not locked into the Arctic being ice-free year round, or Greenland melting."
Last month, 600 scientists wrote to Congress saying that it's time to act.
Some paleontologists have suggested that the world already is witnessing a sixth mass extinction, after five others known from the fossil record. The fifth was the end of the dinosaurs and some 70 percent of other species 65 million years ago. Some of the projections of a do-nothing trend on global warming suggest that 70 percent of all living things could become extinct again.
Douglas Inkley, a senior science adviser at the National Wildlife Federation, said it was impossible to say precisely what percentage of the Earth's plants and animals would be at risk, but that there was no question about the seriousness of the risk from such a large and rapid change to the climate.
"It is clear that the extent we can address climate change by reducing global-warming pollution will reduce the worst-case scenarios of species becoming extinct," he said.
The World Conservation Union says that the rapid loss of species today is 1,000 to 10,000 times higher than the natural rate of species loss over the past millions of years. The causes of the current threat go back through human history: habitat destruction for development and agriculture, overexploitation, diseases and invasions of alien species. Climate change adds to the pressure on vulnerable species.
The group's "Red List" comprises 16,306 plant and animal species that the group says are threatened with extinction. It also says that there's evidence from the effects of climate change on species around the world that rising temperatures will be "catastrophic for many species."
Biologist Camille Parmesan of the University of Texas at Austin, who participated on the Intergovernmental Panel on Climate Change, said that some 40 scientific studies found that the risk at higher temperature increases, 7 to 9 degrees Fahrenheit, could range from an extinction rate of more than 40 percent to more than 70 percent.
Some, such as the Bengal tiger, would be lost when rising seas swamped their territories. Salmon and trout couldn't survive in warmer streams. Some marine species could perish in increasingly acidic oceans. Even seemingly small temperature increases already are changing habitats everywhere, from tropical waters to polar caps, from lowland forests to alpine meadows.
Earth's average temperature has increased by about 1 degree Fahrenheit since 1890, and roughly another degree is expected from the greenhouse gases that already have been released, because of a lag in the climate system and because carbon dioxide, the main greenhouse gas, stays in the atmosphere for hundreds of years.
Many scientists say that the world would be in trouble with any warming of more than 2 degrees Celsius — about 3.5 degrees Fahrenheit — above 19th-century levels. At that temperature, the panel's report said, up to 30 percent of species would be at increasing risk of extinction and most corals would be bleached.
The report found that temperatures could be kept below that level by reducing greenhouse gas emissions by about 80 percent by 2050, or about 2 percent per year. They're now increasing by about 3 percent per year.
Thomas Lovejoy, a conservation biologist who heads the Heinz Center for Science, Economics and the Environment, said he thought that the world would see serious disruptions in ecosystems before global warming hit 3.5 degrees. But he said that the high level of extinction the panel projected wasn't inevitable if society greatly reduced fossil fuel pollution and invested more in wildlife protection.
Lovejoy, Parmesan and some 600 other scientists sent a letter last month urging Congress to pass legislation to curb U.S. global-warming pollution and invest more in protecting wildlife and habitats.
The National Wildlife Federation sent a similar appeal this week from 670 hunting and fishing organizations in all the states.
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More information on the variety of life on Earth and tips for protecting it, from the National Zoo in Washington.