Roughly 251 million years ago, an estimated 70 percent of land plants and animals died, along with 84 percent of ocean organisms—an event known as the end Permian extinction. The cause is unknown but it is known that this period was also an extremely warm one. A new analysis of the temperature and fossil records over the past 520 million years reveals that the end of the Permian is not alone in this association: global warming is consistently associated with planetwide die-offs.

"There have been three major greenhouse phases in the time period we analyzed and the peaks in temperature of each coincide with mass extinctions," says ecologist Peter Mayhew of the University of York in England, who led the research examining the fossil and temperature records. "The fossil record and temperature data sets already existed but nobody had looked at the relationships between them."

Pairing these data—the relative number of different shallow sea organisms extant during a given time period and the record of temperature encased in the varying levels of oxygen isotopes in their shells over 10 million year intervals—reveals that eras with relatively high concentrations of greenhouse gases bode ill for the number of species on Earth. "The rule appears to be that greenhouse worlds adversely affect biodiversity," Mayhew says.

That also bodes ill for the fate of species currently on Earth as the global temperatures continue to rise to levels similar to those seen during the Permian. "The risk of future extinction through rapid global warming is primarily expected to occur through mismatches between the climates to which organisms are adapted in their current range and the future distribution of those climates," Mayhew and his colleagues write in Proceedings of the Royal Society B: Biological Sciences, though it may also be that warmer temperatures lead to less hospitable seas, he adds.

That is not to say that global warming was the cause of this Permian wipeout or that all mass extinctions are associated with warmer worlds—witness the disappearance of 60 percent of different groups of marine organisms during the cooling at the end of the Ordovician period roughly 430 million years ago. But these scientists argue that the evidence of a link between climate change and mass extinctions gives reason to be concerned for the future. "We need to know the mechanism behind the associations and we need to know if associations of this sort also occur in shorter-term climatic fluctuations," Mayhew says. "That will help us decide if this is really a worry for the next generation or if the threat is merely a distant future threat."

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