65 million years ago, it is now widely believed, an asteroid slammed into the Earth, abruptly changing climatic conditions and leading to the extinction of much of our planet's life, including the dinosaurs.
An earlier, even bigger mass extinction -- this one 250 million years ago -- has also been blamed on a comet or asteroid. But now evidence is emerging that the cause may have been something familiar from today's headlines: global warming.
In some of the latest research, a group led by Professor Peter Ward of the University of Washington in Seattle studied both geological and fossil evidence. He says that record of the mass extinction 250 million years ago looks very different from the one left when the dinosaurs went extinct. Certain mineral evidence of an asteroid impact does not appear in 250 million-year-old rocks. The timetable was also very different. The earlier extinction, 250 million years ago, started with slow changes over millions of years, then speeded up.
"The highest rate of extinction seems to be occurring over a period of about 10,000 years," says Professor Ward. "Now, if you say 10,000 years to a geologist, they go, 'Wow! That was fast!' Well, that's about the amount of time it took to do this really nasty mass extinction. At the end of the Age of Dinosaurs, the extinction may have taken months, not 10,000 years."
What sort of life forms went extinct 250 million years ago? Professor Ward says all kinds of plants and animals -- the most noticeable being what scientists technically classify as pre-mammals.
"But were we to see them," he explains, "I think we would recognize them as being mammals. They're probably hairy. They look very much like dogs or wolves or sheep. And there were a lot of them. They are the fauna that gets slaughtered. And what takes their place are dinosaurs. So we are seeing, really, an age of mammals at 250 million years ago that is stopped dead in its tracks, literally and figuratively, by what I consider a global warming catastrophe."
Working in the Karoo Basin of South Africa, Professor Ward found no evidence of a comet or asteroid impact. The absence of evidence, of course, doesn't prove that there was no impact. But what Mr. Ward and his colleagues did find was evidence that the climate was changing, with less oxygen and more carbon dioxide in the atmosphere.
"Instead of cars back then, there were enormous volcanic eruptions, but on a scale that would just be unprecedented today," says Mr. Ward. "These are big fissures in the Earth where liquid lava just pours out. And, as it does so, enormous amounts of carbon dioxide go into the sky."
About one-third of Siberia, for example, is covered with lava from this period, suggesting the extent of these eruptions. The other source of CO2 resulted from a drop in sea level. That exposed sea sediment, which released methane -- a powerful greenhouse gas -- and also reduced the amount of oxygen in the atmosphere.
Although policymakers and scientists today continue to debate the threat of global warming, the atmospheric concentration of greenhouse gases such as carbon dioxide is increasing. Professor Ward says the life forms most vulnerable to climate change include food crops -- which have been bred for productivity, not necessarily for survival in a changing climate.
"Global warming affects plants. The plants we care most about are our crops. We'd better damn be sure that we do not perturb crop yields," says University of Washington Professor Peter Ward. His paper has just been published in the online journal Science Express, and will be coming out soon in the print edition, Science.