About 100 million years ago, during the age of dinosaurs, a warming spell caused cloud cover to drastically decrease, helping drive temperatures even higher, according to a new study.
Average tropical temperatures during that era of the Cretaceous exceeded 100 degrees Fahrenheit (38 degrees Celsius), and the Polar regions were in the 50-degree-Fahrenheit (10-degree-Celsius) range. Palm trees grew in Canada.
Atmospheric levels of the greenhouse gas carbon dioxide were four times higher then than they are today, scientists estimate.
While high, though, "that doesn't seem to be sufficient to get the type of warmth that the temperature data suggest," said Lee Kump, a geologist at Pennsylvania State University.
Writing in tomorrow's issue of the journal Science, Kump and colleague David Pollard suggest a reduction in cloud cover provided the temperature boost.
Scientists are interested in the ancient warming event because its greenhouse gas concentrations resemble what's possible if humans burn all known fossil fuel reserves.
"We're taking the atmosphere into a state it hasn't experienced for maybe 50 million years," Kump said.
(Related: "Sea Levels to Plunge Long Term, Study of Dino Era Says" [March 6, 2008].)
Cloud Formation
Kump and Pollard suggest the reduction in Cretaceous cloud cover stemmed from a drop in cloud condensation nuclei, tiny particles around which water droplets form.
Today about half of such nuclei are human-produced pollutants. During the Cretaceous, the particles were derived mostly from plant material.
According to some climate models, warm global temperatures during the Cretaceous caused a decrease in plant growth.
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