According to the analysis, leaf damage during the Paleocene ranged between 15 and 38 percent at first, spiked to 57 percent during the maximum, and dropped back to 33 percent during the Eocene.
Currano said the warmer temperatures of the PETM allowed insects from the tropics to move north into the more temperate latitude of what is now Wyoming.
Insects do better generally when it's warmer out, she added.
"They have a quicker life cycle [and] you don't have freezing nights or winters, so you might anticipate there are higher insect population numbers," she said.
In addition, plants become less nutritious as carbon dioxide increases in the atmosphere, because the plants need fewer enzymes to fix carbon in their leaves.
Enzymes are rich in nitrogen, which insects need for energy.
"So in order for an insect to get all the nutrients it needs, it's going to have to eat more, and so therefore the more plant gets consumed," Currano noted.
Scientists believe a large injection of the greenhouse gas carbon dioxide into the atmosphere caused the PETM, which saw global temperatures rise at least 9 degrees Fahrenheit (5 degrees Celsius).
(Related: Ancient Warming Caused Huge Spike in Temps, Study Says [December 19, 2007].)
The new findings sound a cautionary note for the immediate future, as increasing amounts of carbon dioxide are pumped into the air by humans, according to the researchers.
The Intergovernmental Panel on Climate Change predicts global temperatures are most likely to rise between 3.2 and 7.1 degrees Fahrenheit (1.8 and 4 degrees Celsius) by the year 2100.
Human-caused emissions of carbon dioxide are "very likely" to blame for the increase, the panel concluded.
(Related: Global Warming "Very Likely" Caused by Humans, World Climate Experts Say [February 2, 2007].)
Speculating on the implications of this research for the current bout of warming, Currano said: "Based on what we've seen, there will be more plant damage, and it will hurt plant fitness."
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