Researchers who studied the impact of this earlier warming period on prehistoric foliage found it coincided with increased damage to the plants due to greater insect feeding. Prehistoric plant life appeared to have been subjected to an intense assault by an unusually abundant and voracious insect population. Investigators believe rising temperatures allowed insects from the tropics to migrate to new habitats in traditionally cooler latitudes, while higher carbon dioxide levels made it harder for them to get the nutrition they needed from the plants.
The Paleocene-Eocene Thermal Maximum (PETM) is the name given to an abrupt warming period that occurred about 56 million years ago and coincided with a temporary tripling of carbon dioxide in the atmosphere. Temperatures spiked by four, and in some places as much as 10, degrees Celsius. Scientists say this earlier period of climate change is comparable to the contemporary global warming phenomenon, which is driven in large part by greenhouse gas emissions. The evidence suggested that a greater diversity and number of predators were feeding on the plants -- and feeding harder -- than before or after. Previous research has shown that animals extend their ranges as temperatures rise. It has also shown that plants grown under higher concentrations of carbon dioxide are less nutritious, so insects must eat more.