Ancient Global Warming Gave Bugs the Munchies

John Roach
for National Geographic News
February 11, 2008
A temperature spike about 55 million years ago gave bugs the munchies, according to a new study.

If modern temperatures continue to rise as anticipated in the coming years, researchers add, the planet could see a similar increase in insect damage to crops and other plants.

The finding is based on an analysis of more than 5,000 fossil leaves that were ravaged by voracious insects dating to before, during, and after the ancient bout of global warming.

"We see a huge increase in the percent of plants that are being attacked," said Ellen Currano, a graduate student in geosciences at Pennsylvania State University in University Park.

The research appears this week in the journal Proceedings of the National Academy of Sciences.

Michael Storey is a geochronologist at Roskilde University in Denmark and an expert on the ancient temperature spike, called the Paleocene-Eocene Thermal Maximum (PETM) because it straddles the two geologic eras. (See a geologic time line.)

Storey said that the PETM is "the best natural analog" to modern warming and that studies like Currano's "reveal more and more about what potentially will happen when we change the climate."

Less Filling

Currano and colleagues collected the fossil leaves from the badlands of Wyoming.

Some leaves have giant holes chewed through them; other fossils are just the leaf skeleton—the veins—because all the other tissue was eaten.

Some of the leaves had galls, which are tumor-like growths that form around insect eggs laid inside the leaves. Another type of damage, called leaf mines, occurs when eggs laid in the leaf hatch and the larvae chew passages on their way out.

"You can see that trail where there's no leaf tissue inside anymore, and in some cases you can see the fossil poop that the insect has left behind," Currano said.

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.

Cautionary Note

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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