Researchers found that the numbers of leaf-eating insects are likely to surge as a result of rising levels of CO2, at a time when crop production will have to be boosted to feed an extra three billion people living at the end of 21st century. Scientists found that, during one of the last great episodes of global warming 55.8 million years ago, there was a significant increase in both the amount of damage caused by leaf-eating insects and the variety of injuries they inflicted on plants.

They believe that the 5C rise in global temperatures caused by a tripling of CO2 levels during the palaeocene-eocene thermal maximum (PETM) period sent insect numbers soaring and left an indelible impression on the fossilised leaves preserved since that time.

The percentage of leaves that suffered extensive insect damage rose dramatically during the PETM as foraging became more intensive. The researchers warned that the same effect might be seen during the present period of global warming caused by man-made emissions of CO2, which could double the pre-industrial concentration of the gas by the end of the century.

Ellen Currano of Pennsylvania State University, the lead author of the study, said that although the global warming experienced during the PETM occurred tens of millions of years ago, it is still the best analogy we have for what may happen in the future.

"By looking at the fossil record, we can observe the long-term – thousands to millions of years – response of ecosystems to abrupt warming and increased atmospheric CO2," she said. "Our study shows [that] ... when temperature increases, the diversity of insect-feeding damage on plant species also increases."

The scientists studied the five geological sites dating back to around the PETM that have well-preserved fossilised leaves. All five sites are in the Bighorn Basin, Wyoming.

The study found that the leaf damage recorded in about 5,000 different fossils jumped from between 15 and 38 per cent prior to the PETM to 57 per cent during the period of global warming. Leaf damage then returned to about 33 per cent after temperatures fell.

"We think that the warming allowed insect species from the tropics ... to migrate north," Ms Currano said.

In addition to migration from tropical regions, the scientists believe that insects had to eat more because the rising concentrations of CO2 in the atmosphere made leaves less nutritious because they contained relatively smaller concentrations of nitrogen. "With more CO2 available to plants, photosynthesis is easier and plants can make the same amount of food for themselves without having to put so much protein in their leaves," Ms Currano said.

Consequently, when CO2 increases, leaves have less protein and insects need to eat more to acquire the nutrients they need. Plants can grow faster when CO2 levels rise, but they suffer from a disproportionate increase in damage, she said.

There is still debate over what caused the global warming during the PETM. One theory is that it resulted from a massive release of methane from frozen deposits under the seabed. Another is that it was caused by huge volumes of CO2 pumped out from volcanoes.

(By Steve Connor, The Independent, 12/02/2008)