The threats from global warming, at least in the popular imagination, usually involve flooded coastal cities, parched croplands, and extreme weather events. Some researchers at Pennsylvania State University have identified another potent challenge if temperatures rise another few degrees. As described in an article in The Independent, Insect explosion 'a threat to food crops', leaf-eating bugs may ravage plants across the globe, following a pattern documented in fossils from the palaeocene-eocene thermal maximum (PETM) period. Science editor Steve Connor wrote:
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.
The pattern noted by the lead author of the study, Ellen Currano, included hungrier insects and less nutritious plants.
"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.
No one is quite sure what triggered global warming in the PETM era. But, we do know man-made emissions are causing our present-day temperature increases.