The costs are likely to have been partially offset by increased CO2 levels, which stimulate plant growth. But the researchers counter the widespread assumption that a small temperature rise will increase agricultural productivity (see pp 29-31 ).
The researchers compared regional temperature change between 1981 and 2002 with local yields of six main crops. Barley appears to have been most affected, followed by maize and wheat. The effect on rice, soybean and sorghum crops was much smaller.
Any impact of climate change will have been masked by improvements in farming technology, which have increased yields, the researchers note.