Crop scientists in the United States, the world's largest food exporter, are pondering an odd question: could the danger of global warming really be the heat?
For years, as scientists have assembled data on climate change and pointed with concern at melting glaciers and other visible changes in the life-giving water cycle, the impact on seasonal rains and irrigation has worried crop watchers most.
What would breadbaskets like the Midwest, the Central Asian steppes, the north China Plain or Argentine and Brazilian crop lands be like without normal rains or water tables?
Those were seen as longer-term issues of climate change.
But scientists now wonder if a more immediate issue is an unusual rise in day-time and, especially, night-time summer temperatures being seen in crop belts around the world.
Interviews with crop researchers at American universities paint the same picture: high temperatures have already shrunken output of many crops and vegetables.
"We don't grow tomatoes in the deep South in the summer. Pollination fails," said Ken Boote, a crop scientist with the University of Florida.
The same goes for snap beans which can no longer be grown in Florida during the summer, he added.
"As temperatures rise we are going to have trouble maintaining the yields of crops that we already have," said Gerald Nelson, an economist with the International Food Policy Research Institute (IFPRI) who is leading a global project initially funded by the Bill and Melinda Gates Foundation to identify new crop varieties adapted to climate change.
"When I go around the world, people are much less skeptical, much more concerned about climate change," said David Lobell, a Stanford University agricultural scientist.
Lobell was one of three authors of a much-discussed 2011 climate study of world corn, wheat, soybean and rice yields over the last three decades (1980-2008). It concluded that heat, not rainfall, was affecting yields the most.
"The magnitude of recent temperature trends is larger than those for precipitation in most situations," the study said.