CHICAGO — 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,