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 U.S. 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.

"We took a pretty conservative approach and still found sizable impacts. They certainly are happening already and not just something that will or might happen in the future," Lobell told Reuters in an interview.

CONCERNS GROWING

Scientists at an annual meeting of U.S. agronomists last week in San Antonio said the focus was climate change.

"Its impact on agriculture systems, impacts on crops, mitigation strategies with soil management -- a whole range of questions was being asked about climate change," said Jerry Hatfield, Laboratory Director at the National Soil Tilth Laboratory in Ames, Iowa.

"The biggest thing is high night-time temperatures have a negative impact on yield," Hatfield added, noting that the heat affects evaporation and the life process of the crops.

"One of the consequences of rising temperatures ... is to compress the life cycle of that plant. The other key consequence is that when the atmosphere gets warmer the atmospheric demand for water increases," Hatfield said.

"These are simple things that can occur and have tremendous consequences on our ability to produce a stable supply of food or feed or fiber," he said.

Boote at the University of Florida found that rice and sorghum plants failed to produce grain, something he calls "pollen viability," when the average 24-hour temperature is 95 degrees Fahrenheit (35 Celsius). That equates to highs of 104 F during the day and 86 F at night, he said.

The global seed industry has set a high bar to boost crop yields by 2050 to feed a hungry world. Scientists said that the impact of heat on plant growth needs more focus and study.

"If you look at a lot of crop insurance claims, farmers say it is the lack of water that caused the plant to die," said Wolfram Schlenker, assistant professor at Columbia University.

"But I think it's basically different sides of the same coin because the water requirement of the plant increases tremendously if it's hot," he said.

"The private sector understands the threats coming from climate change and have significant research programs in regards to drought tolerance. They focus less on higher temperatures, but that's a tougher challenge," Nelson said.

"We are responding with a number of initatives...the primary one is focusing on drought tolerance," said John Soper, vice president in charge of global seed development for DuPont's Pioneer Hi-Bred, a top U.S. seed producer.

Pioneer launched a conventionally bred drought-tolerant corn hybrid seed in the western U.S. Corn Belt this spring, selected for its yield advantage over other varieties.

"We have some early results in from Texas that show that is exactly how they are behaving. They currently have a 6% advantage over normal products in those drought zones," Soper said.

Roy Steiner, deputy director for agricultural development for the Bill & Melinda Gates Foundation, said the foundation is focused on current agricultural effects of climate change.

"It's amazing that there are still people who think that it's not changing. Everywhere we go we're seeing greater variability, the rains are changing and the timing of the rains is creating a lot more vulnerability," Steiner said.

"Agriculture is one of those things that needs long-term planning, and we are very short-cycled thinking," he said. "There are going to be some real shocks to the system. Climate is the biggest challenge. Demand is not going away."