Half the world's population could face climate-induced food crisis by 2100, a new report by US scientists warns.

Rapid warming is likely to reduce crop yields in the tropics and subtropics, according to Prof David Battisti of the University of Washington.

The most extreme summers of the last century will become the norm, he calculates, using 23 climate models.

We must urgently create crops tolerant to heat and drought if we are to adapt in time, he writes in Science journal.

"The stresses on global food production from temperature alone are going to be huge," said Mr Battisti, a professor of atmospheric sciences.

"And that doesn't take into account water supplies stressed by the higher temperatures."

He collaborated with Professor Rosamond Naylor, director of Stanford University's Program on Food Security and the Environment, to examine the impact of climate change on the world's food security.

Beyond extreme
The duo combined direct observations with projections from 23 global climate models that contributed to the Intergovernmental Panel on Climate Change's (IPCC) 2007 global assessment.

They calculate there is greater than 90% probability that by 2100, the average growing-season temperatures in the tropics and subtropics will be higher than any temperatures recorded there to date.

"We are taking the worst of what we've seen historically and saying that in the future it is going to be a lot worse unless there is some kind of adaptation," said Professor Naylor.

"This is a compelling reason for us to invest in adaptation, because it is clear that this is the direction we are going in terms of temperature and it will take decades to develop new food crop varieties that can better withstand a warmer climate."

In the tropics, the higher temperatures can be expected to cut yields of the primary food crops, maize and rice, by 20-40%, the researchers said.

Rising temperatures also are likely to reduce soil moisture, cutting yields even further.

Currently three billion people live in the tropics and subtropics, and their number is expected nearly to double by the end of the century.

"You are talking about hundreds of millions of additional people looking for food because they won't be able to find it where they find it now," said Professor Battisti.

Crop failures will not be limited to the tropics, the scientists conclude.

As an example, they cite record temperatures that struck Western Europe in June, July and August of 2003, killing an estimated 52,000 people.

In France and Italy, the heatwave cut wheat yields and fodder production by one-third.

Scientists say such temperatures could be normal for France by 2100.

"I think what startled me the most is that when we looked at our historic examples there were ways to address the problem within a given year. People could always turn somewhere else to find food," Professor Naylor said.

"But in the future there's not going to be any place to turn unless we rethink our food supplies."  Continued    Source: BBC - BBC News