Half the world's population could face a shortage of food by 2100 due to climate change, a new report by US scientists warns. Rapid global warming is likely to reduce crop production in the tropics and subtropics  , according to Professor David Battisti of the University of Washington.               "The most extreme summers of the last century will become common," he says. "We must immediately create crops that can bear 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. "And that doesn't take into account water supplies stressed by the higher temperatures."                He worked with Professor Rosamond Naylor on Food Security and the Environment, to examine the influence of climate change on the world's food supplies. They predict there is a 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 powerful 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 bear a warmer climate."               "In the tropics, the higher temperatures could be expected to cut production of the primary food crops by 20-40%," the researchers said. Rising temperatures are also likely to reduce soil moisture  , cutting production even further.               Three billion people live in the tropics and subtropics now, 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 would not be limited to the tropics, the scientists concluded. We can know from the text that   _  .
the number of people living in the tropics is reducing crop failures will be a global problem temperatures will rise most in the tropics new food crop varieties will be developed soon