Global warming is cutting rice yields in many parts of Asia, according to research, with more declines to come.Yields have fallen by 10-20% over the last 25 years in some locations.The group of mainly US-based scientists studied records from 227 farms in six important rice-producing countries such as Thailand, Vietnam, India and China.This is the latest in a line of studies to suggest that climate change will make it harder to feed the world's growing population by cutting yields.In 2004, other researchers found that rice yields in the Philippines were dropping by 10% for every 1C increase in night-time temperature.That finding, like others, came from experiments on a research station.The latest data, by contrast, comes from working, fully-irrigated farms that grow "green revolution" crops, and span the rice-growing lands of Asia from the Indian state of Tamil Nadu to the outskirts of Shanghai.Describing the findings, which are published in Proceedings of the National Academy of Sciences (PNAS), lead researcher Jarrod Welch said:"We found that as the daily minimum temperature increases, or as nights get hotter, rice yields drop."The mechanism involved is not clear but may involve rice plants having to respire more during warm nights, so expending more energy, without being able to photosynthesise.By contrast, higher temperatures during the day were related to higher yields; but the effect was less than the yield-reducing impact of warmer nights.However, if temperatures continue to rise as computer models of climate project, Mr Welch says hotter days will eventually begin to bring yields down.