 Global warming is one of the most significant threats facing humankind, researchers warned, as they unveiled a study showing how climate changes in the past led to famine, wars and population declines. The world's growing population may be unable to adequately adapt to ecological changes brought about by the expected rise in global temperatures, scientists in China, Hong Kong, the United States and Britain wrote in the journal Proceedings of the National Academy of Sciences. "The warmer temperatures are probably good for a while, but beyond some level plants will be stressed," said Peter Brecke, associate professor in the Georgia Institute of Technology's Sam Nunn School of International Affairs. "With more droughts and a rapidly growing population, it is going to get harder and harder to provide food for everyone and thus we should not be surprised to see more instances of starvation and probably more cases of hungry people clashing over scarce food and water." Trawling through history and working out correlative patterns, the team found that temperature declines were followed by wars, famines and population reductions. The researchers examined the time period between 1400 and 1900, or the Little Ice Age, which recorded the lowest average global temperatures around 1450, 1650 and 1820, each separated by slight warming intervals. "When such ecological situations occur, people tend to move to another place. Such mass movement leads to war, like in the 13th century, when the Mongolians suffered a drought and they invaded China," David Zhang, geography professor at the University of Hong Kong, said in an interview on Thursday. "Or the Manchurians who moved into central China in 17th century because conditions in the northeast were terrible during the cooling period," he said. "Epidemics may not be directly linked to temperature (change), but it is a consequence of migration, which creates chances for disease to spread." HALF THE WORLD AT RISK Although the study cited only periods of temperature decline to social disruptions, the researchers said the same prediction could be made of global warming. A report last week said climate change will put half the world's countries at risk of conflict or serious political instability. International Alert, a London-based conflict resolution group, identified 46 countries -- home to 2.7 billion people -- where it said the effects of climate change would create a high risk of violent conflict. It identified another 56 states where there was a risk of political instability. "I would expect to see some pretty serious conflicts that are clearly linked to climate change on the international scene by 2020," International Alert secretary general Dan Smith told Reuters in a telephone interview. Near the top of the list are west and central Africa, with clashes already reported in northern Ghana between herders and farmers as agricultural patterns change. Bangladesh could also see dangerous changes, while the visible decline in levels of the River Ganges in India, on which 400 million people depend, could spark new tensions there. Water shortages would make solving tensions in the already volatile Middle East even harder, Smith said, while currently peaceful Latin American states could be destabilized by unrest following changes in the melting of glaciers affecting rivers. Unless communities and governments begin discussing the issues in advance, he said, there is a risk climate shift could be the spark that relights wars such as those in Liberia and Sierra Leone in west Africa or the Caucasus on Russia's borders. Current economic growth in developing states could also be hit.