WASHINGTON (Reuters) - Global warming (news - web sites) could lead to a big chill in the North Atlantic, at least if history is anything to go by, researchers reported on Friday. They published evidence to support a popular theory that rising temperatures caused a big melt of polar ice 8,200 years ago, causing a freshwater flood into the salty North Atlantic. This would have changed the flow of the balmy Gulf Stream and in just a few years, average temperatures plummeted, ushering in a deep freeze that lasted a century or more, researchers have proposed. Writing in the Dec....