Global warming may be making some people nervous now, but from 1810 to 1819, people worried because the Earth was colder than usual. For an entire decade, the Earth cooled almost a full degree Fahrenheit. In fact, 1816 was known as the year without a summer. And until recently, scientists weren't quite sure why everyone was shivering.
http://www.npr.org/templates/story/story.php?storyId=121377379&ps=cprs