Extracted from the following link:
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"To many people's confusion, these weather events happened against a backdrop of increasing man-made greenhouse gas levels in the atmosphere that are gradually warming the planet. But scientists stress this weather does not mean that those gases are no longer exerting a warming influence. Nor does it go against the grain of basic global warming theory. Cold snaps and bouts of natural cooling that could last years are expected naturally even as the climate continues on a long-term warming trend, forced by man-made emissions."
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For a more detailed explanation, click the link: http://climate.nasa.gov/news/index.cfm?FuseAction=ShowNews&NewsID=270