It happens every winter.
Sometime between Santa's squeeze down the chimney and the Groundhog's peek out of his hole, a cold snap or big snowstorm gives climate-change skeptics another chance to suggest that global warming isn't happening.
Some examples this week: Radio talk show host Rush Limbaugh said the "polar vortex" the cold was blamed on is a new term invented by the media to link the cold wave to man-made climate change. The term has been around for decades.
Donald Trump took to Twitter to write: "Record snowfall & freezing temps throughout the country. Where is Global Warming when you need it?!"
So if it's very cold, then global warming isn't real? Scientists say no: "This week's brutal cold wave was a 1-in-20-year type of event," says meteorologist Jeff Masters of the Weather Underground, "and we will continue to see such cold waves in the future, even as the planet warms."