The most severe cold snap in the U.S. in 20 years (see pictures) has some people questioning whether the Earth's climate is in fact warming.

The conservative website Breitbart.com has called the cold snap evidence of a global warming "hoax," while Donald Trump recently tweeted:

But climate scientists say the weather does not invalidate prevailing climate models, and one says that reactions to this week's polar vortex suggest that "people have forgotten what cold is like."

Over the past few days, some parts of the Midwest dipped to nearly -40°F (-40°C)—with windchill, -60°F (-51°C)—and states as far south as Alabama and Georgia experienced colder temperatures than they've had in years. More than 20 deaths have been reported as a result.

The weather has prompted Texas Senator Ted Cruz, a Republican, to poke fun at climate change advocate Al Gore: "It's cold. Al Gore told me this wouldn't happen."

On social media, people have been forwarding pictures of Al Gore looking frozen. (Jon Stewart poked fun at the controversy on The Daily Show.)

Forgotten Cold

But Gavin Schmidt, a climate scientist with NASA's Goddard Institute for Space Studies in New York City, tells National Geographic that people are missing the big picture.

"People should stick to the basics of what we know, the long-term trends, and realize that what we are talking about are global averages," he says.

Schmidt, who enjoyed a cold-weather run in Central Park on Wednesday morning, says climate data show that North America has been warming steadily over the past 30 years.

"The real story is that people have forgotten what cold weather is like," he says. "It was common 20 years ago."

Schmidt pointed to Minneapolis as one example.

In the 1970s, the city had an average of 14.7 nights with temperature below -10°F (-23°C) each year according to Climate Central, but before the recent cold snap it had only a few days that cold in the past few years. In the decade between 2002 and 2012, the average number of days below -10°F was only 3.8.

"Things have changed quite rapidly, and how surprising people find this cold is because things are becoming warmer," Schmidt says. "We're just not used to these things any more, and that means people aren't prepared."

View Images Man camping out over a steaming street grate in Washington, DC. Photograph by Jahi Chikwendiu, The Washington Post/Getty

Pattern of "Weird Weather"?

On the question of weather, the cold snap is another example of a pattern of "weird weather" that seems to have been affecting the world recently—including unusually warm temperatures in Alaska and Siberia at the moment and a heatwave in Australia. Most scientists say it's difficult to draw straight lines from climate change to particular weather events.

"There is a lot of speculation on how dynamics are affected by x, y, or z, but those are very tricky," Schmidt says. (Some reports have suggested that the snap could be spurred by changes in the jet stream, thanks to global warming, but Schmidt thinks the science is speculative.)

For their part, meteorologists have explained the immediate reason for the cold snap. They say air chilled in northern Canada—called a "polar vortex"—was blown into the U.S. by a kink in the jet stream, a band of strong winds in the upper level of the atmosphere.

On Tuesday, all 50 states saw freezing temperatures at some point, although forecasters said that historically that's not so unusual.

When it comes to thinking about the overall warming climate, Schmidt says it's important not to get distracted by a short-lived cooling effect.