Wind chills in the -20s hit southern Wisconsin this weekend, causing the National Weather Service to issue a wind chill advisory that is effective through today. Similar temperatures are expected to continue through the beginning of the week.

"[The cold weather] has been produced over a period of weeks, then it's been dragged southward over this last week, and here it is, sitting on top of us right now," said Jonathan Martin, professor of atmospheric and oceanic sciences at the University of Wisconsin.

Martin said extremely cold temperatures are common for this region of the United States, but the unusually warm temperatures experienced during December and early January are not.

"We haven't had anything like this in about 11 years," Martin said.

The cold temperatures have put strains on many Madison residents, including students who need to walk to classes or work.

UW junior John Sinclair, who works as a SAFEWalk walker on campus, worked during the extreme temperatures Friday night.

"We were joking that it's literally unsafe to be outside, let alone if there was a prowler," Sinclair said.

If extreme temperatures do continue throughout much of the coming week, some students said they would need to consider whether or not they will attend class.

"I'll drag myself to class, but I'll be miserable the whole time I'm outside, and I'll be walking as fast as possible," UW junior Julia MacCallum said. "It's hard for my heat in my apartment to keep up, so even when I stay inside my apartment I get cold — it's a lose-lose situation."

Sinclair said that, while he will go to class because he has a test, he does not think students should be required to go to class if the frigid temperatures persist.

But Terry Devitt, spokesperson for UW Communications, said it is highly unlikely classes will be canceled due to extreme temperatures.

"We only rarely close the university due to weather," Devitt said. "Students should operate under the assumption that classes are going to be held."

This weekend's cold temperatures have also put a strain on area homeless shelters, some of which have been taking in record numbers of people since the severe weather began.

"We've been getting a lot more people," said Scott Pellicane, an intake worker at the Porchlight shelter in Madison.

Porchlight has seen a record number of people this year, but numbers have been even higher since the extreme cold started.

Martin said it is unlikely the cold weather is being caused by an El Niño effect, as it was very weak this year and has nearly died out. He said a cause could be the increase in the earth's temperatures because of global warming.

"Until now, [global warming] had been left as a point of contention," Martin said. "This is no longer the case — there is now a consensus."

Martin said he hopes the word "belief" leaves the discussion on global warming, as it's now a reality we must deal with. He said scientists are studying the effects of global warming on El Niño, which can affect temperatures and weather patterns.

"[El Niño] may become a little more frequent in a warm plant, and we're trying to understand that better whether that's true," Martin said.