What’s global warming got to do with it?

We're taking a look at the strange weather around the country lately and re-visiting a topic we covered Friday night that lit up interest and protest among some of our viewers.

Here in New York on Saturday it was 72 degrees in Central Park.  While its a little colder here today, it's not by much.

Boston's high Monday was 52: its normally 37. That's a full 15-degrees above normal. And it was 57 today in Philadelphia, 21-degrees above normal.

On Friday we looked into it. We invited Dennis Feltgen, a 30-year veteran weather forecaster on the broadcast and asked him about it. He said "it's not global warming at all...it is El Nino, El Nino, El Nino."

Since then, NBC News Chief Science Correspondent Robert Bazell talked to experts who say there is a relationship between this strange El Nino winter and global warming — and there'll be more where that came from.

Climate scientists say there is no question that the immediate cause of the unusually warm weather in the Northeast this winter is El Niño, a natural warming of the ocean halfway around world.

"El Niño is a cyclical change in the ocean temperatures of the Pacific and it does shift the location and intensity of the jet streams, and that acts to influence the patterns of temperature and precipitation across the U.S.," says Dr. Brian Soden with The University of Miami's Rosenstiel School of Marine and Atmospheric Science.

In most years, the upper jet stream brings arctic air to the Northeast in the winter. In an El Niño year like this one,  it does not.

But what about global warming, which most scientists believe is caused by carbon dioxide and other gases produced by humans? Many experts say it too plays a  part.

"Whatever the natural causes are, they are riding on top of the warming trend that has been induced by humans using the atmosphere as a free place to dump our tailpipe waste," says Dr. Stephen Schneider of Stanford University.

From now on, scientists say, many extreme weather events will result from natural causes enhanced by global warming. That includes heat waves, droughts and hurricanes.

"We all know that humans don't make hurricanes," Schneider says. "Katrina was not produced by global warming, yet Katrina was a little stronger because it went over an ocean that was half a degree warmer than it would have been."

So, the unusual warmth in the Northeast could be partly the result of  global warming. Indeed, even the heavy snow in the Rockies this year might be partly caused by global warming.  El Niño brings the storms, but because the air is warmer than usual, it holds far more moisture, producing much more snow.