How Global Is Global Warming?

(CBS)  Even as La Nina may finally be fading, global warming is regarded by many, if not all, climate experts as a factor in generating longer and stronger U.S. tornado seasons, wildfire seasons and more severe droughts. CBS News Correspondent Jim Axelrod has new and exclusive facts on global warming.

The recent twisters in Nebraska and Colorado did a lot of damage, but it's weather changes you can't see that carry the greatest threat. At least, so says the government's top weatherman.

"If we take the period from April back to January, it's the warmest such period in all of recorded history that we have for the United States, " says James Baker. "That's 106 years."

The January-to-April period was nearly 4 degrees warmer than average. Seventy percent of the country had much warmer than normal surface temperatures. Texas and New Mexico had their 2nd warmest year to date records ever. Idaho, Nevada, Utah, Iowa and Oklahoma had their 3rd. All more evidence, says the government, that the world is warming.

"The scientists are telling us that if they look at the pattern over the last hundred years," says Baker, "they cannot explain that, unless they include a significant amount of human impact. That is, greenhouse gases."

This is where the global warming debate heats up. The question is not if it's warming, but why. Not every sceintist believes carbon dioxide emissions from cars and greenhouse gases are the cause. Nor is the idea that recent warming trends are irreversible an accepted fact.

"Weather varies," says Dr. John Christy of the University of Alabama at Huntsville. "It was our turn to be warm. And for the Sahara Desert to be cool."

Dr. Christy says scientists must look beyond the U.S. to draw global conclusions, to realize that weather—by its very nature—varies.

"To say that this is evidence of an enhanced greenhouse effect, I think, is stretching it a bit," he says. "Because when you look at the globe as a whole, you do not see those high temperatures."

Christy says this cycle will pass. Temperatures will cool. But others see melting icebergs and yet more growing data that seems to support a different view. And they will all disagree.