MARTHA MACCALLUM (ANCHOR): Special breaking coverage tonight of Hurricane Dorian. Dorian is an extremely powerful storm, and we are sure to hear people blame its strength on global climate change. So, should we? Is that correct? Roy Spencer is a meteorologist, principal research scientist at the University of Alabama in Huntsville, and author of the book Inevitable Disaster: Why Hurricanes Can't Be Blamed on Global Warming. So we know where you're coming from on this, Roy. It's good to have you with us tonight. You know, a lot of people look at this and they say that global warming has heated the surface of the ocean and that over time, those trapped gases continue to make that hotter and that that warm water is what leads to the fact that we've had five [Category] 5s in just the last few years. What do you say to them?
ROY SPENCER (METEOROLOGIST): Well, there's all kinds of statistics, Martha, that you could look at. The ones I look at are the long-term, because that's climate. Climate is long-term. And since 1900, out of all of the major hurricanes that have hit Florida, there has been no long-term trend in either their intensity or in the number of major hurricanes -- that is Cat. 3 or stronger -- that have hit Florida. Even if Dorian were to hit Florida as a Category 4, which it looks like now is not going to happen, there still would be no long-term trend. Now yes, we do expect, theoretically, that warmer waters should lead to stronger hurricanes. And the waters out around the Bahamas, where Dorian really intensified, are unusually warm. But if you go back, again, through the last hundred or more years, and look at the sea surface temperatures in that area and the hurricanes that have hit Florida, it turns out that the seven hurricanes -- since 1871 -- the seven hurricanes that went over the most unusually warm water for that time of year, and then hit Florida -- all of them occurred before 1950.
MACCALLUM: That's very interesting. Another statistic out there, and you tell me what you think about it, is that since 1970, you've had a one-tenths of a percent celsius increase every decade. So it's this slow but steady climb, and according to those numbers, that show that over time, the oceans are warming up. What do you say about that?
SPENCER: That's right. The oceans are warming up.
MACCALLUM: What do you say about that?
SPENCER: The oceans are warming up. The atmosphere is warming up. I think, probably based on the theory, over 50% of that is due to humans. But the warming is very slow. And warm waters is only one part of hurricane formation, OK? In order to have a hurricane, most of the tropics -- most of the tropics all the time, during the summer, are capable of producing or supporting a major hurricane. What's usually missing is a preexisting disturbance. For instance, most of the hurricanes we get started out as African easterly waves that come off of Africa, south of the Sahara Desert. That's where most of them come from, the disturbances. So you need that. But you -- even more important, you need low wind shear. So you've got these different ingredients you have to have. It's kind of like tornadoes. You have to have a special mixture of certain ingredients. It's not just sea surface temperature. Otherwise we'd be having major hurricanes all over the tropics, and we don't. They are a relatively rare phenomenon. So, yeah sea surface temperatures are important but statistically, like I said, if you look at the data going back over 100 years, you really can't support increasing hurricane activity with the increasing sea surface temperatures that we actually are seeing.