NPR: "CHANG: So let me ask you, are hurricanes becoming more intense and more destructive in general, because it certainly feels like that? I mean, is it normal to have a new Category 5 storm every year?"
"BERARDELLI: It's not normal. In fact, the chance any one year of a Category 5 is about 20%. We've seen five Category 5s in four years."
"CHANG: Yeah."
"BERARDELLI: And your other question was, are we seeing an increase? Yes, we are - in the strongest of storms. There was a study done, and since 1975, they said there was a substantial and observable increase in the proportion of Category 4s and 5s - in fact, an increase of about 25% to 30% per degree Celsius of global warming, so that's 1.8 degrees Fahrenheit. So, yes, there is a substantial increase in global Cat 4s and Cat 5s because of a warmer climate."
"CHANG: OK. And how does that work? How does climate change affect the severity of a hurricane?"
"BERARDELLI: Every single year, we set new records for ocean heat content. So although the global temperatures may go up-and-down every year, the ocean does not go up-and-down. It just only goes up, and that's because 93% of the excess heat that we are trapping because of greenhouse gases is stored in the ocean - 93%. Well, that has to come out somehow, and that is high-octane fuel for hurricanes. So the more heat that there is in the ocean, especially near the surface of the ocean, the stronger these systems tend to get. And that's what we're seeing."