The evidence linking hurricanes and climate change
Interview with Judith Curry
In recent months, two studies in the journal Science and one in Nature have found that hurricanes are growing fiercer. Because so much research is drawing the same conclusion, one might surmise that humans will eventually experience some negative weather-related consequences due to climate change. But with the devastation of Hurricane Katrina filling nightly television news shows, many critics saw any attempt to link hurricanes with climate change as just another sign of global-warming “hysteria”.
During a television program on MSNBC in late September, conservative talk-show host Tucker Carlson mused about recent debate that climate change is creating more powerful hurricanes. “[S]ome people will always interpret that for political reasons, of course,” he opined.
“So many people have a vested interest in this global-warming thing—all these big labs and research and stuff,” said hurricane expert William Gray of Colorado State University in a recent interview with Discover magazine. “The idea is to frighten the public, to get money to study it more.”
In order to try and make some sense of the media controversy over the science, ES&T decided to ignore the talk-show favorites and editorial writers and speak to one of the scientists who have published a paper on this topic.
Timeline
17 June: Kevin Trenberth publishes “Uncertainty in Hurricanes and Global Warming” (Science 2005, 308, 1753–1754).
31 July: Kerry Emanuel publishes “Increasing Destructiveness of Tropical Cyclones Over the Past 30 Years” (Nature 2005, doi 10.1038/nature03906).
29 August: Hurricane Katrina hits land.
16 September: Peter Webster and colleagues, including Judith Curry, publish “Changes in Tropical Cyclone Number, Duration, and Intensity in a Warming Environment” (Science 2005, 309, 1844–1846).
Judith Curry is the chair of the school of earth and atmospheric sciences at the Georgia Institute of Technology and coauthor of one of the Science papers on hurricanes and climate change. She serves on a variety of panels related to climate, including the National Academies’ space studies board and the National Oceanic and Atmospheric Administration’s climate-research working group.
On October 25, the American Meteorological Society will host a briefing on Capitol Hill about hurricanes. Along with Curry, Kerry Emanuel of the Massachussetts Institute of Technology and Kevin Trenberth of the National Center for Atmospheric Research will present their published findings on hurricanes and global warming.
You had a recent paper in Science that looked at tropical cyclones. Tell me a little about what you found.
Okay. What we looked at was the global data set that is available from 1970 through 2004, and it’s a satellite-based data set, so we’re able to look at every single tropical storm and hurricane. And what we looked at was the frequency, intensity, and number of hurricane days for each ocean basin where they have hurricanes.
We looked concurrently at the sea surface temperature over that same period for each ocean basin. What we find—again, the increase of tropical sea surface temperature in these regions is well known—is that there was an increase in the frequency, almost a doubling, of the most intense hurricanes—the category 4s and 5s. And a similar increase in the number of hurricane days.
Now this is across multiple ocean basins?
This was the Indian Ocean, the South Pacific, the North Pacific, Atlantic . . . the whole works.
People can criticize the paper because you only went back to the 1970s. Can you actually see a pattern with such limited data?
We do not have global data prior to 1970. We have data from 1945 to 1970 from aircraft in the North Pacific and the North Atlantic. Prior to 1945, we only have statistics on landfalling hurricanes.
Now, events in the Atlantic comprise only 11% of global hurricanes, and U.S. landfalling hurricanes only comprise 1%. So trying to draw inferences about global hurricane activity from these statistics just doesn’t work. Using the sampling data from the Atlantic to understand what’s happening globally is like only sampling California voters to try and infer U.S. presidential preferences.
If you look at landfalling hurricanes, the statistics are really just looking at California voters over 65 [laughs]. It’s a sampling error.
Just to give you a counterexample, during the same time period, landfalling hurricanes in Australia have actually gone down. So if we had only relied on landfalling hurricanes we would have a different story. But clearly the Australian story doesn’t tell you anything about what is going on globally.
In recent months, we’ve seen three papers that have come out in science journals on the subject of climate change and hurricanes. Do all these papers line up in saying that we have an increase in hurricane intensity?
Yes, all papers agree on that point.
What are the differences?
Kerry [Emanuel] computes something he calls the destructive power. This involves a calculation, but he uses the number of hurricane days and the intensity—wind speed. We just looked at the more physical parameters, the category—which is related to wind speed—and the number of hurricane days. So it is different ways of presenting essentially the same data.
Kerry did do an adjustment to the wind speed on his data set. Now a criticism has been made that he over-adjusted, and he agrees with that. Everybody agrees that the data needs to be adjusted, but most of that adjustment would occur before 1970. So it doesn’t particularly affect our study.
It’s hard to find anything in the peer-reviewed scientific literature that says something opposite to what these three papers have found. Yet, William Gray told Discover magazine that he sees no link between global warming and hurricanes, and he made the same comments on television and during congressional testimony. Max Mayfield, the director of the National Hurricane Center, has given similar congressional testimony in recent weeks.
These are hurricane scientists who don’t know a lot about global climate. Gray and Mayfield focus on Atlantic hurricanes, for obvious reasons. And their view of hurricanes is biased by the very strong variability in the North Atlantic. You don’t get simple cycles, but you do get variability in the North Atlantic that dominates variations certainly in hurricane frequency. The North Atlantic does not have anything to do with what goes on globally.
So their conclusions are based on their investigation of North Atlantic hurricanes.
[Note: AAAS reports that when questioned by senators, Gray was unable to cite peer-reviewed studies to back up his claims.]
During recent Senate hearings, Senator James Inhofe (R-OK) invited science-fiction writer Michael Crichton to speak about climate change.
The American Association for the Advancement of Science tried to get my coauthor Peter Webster invited, but the majority [Republican] party picks the invitees, and they did not want to hear from Webster. So he was not invited. Webster did—the day before the hearing—brief congressional staffers.
What is interesting is why these people are being brought out to talk to the media. We have a situation in which another hurricane expert, James O’Brien, who directs an atmospheric group at Florida State University, was hosted at the National Press Club by the Marshall Institute, a conservative think tank, to give a talk saying that climate change has not affected hurricanes.
Jim O’Brien is looking at the more limited data set of U.S. landfalling hurricanes. Like I said, this is like trying to draw conclusions from Californians above the age of 65.
Now we have a data set going back into the 1800s, but again, this is like 1% of global hurricanes. In fact, landfalling hurricanes only explain about 16% of the variability in Atlantic hurricanes. So you can’t even explain what’s going on in the Atlantic by looking at landfalling hurricanes, let alone what’s going on all over the globe.
If these people disagree with the increase in hurricane intensity analysis, why don’t they begin to publish in the scientific peer-reviewed literature? You can’t find them there. Only on blogs, in opinion pieces, and on news programs.
They haven’t had a chance to do that. Our papers have just come out. Bill Gray says that he is preparing something that he is going to try and get published. I doubt that Mayfield will try publishing anything.
Gray claims that in our paper, if you include category 3 hurricanes—in other words combine categories 3, 4, and 5—you actually see a decrease. Well, we’ve done this. Category 2 and 3 hurricanes are flat over this period. So by adding them in, you dilute the effect. Instead of an 80% increase, you get something like a 50% increase.
But you still definitely get a strong increase.
So Gray hasn’t done the work, but we’ve already done it. And he claims on his website that he’s going to publish something on this.
Why don’t these people publish?
They like operating in this mode. They like the media, and the media likes a debate. It sells more newspapers or gets more people interested. These people get called on because they are on the other side. Whatever their politics are, they like to operate outside the respected and time-honored [laughs] scientific tradition of presenting their data at conferences or putting it in the peer-reviewed literature.
They just don’t operate this way. And each has his own individual reasons for doing this. They like media attention, and they’re able to get it without doing the hard work.
Another apparent rising media star is Roger Pielke, Jr. I noticed that in a recent news story in Science he was listed as a “climatologist”, and he made no attempt to correct that.
Well, he’s a prolific writer, but he is really a policy person [who] has a qualitative understanding of climate science. He is not a climatologist. I don’t know that he’s ever even taken a class in atmospheric science. In fact, he got his degree at the University of Colorado, where I used to teach, well before we instituted the program in atmospheric and ocean sciences.
So he has no formal training in atmospheric science. His mantra, if you will, is, “Let’s not worry about why we have climate change; let’s look at what we can do to adapt.” This is not bad, but in the situation of hurricanes, it’s been taken to an absurd position.
[Update 10/28/05 -- Read a letter to the editor from Roger Pielke, Jr. and a response from Paul Thacker.]
How have people misconstrued your paper?
People have accused us of linking global warming with Katrina. We didn’t even use the expression “global warming” in the paper. We talked about an increase in global tropical sea surface temperature.
But what is causing that increase?
Well, we did not address that in our paper. The only answer that makes any sense, especially over the last 35 years, is greenhouse warming. Again, you cannot explain it with decadal scale variability, because [hurricanes] are different in every basin.
Do you think that the American public is starting to wake up since Hurricane Katrina? Even if this one hurricane had nothing to do with climate change, it seems like people are at least starting to pay attention to the issue.
With all the confusion from the scientific community . . . the American public doesn’t read scientific journals. They listen to the media, and the media likes a good debate. So they trot out this small minority of people to present their contrarian views. And they are given just as much legitimacy as scientists with strong credentials and who publish in the peer-reviewed literature. The media gives equal weight to both sides of this.
So the American public gets confused, but at the same time people are starting to be worried about this now.
Do you think that people are starting to understand the science a little better?
I think so. Ten years ago we were fighting the battle, “Is the globe warming or not?” [Now,] nobody is questioning that. I think it’s irrefutable that the globe is warming. It’s evident from what’s happening in the Arctic, the warm sea surface temperatures in the tropics, the melting of the Peruvian glaciers. Even the greenhouse skeptics are no longer debating if the world is warming.
The issue is whether we blame it on human activity or not. And . . . I think even the skeptics would agree there is some component of greenhouse warming in what we see. But then they claim that natural variability swamps it out.
Before, people said the world is not warming, now they say you can’t tie it to hurricanes. But other changes are occurring such as the melting of glaciers and ice packs in the Arctic and the acidification of the oceans. Aren’t hurricanes just one issue?
Exactly. But you can’t use hurricanes to prove that there is global warming. What you can do is show an unambiguous link between the increase in hurricane intensity and the warming sea surface temperatures. And if you look for why the sea surface temperatures are warming since the 1970s, you don’t have any explanation other than greenhouse warming.
