As we approach the peak of this year’s hurricane season, one question that is frequently asked – particularly in the wake of this summer’s succession of extreme weather events – is whether hurricanes are becoming, or will become, more frequent or stronger as climate change strengthens its grip.
It’s a question with which atmospheric scientists have been grappling for a relatively short amount of time, and as such the answer is something of a moving target. Were we to consult a climatological Magic 8 Ball, we might get a response like, ‘Outlook Uncertain, But Becoming Clearer.’
So we decided to do better and turn to one of the foremost authorities on the subject, Professor Kerry Emanuel of the Department of Earth, Atmospheric and Planetary Sciences at the Massachusetts Institute of Technology. Named one of Time Magazine’s 100 Most Influential People in 2006, Prof. Emanuel has published extensively on the possible linkages between tropical storms and a warming planet, and he guided us through the complex haze of theory, modeling and observation.
Kerry Emanuel: Most of us think that we are seeing a climate change signal in the North Atlantic, which is by far the best observed and has been observed for the longest period of time; but I hasten to add that only about 12 percent of the world’s tropical cyclones occur in the Atlantic. The other parts of the world are not so well observed.
What we expect from a combination of theory and modeling is that as the climate warms, the actual total number of these storms should decline globally, but the incidence of the severe Category 3, 4 and 5 storms is expected on the other hand to go up. And we do see some indication that the proportion of hurricanes that are intense around the world has been going up, although our data is a bit tenuous and is not for very long, so nobody has a great deal of confidence in it.
Let me add that, historically it’s the Category 3, 4 and 5 hurricanes that do the vast majority of damage, at least in developing countries, so those are the ones we’re concerned about the most.
Kerry Emanuel: It’s almost fair. What drives hurricanes is the flow of heat from the ocean to the atmosphere, and that is proportional to the difference between the heat content of the two, rather than the absolute temperature in either of them. But it turns out that if you add greenhouse gases to the atmosphere – or indeed, if you were just able to increase the amount of sunlight coming in, one way or the other – that increases the difference and so increases the potential for hurricanes, so that they could become stronger, at least theoretically.
DNews: And what would be the reason for the overall number of storms decreasing?
Kerry Emanuel: This is an interesting question. It bears on a related question, which is, ‘Why don’t we have hurricanes everywhere all the time?’ And the fact is that hurricanes are, fortunately for us, fairly rare. And yet, the conditions for hurricanes are prevalent over much of the tropics through much of the year. We’ve learned in recent decades that what stops most ordinary, run-of-the-mill disturbances from turning into hurricanes is the relative dryness of the atmosphere a couple of miles above the surface.
Normally, it’s pretty dry there, but the relevant quantity is the difference between how much water is there, and how much water could be there if the air were saturated, and we call that the saturation deficit. And that deficit increases with temperature.
And because of that, as the temperature gets warmer and warmer, ironically it becomes more difficult to start a hurricane, even though once you start a hurricane, potentially it can become more intense, so you have these two contradictory things going on.
Kerry Emanuel: I wish I could answer that question. I think the data in certain other places, like the western part of the North Pacific Ocean, at times in the past was better than today. We surveyed a lot of those storms with aircraft in the period between about 1945 and 1987, when that stopped for budgetary reasons. And we don’t see such a tight connection between hurricane power and temperature in the western North Pacific that we see in the Atlantic.
We’re not quite sure what is so special about the Atlantic. There are some indications that in the Atlantic kind of an alignment goes on. There are a lot of different things that affect hurricanes, not just temperature. The change in the incidence and intensity of hurricanes in the North Atlantic has been dominated more by temperature, by thermodynamics, whereas in other parts of the world some of these other factors, which are varying quite differently, may be more influential.
I should emphasize that it’s a young science, this connection between hurricanes and climate. We’re making progress and we are beginning to see a consensus developing in certain parts of the problem, but there is still a lot of it we don’t understand.
Kerry Emanuel: Well, the answer is a guarded yes. I think when it comes to the global levels of activity around the world, I can see light at the end of the tunnel, that we may indeed arrive at a consensus. When, on the other hand, it comes down to measures of hurricane activity that people care about – for example, the frequency of landfall of intense hurricanes in North America – whenever you get down to that telescopic level of detail, the models inevitably disagree violently with each other, and so scientists are left without much to go on.
And so when it comes to forecasting the things that people really care about, I don’t think there’s going to be much consensus about that for a very long time.
We’re going to form a very strong consensus maybe about things that don’t really matter to people. After all, who really cares how many hurricanes occur in the Atlantic Ocean? We’re really only concerned at the end of the day with landfalling intense storms, and when you get down to that level, all bets are off at the moment.
DNews: My final question then is one you’re probably not comfortable with answering: Assuming a business-as-usual scenario of fossil fuel emissions, do you feel confident in predicting how the intensity and frequency of hurricanes in, say, 2050 might look compared to 2012, or is that a prediction you’re not comfortable with making?
Kerry Emanuel: I’m not comfortable with any predictions. Seeing into the future is pretty tough. I think we have to look at this problem from the point of view of ‘what does society do, and how do we react to this’? As a problem of risk. There’s a rsk that we’re going to have more intense hurricanes. We do know that there is terrific year-to-year volatility with hurricanes; that’s true in the present climate, it will be true in a future climate. So much so that, even if there were a strong global warming signal, we might see it right away in looking at metrics over the entire North Atlantic region, but when we look at metrics that we care about, like hurricane damage – which is caused by a tiny fraction of those events – we would need to wait decades before we see a signal in that.
So we’re in an awkward position. I think hurricane scientists are becoming better and better and better at looking for keys under the lamp, and eventually we’ll find them. But the things people care about aren’t under the lamp, and it will be a long time before we find those.
Let me mention one more thing, and that is that one aspect of hurricanes that people don’t talk about enough is rain. Historically, that’s been a very big killer: for example, the second-most-deadly hurricane in the western hemisphere was Mitch in 1998, and that (deadliness) was entirely due to freshwater flooding. We’ve seen recent examples in the US with Irene last year and Isaac this year. There’s a uniform consensus – one of the few things that we all agree on in hurricane science – that warming the atmosphere will increase the rainfall from hurricanes, and that should be a major concern.
Hurricane Michael — seen here on Sept. 6, 2012, with winds of 115 mph, in high resolution infrared imagery from the NOAA/NASA Suomi NPP satellite, was the first Atlantic storm of the 2012 season to reach Category 3 intensity. (NOAA)
Hurricane Isaac video grab (Whitney Shefte/The Washington Post)