Monday, September 26, 2005

Katrina, Rita, and climate change: Is there a connection?

With Katrina and Rita dominating the news recently, there's a good deal of talk out there about the relationship between hurricanes and climate change (or global warming). And it comes down to this: Are these larger, more powerful hurricanes related to climate change, or not? In other words, has climate changed caused these larger, more powerful hurricanes? (Of course, there's also the lingering question of whether climate change is myth or reality, but, to me, this is a no-brainer akin to evolution -- yes, it's a reality, however much head-in-the-sand naysayers insist on living in denial and avoiding one of the major problems of our time (or any time).

According to USA Today, Admiral James Watkins, chairman of the U.S. Commission on Ocean Policy, finds two major culprits, the natural cycle of hurricane frequency, which is currently on the upswing, and climate change, which is raising sea levels and ocean temperatures, thereby intensifying hurricanes. In other words: "The recipe for a hurricane is simple. Conditions are ripe whenever large thunderstorms occur over tropical seawater heated to at least 80 degrees. Essentially, hurricanes are circling weather machines, sucking the heat out of the ocean and turning that energy into high waves and heavy rains."

Reuters: "Scientists say it's not easy to tell if global warming caused hurricanes Katrina and Rita but on Monday they forecast more unpredictable weather as Earth gets hotter. Even skeptics agree that global warming is under way and that human activity is at least in part responsible. Climate experts also agree that this warming is likely to make the weather more extreme -- colder in some places, hotter in others, with droughts and severe rainstorms both more common. 'Global warming, I think, is playing a role in the hurricanes,' said Kevin Trenberth, a senior scientist at the National Center for Atmospheric Research in Boulder, Colorado. 'But a lot of what is going on is natural. What global warming may be doing is making them somewhat more intense,' said Trenberth, a member of the U.N. Intergovernmental Panel on Climate Change.'"

And there are calls for something to be done. The Boston Globe's Derrick Jackson:

As the media screams about the one-two punch of Hurricanes Katrina and Rita, the question becomes how many more times does America need to be knocked to the canvas before we answer the bell on global warming...

In this tragic season of hurricanes, research continues to increasingly tie global warming to an increase in the intensity of tropical storms.

One was published last month in the journal Nature by Kerry Emanuel, a professor of atmospheric science at the Massachusetts Institute of Technology. Another was published last week in the journal Science by atmospheric researchers at Georgia Tech and the National Center for Atmospheric Research.

While there has been no increase in the actual number of storms worldwide, the Georgia Tech/NCAR study found the number of hurricanes that reached categories 4 and 5, with winds of at least 131 miles per hour, have gone from comprising 20 percent of hurricanes in the 1970s to 35 percent today. This is with only a half-degree centigrade rise in tropical surface water temperatures.

The percentage of big storms in the North Atlantic has increased from 20 percent to 25 percent...

In the 1970s, no ocean basin saw more than 25 percent of hurricanes become a 4 or 5. Today, that percentage is 34, 35, and 41 percent, respectively, in the South Indian, East Pacific, and West Pacific oceans. The biggest jump was in the Southwestern Pacific, from 8 percent to 25 percent.

Emanuel, who formerly doubted that hurricane intensity was tied to global warming, said that he was stunned when his research showed that just that half-degree rise in tropical ocean temperatures has also seen a 50 percent rise in average storm peak winds in the North Atlantic and East and West Pacific in the last half century.

Meanwhile, the right-wing (head-in-the-sand, denial-inhabiting) Washington Times has unsurprisingly come out against any link between climate change and larger, more powerful hurricanes like Katrina and Rita. Admittedly, it bases its case largely on the testimony of the director of the National Hurricane Center, Max Mayfield, who blames the natural cycle of hurricane frequency, but its presentation is decidedly (and characteristically) one-sided -- the case for climate-change (and for its impact on hurricane frequency and strength) is reduced to John Lawton, chairman of Britain's Royal Commission on Environmental Pollution, who is quoted to look like an idiot.

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Regardless (why dwell on the hopelessness of the right?), my friend Grace Miao sent me a link to RealClimate, "a commentary site on climate science by working climate scientists for the interested public and journalists". A recent post -- "Hurricanes and Global Warming -- Is There a Connection?" -- is a must-read. Here's an excerpt:

Katrina was the most feared of all meteorological events, a major hurricane making landfall in a highly-populated low-lying region. In the wake of this devastation, many have questioned whether global warming may have contributed to this disaster. Could New Orleans be the first major U.S. city ravaged by human-caused climate change?

The correct answer -- the one we have indeed provided in previous posts (Storms & Global Warming II, Some recent updates and Storms and Climate Change) -- is that there is no way to prove that Katrina either was, or was not, affected by global warming. For a single event, regardless of how extreme, such attribution is fundamentally impossible. We only have one Earth, and it will follow only one of an infinite number of possible weather sequences. It is impossible to know whether or not this event would have taken place if we had not increased the concentration of greenhouse gases in the atmosphere as much as we have. Weather events will always result from a combination of deterministic factors (including greenhouse gas forcing or slow natural climate cycles) and stochastic factors (pure chance).

Due to this semi-random nature of weather, it is wrong to blame any one event such as Katrina specifically on global warming -- and of course it is just as indefensible to blame Katrina on a long-term natural cycle in the climate.

Yet this is not the right way to frame the question. As we have also pointed out in previous posts, we can indeed draw some important conclusions about the links between hurricane activity and global warming in a statistical sense. The situation is analogous to rolling loaded dice: one could, if one was so inclined, construct a set of dice where sixes occur twice as often as normal. But if you were to roll a six using these dice, you could not blame it specifically on the fact that the dice had been loaded. Half of the sixes would have occurred anyway, even with normal dice. Loading the dice simply doubled the odds. In the same manner, while we cannot draw firm conclusions about one single hurricane, we can draw some conclusions about hurricanes more generally. In particular, the available scientific evidence indicates that it is likely that global warming will make -- and possibly already is making -- those hurricanes that form more destructive than they otherwise would have been.

The key connection is that between sea surface temperatures (we abbreviate this as SST) and the power of hurricanes. Without going into technical details about the dynamics and thermodynamics involved in tropical storms and hurricanes (an excellent discussion of this can be found
here), the basic connection between the two is actually fairly simple: warm water, and the instability in the lower atmosphere that is created by it, is the energy source of hurricanes. This is why they only arise in the tropics and during the season when SSTs are highest (June to November in the tropical North Atlantic).

SST is not the only influence on hurricane formation. Strong shear in atmospheric winds (that is, changes in wind strength and direction with height in the atmosphere above the surface), for example, inhibits development of the highly organized structure that is required for a hurricane to form. In the case of Atlantic hurricanes, the
El Nino/Southern Oscillation tends to influence the vertical wind shear, and thus, in turn, the number of hurricanes that tend to form in a given year. Many other features of the process of hurricane development and strengthening, however, are closely linked to SST.

Hurricane forecast models (the same ones that were used to predict Katrina's path) indicate a tendency for more intense (but not overall more frequent) hurricanes when they are
run for climate change scenarios.

Fascinating stuff, and a balanced assessment of the relationship between climate change and hurricane frequency and strength. There may be more to the story than climate change, and it may be true that we are witness the upswing of a natural cycle, but it would be wrong to claim that climate change has had nothing to do with what's been going on. Indeed, such claims are nothing if not grossly ignorant grotesquely irresponsible.

Which is yet one more reason why the problem of climate change must be tackled. Now.

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Previous posts on climate change at The Reaction:

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