With Hurricane Harvey’s rainfall records and now Irma’s stunning combination of extreme strength and longevity, the hurricane-global warming debate is back in full swing. And it has evolved a good bit since 2005, when the United States was severely damaged by superstrong hurricanes (Katrina, Rita, Wilma).
Actually, there’s a case to be made that the climate change-hurricane discussion should be broadened — a move that will give a sense both of how consequential and how uncertain the issue actually is.
The public debate over hurricanes and global warming generally gets confined to a few issues: Will hurricanes be increasingly intense, like Irma was? Will they rain more, like Harvey did? Will they drive worse inland storm surges because of sea-level rise? Will they be more or less numerous?
That’s understandable: These questions have been more heavily studied — and in some cases, especially when it comes to sea-level rise (which is clearly happening), are more easily answered.
But once you take a step back and think about how hurricanes nestle into the broader climate system — as researchers like MIT’s hurricane and climate expert Kerry Emanuel, a number of whose papers are cited below, tend to do — you realize there is a broader picture, although it’s also a less certain one.
Hurricanes, Emanuel and others have shown, are massive thermodynamic systems that withdraw energy from the oceans and expel it into the atmosphere. That means many of their patterns and traits could change with warmer seas — even if some of those traits are harder to talk about because the research remains less definitive.
So let’s consider some less-discussed storm attributes that could plausibly change in a warming world. We don’t know definitively that these things are happening — again, the research here is thinner — but they’re well worth wondering about.
Season length. Hurricanes follow seasonal patterns. They occur in the summer and fall, and this, too, reflects the temperature of the oceans (among other factors). But as the climate warms, could hurricanes be more likely to occur out of season — either in the late spring or late fall/early winter?
Basically, that would amount to a thickening of the ends of this famous NOAA chart on the seasonal occurrence of Atlantic hurricanes (which demonstrates that we are at peak season right now and probably have a long way to go):
There’s at least some suggestive evidence of season lengthening. In a 2008 study on the Atlantic hurricane season, for instance, James Kossin of NOAA and the University of Wisconsin at Madison found “an apparent tendency toward more common early- and late-season storms that correlates with warming [sea surface temperature], but the uncertainty in these relationships is high.”
However, the question of season length is complex, and other research has yielded more ambiguous results. “Ultimately, there is not yet a consensus on how the length of the [tropical cyclone] season will change as a result of anthropogenic effects,” a 2015 computer modeling study by Columbia University’s John Dwyer and co-authors concluded.
Meanwhile, though the science remains unresolved, it’s hard not to miss real-world storms that appear to fit the pattern. This very year, the first named storm in the Atlantic, Tropical Storm Arlene, formed in April, far outside the bounds of the traditional hurricane season.
Regions of formation and intensification. If the globe’s oceans are warming in general, that could also mean that the regions in which hurricanes (or typhoons or tropical cyclones) can form — currently, seven major “basins” across the globe — could shift. Or, it could mean that these storms — generally creatures of the tropics — will be able to maintain their strength in new places, farther from the equator.
Any general shift in hurricane formation or arrival regions could have large implications because it could subject coastlines that aren’t accustomed to storms to their punishment.
Once again, there is at least some evidence this is happening or could happen. Kossin and two colleagues (MIT’s Emanuel and Princeton and NOAA’s Gabriel Vecchi) published a 2014 study in Nature finding “a pronounced poleward migration in the average latitude at which tropical cyclones have achieved their lifetime-maximum intensity over the past 30 years.”
Hurricanes were moving out of the tropics “a rate of about one degree of latitude per decade,” the researchers added. The study linked the change to a broader “tropical expansion” that, in turn, appears tied to human alteration of the planet’s atmosphere.
Rapid intensification. Hurricane Harvey epitomized a number of dangerous storm traits, one of which was increasing in strength very quickly as it approached the Texas coast. This is a nightmare scenario for forecasters and emergency planners because it gives little time for people to evacuate.
In general, rapid intensification is something that we have seen a lot of lately, including in super-intense storms Wilma (2005) and Patricia (2015). Wilma’s maximum sustained winds increased by 110 mph in just 24 hours, and Patricia’s beat that record, increasing by 120 mph in the same time frame.
So will storms be more likely to rapidly intensify as the climate warms? Emanuel, at least, thinks the answer is yes.
He just published a study in the Bulletin of the American Meteorological Society finding that the “incidence of storms that intensify rapidly just before landfall increases substantially as a result of global warming.” He reached this result by creating thousands of synthetic hurricanes in a computer simulation and then comparing how they behaved with and without a changing climate.
This area remains little researched, so this one computer modeling study shouldn’t be taken as a final answer. But it’s still worth pondering.
Storm size. And then we come to one of the most uncertain changes of all.
Separate from the matter of their wind speeds, overall hurricane sizes also vary greatly, from the relatively small Andrew up to the massive Katrina and beyond. So would a changing climate have any effect on this?
It’s very unclear. But a trend toward bigger storms, like Katrina and Sandy, could be just as much of a problem as a trend toward stronger storms when measured by wind speeds.
In Emanuel’s work you find a hint of this idea. A 2014 paper that he wrote with MIT’s Daniel Chavas wondered “how the distribution of storm size may differ in other climate states.” But the study made clear that before answering that question, it would first be necessary for scientists to achieve a better understanding of what controls hurricane size in the first place.
That hasn’t stopped some top hurricane gurus from speculating, though. Contemplating the massive size of Hurricane Sandy, the Weather Underground’s Jeff Masters wrote, “We have pushed our climate system to a fundamentally new, higher-energy state where more heat and moisture is available to power stronger storms, and we should be concerned about the possibility that Hurricane Sandy’s freak size and power were partially due to human-caused climate change.”
Still, this should be considered a frontier — while it’s not absurd to think that storm sizes could change in some way as the climate does, we don’t know what’s going to actually happen. And it’s possible that here, and elsewhere, there could be a trade-off — larger storms but fewer of them, perhaps. (We shouldn’t assume every change is an unmitigated negative.)
So, in sum, there is much more to be said about changing hurricane traits than the usual mantra that they will probably be more intense, will rain more, will ride atop higher seas, but could be less numerous overall.
As scientists dig into these other questions, we will probably continue to see large storms, rapidly intensifying storms, out-of-season storms and suspiciously placed storms. Those should be regarded as anecdotes — not proof of anything. But we should remember that as the climate changes, all of the different ways that hurricanes extract energy from the tropical oceans could change, too.