With Hurricane Harvey's rainfall records and Irma's stunning combination of extreme strength and longevity, the hurricane-global warming debate is back in full swing.
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?
Here are some less-discussed storm attributes that could plausibly change in a warming world:
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?
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."
Though the science remains unresolved, real-world storms appear to fit the pattern. This year, the first named storm in the Atlantic, Tropical Storm Arlene, formed in April, far outside the bounds of the traditional hurricane season.
Areas of formation, intensification
If the globe's oceans are warming in general, that could also mean that the regions in which hurricanes can form — currently, seven major "basins" across the globe — could shift. Or, it could mean that these storms 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.
Kossin and two colleagues 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."
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. So will storms be more likely to rapidly intensify as the climate warms? Kerry Emanuel, MIT's hurricane and climate expert, thinks the answer is yes.
He 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.
Storm size
Separate from the matter of their wind speeds, overall hurricane sizes also vary greatly. So would a changing climate have any effect on this?
It's unclear. But a trend toward bigger storms could be just as much of a problem as a trend toward stronger storms when measured by wind speeds.
Contemplating the 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 — we don't know what's going to actually happen.
In sum, there is more to be said about changing hurricane traits than 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 other questions, we will probably continue to see large storms, rapidly intensifying storms, out-of-season storms and suspiciously placed storms.
