Late last week, one of the strongest tropical cyclones on record in the South Pacific made a direct hit on the island nation of Vanuatu, leaving more than 20 people dead and massive destruction in its wake.
Tropical Cyclone Pam’s sustained winds of 165 mph and gusts nearing 200 ripped trees from the ground and flattened homes. In the course of a day, Tropical Cyclone Pam intensified from the equivalent of a category 2 hurricane to a category 4, before going on to become just the second category 5 on record to directly hit an island in the South Pacific. At the time, Pam was the strongest of four concurrent cyclones in the western Pacific and Indian oceans.
It was “one of the largest and most intense cyclones” the region has seen, says Greg Holland, a senior scientist at the National Center for Atmospheric Research who has specialized in South Pacific tropical storms. “Taken together I have not seen a storm with higher damage potential in the region,” Holland told The Washington Post, “and this shows in the extensive damage that Vanuatu has suffered.”
Now, as the death toll grows and the people of Vanuatu pick up the pieces of their devastated lives, scientists are pondering what role Earth’s changing climate may have had in the destructive potential of the storm.
In a post on the climate science blog RealClimate, MIT meteorologist Kerry Emanuel dissects the science embodied in the question, coming to the conclusion that “while Pam and Haiyan, as well as other recent tropical cyclone disasters, cannot be uniquely pinned on global warming, they have no doubt been influenced by natural and anthropogenic climate change and they do remind us of our continuing vulnerability to such storms.”
Ocean surface temperature is often the first connection that comes to mind when considering how global warming could impact tropical cyclones. But more important to how strong a tropical cyclone’s winds could become — its potential intensity — is the difference between ocean temperature and the surrounding, high-level air temperature, and global warming impacts both.
Emanuel dug into past ocean and atmospheric temperature and found that potential intensity has been increasing in the region where Tropical Cyclone Pam formed. He found about a 5.5-mile-per-hour increase in potential intensity per decade in the region — meaning that with every passing decade, the upper limit on how intense tropical cyclones can become has increased by over 5 mph. Emanuel’s results were similar to other studies, as well.
Since there aren’t many years of cyclone data in the South Pacific, Emanuel turned to eight climate models to calculate future trends in potential intensity through the year 2100. “Of these, two models showed insignificant trends in the region in which Pam developed, and the rest showed positive trends averaging around [1.1 mph] per decade, considerably less than the observed trend over the last 30 years.”
While the computer model estimate is not as high as the 5 mph per decade that has been observed, Emanuel says it’s high enough to not be dismissed.
“The disparity … suggests either that most of the observed increase in potential intensity (and actual intensity of high-category storms) is due to natural variability, that decreasing anthropogenic aerosol loading over that period may have played a role, or that the model projections are too conservative,” Emanuel writes. “Yet the projected increase is not insignificant, amounting to about [11 mph] over 100 years.
Emanuel’s results are consistent with what we think we know about hurricanes and global warming, and the ever-growing consensus that there is, in fact, a relationship:
Basic theory and a variety of numerical simulations support this, as well as the projection that tropical cyclones should produce substantially more rain, owing to the increased moisture content of the tropical atmosphere. This is important because most destruction and loss of life are caused by high category storms and their attendant storm surges, and by freshwater flooding from torrential rains.
Kevin Trenberth, a climate scientist at the National Center for Atmospheric Research in Boulder, Colo., agrees. “The atmosphere all around there has some 10 to 20% more moisture in it than a comparable storm in the 1970s would have had,” Trenberth told Mashable’s Andrew Freedman. “The high sea surface temperatures and the water vapor fueled the storm and undoubtedly increased its intensity and size. The winds are stronger, the storm surge is greater on higher sea levels.”
In the end, Emanuel concedes that humans may have a distinct ability to adapt to a different climate, but that doesn’t necessarily mean we’ll be able to keep up with the pace of the change, particularly when it comes to extreme natural disasters like hurricanes.
“This human adaptation time scale may be longer than the time over which climate change affects storms, so that comparatively small changes in the frequency of generational events can have large social consequences,” Emanuel writes. “When a 100-year event becomes a 50-year event, it may take a few destructive hits before we adapt to the new reality.”