Highly Unusual Upward Trends in Rapidly Intensifying Atlantic Hurricanes Blamed on Global Warming

Atlantic hurricanes showed “highly unusual” upward trends in rapid intensification during the period 1982 – 2009 that can only be explained by including human-caused climate change as a contributing cause, according to research published last week in Nature Communications. The study, led by NOAA/GFDL hurricane scientist Kieran Bhatia, was titled, Recent increases in tropical cyclone intensification rates.

The paper used two different data sets to study historical tropical cyclone intensification rates: a relatively coarse-resolution satellite data set (HURSAT), and a higher-resolution “best track” data set (IBTrACS) that included all available data, including satellite and hurricane hunter data. Both data sets found that for the Atlantic, there was a significant increase in the proportion of 24-hour intensification rates greater than 30 knots (35 mph) between 1982 and 2009. The greatest change was seen for the strongest 5% of storms, whose 24-hour intensification rates increased by 3 – 4 knots per decade.

For tropical cyclones across the entire globe, the two data sets disagreed. The “best track” data set showed a significant increase in 24-hour intensification rates, while the satellite-only data set did not. The authors theorized that the satellite-only data set was faulty, likely because of well-documented problems judging tropical cyclone intensities during formation of the eye. Due to this discrepancy in the two data sets, the authors were unable to make conclusions on how tropical cyclone intensification rates might be changing globally.

By itself, a 28-year upward trend in one measure of hurricane intensification does not necessarily mean that human-caused climate change is to blame. Natural variability of the climate system, like the decades-long natural cycle in Atlantic hurricane activity called the Atlantic Multidecadal Oscillation (AMO), could be to blame. To see if this was the case, the authors used one of the best global climate models available for studying long-term trends in Atlantic hurricanes, the HiFLOR model.

HiFLOR has one of the highest resolutions of any global climate model used to simulate hurricanes (28 km), has an ocean model it is coupled to, and has been shown to skillfully reproduce the observed year-by-year variations of the frequency of Category 4 and 5 hurricanes--something very difficult to achieve in a climate model. Using HiFLOR, the researchers showed that natural variability could not explain the magnitude of the observed upward trend in intensification rates in the Atlantic. Thus, human-caused climate change must be responsible for a significant portion of the observed increase in Atlantic hurricane intensification rates.

Figure 1. VIIRS image of Super Typhoon Haiyan at 1619 UTC November 7, 2013. At the time, Haiyan was about to make landfall near Tacloban in the Philippines as the strongest landfalling tropical cyclone in world recorded history, with 190 mph winds. Image credit: NOAA/CIRA.

Agreement with other research

The results of the study agree with a 2018 paper by Dr. Bhatia, Projected Response of Tropical Cyclone Intensity and Intensification in a Global Climate Model, which used the same high-resolution global climate model (HiFLOR). The 2018 study predicted a dramatic increase in the global incidence of rapid intensification due to global warming, and a 20% increase in the number of major hurricanes globally. For the Atlantic, the model projected an increase from three major hurricanes per year in the climate of the late 20th century, to five major hurricanes per year in the climate of the late 21st century.

HiFLOR also predicted a highly concerning increase in ultra-intense Category 5 tropical cyclones with winds of at least 190 mph—from an average of about one of these Super Typhoon Haiyan–like storms occurring once every eight years globally in the climate of the late 20th century, to one of these megastorms per year between 2081 to 2100—a factor of eight increase! Even more concerning is that the results of the study were for a middle-of-the road global warming scenario (RCP 4.5), which civilization will have to work very hard to achieve. Under the business-as-usual track we are currently on (a scenario called RCP 8.5), the model might predict an even greater increase in ultra-intense tropical cyclones.

A 2012 theoretical study by MIT’s Dr. Kerry Emanuel, and his 2016 computer modeling study, “Will Global Warming Make Hurricane Forecasting More Difficult?”, also predicted that a future warmer climate should result in an increase in rapidly intensifying hurricanes. In an email, Dr. Emanuel said: “My own work shows that rates of intensification increase more rapidly than intensity itself as the climate warms, so that rapidly intensifying storms like Michael may be expected to become more common." Moreover, he added, "My work on Hurricane Harvey and a few other heavy rain-producing hurricanes such as Florence, together with much previous work by others on the subject, strongly suggests that hurricane-related flooding will increase as the climate continues to warm.”

Figure 2. The percent difference in annual mean number of days with a major hurricane (Category 3 or stronger) between the HiFLOR 1986–2005 simulation and HiFLOR 2081–2100 simulation. Red grid boxes show where an increase is expected by the end of the century. Data are only plotted in a grid box if there is at least one major hurricane day per year. White ‘‘X’’ marks are located in grid boxes that are not statistically significant. In the Atlantic, a significant increase in days with a major hurricane is predicted along the U.S. Southeast and mid-Atlantic coasts, as well as The Bahamas, Bermuda, Cuba, Puerto Rico, and the Leeward Islands. Image credit: Bhatia et al., 2018, Projected Response of Tropical Cyclone Intensity and Intensification in a Global Climate Model, Journal of Climate, American Meteorological Society.

Commentary: a dangerous increase in rapidly intensifying hurricanes making landfall

As Hurricane Michael sped northwards on October 9, 2018, towards a catastrophic landfall on Florida’s Panhandle, the mighty hurricane put on a phenomenal display of rapid intensification. Michael’s winds increased by 45 mph in the final 24 hours before landfall, taking it from a Category 3 hurricane with 115 mph winds to a catastrophic Category 5 storm with 160 mph winds. Michael had a disturbing air of déjà vu after what had happened just one year earlier. On August 25, 2017, Hurricane Harvey rapidly intensified by 40 mph in the 24 hours before landfall in south Texas, from a Category 1 storm with 90 mph winds to a Category 4 storm with 130 mph winds.

Hurricanes like Michael and Harvey that rapidly intensify just before landfall are among the most dangerous storms there are, since they can catch forecasters and populations off guard, risking inadequate evacuation efforts, high death tolls, and increased damages. It’s very sobering to see that two of the most rapidly intensifying hurricanes before landfall (Michael in 2018 and Harvey in 2017) since 1950 have occurred in the past two hurricane seasons, and that a significant increase in such rapidly intensifying hurricanes is predicted by some of our top computer models. Since 1950, here are the greatest 24-hour intensification rates prior to a U.S. landfall:

Humberto 2007 (65 mph increase, Cat 1 landfall)
King 1950 (60 mph increase, Cat 4 landfall)
Eloise 1975 (60 mph increase, Cat 3 landfall)
Danny 1997 (50 mph increase, Cat 1 landfall)
Michael 2018 (45 mph increase, Cat 5 landfall)
Harvey 2017 (40 mph increase, Cat 4 landfall)
Cindy 2005 (40 mph increase, Cat 1 landfall)

The new research from the HiFLOR model and historical hurricane data is the strongest evidence we have to date that the expected increase in more-intense hurricanes due to human-caused climate change is already occurring. One technique of computing hurricane damage—by using ICAT’s damage estimator to look at all contiguous landfalling U.S. hurricanes between 1900 – 2017 and computing the amount of damage that they would do today by correcting for changes in wealth and population—finds that while Category 4 and 5 hurricanes make up only 13% of all U.S. hurricane landfalls, they cause 52% of all the hurricane damage. Thus, it’s very concerning that one of the best models we have to simulate the current and future behavior of Category 4 and 5 hurricanes—the HiFLOR model—is predicting a large increase in the number of these destructive storms. Even more concerning is the model’s prediction of a global factor of eight increase in catastrophic Category 5 storms with winds of at least 190 mph by the end of the century—under a moderate global warming scenario.

It would be an excellent idea to aggressively pursue the goals of the Paris Agreement on climate change to help stave off a very dangerous potential increase in highly destructive hurricanes.

Related:

Dangerous Rapidly Intensifying Landfalling Hurricanes Like Michael and Harvey May Grow More Common, our October 2018 post.
Extreme Hurricane Rainfall Expected to Increase in a Warmer World, our June 2018 post.
Observed Slowdown in Tropical Cyclone Motion May Portend More Harvey-Like Rainstorms, our June 2018 post
Will Global Warming Make Larger Hurricanes?, our April 2018 post.
Will Global Warming Make Hurricane Forecasting More Difficult?, our January 2017 post.
Top Ten Tropical Cyclone Events of 2016 Potentially Influenced by Climate Change, our December 2016 post.
Hurricane Patricia's 215 mph Winds: A Warning Shot Across Our Bow, our 2016 post.
Katrina-Level Storm Surges Have More Than Doubled Due to Global Warming, our 2013 post.
Damaging Katrina-Level Storm Surges are Twice as Likely in Warm Years, our 2012 post.
Big Money for Hurricane Research, our October 2006 post.

Hurricane scientists Kerry Emanuel, Jim Kossin, Michael Mann and Stephan Rahmstorf wrote an excellent May 30, 2018 realclimate.org post, Does global warming make tropical cyclones stronger?