UPDATE (September 8, 4:20 pm)—For more on this developing storm event, including how it compares to Hurricane Harvey, we’ve posted a round-up of expert and scientist perspectives: UCS Experts’ View of Risk and Preparedness as the Impacts of Hurricanes Harvey and Irma Mount.
UPDATE (September 7, 11:50 am)—Hurricane Irma currently remains a category 5 hurricane, feeding off abnormally warm waters along its path across the northeast Caribbean. The hurricane’s strength is expected to continue and it is forecast to remain a category 4 or 5 storm over the next several days (the National Hurricane Center is maintaining Irma as a category 5 until Friday).
Hurricane Irma’s 180-plus mph winds held for over 24 hours (currently nearly two days), setting a record for an Atlantic hurricane and leaving casualties and destruction on the French island territories, U.S and British Virgin Islands, and Puerto Rico. Early on September 7, Irma’s center was about 95 miles north of the Dominican Republic, moving at about 17 mph and expected to go north of the Dominican Republic and Haiti on Thursday, Turks & Caicos and the Bahamas by Thursday night, and then Cuba on Friday night/Saturday. After that, recent tracks show it heading towards Florida and the Eastern US coast; those tracks will keep being updated.
In a world that is increasingly defined by superlatives, let’s start with this just-released statement from the National Hurricane Center: Hurricane Irma is the strongest hurricane in the Atlantic basin outside of the Caribbean Sea and the Gulf of Mexico in their records, and a potentially catastrophic one, tied for second place as the strongest ever in the Atlantic. And it is following on the footsteps of Hurricane Harvey, which gathered strength very fast and dumped record amounts of rain on Texas and Louisiana.
If that doesn’t give you pause, I don’t know what would.
It is not just that records are being broken, it is the intensity with which they do so
Harvey underwent a very rapid strengthening from tropical storm to category 4 hurricane (about 48 hours), and gained strength as it approached landfall, as opposed to the usual weakening. It had so much rain associated with it that the National Weather Service had to create new colors for their precipitation maps in order to properly depict the amounts. These extreme rain events are becoming increasingly common, with a number of 500-year rain events (i.e., those with a probability of 0.2% of occurring at any one year) that should be rare happening all over the country in a matter of months (Houston alone has now seen three since 2015).
What is behind the intensity of these hurricanes, and the increase in precipitation observed in a variety of weather events?
Sea surface temperatures – the main hurricane fuel – have been on the rise
We know from studying hurricanes that many factors cause and drive them, but their main fuel is a warm ocean. Warm surface waters produce heat and water vapor. Hurricanes feed on and intensify from both, and the amount of rain they ultimately dump can be increased by both the higher availability of water vapor from the warm water and the fact that a warmer atmosphere can hold more of that available moisture (more on this here). Therefore, a trend of both increased intensity and rainfall associated with hurricanes can be expected, and in fact recent studies are in agreement with that (see here and here).
The waters off the coast of Texas when Harvey intensified from a category 2 to a category 4 hurricane were 2.7-7.2°F above average. The sea surface temperature where Irma was located as of the morning of 9/5 appears to be at least 2.7°F above average, which may have had a role in it turning into a category 5.
Should we expect hurricanes to be more intense then?
Studies have detected the influence of human-made global warming both on the near-surface amount of water vapor, and in sea surface temperature in the tropical Atlantic ocean, among other areas such as the Western Pacific and South Asia – and it is worth noting that China has recently had a series of powerful typhoons, Hato and Mawar being the latest ones to wreak havoc in that part of the world, among other areas.
Emissions already in the atmosphere have committed us to a certain amount of warming in the near future, and oceans have been absorbing about 93% of this warming. Therefore, oceans have also locked in a certain amount of warming from the increase in greenhouse gases from human activities. It follows that yes, we may be seeing more intense hurricanes, dropping a lot more rain when they do come ashore.
Preparedness is key!
Heeding hurricane warnings and the directives of authorities, and reducing risk as much as possible when a hurricane is approaching is the first priority. But also important in the long run is for individuals and all levels of government alike to learn from each event and from the latest science; build and upgrade in smarter ways now, reflecting the mounting risks; and when disaster strikes, rebuild in a way that improves community resilience for the long term, and for the next big one. Because the next big one will come. It’s just a question of when and where.