How hurricanes are supercharged by climate change

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15 October 2024

Environment, Faculty of Science

The main impact of climate change on hurricanes arises through the changed environment, especially the warmer oceans.

We may think of the hurricanes as part and parcel of living in the Atlantic basin in peak hurricane season which officially runs from June 1 to November 30, but were the tropical cyclones that have caused mayhem and cut their destructive paths through Florida - first Debby then more recently Helene and Milton - worse than usual? And if so, did their strength and hyperactivity have anything to do with climate change?

I remember being asked that question in the summer of 2004 when four hurricanes made landfall in Florida, and to me it was obvious that it did. I had by then worked extensively on climate change, and had connected the increases in sea surface temperatures, which were clearly and demonstrably linked to global warming, to increases in water vapour in the atmosphere. As long as there is ample water available, as there always is over the ocean, atmospheric water vapour increases at a rate of seven percent per degree Celsius of warming.

Storms reach out and gather in the available water vapour around them, which fuels the storm as it rains harder, releasing the latent heat that went into evaporating the moisture in the first place. I reasoned that the same scientific logic would apply to hurricanes.

None of this was reflected in any official 2004 statements made by the National Oceanic and Atmospheric Administration in the US 20 years ago, which put the extra hurricane activity down to natural variability. Shortly thereafter, I participated in a news conference on the topic, set up by Harvard University, in which I cautiously suggested that yes, global warming was playing some role.

This led to a major outcry from the hurricane meteorologists, and I responded to their criticisms in an opinion piece published in Science magazine in June 2005. It turned out to be a prescient article, as it preceded the record-breaking 2005 hurricane season in which Hurricane Katrina devastated New Orleans. There were many sceptics intent on disproving my argument publicly - but the subsequent events and new research and analyses managed to change some of their minds. I capped this phase of work off with a Scientific American article, "Warmer oceans, stronger hurricanes".

I have also published on this in relation to 2017's Hurricanes Harvey, Rita and Maria. Harvey struck Texas from the Gulf of Mexico, the same year Maria caused devastation especially in Puerto Rico, and Hurricane Irma, which covered 90 percent of Florida (as well as other places), led to the largest US evacuation in its history. That was the year of 17 named storms and 10 hurricanes, six of which were categorised as "major".

Hurricanes form naturally, and come in all sizes, locations, and tracks, and the activity can be manifested in several ways, through the intensity, size, lifetime, and number of the storms ... For tropical cyclones (including hurricanes or typhoons), the general expectation with global warming is for more activity.

My research in 2017 used Hurricane Harvey as a test case, and we were able to convincingly show that total rainfall likely matched the evaporated moisture and the corresponding ocean heat loss. In Harvey, near Houston, rainfalls exceeded 1m over several days and were up to 1.5m in spots.

My argument then, which I still stand by, was that human-induced climate change continued to heat the oceans, which provide the memory of past accumulated climate change effects. The resulting environment, including warmer oceans and higher sea surface temperatures, invigorates tropical cyclones to make them more intense, bigger, longer-lasting, and produces much more rainfall that leads to flooding.

How are hurricanes affected by climate change?

The main impact of climate change on hurricanes arises through the changed environment, especially the warmer oceans. Hurricanes form naturally, and come in all sizes, locations, and tracks, and the activity can be manifested in several ways, through the intensity, size, lifetime, and number of the storms. Good statistics exist for only a few of these metrics.

For tropical cyclones (including hurricanes or typhoons), the general expectation with global warming is for more activity. This can be manifested in several ways: more storms, more intense storms, bigger storms, longer-lasting storms, and all with a lot more rainfall. As the intensity increases, spiral arm bands become wrapped around the eye of the storm and can form a new eyewall farther from the centre. This is what meteorologists call eyewall replacement, where the new eyewall has a much larger radius. This process weakens the storm but makes it larger, and because of the warm oceans, they then recover strength and hence live longer. Such eyewall replacements occurred several times in Irma, resulting in a very large hurricane. This is one way climate change makes bigger, longer-lasting and more intense storms.

Hurricanes occur naturally, and in some parts of the world they are regarded as part of life, but human-caused climate change is supercharging them and exacerbating the risk of major damage.

Planning for such supercharged hurricanes (adaptation) by increasing resilience (for example by better building codes and flood protection) and preparing for contingencies (such as evacuation routes, power cuts, and so forth) is essential but not adequate in many areas. This includes Texas, Florida, and Puerto Rico where Harvey, Irma, and Maria took their toll, and now in the Southeast (especially North Carolina with Helene) and Florida where Milton has caused chaos and destruction.

Dr Kevin Trenberth is Distinguished Scholar at the National Center for Atmospheric Research in the US and honorary academic at the Faculty of Science.

This article reflects the opinion of the author and not necessarily the views of Waipapa Taumata Rau University of Auckland.

This article was first published on Newsroom, How hurricanes are supercharged by climate change, 15 October, 2024

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