By Federico Waisman, Ph.D.
This article, republished with permission, originally appeared on pages 28–31 of the March 2019 issue of Claims magazine. ©2019 ALM Media Properties, LLC. Further duplication without permission is prohibited. All rights reserved.
The first generation of U.S. flood models was introduced into the market only last year. While barely up and running, their developers face serious challenges already as a result of the lack of good data, which is essential in carrying out accurate probabilistic modeling, since it enables a well-constructed and ‑calibrated model to deliver results that match experience over the years.
Unfortunately for the re/insurance sector, even for mature perils such as Florida windstorm, the data is never entirely adequate. Worse still, in the U.S., flood data is almost non-existent. Making the job even tougher is that a model only works with all things being equal. If the severity or frequency of events is changing, modelers may feel their chances of hitting the mark are dwindling.
Weather and climate change
It is easy to presume from hurricanes Harvey and Florence – two of the ten wettest storms ever to make landfall in the U.S. – that climate change is making U.S. hurricanes wetter. Yet what happened in the past may not be a useful guide to the future. Inland flooding due to hurricanes may indeed be growing more severe due to climate change. But, then again, maybe not. The science does not provide a definitive answer.
First, let’s look at some of the evidence that points to an impact. In a November 2018 article in Nature, Patricola and Wehner found that “relative to pre-industrial conditions, climate change so far has enhanced the average and extreme rainfall of hurricanes Katrina, Irma and Maria” and that “future anthropogenic warming would robustly increase the wind speed and rainfall of 11 of 13 intense tropical cyclones.”
The scientists, from the Climate and Ecosystem Sciences Division at Berkeley Lab, calculate that recent hurricanes dropped 10 to 30 percent more rain than similar storms in pre-industrial times; and they predict the rising trend will continue. That may be the case – other academic papers draw similar conclusions.
Another piece of evidence is the fact that the world’s oceans are getting warmer, (with ocean heat fueling tropical cyclones) and so is the atmosphere. Warmer air also holds more moisture: each additional 1C causes a 7% increase in moisture retention.
In another Nature article last year, James Kossin showed that the land speed of cyclones between 1940 and 2016 has gone down 10% globally, and 16% for North American hurricanes. The longer a hurricane remains over land, the higher the total precipitation.
Meanwhile, atmospheric blocks – areas of high pressure that cause hurricanes to remain stationary for long periods and therefore dump more rain – are on the increase, possibly driven by climate change.
All of the evidence seems to show that climate change and global warming might be producing wetter cyclones; however, this may also be a mix of interannual variability and long-term cycles.