Hurricanes: Is the Saffir-Simpson scale still adequate?

The intensity of hurricanes is currently determined on a five-stage Saffir-Simpson scale. But as climate change and hurricanes intensify, it is no longer enough to report the increased danger from such storms, researchers say. In the past nine years alone there have been five hurricanes that should actually be classified as a new level 6 on the Saffir-Simpson scale. With temperatures rising two degrees above pre-industrial levels, the risk of such severe storms will double in the Caribbean and increase one and a half times in parts of Southeast Asia. In order to raise awareness of the risks associated with such hurricanes, adding a hurricane scale makes sense, researchers say.

Hurricanes are caused by warm sea temperatures and rising water vapor: warm, moist air gives them the energy they need and drives their giant rotating cloud eddies. For this reason, the warming of the seas due to climate change encourages the formation of severe hurricanes, hurricanes, etc. “Global warming increases the energy available to intensify tropical cyclones,” explains Michael Weiner of the Lawrence Berkeley National Institutes.
Lab and James Kossin of the University of Wisconsin-Madison. Observational data from the past few decades show that the number of strong hurricanes is increasing, that the hurricane season is lengthening, and that storms are reaching higher latitudes more often than before.

The sixth category instead of the fifth level is open at the top

Hurricane strength has been measured using the Saffir-Simpson scale since the 1970s. This five-stage scale is now based on measurements of maximum wind speed at ten meters above the ground. The highest level of this hurricane scale begins with wind speeds of 70 meters per second – about 250 kilometers per hour. However, this level is open at the top. As a result, much stronger hurricanes may be classified as Category 5 at the maximum — and thus perhaps underestimated in public perception, for example in tornado warnings, Weiner and Kosin explain. Therefore, they support adding level six to the Saffir-Simpson scale. This should then include hurricanes with maximum wind speeds greater than 86 meters per second, or more than 309 kilometers per hour. “This expansion in cyclone range reflects that the strongest tropical cyclones are becoming increasingly stronger as a result of global warming and that this evolution will continue as climate change continues,” they wrote.

Researchers offer three arguments for this additional category. First: In the 42 years since 1980, 197 Category 5 tropical cyclones have occurred. Of these storms, five will be classified as new Category 6 storms – all of which occurred in the past nine years. Typhoon Haiyan, which struck the Philippines in November 2013, is one of the most famous of these “superstorms,” distinguished simply by its high wind speeds. Its wind speed reached 379 kilometers per hour, and the storm and heavy rain claimed the lives of more than 10,000 people. The strongest of the five exceptional storms was Hurricane Patricia, which struck Mexico from the Pacific Ocean in October 2015 – the strongest hurricane ever recorded in the Pacific Ocean. According to researchers, the increased occurrence of such particularly intense hurricanes in the past decade is a result of climate change.

Better representation of increased risks

Weiner and Kossin support this through physical analyzes in which they examined how much energy was transferred to the upper limit of the troposphere through rising warm, moist air in different ocean regions. To do this, they evaluated daily data from the European Center for Medium-Range Weather Forecasting from 1979 and 2019. A trend emerged here too: “The probability that values ​​at a data point exceed the Class VI threshold nearly tripled between 1999 and 2018 compared to the previous period,” the researchers reported. Using a simulation model, they determined that this trend would increase if the climate continued to warm: If warming reached 2 degrees, the risk of a Category 6 hurricane in the Philippines increases one and a half times. “In the Gulf of Mexico, the risk of Category 6 hurricanes increases even further, doubling at two degrees of warming and quadrupling at four degrees of warming,” the scientists wrote.

According to Weiner and Kossin, this development provides sufficient arguments for extending the Saffir-Simpson scale. “Hurricane risk communication is already a much-discussed topic,” they explain. Wind speeds alone are responsible for only part of the devastating consequences of these extreme weather events. However, the heavy rains and storm surges associated with such hurricanes are not included in the Saffir-Simpson scale, even though they often cause much greater damage. “Adding a sixth category to the hurricane scale does not solve this problem,” the researchers say. “But it could raise awareness of the increased danger posed by particularly strong hurricanes.”

Source: Michael F. Wiener (Lawrence Berkeley National Laboratory, Berkeley) and James Kossin (University of Wisconsin-Madison), Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2308901121

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