Hurricanes: Is the Saffir-Simpson scale still enough?

Hurricane

Hurricanes are becoming stronger due to climate change because warmer seas give them more energy. © Elen11/ iStock

The intensity of hurricanes is currently indicated on the five-stage Saffir-Simpson scale. But as climate change and hurricanes intensify, it is no longer sufficient to communicate the increasing danger from such storms, researchers argue. In the last nine years alone there have been five hurricanes that should actually be assigned to a new sixth level of the Saffir-Simpson scale. With warming of two degrees above pre-industrial levels, the risk of such superstorms would double in the Caribbean and increase one and a half times in parts of Southeast Asia. In order to raise more awareness of the dangers associated with such hurricanes, an addition to the hurricane scale makes sense, say the researchers.

Cyclones are driven by warm sea temperatures and rising water vapor: the warm, moist air provides them with the energy they need and drives their gigantic rotating cloud vortices. That's why the warming of the seas due to climate change favors the formation of severe hurricanes, typhoons, etc. "Global warming increases the energy available for the intensification of tropical cyclones," explain Michael Wehner from Lawrence Berkeley National
Laboratory and James Kossin of the University of Wisconsin-Madison. Observational data from the last few decades show that the number of strong hurricanes is increasing, the hurricane season is lengthening and the storms are reaching higher latitudes more often than before.

Sixth category instead of the fifth level, which is open at the top

The strength of a hurricane has been measured using the Saffir-Simpson scale since the 1970s. This five-stage scale is now based on measurements of the maximum wind speed at a height of ten meters above ground. The highest level of this hurricane scale begins with wind speeds of 70 meters per second - around 250 kilometers per hour. However, this level is open at the top. As a result, even much stronger hurricanes can be classified in a maximum of category five - and are therefore possibly underestimated in public perception, for example in hurricane warnings, as Wehner and Kossin explain. They therefore advocate adding a sixth level to the Saffir-Simpson scale. This should then include hurricanes that have maximum wind speeds higher than 86 meters per second - over 309 kilometers per hour. “This expansion of the hurricane scale reflects that the strongest tropical cyclones are becoming increasingly stronger as a result of climate warming and that this development will continue as climate change continues,” they write.

The researchers offer three arguments for this additional category. The first: In the 42 years since 1980, there have been 197 category five tropical cyclones. Of these, five storms would be assigned to the new category six - all of which occurred in the last nine years. Typhoon Haiyan, which hit the Philippines in November 2013, is one of the best known among these “super storms”, which are outstanding simply because of their high wind speeds. Its wind gusts reached speeds of up to 379 kilometers per hour, and the storm and heavy rain claimed more than 10,000 lives. The strongest of the five exceptional storms was Hurricane Patricia, which hit Mexico from the Pacific in October 2015 - it is the strongest Pacific hurricane ever recorded. According to researchers, the increased occurrence of such particularly intense cyclones in the last decade is a consequence of climate change.

Better representation of increasing risk

Wehner and Kossin support this with physical analyzes in which they examined how much energy was transported to the upper limit of the troposphere via warm, moist rising air in various 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 the values ​​at a data point will exceed the threshold for category six has almost tripled between 1999 and 2018 compared to the previous period,” the researchers report. Using a model simulation, they determined that this trend will increase if the climate continues to warm: If warming reaches two degrees, the risk of a category six cyclone for the Philippines increases one and a half times. “In the Gulf of Mexico, the risk of Category Six hurricanes increases even more, doubling at two degrees of warming and quadrupling at four degrees of warming,” the scientists write.

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

Source: Michael F. Wehner (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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