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Photo credit: Space Science and Engineering Center/Greg Anderson Photography Jim Kossin is an atmospheric research scientist at NOAA’s Center for Weather and Climate, part of NOAA’s National Centers for Environmental Information (NCEI). His research focuses on extreme storms—in particular tropical cyclones (hurricanes, typhoons)—and their relationship with climate and climate change. We recently sat down with Jim to look back at the record-breaking 2020 Atlantic Hurricane Season as it draws to a close. The 2020 Atlantic hurricane season was one for the record books. You’ve written many research papers on just about every aspect of tropical cyclones. What about this season stood out to you? The total number of tropical storms that were named this season has broken the all-time record set in 2005, so this is probably the most noteworthy aspect. But we also saw yet more examples of very rapid intensification and very slow moving hurricanes, both of which have recently been linked to climate change. In 2020, to date, there have been a remarkable ten hurricanes that rapidly intensified(Hanna, Laura, Sally, Teddy, Gamma, Delta, Epsilon, Zeta, Eta and Iota)—some of which underwent explosive intensification—and two hurricanes that practically stopped moving as they made landfall (Sally on the Gulf Coast and Eta in Central America). All of these storms had the potential for causing great damage and loss of life because they were so strong and they lingered for so long. What are the reasons some of this year’s storms intensified so quickly? Are we seeing more of these types of storms occurring overall? The main culprit for the hyper-activity this year was warmer-than-average ocean temperatures. This is also the main reason for the longer-than-average season and the numerous rapid-intensification events. The frequency of rapid-intensification events has increased over the past four decades, and this increase has been linked to climate change. The length of the hurricane season increases by about 40 days per degree Celsius of warming (the season starts about 20 days earlier and lasts about 20 days longer, on average). But the link to climate change for this is not as clear because our understanding of how and why hurricanes form is not as strong as our understanding of how and why hurricanes intensify. Based on the changing climate, will the hurricane season need to officially start earlier than June 1? This is a bit tricky. The relationship between global warming and tropical storm formation is less clear than, for example, the relationship between global warming and intensity. There is good consensus that global warming increases intensity, but no strong consensus that warming increases frequency. If there is no clear expectation that global warming increases tropical storm frequency, then there should be no clear expectation that the season will lengthen under global warming. This may seem a bit counterintuitive since the length of the season is influenced by ocean temperatures, as noted just above, and ocean temperatures are influenced by global warming. But this is where things get more complicated. The way that tropical storm formation responds to warming oceans depends strongly on what it was that warmed the ocean. Global warming from greenhouse gases tends to warm the tropics somewhat uniformly and this type of warming is not expected to increase frequency. Regional warming from things like reduced pollution or Saharan dust over the Atlantic ocean does increase frequency. So, in this case, the increased frequency we’ve observed in the Atlantic since around the mid-1990s is most likely due to these regional effects and not so much due to global warming. And if this is true, then we don’t necessarily expect Atlantic storm frequency to continue to increase due to continued global warming. This is one of the frontier research problems that we’re trying to understand better.