Hurricanes Harvey, Irma, and Maria: Did Climate Change Play a Role? (Invited Presentation)

What do climate models and analysis of long-term climate data indicate about a possible role for climate change in the recent powerful Atlantic hurricanes of 2017? Hurricane Harvey's record rainfall totals were primarily due to the storms slow movement over southeast Texas. There is no compelling evidence that climate change is making the occurrence of slowly moving landfalling hurricanes in the region, such as Harvey, more or less likely. Hurricane Irma was an exceptionally intense storm. It likely set a global record for the satellite era as the longest duration that a tropical cyclone has maintained surface wind speeds of at least 185 mph. There is indirect evidence that the severity of some of the impacts of the population of hurricanes in general, specifically storm surge levels and rainfall rates, are being increased by anthropogenic climate change through increased sea level and atmospheric moisture. However, despite strong physical arguments, a caveat is that increased hurricane surge levels and hurricane rainfall rates have not been clearly detected in observed climate data. Similarly, we expect that Atlantic (and global) hurricanes will become more intense on average as the climate continues to warm, but again there is no clear observational evidence to date that such a signal has emerged from the background of natural variability. Climate change is further expected to increase the global frequency of very intense (Category 4-5) storms similar to these hurricanes in the future, but quantifying the effect remains difficult, and there is only low confidence for such an increase in the Atlantic basin. Increased occurrence of very intense hurricanes in models is a mixture of increasing average intensity, partially offset by decreasing overall frequency of hurricanes, leading to less overall confidence in the change.