Hurricane season lasts for a few more weeks. The US has been fortunate over the past few years because there haven’t been many direct hits on the mainland, but this year the hurricanes just keep coming. Sustainability commentator Bob Brecha takes a look at what we should expect for the future with warming oceans.
It’s been a strange hurricane season already. Harvey hovered over Houston and unleashed unimaginable amounts of rainfall. The astonishing thing about Irma is not just that the maximum wind speeds made the storm a strong Category 5 hurricane, but also how long those 185 mile per hour winds were maintained. Right after Irma, Jose also reached Category 5 strength, then Maria did the same soon afterwards, and the weird storm named Katia parked off the coast of Mexico for days, gathering strength before dashing toward land.
There’s been a lot of discussion about what we should expect to happen with hurricanes in a changing climate. It’s really complicated since “climate” is defined as an average over longer time periods and larger areas, trying to figure out the behavior of relatively localized events is hard.
There are a few things that are clear from basic physics, however. The first one is often quoted in media reports – for every degree Fahrenheit the air temperature increases, the amount of water vapor increases by a few percent. That’s why the same relative humidity in summer results in a lot more water in the air than in the winter. Likewise, as the earth warms, there will be more capacity for the atmosphere to hold water. And warmer air over the ocean also means that more water can evaporate from the surface, fueling storms even further.
But there’s one research result that I came across recently that seems to have really expressed the relationship between hurricanes and ocean temperatures concisely, and gives us a picture of what might be expected in the future. Zeke Hausfather and Robert Rohde of Berkeley Earth posted graphs showing the probability of different hurricane strengths for different ocean surface temperatures. The key point seems to be this - hurricanes form over warm ocean waters, and there is essentially never a Category 4 hurricane if the ocean temperature is less than 81 degrees Fahrenheit, and almost never a Category 5 hurricane if the ocean temperature is less than 82 degrees Fahrenheit.
Scientists are uncertain as to whether the total number of hurricanes will increase with further global warming, but most who study the issue seem to think it won’t change much. What will change is the number of intense hurricanes, and that ties right back to the warming oceans. There will always be more Category 1 and 2 hurricanes than there will be Category 4 and 5 hurricanes. But with climate change, it’s more likely that those most powerful storms will develop. Add to that the fact that sea levels are rising, meaning that the storm surges from hurricanes will likely be more damaging, and coastal property starts to look like a risky investment over the next few decades.
Ocean temperatures in the Atlantic are well above average, and we know that a primary cause of the temperature increase is carbon dioxide emissions from coal, oil and natural gas. Although we can never say that a particular extreme weather event is caused directly by climate change, scientists are getting better at figuring out the chance that a given event could have happened without climate change, and it’s becoming increasingly clear how often the determining factor of so-called natural catastrophes is actually due to humans.
The world is beginning to use fossil fuels at a somewhat slower rate – emissions have been flat for three years. That’s a good start, but more is needed to minimize the warming of the oceans, and, I hope, to minimize the number of times we have to read about hurricane disasters like Irma and Harvey.
Bob Brecha is a professor of Physics and Renewable and Clean Energy at the University of Dayton, and Research Director at UD's Hanley Sustainability Institute. Follow him on Twitter: @BobBrecha