Did climate change impact hurricanes Irma and Harvey?

JACKSONVILLE, Fla. – The question is going to be asked. Many times. By many people. Did global warming cause or have any impact on Hurricane Irma? What about Harvey? 

While there is tremendous scientific agreement among scientists about the basic science of climate change, politics and social media have created an atmosphere where a lot of incorrect information is being bantered about.

Here's the truth about the relationship between global warming and Hurricanes Irma and Harvey:

Scientists are uncertain about whether or not global warming will cause more hurricanes. In fact, some scientists say that a warmer planet will actually result in fewer hurricanes. However, that is not the entire story.  A warming world means that ocean temperatures increase, too. And we all know that warm ocean water is the fuel that power hurricanes. So, even if the coming decades show a decrease in overall hurricanes, those hurricanes that do develop in otherwise favorable conditions will likely be stronger. The bottom line is that our future may be one of fewer but stronger hurricanes. Is that good or bad news? Fewer hurricanes means a lower chance that one hits Florida. But a hurricane that does form and heads our way will likely be stronger, with a more severe storm surge, and higher wind.

Something else to consider is that, as the world warms, more ocean water evaporates into the atmosphere (which has been documented...it's happening). This water vapor is what hurricanes turn into rainfall. Our warmer world with higher atmospheric humidity means that tropical weather systems have more moisture to tap into, which means greater potential for increased rainfall. Statistics show that extreme precipitation events nationwide are increasing, and this also applies to tropical systems. What most people don't realize is that, as summer wanes and we transition into fall, hurricanes are the atmosphere's natural mechanism to return the summertime water vapor back into the ocean. So, hurricanes are necessary to balance the planet's water budget. The only problem is that people get in the way, and hurricanes of the future likely will have higher rainfall, and increased potential for catastrophic flooding events (also due to higher storm surges resulting from higher sea levels from the warming climate).

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So let's now apply all of this to Hurricanes Irma and Harvey:

Did global warming cause Irma or Harvey?

No. Both hurricanes could have developed regardless of the warming climate. 

Could global warming have affected Irma’s record strength, or Harvey's rapid intensification just before landfall? 

Harvey strengthened rapidly from a Category 1 to a Category 4 storm in just over a day, undoubtedly due to very warm Gulf of Mexico waters it traveled over in otherwise favorable conditions. It was the first Category 4 storm to make landfall in Texas since Hurricane Carla in 1961. Irma became the strongest Atlantic basin hurricane when its winds increased to 185 mph, also due to very warm ocean temperatures and otherwise favorable conditions. There is no question that the warmer ocean temperatures we’re experiencing now have been impacted by the warming climate. So, global warming may have been part of the reason that Harvey rapidly intensified just before landfall, and Irma became a record-breaking hurricane.

Did global warming cause the incredible rain amounts that Harvey produced?

As I explained above, increased atmospheric humidity due to global warming is making storms like Harvey more predisposed to heavier rainfall. However, this is not the only reason Texas saw such extraordinary rain totals. Remember that most hurricanes (such as Irma) come and go...they make landfall, and keep on moving. Hurricane Harvey became a tourist...it hung around for a while and meandered due to weak steering currents. While most people focus on wind, the true destructive power of that storm was water. Bands of torrential rain plagued southeast Texas for over five straight days. Think about that for a moment:  hurricane / tropical storm induced heavy rain bands, day after day. The bottom line is that, while global warming may have provided Harvey with extra water vapor to generate increased rainfall, the bigger reason for the catastrophic rain amounts was the storm's unusually slow movement. And by the way, there is no way to know based upon our current knowledge whether or not global warming had any impact on Harvey's slow movement. Hopefully, future research will answer that question.

Did global warming have anything to do with the incredible storm surge flooding Irma caused? It's documented fact that sea levels are rising. While most people know part of the reason -- melting global ice sheets -- most don't know that about one-third of all sea level rise is caused by the rising ocean temperatures. You see, water is a very interesting compound: when it's warmed, it expands. So just the simple fact that the world's oceans are warming is causing some of the sea level rise. Higher sea levels mean that strong winds pushing water inland have a higher starting point, resulting in those waters being pushed farther inland. So it is likely that Earth's warming climate is part of the reason that Irma's storm surge was so devastating.