Hurricane Irene: Climate Connection?

“Don’t wait, don’t delay, we all hope for the best and prepare for the worst.” President Obama’s statement on Hurricane Irene urges the public to take precautions before one of the most significant northeast hurricanes in recent history. Mandatory evacuations have been ordered for much of the Atlantic seaboard, including coastal areas of New York City. All lanes of one major highway in New Jersey are headed in one direction only – west. The safest course of action is always to get out of the way of an approaching storm – to minimize the risk of harm when you can.

Hurricane Irene off the Carolinas Credit: NASA Goddard MODIS Rapid Response Team

People will invariably ask “Did climate change cause Hurricane Irene?” But there is a problem with that question: Since climate is defined as the average of many events, anybody who says that climate change caused one event, like Hurricane Irene, is not speaking from a basis in science – which is equally true for anybody who says that climate change had nothing to do with Hurricane Irene. To understand the effect climate change has on a type of weather event, you have to look at averages. So reasonable questions to ask are: Do we expect there to be more or fewer hurricanes because of climate change? Do we expect hurricanes to become stronger or weaker on average because of climate change? Do we expect hurricanes to drop more or less rain because of climate change? But the most useful question is what is our current risk exposure to hurricanes and how is this risk changing due to climate change?

The effect of climate change on hurricane activity has been the focus of a lively discussion among researchers since before Hurricane Katrina in 2005. Although there are a variety of views, scientists have reached a consensus on what the current evidence shows and does not show (A review of the current consensus appears here). There is good evidence that climate change will increase the average strength of hurricanes by increasing sea surface temperatures.1 For example, sea surface temperatures are currently running well above normal in the vicinity of Irene, and warmer seas serve as fuel for the storm. Moreover, it is well established in the scientific literature that climate change is likely to increase the amount of rainfall that hurricanes produce. This is because warmer atmospheric temperatures mean the air can hold more moisture – the same mechanism that has led to an increase in extreme rainfalls and flooding events in recent years. On the other hand, there is evidence that changing wind patterns could decrease the total number of hurricanes that form, even though those that do form will likely be stronger on average. 

The evidence for changes in tropical storm activity is strongest when examined globally. Projecting changes in an individual basin, such as the North Atlantic, is considerably more difficult, especially since the cycles over which ocean temperatures influence hurricanes in one basin can last decades, obscuring the climate signal.

That said, based on trends in coastal property and infrastructure exposure and current scientific understanding, the risk of hurricane damage can be expected to increase in the future due to climate change. Since current climate change is largely due to human activity, extreme weather events like Hurricane Irene remind us that we need to reduce our greenhouse gas emissions and prepare to adapt to the climate change our previous emissions have already committed us to – to minimize the risk of harm when we can.

1 Knutson, et al.  Tropical Cyclones and Climate Change, 2010