The University of Arizona

Hurricane Irene – herald of a warming world, or just another storm?

August 29, 2011
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Figure 1 Hurricane Irene passes over the mid-Atlantic coast as a Category 1 hurricane. Image from a great animation at Weather Underground.

As I write this, over 3 million people are without power in the Carolinas; waves are breaking in Battery Park on Manhattan; and at least 9 have died, all in the wake of Hurricane Irene. The storm peaked as a category 3 hurricane and hopscotched up the eastern seaboard as it weakened. Even as a tropical storm, it continues to create havoc in the northeast. Can we blame this on global warming? Or is it just another storm?

The answer, of course, is probably a little bit of both. Tropical storms have menaced the eastern seaboard since well before weather records were kept, even as far north as New England. As a geology undergraduate in Rhode Island, I remember field trips where we cored into marsh sediments punctuated by sandy hurricane deposits from the 1930s. A few years later, I body-surfed the swells of Gloria on Cape Cod (stupid, perhaps, but memorable). Tropical storms may not occur every day in the northeast, but they are nothing new.

And yet…

Tropical storms feed on warmer waters and atmospheric moisture. The atmosphere’s temperature is rising, and consequently it can hold more vapor. Observations bear this out; the amount of vapor in the atmosphere has risen by about 4%. The Atlantic has been unusually warm along the area where tropical storms grow and develop into hurricanes. The combination of warmer waters and more vapor provides more energy to storms than they would have in a cooler world.

Figure 2: Deviation from normal ocean surface temperatures in the week leading up to Irene’s transition from storm to hurricane (Aug. 14-20). Image from Columbia University/IRI Data Library.

Other factors come into play also. El Niño events – a warmer tropical Pacific – reduce the number and intensity of Atlantic hurricanes, by generating winds high in the atmosphere that essentially blow the tops off tropical storms, keeping them from reaching hurricane force. We are now in a relatively neutral, bordering on cool, Pacific phase, so the Pacific is probably playing a minor role in this storm. (Stay tuned; La Niña years tend to favor a strong Atlantic hurricane season). The Atlantic undergoes its own warm and cold phases that alternately fortify and damp hurricane strength; we are currently in a warm interval.

So which is it? Is Irene natural, or a consequence of global warming? Well, it’s a little bit of both. Our climate system includes both significant natural variations and an intensifying human fingerprint. Climate events simply cannot develop independently of either one of these influences. For example, warm phases in both the Atlantic and Pacific are warmer than they used to be because of human causes. Instead, what we experience is the combination of Mother Nature and humankind’s greenhouse experiment.

Let’s ask the question differently: Are strong storms more likely to happen in a warmer world? Here, climate science clearly suggests yes.

As the world continues to warm, the human influence on hurricanes will likely become more apparent. Ocean temperatures and atmospheric water vapor will continue to rise. We are likely to see more of the strongest hurricanes (like Katrina), although perhaps not more of the weaker ones. Rising seas will make coastal damages worse, and higher rainfall rates within storms will exacerbate flooding. As ocean temperatures warm, and the tropical zone expands, will a larger swath of the Earth be in the hurricane zone? It seems to make sense, and maps of future storm potential suggest this expansion.

HOW we experience these storms – the rising costs of storm damage – is another matter. The accelerating toll of tropical storms, and other natural disasters, over the past few decades cannot be attributed to either natural or human-caused climate changes. Instead, they are mostly the result of a more immediate human folly – the increasing tendency to place investments and lives in harm’s way.

We have the capacity to reduce the damage from storms. In the short term, effective evacuation planning and communications are critical. In the medium term, smart zoning for coastal development that incorporates future changes (rising seas, expanding tropical storm zones, and greater population pressures) can protect lives and investments. In the long term, the challenge will be easier and less costly if climate change is kept to a minimum through real cuts in greenhouse emissions.

I recommend a couple of web resources below for further exploration of the relationship between global warming and tropical storms: 

http://www.gfdl.noaa.gov/global-warming-and-hurricanes

http://wind.mit.edu/~emanuel/Papers_data_graphics.htm