The Climatology of Hurricanes

When I decided to come to Swarthmore, I never considered Philadelphia a place that was ever hit by hurricanes. Now, in the past two years, two major hurricanes or tropical storms have hit the area. This week’s devastating weather may have just been the “Frankenstorm” — a freak, statistics-flaunting event  — to make many sound the alarm that climate change is starting to make this kind of event less a freak of nature and more the norm.

So is this the new normal for Swarthmore? Like with much of climate science, the answer isn’t easy, and it could be months or years until we know for certain whether hurricanes like Sandy and last year’s Irene are infrequent disasters or the beginnings of a trend driven by higher oceanic temperatures and melting polar ice, other byproducts of climate change. And, although the scientific evidence backing current theories of climate change is indisputable at this point, there is current disagreement among climate scientists about how hurricane activity will change the future of our planet.

Hurricanes begin as a cluster of thunderstorms in the sea. If the seawater is above a critical threshold of 26 degrees Celsius, the storms can become concentrated and create a vortex of humid air, which constitutes a hurricane. Hurricanes typically form during the hurricane season, which in the Atlantic runs from May to October. The worst time of this season is typically in September, when the difference between air temperature and sea temperature is greatest.

Part of the reason the Frankenstorm was so unique was that the storm came so late in the season. Not only are temperatures usually not right for hurricanes in late October, but ordinarily storms arriving from Sandy’s location end up missing the mainland and careen harmlessly out to sea. This is since strong cold fronts blowing off of the East Coast typically cause hurricanes to miss the mainland United States. This year, however, there was an unusually intense high-pressure system in the Northern Atlantic. The storm was turned by the pressure disparity to instead make landfall in the mid-Atlantic United States.

The cause of this odd high-pressure system is a mystery to climate scientists at the moment.  Some have suggested that the melting of Arctic sea ice, which is currently happening at record-setting rates, has caused the seawater in the northern Atlantic to be heated more than usual by the sun.

Others have suggested that the melting of Arctic ice is affecting the Earth’s climate in more complex ways, altering wind and weather patterns across the entire Northern Hemisphere.  These changes are due to what is known as “upper level air flow,” or the wind currents that circle the Earth high above us. These currents have been shown to have a slower eastward progression than they once did, which could be responsible for more persistent weather patterns, causing more severe droughts, heat waves, and cold waves, as well as unseasonable weather events such as the Frankenstorm.

The frequency of reported hurricanes has risen steadily for the past century, and over the past decade or so has increased even more rapidly. From the mid-1970s to the mid-1980s, there were consistently eight to nine tropical storms and hurricanes named by meteorologists. Since 1992 however, the number of namings has been above 11 storms each year, along with the record-breaking 2005 hurricane season in which 31 storms were named, including the most devastating hurricane on record, Hurricane Katrina. (For reference, there have been a total of 19 tropical storms or hurricanes this year — the third year in a row there have been 19 or more named storms).

Activists point to this alarming and unprecedented spike in the number of hurricanes as a demonstration of how climate change is already taking its toll on society. However, scientists have been slow to argue for any connection between global climate change and the increased number of storms. One reason behind this skepticism is that we may be naming more storms because we have become better at detecting and defining hurricanes and tropical storms. Our records since the advent of satellite monitoring in the 1970s are not as accurate and precise as the records we are taking today, and so it is hard to understand how unique the recent increase in hurricane activity really is. There has also been a mixed bag of results in what climate models suggest about future hurricane activity — one published study says that further warming will increase hurricane frequency, while another argues that hurricane frequency will decrease in changing climatic conditions.

There may still be good questions about how tropical storm frequency is being affected by climate change, but scientific evidence is clear that climate change will increase the intensity of hurricanes in the future. Since the 1970s, there has been a steady increase in the measured “Power Dissipation Index” of hurricanes, which measures the intensity and duration of a storm.  One study showed that hurricanes in the 21st century last 60 percent longer, with wind speeds 15 percent higher than hurricanes thirty years ago. Storms get their energy from the water, so there is likely a connection between higher water temperatures and the strength of the hurricanes that form above. Warming also increases the amount of water vapor in the air, which develops into more concentrated, intense precipitation both on water and on land.

Before Hurricane Sandy arrived in New York City, hundreds of New York City residents took to the streets in Times Square, protesting the recent lack of political discourse about global climate change, which has been dubbed “climate silence.” This silence has been reaching new levels with the recent presidential campaigns, in which neither major candidate has talked seriously about global climate change and the damage it could do.

While extreme weather isn’t a new phenomenon, extreme weather events may increase in scale and scope as global climate change becomes more rampant. The amount of damage it could do and the number of lives it could take will only be apparent several days after the storm dissipates, but the best strategy to ensure the damage done is minimized is to bring our carbon emissions under control and to prepare for a future world with a much different climate. This means rebuilding our cities and infrastructure to be able to withstand extreme weather events, and to do the best we can to plan new developments with an eye on the future.

We are extremely lucky damage has been minimal here at Swarthmore from hurricanes Sandy and Irene, but we cannot afford to sit and wait for another  extreme weather pattern to develop — mitigation as a campus and as a planet must begin today.

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