Friday, February 01, 2013

Climate Change and the Adairsville Tornado


I woke up last Wednesday morning to the sound of the Emergency Broadcast System on my radio alarm clock issuing a tornado warning for North Georgia, including the greater Atlanta region.  The skies outside were dark, even for that early hour of the morning, but it didn't really look like tornado weather, or even severe thunderstorm weather.  The warning was called off before 10 am and before the incoming cold front hit Atlanta.

The cold front finally did arrive around 2:00 in the afternoon, and it rained for about an hour as hard as I've ever seen it come down.  The picture above is of Interstate 75 traffic about a half-mile from my home as the rain was letting up - at its peak, visibility was so low you wouldn't have been able to see as far as the highway signs, much less the skyline.

As you've probably heard, the storm included a tornado that reportedly reached 900 yards wide and tore across northwest Georgia for nearly 22 miles.  The tornado reached an estimated peak wind speed of 160 mph when it slammed into Adairsville, causing staggering damage and killing one person.  

The fatality was the first U.S. tornado-related death in 220 days, ending the longest streak of days on record without a tornado-related fatality in the nation.  The streak was due both to the low number of tornadoes that the U.S. saw during 2012, and the fact that the strongest tornadoes that did occur tended to miss heavily populated areas.



It's very unusual, even in Georgia, to see weather like this in January.  Big spring thunderstorms and associated tornadoes usually don't occur until at least the month of March.  Wednesday's tornado was part of a three-day outbreak of severe weather associated with a sprawling storm system that first brought spring-like temperatures and humidity to the Midwest and East Coast, followed by howling winds and cold weather on Wednesday night.  The early appearance of a tornado raises the question of what role, if any, climate change had with the storm.




Tornadoes require a particular combination of ingredients in order to form, and the number of tornadoes is highly dependent on the prevailing weather patterns. As explained by science writer Andrew Freedman at Climate Central, the same weather pattern associated with the record heat and drought of 2012 also stifled tornado activity by keeping a very hot, dry, and stable air mass in place across Tornado Alley.  The drought was most intense in the heart of Tornado Alley - Nebraska, Oklahoma, Missouri, Iowa, and other states where spring and summer twisters are typical. 

While natural climate variability likely played a major role in initiating the drought, climate scientists said global warming may have made the drought worse by making conditions hotter, and therefore drier, than they otherwise might have been. So climate change may have contributed as much to the low number of tornadoes observed in 2012 - and the record 220 days without a tornado-related fatality - as to the generation of Wednesday's intense storm.  

With the Arctic warming up, the temperature differential between the Gulf and Arctic air masses has decreased and will decrease further. That differential is what drives tornadoes and makes North America the tornado capital of the planet, so the good news is that climate change means that tornadoes will likely decrease in number as the climate continues to change. 

The bad news is that with the warming planet, there is much more energy in the climate system and we can expect more powerful storms in general. Tornadoes might become less frequent, but they will tend to be larger and more intense when they do occur.

The warming Arctic may have triggered the storm front that caused the Adairsville tornado.  A sudden warming event that took place high in the stratosphere above the Arctic Circle in late December and early January may be drawing cold air away from the Arctic and into the northern mid-latitudes, while the Arctic enjoys relatively mild temperatures. Although the physics behind sudden stratospheric warming events are too complicated for me to explain (a good explanation can be found here), we can expect colder and possibly stormier weather for four to eight weeks after the event, meaning that after a mild start, the rest of this winter could be the coldest yet for parts of the U.S., along with a heightened chance of snow.

Sudden stratospheric warming events take place in about half of all Northern Hemisphere winters, but they have been occurring with increasing frequency during the past decade, possibly related to the loss of Arctic sea ice due to global warming.  Arctic sea ice declined to its smallest extent on record in September 2012. 




Storms like this are also powerful reminders of impermanence.  As a society, we put such great emphasis on our material belongings, and we define ourselves as much on what we own as on what we do.  But one large tornado, or an earthquake, or a tsunami, or a storm surge, can wipe it all away in a matter of minutes, causing us such great grief and suffering, raising the question of why we put such value on that which we can't hold on to anyway (including our own selves).


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