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First, let me express my sympathy for all those caught in the devastating path of tornadoes. To be caught in such devastating power of nature, recourse totally beyond one’s ability for action, is beyond my emotional imagination. I can only begin to imagine the pain for the families of the dead and for those whose lives were otherwise struck by these storms.

Second, we should clearly understand, at this time, it is absolutely impossible to tie any single weather event, any specific ‘abnormal’ temperature, any storm to the effects of Global Warming.

Thus, it is impossible to state that “Global Warming caused the tornadoes” with any confidence … but that is not the end of the conversation.

In fact, we now face an opposite challenge. As per Bill McKibben’s Eaarth, we have changed our planet such that we now in a different world than that in which modern civilization evolved. While Climate Disruption doesn’t cause any weather event, no weather event occurs any longer outside the context of the changed climate.

Just as it is impossible to state that “Global Warming caused the California fires” of 2007, even if Global Warming did fan the flames (see also Forest Chief Warns of Global Warming threat) nor is possible to say that Moscow’s clouded skies last year were due to Climate Disruption nor the Texas drought nor the high temperatures around the globe last year nor …, perhaps the correct statement is that Global Warming did not cause these tornadoes, even if Climate Disruption fostered the atmospheric stew that led to the tornadoes over the last few weeks … and the deadly tornadoes of February 2008.

From Jeff Masters just over two years ago,

Violent tornadoes rampaged across the South last night, killing at least 52 people, injuring hundreds, and destroying thousands of buildings. The death toll from the 2008 Super Tuesday Tornado Outbreak makes it the deadliest tornado outbreak in the past 23 years. The last time tornadoes killed so many people in the U.S. was on May 31, 1985, when 88 people died in a tornado outbreak that hit Ohio, New York, and Pennsylvania. What is really unusual about yesterday’s Super Tuesday Outbreak is that it occurred in early February. Only one other tornado outbreak in the past century killed so many people so early in the year–the great Warren, Arkansas tornado outbreak of January 3, 1949, which killed 60 people.

In 2008, there was a devastatingly serious tornado strike with over 50 dead. But this shows the problem of isolated data. Over 60 years ago, there was an outbreak that killed 60 people. And, that was in a United States with less than half today’s population but also, we might presume, less adequate weather reporting and warning systems. How to rate death statistics?

Thus, is this unusual?

Tornado outbreak fueled by record warm temperatures
Yesterday’s outbreak was fueled by record warmth over the South. Record high temperatures were recorded in Little Rock, Arkansas (75), Shreveport, LA (78), El Dorado, AR (77), Memphis, TN (75), Jackson, MS (81), and Charleston, SC (79), to name a few locations. A strong cold front associated with a powerful winter storm over the north central U.S. pushed into this warm, unstable air mass, triggering Tuesday’s bout of violent weather.

“Record warm temperatures …” Now, that is starting to point more directly to a linkage with Global Warming, at least to make this tornado outbreak ‘consistent’ with the types of changes we might see in the globe due to Global Warming.

As highlighted by Chris Mooney,

when it comes to tornadoes, last year the definitive source of climate information, the U.N.’s Intergovernmental Panel on Climate Change, explicitly stated that there wasn’t enough proof to claim that they had been changing:

There is insufficient evidence to determine whether trends exist in the meridional overturning circulation (MOC) of the global ocean or in small-scale phenomena such as tornadoes, hail, lightning and dust-storms.

As Chris has also stated, a key challenge in this arena is inadequacy of the data to understand, fully, trend patterns. That we can’t prove the linkage is true, but that absolutely does not mean that linkages are even close to being disproved.

If we look to NOAA,

“Tornado season in the United States generally starts in March and continues through the summer months but winter tornadoes have become an almost annual occurrence, according to Harold Brooks of the National Oceanic and Atmospheric Administration.”

Just to be clear, the Super Tuesday tornados were far from the first to hit in 2008. There were the 10 Jan 2008 Mississippi tornadoes. And, the same day, a “rare” (time of year, NE US) tornado touched down in Vancouver, Washington.

Hmmm … Doesn’t “tornado season in the United States generally start in March”? But what about these January and February storms? Does that suggest that something is changing? And, what could be driving those changes? Correlation does not mean causation, necessarily.

When asked about this, an acquaintance with far more education in this domain responded with a basic tutorial and comment.

Necessary conditions for tornadoes:

• A deep layer of mid-atmospheric dry air above a moist surface layer
• Steep moisture and temperature gradients
• High surface temperatures
• Low level convergence and upper level divergence

Vertical windshear (change in wind direction and speed with height)Atmospheric instability (air continues to rise once it starts rising)

Global warming has increased surface temperatures (“surface air temperatures over land have risen at about double the ocean rate after 1979 (more than 0.27°C per decade vs. 0.13°C per decade), with the greatest warming during winter (December to February) and spring (March to May) in the Northern Hemisphere”), increased atmospheric moisture content (“increases in temperature lead to increases in the moisture-holding capacity of the atmosphere at a rate of about 7% per °C”), wind shear (“Mid-latitude westerly winds have generally increased in both hemispheres”).

Certainty, no, but something suspicious? In another piece of correspondence, he added information and reflected on the implications:

We certainly don’t know precisely how they are being shaped but we are starting to have a clearer picture of what the regional climate regimes will be in a significantly warmer world.

He concluded with these points.

• This is a climate disaster;
• Freakishly warm weather was a precondition for this climate disaster;
• It doesn’t help that much to be in a rich country when a climate disaster hits your town;
• The Bush Administration decimated emergency response mechanisms and planning and research [which the Republican House plans on defunding

• Freakishly warm weather is guaranteed to be on the rise due to global warming.

This logic seems clear to me. You?

In any event, let us return to the question of the tornadoes.

Reality is that Global Warming is changing the atmosphere, the oceans, having an impact on the land (although less, at this time, than humanity’s direct actions), and is changing the environment in which weather and weather events occur. That Global Warming will have some form of influence on tornadoes seems clear, what that influence might be seems far less clear in the science.

Thus, we absolutely cannot say, with any justifiable basis that I am aware of, that Global Warming caused the devastating 2008 Super Tuesday tornado outbreak, but we can, it seems, question whether and to what extent Global Warming/climate change fostered the atmospheric stew that resulted in the outbreak.

THUS … above is a repost of a 2008 discussion of tornadoes. As FishOutOfWater discussed in the (excellent) eSci: Fires, Tornadoes & Drought, Climate Collapse Slams the Southern States+*, “this year likely has the highest number of tornadoes ever reported by April 20” while the Southern United States is facing a situation of “historic proportions” in which the conditions in North Carolina and its devastating tornadoes occurred amid humanity-driven Climate Disruption even if we can’t say that climate change caused any specific weather situation.

Re the recent tornadoes, in addition to FOOW’s excellent piece, see:

WeatherDude, “Town is Gone … Pavement Scoured Out of Ground” … Tornado Outbreak Liveblog Updated x11+, 25 Apr 2011

SmoothNMellow, Breaking: Tornado Damages Lambert St. Louis International Airport (updated 5x), 22 Apr 2011

Got a Grip, A Tornado is Coming at YOU! What Should You Do?, 20 Apr 2011

And, as a final note, Kevin Trenberth, head of the Climate Analysis Section at the National Center for Atmospheric Research, on the question:

I find it [Climate Change] … often gets underplayed by my fellow scientists. Because one of the opening statements, which I’m sure you’ve probably heard, is “Well you can’t attribute a single event to climate change.” But there is a systematic influence on all of these weather events now-a-days because of the fact that there is this extra water vapor lurking around in the atmosphere than there used to be say 30 years ago. It’s about a 4% extra amount, it invigorates the storms, it provides plenty of moisture for these storms and it’s unfortunate that the public is not associating these with the fact that this is one manifestation of climate change. And the prospects are that these kinds of things will only get bigger and worse in the future.