Erin A. Thead | May 16, 2016
Atmospheric scientists have long suspected that climate change produces an increase in weather extremes of all varieties, but tornadoes are an unusually tricky case. A recent publication from the National Academy of Sciences summarizes the state of the art in the new discipline of event attribution, finding that that, although tornadoes are among the most difficult extreme weather events attribute to anthropogenic climate change, improvements in modeling and climate-weather model coupling have made possible some degree of probabilistic attribution. At present it seems likely that the influence of climate change on tornadoes is indirect, manifested largely by more direct influences on natural climate cycles such as the amplitude of waves in the jet stream that bounds the polar vortex and the El Niño-Southern Oscillation (ENSO), with which severe tornado seasons and their predominant locations have been loosely linked. Researchers are not yet in a position to say for sure what if any role climate change has played in the increases in tornado frequency and severity we have seen over the past 50 years.
However, we need not wait until these issues are sorted out to begin working to protect vulnerable populations. In what follows, I first give some background on the increasingly significant threat posed by tornadoes and then outline some proactive steps governments and other entities can take to keep people safe.
A Disturbing Trend
A disturbing trend has already developed concerning tornado fatalities. After several decades of decline that can largely be credited to a great increase in forecasting skills and warning lead time, the United States fatality rate for tornadoes has leveled off, although there may have been a slight increase in recent years. Figure 1 shows United States tornado fatalities since 1940; Figure 2 shows the trend line for 1985-2015. The figures also show linear (black) and nonlinear (red) trend lines together with R2 values (the higher the number, the better the fit to the underlying data). As the charts indicate, the long-term trend is a decrease, but in more recent years, the statistical trend is for a slight increase, as is made clear by the better fit of the nonlinear curve to the 1940-2015 data. Further statistical analysis of the 1985-2015 data shows that the statistical uptick is caused by the outlier year of 2011. Removing this data point causes the trend lines for 1985-2015 to become flat. (There has also been a “tornado drought” from 2012-2015, with each year being below the climatological average in total annual tornado count.) However, even if 2011 is considered an outlier, it is evident that tornado fatalities in the United States are no longer in a decline.
The NOAA Storm Prediction Center has also tracked the circumstances of tornado fatalities since 2008. What tornado fatality data from 2008 to 2015 makes clear is that fatalities occur in markedly different locations in violent (EF4 and EF5) tornadoes than in less intense tornadoes. This data pool includes the modern-era record year of 2011. Although that year was a statistical outlier, it still provides a large sample to examine the factors that now lead to tornado deaths in the United States, particularly in the Southeast, Midwest, and Atlantic states—areas that do not fall into the traditional designation of “tornado alley.”
Figure 3 shows 2008-2015 fatality circumstance information for EF0-EF3 tornado deaths, taken directly from the Storm Prediction Center. Figure 4 shows this information for EF4 and EF5 tornadoes. The first graph demonstrates what decades of education and awareness have taught us, namely, that mobile homes account for a disproportionately high death rate in weaker (relatively speaking—EF3 tornadoes have up to 165 mph winds) tornadoes compared to their frequency of usage as dwellings. The second graph indicates a very different pattern. Almost any type of residential structure can be leveled by violent tornadoes. A clear majority of deaths in violent tornadoes occur in anchored buildings, whether residential or commercial.
Dangerous Amateur Videography
One interesting point is that the percentage of deaths in vehicles and outdoors is similar between EF0-EF3 and EF4-EF5 tornadoes. (The “unknown” category of fatalities may include even more vehicular deaths.) This is an indication that it is dangerous to be in a vehicle or outside in any tornado, a fact that has been known for years. However, there is a specific activity associated with unprotected locations like these that is becoming more common: dangerous amateur videography. It has only become a societal trend since the era of social media began, so it is not completely clear yet what effect it is having on tornado injuries or fatalities. However, there is ample cause for concern. Thousands of tornado videos are online, and many of them were not taken by experienced storm chasers or security cameras—or by people who were a safe distance away. Tornado videos from Tuscaloosa, AL (April 27, 2011) and Rochelle, IL (April 9, 2015) were shot by people who drove into outer vortices and had car accidents while taking the video. A quick search of YouTube or other video websites will easily uncover many more such dangerously filmed videos, most of them with tens of thousands of views.
Although there are some irresponsible storm chasers, over the past decade there has been awareness in the storm chasing community of irresponsible behavior and group censure of it when it occurs. Respected storm chasers do not condone dangerous video filming, especially by amateurs. This particular trend was generated by social media culture, similarly to the trend of taking “selfies” in various hazardous conditions (which has definitely resulted in fatalities). Responsible storm chasers and weather spotters make on-site reports, raising awareness and providing information about storms. On the other hand, many people who happen to see a tornado now decide at once to get video of it, and the videographers often are unaware of how far away they should be or where to go if the tornado shifts its path. The people filming these videos sometimes say this in the footage, in fact. Amateurs cannot be prevented from shooting videos in dangerous conditions, but newscasters should not air them, as it implicitly encourages the behavior.
Urban and Suburban Storm Shelter Options
This, however, is a comparatively easy way to reduce tornado fatalities. A much more difficult one involves storm shelters for buildings, and it is difficult both because of cost and geographical scope.
One ugly lesson forced on us in 2011 is that, contrary to long-standing cultural myths about tornadoes mostly hitting rural trailer parks and prairie farmsteads, cities can be hit too, and the safety options for urbanites are arguably more limited. Even in the age of high-resolution radar, real-time reports, live coverage, and long lead times for warnings, we now know that an EF5 tornado striking a moderately sized town can result in a triple-digit death toll, as happened in Joplin, MO. It is easy, in retrospect, to understand why a densely packed urban area may be the worst possible place to be. Other than the very center of high-rise office buildings, there is no safe place above ground. High-rises, according to the EF-scale, will not be demolished even in an EF5; the maximum expected damage is “significant structural deformation.” However, directing everyone to the nearest tall office building is completely infeasible, needless to say.
In medium-sized urban areas like Joplin, many buildings in the central business district, such as restaurants and small offices, are not constructed to withstand a tornado. Big-box retailers will contain very heavy stock that is piled high, creating potentially deadly missiles and collapses. Vehicles fill the roads and parking facilities. There is no easy way to get out of danger; traffic congestion will occur if people try to evacuate en masse, possibly putting people in even greater danger than they would have been if they had stayed put. And, of course, cities will generate more debris than any other type of community. The volume of debris in Joplin was unprecedented.
Communal tornado shelters, which some smaller communities do have, would be useful in cities only if people flocked to these sites well in advance, because congestion on the roads could result in mass fatalities. Indeed, this very situation almost unfolded in Oklahoma on May 31, 2013, when a newscaster urged people to “get out”—and they did. Major highways became “parking lots,” and professional meteorologists estimated that the fatality count for the tornado could have been in the hundreds if it had passed directly over these congested roads. Traditional storm cellars are all but nonexistent in cities, and basements are directly beneath the houses, which puts anyone taking shelter therein at risk of exposure to tornadic winds and suction if the house is removed.
Nonetheless, being in a basement is better than being above ground, so their construction should be promoted in cities where they are uncommon in homes. Furthermore, a basement can be made very safe by installing an engineered safe room.
It is rare for cities to be struck by violent tornadoes, but it can happen. The only reason why most cities in tornado-prone areas do not get struck is that they do not occupy much land space. With an increase in urban sprawl, this is changing. When cities are hit, the buildings do not provide friction-based wind resistance that would mitigate violent winds; in fact, wind engineering analyses have shown that a wind-tunnel effect actually occurs, which may increase the wind speed to which residents will be subjected. The best suggestion for urban environments consists of promoting structure designs and retrofits that offer increased resilience to natural phenomena.
Violent tornadoes, those rated EF4 and EF5, will utterly demolish well-built houses, leaving only a pile of debris over a foundation (EF4) or a bare foundation altogether (EF5). This fact readily accounts for the high percentage of fatalities in anchored buildings, as shown in Figure 4.
Unfortunately, a majority of houses in the South and Midwest do not have basements or storm cellars. Storm cellars are generally preferable to basements, unless a basement contains an engineered safe room. There are enough accounts of people who took shelter in open basements and were pulled out (e.g., the Parkersville, IA EF5 tornado of 2008, as well as several of the 27 April 2011 tornadoes) that open basements cannot be equated with storm cellars or basements with safe rooms. Storm cellars where the entrance is not directly above the main room, but is horizontally removed from it by a small underground passage, are even better. Firmly anchored handrails in the main room are also advised, in case the door is torn away. This event has been documented in damage photographs of EF5 tornadoes, including the Hackleburg tornado of 27 April 2011.
Above-ground safe rooms are another shelter option that is less than ideal. These structures are engineered, but they are vulnerable on two counts. First, they must be designed so that they will not be undermined from below and will withstand the brunt of horizontal winds without breaking loose of their anchorings. Considering that EF5 tornadoes can rip masonry walls from foundations even when the walls are anchor-bolted, this is a tall order. Second, the engineering is based on impacts from a flying missile the size of a two-by-four with a speed of 100 mph. EF5 tornadoes have wind speeds upwards of 200 mph and have even been clocked as high as 300 mph. In Jarrell, TX, in 1997, a slow-moving F5 tornado tragically destroyed an entire subdivision, including obliterating one house with stone walls two feet thick.
Most tornadoes are not EF4 or EF5, and it is better, of course, to have an engineered safe room than not, even an above-ground one. However, these shelters are safest when constructed in a basement. Even FEMA documents concerning safe rooms acknowledge that “the likelihood of wind-borne debris entering the basement is lower than for above-ground spaces” and that “repairs to the walls, ceilings, and door of a safe room may be necessary after an extreme-wind event.”
The engineering analysis assumed that 100 mph is a typical projectile velocity in a tornado with wind speeds of 250 mph, but even if that is the case in a real scenario, the size of the tested projectile is not an upper bound on what is possible. In Smithville, MS, the town’s water tower was dented 120 feet above ground by an SUV that became airborne. In EF5 tornadoes in El Reno, OK in 2011 and Moore, OK in 2013, large metal tanks for oil and water storage were hurled over a mile from their original sites. Video exists of a Canadian F5 tornado in which a whole house is airborne at a great height before it disintegrates. There is, in short, a good reason why the National Weather Service has for decades advised people to go underground in tornadoes.
It’s easy to say that everyone should have an underground shelter. It is much more difficult to make it reality. This must be a matter of personal responsibility rather than a mandate on individuals, which would be difficult to pass given political gridlock. The decision to install a storm shelter probably needs to be rewarded with a tax rebate or credit. Such credits have been offered in the past, usually to specific regions after particularly high-profile and destructive weather events; to encourage their adoption, they should be permanent and universal. Disaster preparedness should be encouraged before any disaster has ever struck, instead of being limited to communities that have already been affected.
These are just a few suggestions about what types of measures might be taken to reduce tornado fatalities and reverse the beginnings of the unwanted trend we are now starting to observe. Undoubtedly others will focus on other possibilities, but one thing is certain: as the climate continues to change, communities will find themselves at greater and greater risk from extreme weather. It is best to make preparations now.
Erin A. Thead is a Graduate Research Fellow at the Climate Institute.
PDF with references available here.