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The 2012 tornado season got off to a rousing start. Between February 28th and March 3rd, two deadly storm systems developed in the southern United States. The storms spawned numerous tornadoes that together killed at least 52 people. This kind of extreme tornado activity, so early in the year, has fueled fears that global warming will increase the severity and duration of the tornado season. But, scientific studies show that this is not necessarily to be expected.
Early tornadoes are not unheard of. For example, on February 29 in 1952, two tornadoes caused severe damage in the south-eastern US. But this year, the number of early tornadoes has been much higher. The National Oceanic and Atmospheric Administration reported that in January of 2012, the tornado total was 95, much higher than the 1991–2010 average of 35. And the five-day total for February 28 to March 3 could rank as the highest ever since record-keeping began in 1950, according to meteorologist Dr. Jeff Masters, co-founder of the Weather Underground. With such a record-breaking start, it is not surprising people worry that a more severe 2012 storm season is ahead, and that global warming is to blame.
Tornadoes form when warm and moist air from the Gulf of Mexico meets with very cold and dry air above, which was brought south from the arctic. The collision of these air masses, which have different densities, as well as speeds and directions of motion, forces them to want to switch places very rapidly. This creates updrafts of warm and wet air, which produce thunderstorms. And, as the updrafts climb through the atmosphere, they encounter fast- moving jet stream winds, which change speed and direction with altitude. These changes give the updraft a strong twisting motion that spawns tornadoes.
The severity of tornadoes is rated on the Fujita Scale, which examines how much damage is left after a tornado has passed: F0-F1 tornadoes produce minor damage and so are considered weak, F2-F3 tornadoes produce significant damage and are considered strong, and F4-F5 tornadoes produce severe damage and are considered violent. The problem with this ranking is that it is related to a human-based assessment of damage; you need something (buildings, vegetation, etc.) to be destroyed and someone to see the damage. So, a severe tornado that occurs somewhere where there is nothing to be destroyed would be classed as weak, and one that occurs where there is no-one to see the damage wouldn’t even be counted.
Still, tornado awareness and volunteer reporting programs, along with good record-keeping, have significantly improved our understanding of tornadoes and their frequency. Surprisingly, the Storm Prediction Center’s tornado database, which goes back to 1950, does not show an increasing trend in recent tornadoes. This finding is confirmed by Dr. Stanley Changnon from the University of Illinois at Urbana-Champaign, whose study of insurance industry records was published last year. Dr. Changnon’s work shows that tornado catastrophes and their losses peaked in the years between 1966 and 1973, but have shown no upward trend since that time. In fact, the number of the most damaging storms, those rated as F2 to F5 has actually decreased over the past 5 decades. So, it does not appear that global warming is increasing the number of tornadoes that occur.
This is actually not as surprising as it seems. While a local increase in temperature and humidity, whether caused by global warming or not, would be expected to create more thunderstorms, it is not clear that these thunderstorms would spawn tornadoes. The reason is that global warming does not increase temperatures the same everywhere. Warming at the poles is expected to exceed warming at more southern latitudes. This means that cold polar air will be much less colder than before and warm Gulf of Mexico air will only be slightly warmer. When these two air masses meet above the southern US, the temperature difference between them will not be so great and their drive to swap places will be much less intense. The result will be a significantly slower moving updraft of warm air that is not expected to produce as many extreme thunderstorms or spawn as many tornadoes.
So, global warming is not expected to increase the total frequency of tornado activity. However, warming global temperatures will mean an earlier spring and the potential for earlier tornadoes. In fact, the early tornado numbers we’ve seen so far this year may be a sign of a global warming-induced shift in the tornado season, according to Dr. Masters. If this is the case, the tornado season may start earlier, but it will also end earlier. As meteorologist Harold Brooks from the National Severe Storms Laboratory in Norman, Oklahoma, points out, this record start to the 2012 tornado season does not necessarily mean the rest of the season will be severe.
Recap of deadly U.S. tornado outbreak February 28-March 3, 2012, M. Daniel, EarthSky Mar 5, 2012.
NASA Earth Observatory, March 5, 2012.
Temporal distribution of weather catastrophes in the USA, S.A. Changnon, Climatic Change 106 (2), 129-140, 2011, doi: 10.1007/s10584-010-9927-1.
Does Global Warming Influence Tornado Activity? Diffenbaugh et al., EOS 89 (53), 553-554, 2008.