Are tornadoes getting stronger and more frequent?

Are tornadoes and severe thunderstorms getting more numerous and more extreme due to climate change? To help answer this question, let's restrict our attention to the U.S., which has the highest incidence of tornadoes and severe thunderstorms of any place in the world. At a first glance, it appears that tornado frequency has increased in recent decades (Figure 1).


Figure 1. The number of EF-0 (blue line) and EF-1 and stronger tornadoes (maroon diamonds) reported in the U.S. since 1950. There is not a decades-long increasing trend in the numbers of tornadoes stronger than EF-0, implying that climate change, as yet, is not having a noticeable impact on U.S. tornadoes. However, statistics of tornado frequency and intensity are highly uncertain. Major changes in the rating process occurred in the mid-1970s (when all tornadoes occurring prior to about 1975 were retrospectively rated), and again in 2001, when scientists began rating tornadoes lower because of engineering concerns and unintended consequences of National Weather Service policy changes. According to Brooks (2013), "Tornadoes in the early part of the official National Weather Service record (1950-approximately 1975) are rated with higher ratings than the 1975 - 2000 period, which, in turn, had higher ratings than 2001 - 2007." Also, beginning in 2007, NOAA switched from the F-scale to the EF-scale for rating tornado damage, causing additional problems with attempting to assess if tornadoes are changing over time. Image credit: Kunkel, Kenneth E., et al., 2013, "Monitoring and Understanding Trends in Extreme Storms: State of Knowledge," Bull. Amer. Meteor. Soc., 94, 499–514, doi: http://dx.doi.org/10.1175/BAMS-D-11-00262.1

However, this increase may be entirely caused by factors unrelated to climate change:

1) Population growth has resulted in more tornadoes being reported.

2) Advances in weather radar, particularly the deployment of about 100 Doppler radars across the U.S. in the mid-1990s, has resulted in a much higher tornado detection rate.

3) Tornado damage surveys have grown more sophisticated over the years. For example, we now commonly classify multiple tornadoes along a damage path that might have been attributed to just one twister in the past.

Given these uncertainties in the tornado data base, it is unknown how the frequency of tornadoes might be changing over time. The "official word" on climate science, the 2007 United Nations IPCC report, stated it thusly: "There is insufficient evidence to determine whether trends exist in small scale phenomena such as tornadoes, hail, lighting, and dust storms." Furthermore, we're not likely to be able to develop methods to improve the situation in the near future.The current Doppler radar system can only detect the presence of a parent rotating thunderstorm that often, but not always, produces a tornado. Until a technology is developed that can reliably detect all tornadoes, there is no hope of determining how tornadoes might be changing in response to a changing climate. According to Doswell (2007): I see no near-term solution to the problem of detecting detailed spatial and temporal trends in the occurrence of tornadoes by using the observed data in its current form or in any form likely to evolve in the near future.

Are strong tornadoes increasing?
Stronger tornadoes (greater than EF-0 on the Enhanced Fujita Scale, or F0 on the pre-2007 Fujita Scale) are more likely to get counted, since they tend to cause significant damage along a long track. Thus, the climatology of these tornadoes may offer a clue as to how climate change may be affecting severe weather. Unfortunately, we cannot measure the wind speeds of a tornado directly, except in very rare cases when researchers happen to be present with sophisticated research equipment. Tornadoes are categorized using the Enhanced Fujita (EF) scale, which is based on damage (note that the EF scale to rate tornadoes was adopted in 2007, but the transition to this new scale still allows valid comparisons of tornadoes rated, for example, EF-5 on the new scale and F-5 on the old scale.) So, if a strong tornado happens to sweep through empty fields and never destroy any structures, it will never be rated as a strong tornado. Thus, if the number of strong tornadoes has actually remained constant over the years, we should expect to see some increase in these twisters over the decades, since more buildings have been erected in the paths of tornadoes. However, if we look at the statistics of U.S. tornadoes stronger than EF-0 or F-0 since 1950, there does not appear to be any increase in their number. Not surprisingly, a study accepted for publication in Environmental Hazards (Simmons et al., 2012) found no increase in tornado damages from 1950 - 2011, after normalizing the data for increases in wealth and property (note, though, that I am suspicious of studies that normalize disaster data, since they are prone to error, as revealed by a 2012 study looking at storm surge heights and damages.)

The future of tornadoes
An alternate technique to study how climate change may be affecting tornadoes is look at how the large-scale environmental conditions favorable for tornado formation have changed through time. Moisture, instability, lift, and wind shear are needed for tornadic thunderstorms to form. The exact mix required varies considerably depending upon the situation, and is not well understood. However, Brooks (2003) attempted to develop a climatology of weather conditions conducive for tornado formation by looking at atmospheric instability (as measured by the Convective Available Potential Energy, or CAPE), and the amount of wind shear between the surface and 6 km altitude. High values of CAPE and surface to 6 km wind shear are conducive to formation of tornadic thunderstorms. The regions they analyzed with high CAPE and high shear for the period 1997-1999 did correspond pretty well with regions where significant (F2 and stronger) tornadoes occurred. The authors plan to extend the climatology back in time to see how climate change may have changed the large-scale conditions conducive for tornado formation. Riemann-Campe et al. (2009) found that globally, CAPE increased significantly between 1958 - 2001. However, little change in CAPE was found over the Central and Eastern U.S. during spring and summer during the most recent period they studied, 1979 - 2001. A preliminary report issued by NOAA’s Climate Attribution Rapid Response Team in July 2011 found no trends in CAPE or wind shear over the lower Mississippi Valley over the past 30 years. However, preliminary work by J. Sander of Munich Re insurance company, presented at the December 2011 American Geophysical Union meeting in San Francisco, found that the number of days with very high CAPE values over the eastern two-thirds of the United States between 1970 and 2009 did increase significantly.

Del Genio et al.(2007) used a climate model with doubled CO2 to show that a warming climate would make the atmosphere more unstable (higher CAPE) and thus prone to more severe weather. However, decreases in wind shear offset this effect, resulting in little change in the amount of severe weather in the Central and Eastern U.S. late this century. The speed of updrafts in thunderstorms over land increased by about 1 m/s in their simulation, though, since upward moving air needed to travel 50-70 mb higher to reach the freezing level. As a result, the most severe thunderstorms got stronger. In the Western U.S., the simulation showed that drying led lead to fewer thunderstorms, but the strongest thunderstorms increased in number by 26%, leading to a 6% increase in the total amount of lighting hitting the ground each year. If these results are correct, we might expect more lightning-caused fires in the Western U.S. late this century, due to enhanced drying and more lightning.

Using a high-resolution regional climate model (25 km grid size) zoomed in on the U.S., Trapp et al. (2007) and Trapp et al. (2009) found that the decrease in 0-6 km wind shear in the late 21st century would more than be made up for by an increase in instability (CAPE). Their model predicted an increase in the number of days with high severe storm potential for almost the entire U.S., by the end of the 21st century. These increases were particularly high for many locations in the Eastern and Southern U.S., including Atlanta, New York City, and Dallas (Figure 3). Cities further north and west such as Chicago saw a smaller increase in the number of severe weather days.


Figure 3. Number of days per year with high severe storm potential historically (blue bars) and as predicted by the climate model (A2 scenario) of Trapp et al. 2007 (red bars).

Summary
We currently do not know how tornadoes and severe thunderstorms may be changing due to changes in the climate, nor is there hope that we will be able to do so in the foreseeable future. At this time, it does not appear that there has been an increase in U.S. tornadoes stronger than EF-0 in recent decades. Preliminary research using climate models suggests that we may see an increase in the number of severe storms capable of producing tornadoes over the U.S. late this century. However, this research is just beginning, and much more study is needed to confirm these findings.

References
Brooks, H.E., 2013, "Severe thunderstorms and climate change," Atmospheric Research, Volume 123, 1 April 2013, Pages 129–138, http://dx.doi.org/10.1016/j.atmosres.2012.04.002.

Brooks, H.E., J.W. Lee, and J.P. Craven, 2003, "The spatial distribution of severe thunderstorm and tornado environments from global reanalysis data", Atmospheric Research Volumes 67-68, July-September 2003, Pages 73-94.

Doswell, C.A., 2007, "Small Sample Size and Data Quality Issues Illustrated Using Tornado Occurrence Data", E-Journal of Severe Storms Meteorology Vol 2, No. 5 (2007).

Del Genio, A.D., M-S Yao, and J. Jonas, 2007, Will moist convection be stronger in a warmer climate?, Geophysical Research Letters, 34, L16703, doi: 10.1029/2007GL030525.

Kunkel, Kenneth E., et al., 2013, "Monitoring and Understanding Trends in Extreme Storms: State of Knowledge," Bull. Amer. Meteor. Soc., 94, 499–514, doi: http://dx.doi.org/10.1175/BAMS-D-11-00262.1

Marsh, P.T., H.E. Brooks, and D.J. Karoly, 2007, Assessment of the severe weather environment in North America simulated by a global climate model, Atmospheric Science Letters, 8, 100-106, doi: 10.1002/asl.159.

Riemann-Campe, K., Fraedrich, K., and F. Lunkeit, 2009, Global climatology of Convective Available Potential Energy (CAPE) and Convective Inhibition (CIN) in ERA-40 reanalysis, Atmospheric Research Volume 93, Issues 1-3, July 2009, Pages 534-545, 4th European Conference on Severe Storms.

Simmons, K.M., Dutter, D., and Pielke, R., 2012, "Normalized Tornado Damage in the United States: 1950-2011," DOI: 10.1080/17477891.2012.738642

Trapp, R.J., N.S. Diffenbaugh, H.E. Brooks, M.E. Baldwin, E.D. Robinson, and J.S. Pal, 2007, Severe thunderstorm environment frequency during the 21st century caused by anthropogenically enhanced global radiative forcing, PNAS 104 no. 50, 19719-19723, Dec. 11, 2007.

Trapp, R. J., Diffenbaugh, N. S., & Gluhovsky, A., 2009, "Transient response of severe thunderstorm forcing to elevated greenhouse gas concentrations," Geophysical Research Letters, 36(1).

Jeff Masters