By Marshall Shepherd
Tornadoes have devastated the United States over the past few weeks. Rural and urban areas have not been spared from devastating spring storms. According to Weather.com writer Jonathan Erdman, “The United States is experiencing the most active prolonged period of tornadoes since the April 2011 Super Outbreak.” With reports of tornadic storms dominating news cycles and social media, it was inevitable that questions linking (or denying linkage) to climate change would surface. Dr. Victor Gensini, a professor at Northern Illinois University, is one of the top tornado experts in the world. He provided succinct Twitter thread on the topic worth dissection.
Before delving into the climate change question, some historical context for the current tornado activity is in order. Patrick Marsh is the Warning Coordination Meteorologist at the National Weather Service Storm Prediction Center. At the time Marsh tweeted the information below, there were approximately 500 filtered “eyewitness” tornado reports in the past 30 days. This number has likely increased given the widespread tornadic activity in Kansas and the Northeast U.S. yesterday.
Ok, what about climate change linkages? In 2016, The National Academy of Sciences released one of the most definitive reports to date on a new area of climate science called Attribution. The report, entitled “Attribution of Extreme Events in the Context of Climate Change,” provided a critical analysis of what scientific studies say about this generic question: “Was it caused by climate change?” I always get nervous with that question because as I wrote previously in Forbes, this comes next:
Person X: “This event is clearly caused by climate change…..blah blah blah”
Person Y: “See they say every extreme event is caused by climate change, but the climate changes naturally and there were always extreme events…..blah blah blah”
A better way of framing these discussion is in terms of how likely extreme events are increasing in frequency and intensity. If you review the findings of the 144-page report, it says scientific studies find that certain extreme events are more likely to have a climate change signal than others. The graphic below illustrates that contemporary heat waves, drought, extreme rainfall, and lack of cold events (relatively speaking) are natures versions of “home runs” that likely have “climate change steroids” on top of them based on historical data, model replication, and physical science understanding. It is important to point out that just because an event is low on the scale that doesn’t mean there is no climate change influence. It simply means scientific evidence is not strong enough at this time to draw stronger conclusions. Severe convective storms, including tornadoes, falls into this category.
Dr. Gensini has been studying storms for nearly a decade and has even developed techniques to predict seasonal activity. His Twitter thread did a great job of conveying what we know about tornado-climate change linkages. Gensini starts with:
No, climate change did not cause the recent rash of US tornadoes. Climate change does not cause any given extreme weather event. It does alter background probabilities of the PDF curve.
He points out, using the baseball analogy, that it is hard to attribute any particular home run to steroids because that hitter could likely hit home runs without using a performance enhancing drug. However, it becomes easier to see the influence of steroids in seasonal home run averages, length, and so on. Gensini goes on to say:
Most of the literature has focused on severe convective storms in the aggregate sense due to the lack of a good discriminator among hazards using regional climate models….These studies suggest that severe weather environments and surrogate severe will increase in the spring and become more variable from year to year. This is not (yet) specific to any specific severe convective system.
In other words, smaller scale models feed with global climate model data suggests environments and associated proxies associated with severe weather (atmospheric stability, shear, jet stream patterns) will increase in the spring and exhibit variability from year to year. However, such studies do not predict a specific number of tornadoes or outbreaks as climate warms. Gensini conducted some of these studies as a part of his doctoral studies with Tom Mote at the University of Georgia.
Gensini also captures several other important points about natural variability and anthropogenic forcing:
@hebrooks87 and I have shown some interesting spatial trends in activity that could lead to increasing risk for more vulnerable populations. It is not clear if these trends are natural variability or climate change induced trends. (Link to the study here)
He also mentions work by his colleagues Walker Ashley and Stephen Strader. Their research suggests the expanding urban footprints inevitably means more tornado impacts in highly populated areas. Columbus, Dayton, New York, Jefferson City, Oklahoma City and Kansas City have all dealt with tornadoes in the past week. While such activity should dispel the myth that tornadoes avoid cities, the bigger concern the “expanding” urban “bull’s eyes” all over the country.
Roger Pielke, Sr. injected an important reminder into the Twitterverse about the role of the Southern Oscillation on tornadic activity. Ashton Robinson, now with NOAA’s Storm Prediction Center, published is doctoral research in the Journal of Applied Meteorology and Climatology, on the linkages between El Nino-Southern Oscillation and tornado activity. Robinson found that the La Nina phase tends to be most correlated with increased tornado activity. However, we are currently not in La Nina. Such climate processes affect jet stream patterns in the U.S. (teleconnections). Gensini’s team analyzes sub-seasonal indicators and jet stream patterns to predict tornadic activity weeks in advance. He writes:
Finally, the recent period of intense severe weather with virtually no break was anticipated. Our team began discussing this possibility on April 20th, a month before this pattern emerged.
For more information on his methods for forecasting tornado activity (and it has its critics and supporters alike), I highlighted the technique in a 2016 Forbes piece.
Given the inevitable and often incorrect characterizations of climate change-tornado relationships, I thought this essay would be useful.
Dr. Marshall Shepherd, Dir., Atmospheric Sciences Program/GA Athletic Assoc. Distinguished Professor (Univ of Georgia), Host, Weather Channel’s Popular Podcast, Weather Geeks, 2013 AMS President
This article appeared on the Forbes website at https://www.forbes.com/sites/marshallshepherd/2019/05/29/climate-change-and-tornadoes-a-succinct-guide-to-what-we-know/#3b7c7352709c