When it comes to the connection between anthropogenic (human-caused) climate change and extreme weather, the gap gets smaller each year. There are clear links to drastic changes in hurricanes, floods, droughts, heat waves, fires, and winter extremes.
What about severe weather? We don't hear much about bigger and more dangerous tornadoes... that's because the connection between a warming world and severe weather like thunderstorms and tornadoes is much wider than with the others.
To dig into the details on this and find out what scientists do know relating to climate change and severe weather, I spoke with a leading expert in such research.
Harold Brooks was born and raised in St. Louis, Missouri. These days he lives in Norman, Oklahoma working as a senior research scientist for NOAA and the National Severe Storms Laboratory. He specializes in severe weather climatology.
"Well, I think we understand that the climate is warming, and that it's because of human activity. That's a solved question," Brooks explained nonchalantly. "One of the things that we're now trying to figure out, and this is a really hard question, is exactly what does that mean for day-to-day weather?"
That is perhaps one of my favorite things about explaining anthropogenic climate change. This is science. We don't have all the answers yet. You can bet we've had basic answers for decades and we've been expanding on that science and digging deeper each year.
SHIFTING INGREDIENTS
One of the research topics being investigated by researchers in the field is the shifting dry air.
Dry air is a main ingredient in severe weather. Just as warm air and cold air must meet, so must moist air and dry air. Therefore, if the dry air shifts, so too will the severe weather placement. Brooks gives us a history lesson on the finding of this dry air and its importance.
"This actually goes back into the mid-19th century, John Wesley Powell, who was the first European to go down the Colorado River. As he went across the West, he noted what we frequently call now the 100th meridian or the dividing line between relatively moist conditions and drier conditions to the west. And this is, as you go across the state of Kansas and into Colorado, you notice really large changes in what kind of vegetation grows. And that's associated with the frequency of how much rain you get places and how wet things are. That location has moved eastward by a couple hundred miles over the last 50 years. It's again, it's not a real huge change in some ways. The climate models are good at representing the change that we would expect to occur as the planet warms. Essentially, you can think of it as the Desert Southwest heat dome expanding eastward. And that forces that drier further east."
A NEW TORNADO TREND
We don't hear much about a connection between more deadly tornadoes and climate change. Yet, we hear about more deadly and damaging wildfires, heat waves, cold snaps, floods, hurricanes, etc. This is because the connection is not as clear. The factors that go into creating a tornado are not the same as what goes into making the other events more extreme. When it comes to severe weather, it is more intricate. And that means we need far more data and research to determine how a warming world affects severe weather.
Don't get me wrong, we do see changes happening; it's just more complicated than with the other weather events.
"And when we think about tornadoes," Brooks explains, "if you look over long term averages, and we eliminate the F0 tornadoes that have changed a lot in the reporting over the years, and only look at the F1 and stronger tornadoes over the years, the average number of tornadoes per year, if you look on a decadal basis, has stayed about the same over the last 60 to 70 years. It's about 500 a year in the United States."
Brooks continues, "What has changed is that things have become more variable in last 40 years."
When you think about the term "more variable" think about extremes. It means that we are now seeing more extremes more often. That means extreme ups and downs. It isn't as linear as in the past.
Here's a great example from Brooks: "We set a record from April of 2011 through March of 2012 for the most number of tornadoes observed in a 12 month period. And then from the next 12 months we set the record for the fewest number of observed F1 and greater tornadoes immediately after that."
That is an extreme swing. We are seeing this in our weather conditions even from month to month at times. In 2018, Missouri went from the driest April and top ten coldest April to the absolute hottest May the following month. More recently, in February 2021 mid-Missouri went from a very warm December and January to the 9th coldest February (due to an ejection of the polar vortex which caused more death in Texas than hurricane Harvey) and then to the 11th warmest March on record and the 9th wettest.
This increase in variability, or the increase in extremes, has been well connected to anthropogenic climate change, and according to meteorologist and climatologists alike, until we stop emitting fossil fuels it will keep getting more variable.
Back to tornadoes...
We see in the data that days with tornadoes are going down. Yes, we are seeing, on average, fewer tornado days.
However, days with multiple tornadoes are increasing. This means that even though tornado days are decreasing, tornado outbreaks are increasing.
Brooks continues, "But we don't have all of the physical changes figured out for why we are getting more tornadoes on big days. It says there's probably a change in the in the shape of large scale atmospheric waves. But that's really hard to figure out. And the biggest days still don't happen so often that we have really big statistics on."
Some might think we simply have all the data on every tornado that touches down. The truth is, we don't. On tornado days, they can happen over a large surface area. While we catch most of them now, the technology for advanced tornado detection was only introduced 10 years ago. The science community has relied on spotters and civilians to help us find these tornadoes and document them. Plus, most tornadoes don't hit a town or structures and spin through fields. This also makes it hard to research tornadoes because it is difficult to know exactly how strong they are if they don't hit something.
A tornado is generally ranked on the destruction it causes. Did it break a few branches on a tree or did it blow out every window of a hospital? If a tornado doesn't hit anything, it can be difficult to document accurately. Thanks to technological advances, it is getting easier. But the data is still new.
When it comes to the ingredients for tornado development, such as the dry air mentioned above, research shows them expanding over a wider area. Brooks says this may be due to a change in the jet stream due to warmer poles. In order to get a strong jet stream (upper-level winds that steer weather patterns and cause supercells to spin) we need a big difference in temperatures, not just at a local level, but large-scale too.
An example of how our jet stream is weakening is the ejection from the polar vortex. You see, the polar vortex is always over the north pole (there is one over the south pole, too). The jet stream acts as a wall keeping it in. But when temperatures warm in the poles, lessening the cold air, it weakens that wall. At times, the wall may break and a piece of the polar vortex is then allowed further south. We saw this in February 2021.
If the jet stream is weakening more often, this may be a reason overall tornado days are going down. This is because with severe weather you need all the ingredients to come together, or nothing happens. It is more often an all-or-nothing scenario.
Brooks also mentions, "Again, we see more variability occurring in the future. There was a very nice study that Victor Gensini at Northern Illinois University did a few years ago, that showed that, in the last decade of the 21st century, we'd probably see on average an increase of 10 to 15% in the number of severe thunderstorms that are occurring on an annual basis.
"The really interesting part about it, though, was that he counted the number of events in each year. And what he found was that if we looked at the last decade of the 21st century, compared to last decade of the 20th century, both the highest and the lowest years, if we took all those 20 years together, would have been in the last half of the 21st century. So we'd have some incredibly big years and then some years where just nothing at all happened. The 'nothing at all happened' years are interesting in the Plains of the United States, because the reason you get years that have basically no hail is that you have drought and there's just no storms occurring at all. And so it's a challenge for the growing areas in the Midwest where they're likely going to either have lots and lots more hail, or you don't get any precipitation at all, in the growing season. And neither one of those things are particularly good."
So, we have severe weather ingredients covering a wider area. When the jet stream gets its act together this means there is a greater chance for more tornadoes. It's like when you finally bake the perfect cake after a few tries when you used salt instead of sugar or forgot the baking powder.
"Our basic understanding of the environmental change as the planet warms is that the energy will increase. But that shear term, the organizational term, will probably decrease a little bit. And so there's this balance between the two that we have to deal with."
One ingredient we are seeing more of, according to Brooks, is CAP. This ingredient actually inhibits storms from forming.
"One of the things for people in the middle part of the country that's always a big deal in forecasting of severe storms is the presence of what we call the CAP: convective inhibition that will stop storms from forming. And we've recently published a paper that shows that that has increased in the United States over the last 40 to 50 years in the middle part of the country. And that's one of those things that we really didn't have the kinds of models that allowed us to even look at that 20 years ago. We just couldn't resolve the presence of the CAP very well. And so we knew that the energy available would increase, and we had a pretty good idea that the organizational term, the shear term would decrease. But we really didn't know anything about how this inhibition would change until just the last couple of years. And I think that's a really interesting result that we now have to figure out. Well, what does that do to things because on individual days it probably changes the timing of when storms form. And that has, you know, implications we can't even figure out yet."
Moreover, Brooks says, "What we're seeing is that we don't get the ingredients together as often. But when they do, we are getting more of them. And that means mostly that we're getting them over a little bit larger areas. And so the area that's favorable on a day that's favorable for tornadoes has increased a little bit in size. And so that leads to more storms forming on those days and more opportunities for tornadoes to occur."
To be clear, we haven't seen bigger tornadoes.
"One of the things we haven't seen change at all really is the intensity of tornadoes. We don't really see much of a change as we go through time. That would probably take a very long time to see anything happening."
SEVERE THUNDERSTORMS: HAIL & WIND
The reporting data on wind and hail is also over large surface areas and is therefore difficult to determine, making it difficult to research. In fact, Brooks told me the database for wind and hail is even more dire than tornadoes.
As for hail, Brooks says there are a few trends that may be coming into play.
"As the planet is warmed, the height at which the temperature gets to freezing in the atmosphere has also gone up, and so hailstones melt as they fall, and small hailstones will melt a lot, and so we may lose the smallest hailstones. Large hailstones, when we start getting down to two inch in diameter or so, they fall at almost 100 miles an hour. They don't have very much time to melt as they fall. And so they don't tend to see this as much. It's happening probably, it's just not as easy to see."
Brooks also says there is some evidence of getting more of the extremely large hail, but that is also difficult to pinpoint exactly.
Meanwhile, Brooks says there is some evidence that some areas, "particularly in Colorado has been where people started to look at extremely large amounts of small hail, the kind of stuff you have to snowplow to get off the road because it may drift to three feet deep. And that's a little hard to deal with."
As for damaging wind, Brooks says the decrease in shear (discussed above) will likely lead to more damaging wind events.
"We do know that on average wind storms currently occur in environments which have less of that organizational wind shear term, the tornadic and hail storms have, and the suggestion that that organizational term will decrease in the future gives me relatively high confidence that the number of non-tornadic wind events will increase in the future. The climate model runs all suggests that's going to be a big, big problem; a much bigger problem by the end of the century than it is currently."
EXTRA, EXTRA: St. Louis Tornadoes
Brooks is already an expert in U.S. tornado history, but being from St. Louis, Brooks has done ample research on the history of tornadoes in the city. During our hour-long conversation we went down a rabbit hole about this history, as scientists do.
We discussed the top three most expensive tornadoes in the United States, if they occurred today. All three of which have touched Missouri, and two of which went through St. Louis.
"Number three is the 1927 St. Louis tornado. It went through the northern part of St. Louis, actually took down the the flagpole and the right field pavilion at Sportsman Park with three days to go in the baseball season. They actually played the day after the tornado with rubble in front of the stadium and 200 people attended the St. Louis Browns doubleheader. And the St. Louis Cardinals finished one game out of first place, otherwise the World Series would have been scheduled to open four days after the tornado at Sportsman's Park. I don't know what they would have done.
That September tornado is also a really oddball in terms of the timing, because as you know, September 29, that's not a real (big time for severe weather in Missouri), that was at one o'clock in the afternoon. If it wouldn't have rained so much the day before to where the field was wet - they actually called the Browns game off early in the morning, because there was standing water in the field - that tornado would have come through as they were trying to start the game.
Now, it was the St. Louis Browns, the last week of the season in 1927. So you know... they were averaging less than 500 in attendance.
I couldn't believe they actually played the doubleheader with the Cleveland Indians the day after the tornado, with tarps apparently covering the damage in the stands. There were only eight teams in the American League then. This wasn't like this was to determine who's going to the World Series. It was between the teams in like eighth and sixth place. These games meant absolutely nothing. The attendance is listed as 200 and newspaper reports make it sound like that was an optimistic estimate of the actual crowd there.
The number two is the Tri-State tornado. There's lots of small towns that get hit very badly, but it doesn't hit any really large areas. It just accumulates an incredible amount of damage.
Tri-State really represents a sea change in how tornadoes are understood by the public. If you read old newspaper coverage there is almost this, 'this place got hit, this place got hit' there's a - they call it 'the finger of God.' But with Tri-State there's a real notion of path. In the New York Times on the Sunday after the Wednesday tornado there is a opinion piece that talks about, you know, we've got all these telegraph and telephone lines, why aren't we telling people downstream this thing is coming. Murphysboro, Illinois, which was the town that had the most fatalities in it - the first fatalities from this tornado are over an hour and a half before the tornado gets to Murphysboro. The whole notion of the thought that we could communicate information about tornadic threats began.
And it looks like perhaps the hints of spotter networks start to grow out of Tri-State and the St. Louis 1927 tornado. Spotter networks really got going in the St. Louis area. It was the first place they existed in World War II because of ammunition factories. It was considered to be a bad thing to have lightning hit an ammunition dump. They were originally really just looking for storms and trying to communicate that to get protection for ammunition.
The number one most damaging tornado in U.S. history is the St. Louis 1896 tornado. And I don't think there's a whole lot of question of that. For people who are familiar with St. Louis, the tornado went across the south side of the city, went through Tower Grove Park and then Lafayette Park and then crossed the Mississippi River where the Arch is now. If that tornado occurred now it's damage would probably be on the order of $6 billion. It's the only tornado that shows up when you start looking at the fraction of the U.S. economy damaged. It's the only tornado that shows up in the top 25 weather events. It was a really amazing event.
Another amazing thing about it was that it happened five days after what was described as an unprecedented hail event is the history of St. Louis. It damaged almost all of the glass panes at the Missouri Botanic Gardens. And then five days later, the tornado happens and, where the tornado was almost certainly hail fell where the Botanic Gardens was but there was nothing to damage because they had not repaired anything yet."
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