Are there more tornadoes because of global warming?

There are good reasons to believe that global warming leads to more storminess, but the exact nature of that transition is unclear and hard to measure. Part of the reason for this difficulty is that a given type of storm may become more likely under certain conditions caused by climate change, while a different kind of storm may become less likely, with the "storminess" overall increasing but doing so indifferent ways across time. Also, the most severe, and thus possibly the most important, weather events are infrequent so it is difficult to see changes over time with any statistical confidence. I address many of these issues here and here.

Looking at the raw data, it is clear that there are "more tornadoes" over time in the US. Have a look at this graph:

Annual number of tornadoes for the period 1916-1995; the dashed line connecting solid circles shows the raw data, the red heavy solid line is the result of smoothing. Also shown in the green light solid line is the number of tornado days (i.e., days with one or more tornadoes) per year. Annual number of tornadoes for the period 1916-1995; the dashed line connecting solid circles shows the raw data, the red heavy solid line is the result of smoothing. Also shown in the green light solid line is the number of tornado days (i.e., days with one or more tornadoes) per year.

At first glance, his graph makes it look like there are a lot more tornadoes, but there is a strong effect of observer error; earlier tornadoes were simply missed much of the time, so the big increase you see here, while it may reflect an underlying increase in number of tornadoes, is not reliable and cant' be taken as evidence. However the later years shown here, from 1950-something to the 1990s, seems to show an increase that could be taken as meaningfull

However, when people speak of tornadoes they often show this graph as evidence that there are not more of them over time:

Looks like the number of tornadoes does not go up over time. Looks like the number of tornadoes does not go up over time.

Looking only at this graph it looks like the number of tornadoes per year in the US is pretty variable but not increasing, as one would expect if global warming was causing more of them.

There is a problem with this graph, however. Actually, a couple of problems (other than those pointed out here). The main problem is that the most frequent tornadoes are left off this graph. If we look at F0 grade tornadoes, not included here, we see that they have actually increased in frequency over time. If we include ALL tornadoes, and not just the kinds that don't seem to increase in frequency over time, we get this graph:

Huh.  Maybe the number of tornadoes DOES increase over time! Huh. Maybe the number of tornadoes DOES increase over time!

Compare the scales of the last two graphs. It turns out that the number of tornadoes at the smaller end of the scale goes up quite a bit. It might be hard to see. The upper graph goes up to 900, the lower graph goes up to 1900. So, if we add all the data instead of just select data, we get many hundreds more tornadoes per year.

The proportion of tornadoes that are F0 increases over time as shown here:


... and the overall distribution of tornadoes by strength changes over time as shown in this very cool graph:

It isn't just the F0 tornadoes changing over time.  The overall pattern of tornadoes shifts with time. It isn't just the F0 tornadoes changing over time. The overall pattern of tornadoes shifts with time.

As I point out here, one of the contributing factors to variation over time in tornado frequency is the fact that we have somewhat arbitrary boundaries in which we measure them. For instance, the US-Canada border provides an arbitrary line across our data set. By not counting all North American tornadoes the same way, we may be adding unnecessary variability to the data. To demonstrate this, have a look at this graph showing tornado frequency per year in France and Germany, two countries that are right next to each other:

Frequency of tornadoes in France and Germany ... seems to be uncorrelated. Frequency of tornadoes in France and Germany ... seems to be uncorrelated.

This shows a few things. For one thing, they don't have too many tornadoes in that part of the world. For another thing, there is an increase in overall frequency over time, and this is not because of lack of reporting. The reporting problem in the US is partly because the western and central states were relatively empty in the old days, and also more technology was available for spotting tornadoes later. But the European and US data have the same shape over a similar time span, but France and Germany do not have the missing observations owing to vast unoccupied (sort of) territories.

But the main thing I want to demonstrate with this graph is the fact that dividing a largish area of land up into arbitrary units can cause your data go go all flooey. Increased variability in data owing to partitioning is a well known phenomenon and this is what it looks like.

Another part of the problem is that the largest storms, which may be the most important ones, have a great deal of variation in their occurrence. Compare any of the graphs above of all tornadoes or all excluding the F0 tornadoes of this graph of just the largest storms:

Pay attention to the vertical scale, but note that there is a lot of variation over time in these large events.  This kind of data almost has too much variability to track change over time meaningfully Pay attention to the vertical scale, but note that there is a lot of variation over time in these large events. This kind of data almost has too much variability to track change over time meaningfully

Not only is there a lot of variation in numbers of tornadoes at the larger end of the scale, but I suspect there is a lot of variability among the tornadoes in each class in terms of overall energy represented. An F4 tornado that lasts five minutes compared to an F4 tornado that lasts 20 minutes are hugely different, but this is not reflected in this sort of data.

Here is a graph showing the amount of storm damagein adjusted dollars over time in the US (pink) with average temperature (blue). Clearly, the total amount of damage goes up, and probably for a number of reasons including there being more stuff to damage, but also, likely overall increases in storminess including hurricanes, tornadoes, severe thunderstorms, etc.

More storm damage over time More storm damage over time

Here is another graph that shows something similar:

Increasing bad stuff over time. Increasing bad stuff over time.

There are many who do not want to link increases in severe weather to global warming. They are probably wrong. Global warming seems to increase severe weather overall. The best way to deny this is to cherry pick the data by ignoring variability across space, leaving out entire categories of storms, or focusing on just some kinds of storms. I suspect the size and severity of tornadoes at the larger end is increasing now, but did not start increasing until recently; time will tell if this is right. But overall tornadoes are so variable across time and space that they are not a reliable canary, as it were. But overall storminess seems to be on the increase, in accordance with expectations from the basis physics of climate, under warming conditions.

Photo Credit: Vvillamon via Compfight cc

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Here are the stats (not including 2013 tornadoes) for all recorded Fujita 5 tornadoes (much less likely to be missed at any time) since the 1950s from the Tornado History Project:
1950s: 12 category 5 tornadoes (464 deaths)
1960s: 11 category 5 tornadoes (130 deaths)
1970s: 14 category 5 tornadoes (245 deaths)
1980s: 3 category 5 tornadoes (27 deaths)
1990s: 16 category 5 tornadoes (147 deaths)
2000s: 2 category 5 tornadoes (20 deaths)
2010s: 6 category 5 tornadoes (290 deaths)
During May in this 63-year time frame:
- In the first 30 years, there were 37 cat 5 tornadoes; in the next 33 years there were 21.
- In the first 30 years, there were 839 deaths; in next 33 years there were 484 deaths

By David Stern (not verified) on 21 May 2013 #permalink

David, I'm not sure what your point is other than to repeat one part of what I said and showed in the post. Or are you just trying to underline the part that supports a particular conclusion, excluding or avoiding other conclusions. Which is what you seem to have done.

The reason we use "selected" data is because we know that the F0 tornadoes are more susceptible to non-meteorological factors in reporting than stronger tornadoes. If you can come up with a plausible physical mechanism why short-lived, non-damaging tornadoes should increase by an order of magnitude in 60 years while there's no change in F1+, I'd be very interested in that process. At the high end, we know that the pre-1975 tornadoes, which were given ratings in the late 1970s, are rated stronger than tornadoes in the 1975-1999 period were. If you believe the F0 numbers represent something physically real, then I don't see how you can believe the decrease in reports of F4 and F5 tornadoes isn't physically real. Having spent a quarter of a century working with the tornado database, I don't believe either of those changes are physically real.

By Harold Brooks (not verified) on 21 May 2013 #permalink

Where did I say the f4 and f5 numbers are not real? I advise against putting words in my mouth in order to disagree with them. Also, thes is not about competing beliefs or competing incredulities; that s not a framework for analytical discussion. Finally, read the companion post.

Fine, in your other post you said there are more "bad tornadoes." If you believe the reported record to be an accurate reflection of physical reality, then you are saying that the number of violent tornadoes (F4+) has dropped dramatically since 40 years ago.

In the 1950s, there were 124 (and the real-time record keeping didn't get going until mid 1953), in the
60s 117
70s 143
80s 70
90s 96
00s 47
10s 44 (through 20 May)

If you restrict it to F5, it's 12, 14, 17, 3, 10, 2, 7 (5 of which were on one day).

The literature strongly supports the idea that there have been large changes in rating practices for strong tornadoes. I don't think there's any physical reason to believe the number of violent tornadoes has dropped dramatically since 40 years ago, but that's what the record suggests, if you think it's an accurate reflection of reality.

F4 and F5s have become much rarer in the reported record than they used to be. I don't think you'll find anyone in the tornado/severe thunderstorm community who thinks that the atmosphere only produced 30% as many violent tornadoes in 2000-2009 as it did in 1970-1979.

I also don't think you'll find anyone who thinks that there are an order of magnitude more weak, short-lived, non-damaging tornadoes now than in the 50s.

You need to read the literature on how rating practices have changed over the years and how NWS verification practices have changed over the year. The tornado database exists primarily to verify warnings, not to create climatologically-accurate information.

By Harold Brooks (not verified) on 22 May 2013 #permalink

It would indeed be a shame if the tornado database was essentially useless. At present, the database does indicate that the tornado situation has gotten worse in recent years, but perhaps that is just because data collection practices have been haphazard or the methods have changed so much as to make the information very low value.

However, we do have to be very careful about two things. First, something I've not directly addressed in any of these posts: assuming that an absence of evidence (caused by the data being useless) can be used to draw ANY conclusion, including that tornadoes have not changed in response to changing climate. The second, which is the point of this and the other posts, is that we should not believe a priori that climate change translates into storm-type-specific changes rather than overall energy for storms in general.

Also, Harold, you seem to be saying that there is a 40 year time frame for change if there is a change. I don't see why and I don't agree. Some effects of greenhouse gas related warming could be seen in the early part of the 20th century, others much later. There is no particular reason to formulate the hypothesis that climate change causes an uptick in tornado activity or severity from -40 onwards but not before or after.

We already know that the time for drought is different on different continents, it seems that the time frame for extensive fire post-dates drought, that the time frame for sea level rise so far is very different from either of those, the time frame for rapid arctic sea ice melt is pretty recent, and if things continue as they are, the time frame for an increased melting of greenland ice sheet is later than that. We may never know if it is really true that the people who record tornadoes are random in their data collection methods, but the time frame for changes in tornado activity would not be expected to link to a particular time period.

Just stick your head further up your butt and make more ridiculous predictions with phony facts. The government and most all honest scientists recognize that tornado activity has decreased significantly; you are so wrapped up in your fraudulent religion and commie redistributionist philosophy you are now legally crazy. May God have mercy on your worthless soul and hopefully soon.

I see what you did there! Hiding the death threat at the very end in the second half of that sentence.

The database is not useless, but it must be interpreted in light of changes in technology. F0 (i.e., tiny) tornadoes are increasing substantially over the past decades. Due to the fact that meteorologists have much more sensitive technologies for detecting tornadoes, one would expect an increase in reporting of F0 tornadoes, even if their actual frequency was unchanged. Thus, an increase in F0 tornadoes is likely due to detection bias; and the fact that large tornadoes have not changed in frequency more likely reflects reality.

By David Stern (not verified) on 23 May 2013 #permalink

First, you take a graph I modified with yellow highlighter and then very badly misinterpret its meaning:

Second, calling it "this graph" you link the very article I wrote that contained the yellow highlighted graphic...
(… )
... when, apparently, seeking to link to a NOAA source graphic of tornado counts from 1950-2012 located here:…

Then, in keeping with your poor analysis of the whole subject, badly misinterpret that graphic to.

Read Dowell III

They clearly explain why the number of small tornadoes (F0-F2) increased after 1952 because of the SELS program.

A graph from that you do not show:…
Clearly shows that the number of violent tornadoes/year before 1974 was much higher than after 1974.
and why the number of large tornadoes (F3-F5) have NOT increased.

azleader, Why does the graph "violent tornado" not say "
F3-F5 " yet you claim that is what it means?

The problem spans in the United States is in part because the Western and Central States were relatively empty in the old days

By william hamel (not verified) on 25 Sep 2013 #permalink

Every article I have read confirms less stronger tornadoes. Somehow you equate smaller lest detectable tornadoes on global warming . You should be embarrassed.

By Marty Thomas (not verified) on 29 Sep 2013 #permalink

Greg Laden is a biological anthropologist. Why is he even commenting on climatology?

By 18t street (not verified) on 17 Jun 2014 #permalink

18th, there are several reasons. First, I am a palaoanthropologist who has studied ancient climates, at on the faculty of a climate research center, and have taught and read on the subject extensively. Perhaps you are unaware that biological anthropologists do this. Second, I am a science writer and although I try to write mainly in areas where I have training science writers write bout all sorts of things. Third, you ask why I can comment on something, but you have not provided credentials for why you can make the comment you just made. This is preposterous!

What a joke of a blog. You are a shill, nothing more.

By Derek Shaffer (not verified) on 11 Jul 2014 #permalink

A shill for who, because don't shills usually get paid? Where is my shill-check!


By ManBearPig (not verified) on 20 May 2015 #permalink