Tornadoes, extreme weather, and climate change

April sets record for tornadoes in any month and in any 24-hour period. But what caused “The Katrina of tornado outbreaks”?

UPDATE:  It is unfortunate that NYT blogger Andy Revkin has chosen to inappropriately shorten and then take out of context the nuanced statements of a number of actual scientific experts, like Trenberth (cited bel0w).  Ironically, Revkin supports a too-little, too-late energy technology development strategy can’t possibly avert catastrophic global warming — nor can it generate funds needed for adaptation.  So it is hypocritical of him to attack others for not constantly saying how much we need to improve housing for those in tornado alley.  Obviously we do, and that’s a great thing for blogs that don’t focus on climate to write about.  Just as obviously we need an aggressive strategy for reducing GHGs that also supports real adaptation.

Revkin’s argument that we shouldn’t talk about the impact of global warming on extreme weather if we don’t propose efforts to reduce the devastation caused by extreme weather today would be like saying we shouldn’t talk about the impact of global warming on the poor unless we propose solutions to poverty today.  It the Bjorn Lomborg two-step.

Stu Ostro, Weather Channel Senior Meteorologist, “The Katrina of tornado outbreaks“:

The atmosphere was explosively unstable with summerlike heat and humidity, interacting with a classic wind shear setup as a strong jet stream and upper-level trough crashed overhead….

The atmosphere is extraordinarily complex, and ultimately what’s happened the past month is probably a combination of influences, including La Nina, other natural variability, and anthropogenic global warming.

UPDATE:  “Persistent, heavy rains have helped swell the Mississippi and Ohio rivers to the highest levels ever recorded,” CNN reports.  And the rivers are still rising.

The Effect of Climate Change on Tornado Frequency and Magnitude:  “There is an obvious increase in tornado frequency between 1950-1999. This could be due to increased detection. Also this could be due to changing climatic conditions.”

For decades, scientists have predicted that if we kept pouring increasing amounts of heat-trapping greenhouse gases into the atmosphere, we would change the climate.   They specifically predicted that that many key aspects of the weather would become more extreme — more extreme heat waves, more intense droughts, and stronger deluges.

As far back as 1995, analysis by NOAA’s National Climatic Data Center (led by Tom Karl) showed that over the course of the 20th century, the United States had suffered a statistically significant increase in a variety of extreme weather events, the very ones you would expect from global warming, such as more “” and more intense “” precipitation. That analysis concluded the chances were only “5 to 10 percent” this increase was due to factors other than global warming, such as “natural climate variability.” And since 1995, the climate has gotten measurably more extreme.

Multiple scientific studies find that indeed the weather has become more extreme, as expected, and that it is extremely likely that humans are a contributing cause (see “Two seminal Nature papers join growing body of evidence that human emissions fuel extreme weather, flooding that harm humans and the environment” and links therein).

Beyond that, as Dr. Kevin Trenberth, head of the Climate Analysis Section of the National Center for Atmospheric Research, explained here last year: “There is a systematic influence on all of these weather events now-a-days because of the fact that there is this extra water vapor lurking around in the atmosphere than there used to be say 30 years ago. It’s about a 4% extra amount, it invigorates the storms, it provides plenty of moisture for these storms.”  He told the NY Times, “It’s not the right question to ask if this storm or that storm is due to global warming, or is it natural variability. Nowadays, there’s always an element of both.”

I have tended to focus on the extreme weather events for which the causal chain is clearest and which will do the most damage to the most people in the foreseeable future.  Dust-Bowlification is probably at the top of that list (see NCAR analysis warns we risk multiple, devastating global droughts even on moderate emissions path).

But tornadoes are among the most visibly and viscerally destructive events — though I do find it interesting how much media coverage these tornadoes have gotten compared to, say, Tennessee’s 1000-year deluge aka Nashville’s ‘Katrina’.   So it was inevitable that scientists would be asked the obvious question of whether the two recent remarkable outbreaks are connected to human-caused climate change — and they were indeed remarkable.

NOAA and the National Weather Service reported yesterday the astonishing statistics:

NWS’s preliminary estimate is that there have been more than 600 tornadoes thus far during the month of April 2011.

  • The previous record number of tornadoes during the month of April was 267 tornadoes set in April 1974.
  • The previous record number of tornadoes during any month was 542 tornadoes set in May 2003.

Meteorologist Dr. Jeff Masters notes that “the period from 8am April 27 – 8am April 28 during last week’s outbreak has a good chance of breaking the record for most tornadoes in a 24-hour period, which is currently 148.”  NOAA’s preliminary report says that there were “a total of 226 tornadoes” during that 24 hours!

Masters also points out “remarkably, two of the top four outbreaks in history occurred within two weeks of each other.”  Indeed, the other two were May 2003 and May 2004, which means the top 4 were in the past 8 years.

[Comparing fatalities over time is not germane for reasons that I would have thought obvious, and Peter Gleick rightly slams Roger Pielke, Jr. for “gross misuse” of that data on Pielke’s website.  My thoughts are with those who suffered through these storms and lost loved ones.  As someone with a family member who lost his home during Katrina, I know it is an unimaginably traumatic event.]

Many scientists have weighed in on the climate-tornado link.  Two of the people who have done more research and publication on extreme weather and climate change than most are Trenberth and Karl, now director of NCDC.  I emailed Karl for his thoughts and here is what he wrote:

Best info we have on the relationship between a warmer world and severe convective storms that can produce tornadoes is in the 2008 Synthesis and Assessment Report of the US Global Change Research Program. Chapter three of that Weather and Climate Extremes Assessment indicates that several studies do show that environmental conditions favorable for convection are more likely with more greenhouse gases, but results are not conclusive.

We now have improved resolution models running at our Oak Ridge Supercomputer thanks to the Stimulus funding.  We may be  able to make more definitive statements (one way or the other)after these get analyzed over the next few years. Meanwhile, we know that La Nina years tend to have a greater chance of severe outbreaks.  So as usual, there are natural factors that have to be considered, and any human made factors would be confounded within these naturally occurring events making our attribution much more difficult.

Joe, what we can say with confidence is that heavy and extreme precipitation events often associated with thunderstorms and convection are increasing and have been linked to human induced changes in atmospheric composition.

You can find that 2008 Report here.  I wrote about it here (see Sorry, deniers & delayers, Even Bush Administration says human emissions are changing the climate).

Trenberth made clear to me a year ago in an extended interview that he was dismayed by the media coverage of extreme weather, especially extreme deluges, that made no mention whatsoever of global warming:

I find it systematically tends to get underplayed and it often gets underplayed by my fellow scientists. Because one of the opening statements, which I’m sure you’ve probably heard is “Well you can’t attribute a single event to climate change.” But there is a systematic influence on all of these weather events now-a-days because of the fact that there is this extra water vapor lurking around in the atmosphere than there used to be say 30 years ago. It’s about a 4% extra amount, it invigorates the storms, it provides plenty of moisture for these storms and it’s unfortunate that the public is not associating these with the fact that this is one manifestation of climate change. And the prospects are that these kinds of things will only get bigger and worse in the future.

I emailed Trenberth to check his quote in ThinkProgress. He stands by the quote with a little to clarification of the context:

It is irresponsible not to mention climate change in stories that presume to say something about why all these storms and tornadoes are happening.

The environment in which all of these storms and the tornadoes are occurring has changed from human influences (global warming). Tornadoes come from thunderstorms in a wind shear environment. This occurs east of the Rockies more than anywhere else in the world. The wind shear is from southerly (SE, S or SW) flow from the Gulf overlaid by westerlies aloft that have come over the Rockies. That wind shear can be converted to rotation. The basic driver of thunderstorms is the instability in the atmosphere: warm moist air at low levels with drier air aloft. With global warming the low level air is warm and moister and there is more energy available to fuel all of these storms and increase the buoyancy of the air so that thunderstorms are strong. There is no clear research on changes in shear related to global warming. On average the low level air is 1 deg F and 4 percent moister than in the 1970s.

Just because attribution is difficult doesn’t mean that the subject of global warming should be avoided entirely when talking about tornadoes.  Equally important, when discussing extreme weather and climate, tornadoes should not be conflated with the other extreme weather events for which the connection is considerably more straightforward and better documented:

TP quotes climate scientist Gavin Schmidt, climate modeller at the NASA Goddard Institute for Space Studies, who said:

It is a truism to say that everything has been affected by climate change so far and therefore this latest outbreak must in some sense have been affected, but attribution is hard and the further down the chain the causality is supposed to go, the harder this is. For heat waves it is easier, for statistics on precipitation intensity it easier – there are multiple levels of good modelling, theory and observations to back it up. But we have much less to go on with tornadoes.

I thought the NY Times story Thursday was pretty good.  There are lots of posts on this.  Michael Tobis pointed out Judith Curry’s post, which I wouldn’t normally link to given her general abandonment of science, but she pointed out the study I cited at the top, The Effect of Climate Change on Tornado Frequency and Magnitude:

research project by Michael Pateman and Drew Vankat found that the frequency of tornadoes had increased between 1950 and 1999″”though better detection likely played a significant role in those statistics. But if there’s strong evidence that climate change and tornadoes are connected, researchers have yet to uncover it….

The researchers themselves found:

There is an obvious increase in tornado frequency between 1950-1999. This could be due to increased detection. Also this could be due to changing climatic conditions. Looking at the raw data we have seen that there are generally less tornadoes in El Nino years compared to La Nina Years. But, since we were unable to get climate data, we were unable to see if the change in the frequency was due to climate factors.

Our data has failed to show a strong correlation in increase in tornado frequency and magnitude during El Nino and La Nina events.

The jury is out.

For more data on the increase in frequency, which is certainly due in large part to better detection, here is NCDC’s State of the Climate:  Tornadoes Annual 2010:

A NYT blogger directs us to this chart:

tornado trends

There is no apparent trend in the strongest tornadoes (F5 is the most destructive).  The NYT blogger quotes Harold Brooks of the National Severe Storms Laboratory:

The primary changes appear to occur ~1975, most likely as a result of the retrospective rating process that assigned ratings to tornadoes prior to the near-real-time ratings that began when the [National Weather Service] adopted the F-scale operationally in the mid-1970s, and ~2000, for reasons that aren’t completely clear, but are likely due to an increased emphasis on examining construction details and policies that changed the nature in how the ratings are created for the strongest tornadoes. Both have lead to a decrease in probability of a tornado being very strong, given that it’s strong. It’s possible that there’s a meteorological component, but the reporting practice changes are large enough that I don’t think we can pull a physical signal out, even if it’s fairly large.

So it may simply be that the data is simply is too confused by the reporting practices for analysis to draw any strong conclusions.  That doesn’t mean the question shouldn’t be asked or that scientists shouldn’t give their best answer.

In general I do think it’s best to avoid statements like “global warming is to blame for” or “global warming caused” or “this is evidence of global warming,” especially in regards tornadoes.

Finally, while tornadoes will continue to grab the headlines wherever they flatten cities and take lives, it is virtually certain that other extreme events — and ultimately the permanently changed climate — will cause the greatest harm attributable to human emissions of greeenhouse gases:

Related Posts:

“¦the climate change that is taking place because of increases in carbon dioxide concentration is largely irreversible for 1,000 years after emissions stop”¦.   Among illustrative irreversible impacts that should be expected if atmospheric carbon dioxide concentrations increase from current levels near 385 parts per million by volume (ppmv) to a peak of 450-600 ppmv over the coming century are irreversible dry-season rainfall reductions in several regions comparable to those of the “dust bowl” era

57 Responses to Tornadoes, extreme weather, and climate change

  1. sault says:

    Yes, you can’t tie individual events to climate change, but the trends in rainfall totals, flooding and droughts are clear. When talking to a denier, the burden of proof should be on them to show how in the world pouring billions of tons of carbon into the atmosphere DOESN’T affect the climate. It’s their uncrontrolled atmospheric experiment and we all have to live with the consequences.

  2. Mark says:

    If the Mississippi jumps channels and even half of the predictions in the third link below come to pass, as horrible as that will be to so many people directly affected, maybe we’ll find a silver lining, if that makes more folks acknowledge the need for immediate aggressive climate action

  3. Apparently TV weathercasters think this type of analysis is politically motivated:

    @spann James Spann 
    To those using our suffering to further a political agenda, please stop. Shame on you. #WeAreAlabama
    1 May via TweetDeck

    [JR: To those using their ignorance to further an anti-science agenda, please stop. Shame on you.]

  4. P.S. Spann’s spew was inexplicably retweeted by Bob Ryan, a former president of the American Meteorological Society, who should know better.

  5. Crank says:

    “There is an obvious increase in tornado frequency between 1950-1999. This could be due to increased detection.”

    Seriously? There used to be the same number of tornadoes in the 50s, it’s just that people didn’t notice?

  6. MapleLeaf says:

    Cross-posted form another thread at CP.

    Re possible trends in severe weather in the US. I promised one by Brooks, but this one is perhaps more relevant–from Van Klooster and Roebber (2009, J. Climate):

    “In this work, the authors present a “perfect prog” approach to estimating the potential for surface-based convective initiation and severity based upon the large-scale variables well resolved by climate model simulations. This approach allows for the development of a stable estimation scheme that can be applied to any climate model simulation, presently and into the future. The scheme is applied for the contiguous United States using the output from the Parallel Climate Model, with the Intergovernmental Panel on Climate Change third assessment A2 (business as usual) as input. For this run, relative to interannual variability, the potential frequency of deep moist convection does not change, but the potential for severe convection is found to increase east of the Rocky Mountains and most notably in the “tornado alley” region of the U.S. Midwest. This increase in severe potential is mostly tied to increases in thermodynamic instability as a result of ongoing warm season surface warming and moistening.”

    So there are at least three recent papers (the above, DelGenio et al. 2007 (GRL), and Trapp et al. (2007)), which suggest that the potential for severe storms will increase over the southern and southeastern portions of the US as the planet warms and atmospheric water vapour increases.

  7. David F. says:


    Good post. The weather is clearly becoming more extreme — this has been the wettest year on record to this point at my location. And it continues a trend that begun around the turn of the century towards greater precipitation. It shouldn’t be too much of a surprise, since this is precisely what climate models have predicted. But, of course, the deniers would just have us bury our heads in the sand and pretend like everything is all hunky-dory. The fact is climate change is almost certainly already causing billions of dollars of damage.

    I realize the link between climate change and tornadoes is more tenuous, but the devastating flooding we are seeing — and have been seeing in recent years across the globe and particularly right here at home — are entirely consistent with a warmer globe.

  8. nyc-tornado-ten says:

    The increase in precip and probably storm intensity is generally attributed to the 4% increase in water vapor in the atmosphere under agw. I suspect that the cooler temps in the upper troposphere and stratosphere, combined with warmer surface temps must also be a factor. The increased temperature difference between the surface and upper atmosphere should give the increasingly humid air more lift, which also increases the amount of precip and power in storms. I believe the upper atmosphere cools about as much as the lower atmosphere warms due to CO2, a 1.5 degree increase in the surface temp should increase the difference with the upper atmosphere by 3 degrees, which is significant.

  9. dhogaza says:


    Seriously? There used to be the same number of tornadoes in the 50s, it’s just that people didn’t notice?

    Yes, many form, touch down, and dissipate without causing damage because much of tornado alley is rural, relatively sparsely populated and farm country. This is one reason why RPJr’s use of fatalities as a proxy for number of tornados is flawed – one tornado cruising through a city (such as Tuscaloosa during this last outbreak) can be deadly. Also, given that about 50% of fatalities come from trailer parks … a tornado that by chance rips through one will kill people while if it were to hit a neighborhood with recent homes built with tornados in mind, there might be no fatalities.

    We monitor them now using doppler radar, which misses some, but not as many as were missed before. Also, population in those parts of the country most prone to tornados has grown – more eyes.

  10. Joan Savage says:

    CNN..”Highest levels ever recorded.”

    Spot-checking USGS real-time gages on tributaries to the Mississippi in Arkansas and southern Missouri, the most recent instantaneous flow numbers for this calendar date are “high” to “flood.”

    Some tributary flows are multiples of previous high flow for this date. These are provisional data, subject to revision. Some daily records have been seriously broken, and the day is not over yet.

    USGS gage 07069500 at Spring River at Imboden, AR has 62 years of record.
    14000 cfs Historic Maximum (1973) for May 2.
    61300 cfs Most Recent Instantaneous Value, May 2, 2011 afternoon.

    USGS 07047942 L’Anguille River near Colt, AR
    6580 cfs Historic Maximum (1981) for May 2.
    10900 cfs Most Recent Instantaneous Value, May 2, 2011 afternoon.

    USGS 07058000 Bryant Creek near Tecumseh, MO has 56 years of record.
    4280 cfs Historic Maximum (1995) for May 2
    13400 cfs Most Recent Instantaneous Value, May 2, 2011 afternoon.

    National Weather Service prediction for the Mississippi River at Vicksburg MS: projected to reach major flood stage around 4 am on Saturday, May 7.

    To track the Mississippi flow near the Old River Control Structure, US Army Corps of Engineers data are available with several COE gages near or on the Old River, as well as the Knox Landing gage in the Mississippi.

    USGS and COE jointly monitor the gage at Knox Landing MS near the Old River Control Structure. It is not (yet) showing exceptional spring flow.

  11. Joan Savage says:

    And the information on tornado formation factors is really helpful.

    Thanks, Joe!!

  12. Crank says:

    thanks for the additional info (although I was being a little facetious earlier). It just didn’t seem all that plausible to me that a significant number of tornadoes would go undetected; I stand corrected on that point.

  13. Dano says:

    Mark @4:49 PM:

    maybe we’ll find a silver lining, if that makes more folks acknowledge the need for immediate aggressive climate action

    It never has in the past when people have said this. Has something changed all of a sudden?



  14. Mark says:

    Today, NOAA radio reports that tomorrow the oceans water will boil off, the oxygen atoms in the water will bind with carbon to form CO2, and the hydrogen atoms will escape to space.

    BUT senior scientists caution that it is impossible to say for certain whether these events will be related to climate change.


    Seriously, at what point do we start saying climate change plays a major roll?

  15. Richard Brenne says:

    While this post is excellent and thorough for the sophisticated Climate Progress readers, I don’t think it would work as well for a general audience because they could just as easily come away saying “There’s no problem.” I think for a general audience Kevin Trenberth’s quotes alone would be best. They are the most clear and to the point, and I think he’s the most knowledgeable, authoritative, forthright and candid about this.

    As Stu Ostro pointed out on the Weather Channel, when the tornadoes hit Alabama the temperature was 91 degrees and the humidity was 70%, both high figures for April 27. That supplied much of the fuel and energy for the storms that included the tornadoes. The likelihood of those figures being that high has increased with global warming, it’s that simple.

    [JR: No, not that simple. Also, I don’t have a general audience so there’s no point in my writing for one BUT I wrote this post this way specifically for the general CP audience, so that the important stuff is up front.]

  16. Richard Brenne says:

    As I said many times on the Weekend Open Thread and other posts beginning with the Master’s post on April 27 (the day the tornadoes hit), the problem is the same as the one hydrologists have with seeing flood increases, which long-time head of hydrology for the USGS Robert M. Hirsch said isn’t happening: They’re looking at mostly 20th Century data and not seeing any or much shift, when the trend shift during the 21st Century will probably be immense with up to 6 degrees C increase and 40% more water vapor, or the equivalent to 15 Lake Superiors of additional water vapor in the atmosphere as well as energy added to the system that is the equivalent to the output from almost 2 million nuclear power plants.

    That will leave a mark. It just hasn’t yet.

  17. Chad says:

    It’s not the right question to ask if this storm or that storm is due to global warming, or is it natural variability. Nowadays, there’s always an element of both”

    I actually hate this quote, Joe, because it seems to conflate “natural variability” of weather and “natural variability” of the climate. The former is the dominant factor in any given weather event. The latter is playing no known role at all. Trenberth’s quote can be taken to imply that “natural variability” of CLIMATE is part of the cause of these storms. Yet we do not know of any significant natural cause of climate change, let alone know that it is a type of change that would cause more freak weather events.

    [JR: I think that’s a stretch. Weather has a lot of natural variability. I think that’s clear.]

  18. Richard Brenne says:

    Regarding the end of #16: Yes, simple. Certainly not the entire connection between global warming and tornadoes as you’ve pointed out better than anyone, but the connection I made and in the sentence I made it.

  19. Joan Savage says:

    Richard Brenne (#16 and #19)

    Please would you do a double check on the source of Stu Ostro’s figures?

    The US Weather Service’s ground measurements at four stations in Alabama on April 26 and 27 showed daily highs in the low to mid 80s, occurring in early to mid afternoon, coinciding with the lowest R.H. values of the day, which were typically around 58% R.H. at that time. The average daily R.H. values were close to 70% but that was over 24-hour intervals.


    I’d be curious to hear where Stu Ostro got his numbers, — were they from satellite? what altitude? and place them in the context of what’s going on at different altitudes in tornado development, such as what precedes development of the wind shear.

  20. Peter M says:

    Something of interest over at Climate Central

    Can Catastrophe Galvanize Action on Global Warming?

    Stavins is an optimist. In the past several years the US has experienced a catastrophic economic collapse and many examples of catastrophic flooding, but no significant progress toward reforming financial regulations or land-use policies to address the causes. If catastrophes can’t spur reform in these areas, why would they induce progress on climate policy?

  21. I seem to recall reading a paper recently that has estimated the bias in the F-scale ratings pre-1975-ish when we started rating tornadoes as they happened. I seem to recall it may have been 0.5-1.0 to high on average on the F-scale during the 1950-1975 period. That certainly has implications for any trends…

  22. Michael says:

    There is a major flaw in saying that tornadoes have increased due to climate change – when one looks at strong tornadoes only, there is a very obvious DECREASE.

    Even more so, one would expect that an increase in population would cause an increase in strong tornadoes. Why? Because tornadoes are rated by damage – a tornado that is EF0 in an open field could be an EF5 if it hit a city. Thus, the decrease is likely even stronger than indicated here.

    It also makes sense that tornadoes would decrease in strength when one looks at the conditions – warm and cold air masses plus a strong jet. Since the Arctic is warmer faster than the tropics, one would expect the contrast to decrease, plus it has been found that the jetstream is weakening. Also, it was only a year ago that we had the slowest start to the tornado season on record (and 2009 had a record low number of deaths), likely due to El Nino, which reduces the temperature contrast over the U.S., whereas La Nina increases it, not a coincidence that 2008 also had major tornado outbreaks – or 1974 for that matter (though I have heard that parts of the Gulf Coast and Florida have an increased threat during El Nino). There have also been some recent news articles talking about the connection between ENSO and tornadoes, and as measured by the SOI, we have been having one of, if not the, strongest La Nina on record (the SOI is probably a better index than sea surface temperatures since it shows the atmospheric circulation, plus SSTs can be affected by global warming).

    Anyway, while some weather events may be easy to connect to global warming, I don’t think tornadoes is one of them (increased convection and storms doesn’t necessarily mean more tornadoes either; think about how many tornadoes occur in the tropics where daily storms occur).

  23. Joan #20,
    Ostro was quoting dewpoints of 70°, not 70% RH. Temperatures near 90° and RH of 70% would be off the charts.
    “The Katrina of tornado outbreaks”

  24. Richard Brenne says:

    Joan Savage (#20) – I was hoping (and asked for) someone with your knowledge and the time and inclination to look up those figures in previous comments, so thank you for doing so.

    All I know is what I heard Ostro say, 91 degrees and 70% humidity in Alabama the day the tornadoes hit. It was a mini-panel with he and another expert and a questioner – I forget who the other two were. As you know it is the nature of such TV conversation to not allow anyone to be thorough, so Stu didn’t say where in Alabama or when exactly. He wasn’t allowed the time to make the connection that I made at #19 and elsewhere – that is my own extrapolation from what he said that I feel is accurate. If you can’t say that there is an increased likelihood of record or near-record temperatures and humidity due to global warming, I don’t know what you can say.

    Since you’re an expert I’m sure Stu would answer an e-mail from you. Try him at the Weather Channel (or comment on his most recent blog post) and if you don’t get through I can give you an e-mail introduction.

  25. Richard Brenne says:

    I’m at if you’d like that introduction, Joan. We were actually part of an e-mail group discussing just such things that included not only Stu but NCAR’s Bob Henson, many of the country’s finest TV and print meteorologists, NOAA scientists and others.

  26. Richard Brenne says:

    Michael (#23) – That’s an impressive comment that could counteract much of what I’ve been saying about this if it’s accurate. I’m no expert, but as a journalist, documentary filmmaker and moderator I just try to quote or allow the best experts to speak about this, as Joe has done with his typically excellent piece. I do have my opinions about who the best experts are on this, and I’d go to Kevin Trenberth and Tom Karl as Joe has.

    So rather than respond myself, I’d love to hear how Kevin and Tom would respond to what you say. Regardless of how that turns out, it is well said.

    Maybe Joe could ask Kevin and Tom and a kind of blog-panel could be formed. Thanks to you, Joan, Kevin, Tom and especially Joe for all their work to date on this.

  27. Richard Brenne says:

    And Michael (#23) – One smaller point is that tornadoes can damage crops, fields and forests as well as human infrastructure, and there was more of that in previous decades (but I don’t know how much detective work was done then relative to now). And with big-ag business replacing family farmers, while there is double the overall population nationally, more farmers are now employees who live in towns, so if an F0 tornado slipped through the Doppler system on the huge industrial farms now dominating much of the Tornado Alley landscape, it could often slip through observation more easily than during the family farm era of the 1950s and other previous decades.

  28. dhogaza says:


    There is a major flaw in saying that tornadoes have increased due to climate change – when one looks at strong tornadoes only, there is a very obvious DECREASE.

    Which, of course, is why total outbreak numbers are of real interest, and we’re off the charts here.

    Also, the supercell sizes associated with the outbreak were of record size.

    It also makes sense that tornadoes would decrease in strength when one looks at the conditions – warm and cold air masses plus a strong jet.

    This makes no sense … it’s exactly the conditions that leads scientists (not blog commenters) to suggest that strong tornados will result …

    ikely due to El Nino, which reduces the temperature contrast over the U.S., whereas La Nina increases it

    But you just suggested above that high contrast between warm and cold combined with a strong jet stream – La Niña conditions -would cause tornados to decrease in strength …

  29. Michael says:


    But you just suggested above that high contrast between warm and cold combined with a strong jet stream – La Niña conditions -would cause tornados to decrease in strength …

    You misunderstood what I said – warming is expected the decrease the contrast between warm and cold air masses, which in turn leads to less favorable conditions for tornadoes; another way to think of this is the observation that thunderstorms are most frequent during the summer but tornadoes are most frequent in the spring.

    Also, the decrease in strong tornadoes should be more significant than the trend in all tornadoes, given the factors that I mentioned, since everything else being equal, a increase in tornadoes should also result in an increase in stronger tornadoes. It’d certainly be much easier to see trends if we could actually measure the winds in tornadoes (aside from a few cases), as is the case with hurricanes, which would also eliminate any possible biases due to changes in building construction.

  30. John Mason says:

    I think Kevin Trenberth is on entirely the right track here. The statement, “You can’t attribute a single event to climate change.” has never sat well with me personally. You cannot attribute a weather-event to climate, full stop. Weather events are the result of interactions between synoptic set-ups and local conditions. Climate, however, ALWAYS influences the weather in general: whilst not CAUSING any weather event to happen, it INFLUENCES trends in its properties. Put more energy into the overall system – as we are doing – and that energy has to go somewhere. The question with respect to tornadic supercell outbreaks is – what is that energy getting up to?

    The critical factor for tornadogenesis is deep-layer windshear: you can have massive convective instability with high CAPE but without the shear you will simply see torrential thundery downpours, albeit influenced by the 7% more moisture/degree C extra warmth factor that we often discuss. The critical question must be along the lines of “can climate change influence the conditions that lead to an increased frequency/intensity of wind-shear?”: this requires things to be influenced so that certain synoptic patterns become more prevalent.

    It’s a question I’ve had to answer in live TV interviews over here, such as after the F2 Bow Street Tornado in 2006. The UK is a bit different with respect to tornadogenesis: although we do get classic summer supercells on occasion, many of the twisters we have recorded over here have happened in conditions of low CAPE/instability but with very high attendant shear – especially at low levels – influenced in some cases by wind-topography interactions. A relatively frequent setup here for tornadogenesis, especially in the winter months, occurs along fast-moving active cold fronts in a highly-sheared environment: in such cases, line convection, often with multiple bowing segments, can develop and persist over great distances. Damage reports from such lines have to be examined with great care as they can produce both microbursts and tornadoes, although with experience these can often be distinguished on the ground by damage-patterns. How climate change is likely to influence these situations is poorly-understood and is thoroughly deserving of further research. Off-the-cuff, if anything, the UK has tended to see more blocked setups in the past few years and there have been relatively few tornado reports compared to a few years ago when a more mobile or zonal pattern often persisted for much longer periods. But it is any trend within the mobile patterns that needs to be discerned, as it is these situations that bring about most UK tornadoes.

    Sorry for the latter ramble – just thought people would be interested to see what we are doing WRT to tornadogenesis over here :)

    Cheers – John

  31. Richard Brenne says:

    John Mason (#32) – I was going to say exactly the same thing word for word except for the Cheers part.


  32. John Mason says:

    I’ve visualised what I mean with the graphic linked-to below – as someone who uses computers a lot I often right-click on things to check them out in more depth than you can get from first appearance alone:

    Cheers – John

  33. Dappledwater says:

    Michael @ 30 – ‘You misunderstood what I said – warming is expected the decrease the contrast between warm and cold air masses, which in turn leads to less favorable conditions for tornadoes’

    The decrease in wind shear, is more than compensated by the increase in available convective energy – according to these guys:

    1. Changes in severe thunderstorm environment frequency during the 21st century caused by anthropogenically enhanced global radiative forcing – Trapp 2007

    2. Transient response of severe thunderstorm forcing to elevated greenhouse gas concentrations – Trapp 2009

    3. Will moist convection be stronger in a warmer climate?

  34. Richard Brenne says:

    CapitalClimate (#24) – Thanks so much for catching my mistake. At 90 degrees F, a dew point at 70 F correlates to 52% relative humidity, which is unusually humid. As you correctly state, at 90 degrees 70% relative humidity would be almost unheard of and fatal to many people with diseases like asthma. And 70% relative humility is common only among our Canadian cousins.

    Your link to Stu’s comprehensive blog post about this is great. Stu did a superlative job with this, not only the science but his humanity as well, which I’ve seen in great evidence ever since I met him.

    The information is so good I’m now quoting Stu Ostro, Senior Meteorologist, The Weather Channel:

    “On the one hand there is no decisive trend in overall tornado occurrences, and while in recent years there’s been a rash of outbreaks which have been unusually far north and intense for the time of year (including the one in Wisconsin last month), the one last Wednesday was geographically consistent with April climatology.

    On the other hand, this event needs to be considered in the *context* of the relentless series of severe thunderstorm and tornado outbreaks which started on April 4 and culminated on the 27th. The number of severe weather reports and confirmed tornadoes has been atypical even by April standards, shattering the previous records. Even taking into account limitations of the historical record, the numbers have been stunning.

    As noted above, the combination of instability and wind shear was extreme even by classic tornado setup standards. The temperature in Laredo reached 111 degrees the day prior to the peak outbreak, the hottest on record at that location for so early in the season. Precipitation extremes have been extreme even by extreme precipitation standards, with April rainfall upwards of 20″ in Arkansas and record levels on some rivers in the central U.S., juxtaposed with an exceptionally large amount of Texas being classified in extreme or exceptional drought.

    And all of this is in the context of a relentless series of extreme weather events in the U.S. and other countries during the months preceding April, and many others worldwide during recent years which I’ve documented and which have had apparent a physical connection with a warmer atmosphere.

    The atmosphere is extraordinarily complex, and ultimately what’s happened the past month is probably a combination of influences, including La Nina, other natural variability, and anthropogenic global warming.” (end quote)

    I’d just like Stu to say more clearly as he has many times in the past that this event, like all other weather, is now happening in the context of global warming as well. (He does in so many words, but I’m looking for fewer.)

    Here’s the link to the entire blog entry again, which, when combined especially with Kevin Trenberth’s quotes in the original post here, give us the best picture of how global warming could be influencing tornadic activity:

    And Joe, would you consider Stu’s blog entry as a guest post? And thanks so much, Joe, for being the go-to guy on getting all the best info from all the other go-to guys and gals. If I get a little too testy from time to time it’s always a lover’s quarrel, or it’s platonic equivalent.

  35. Joan Savage says:


    The US Weather Service ground stations in Alabama were reporting what you call “off the charts” conditions.

    I also want to take this opportunity to correct a mistake I made; the RH “lows” were not coinciding with the temperature highs but preceded the temperature highs by hours.

    Here are some cut and pastes.

    April 27, 2011, the day of the tornadoes, maximum daily high temperatures, and R.H.

    Anniston AL

    MAXIMUM 84 250 PM

    HIGHEST 73
    LOWEST 58
    AVERAGE 66

    Birmingham AL

    MAXIMUM 84 237 PM

    HIGHEST 84 100 AM
    LOWEST 58 1100 AM
    AVERAGE 71

    Montgomery AL

    MAXIMUM 88 1226 PM

    HIGHEST 87 600 AM
    LOWEST 57 1100 AM
    AVERAGE 72

    Tuscaloosa AL

    MAXIMUM 84 138 PM

    HIGHEST 87 700 AM
    LOWEST 59 1000 PM
    AVERAGE 73

  36. Colorado Bob says:

    Cape Girardeau’s rainfall total for April was 20.52 inches, according to Pat Spoden, meteorologist for the National Weather Service in Paducah. While he couldn’t confirm the mark was a record for the month, he said it’s difficult to imagine a wetter month in the city.

    Paducah recorded 15.91 inches of rain in April, a clear record amount, Spoden said.
    Cape Girardeau is Rush Limbaugh’s hometown

  37. Merrelyn Emery says:

    Re Dappledwater #31, has NZ ever had a tornado before? They all seemed shocked over there and so am I, and I’ve been around for a long time.

    I appreciate the detailed scientific argument I see in the comments above but it looks like events are outrunning our science. It’s all happening too fast! Looks like a major phase change.

    What it means, to me at least, is that our science needs to adjust, adapt, itself. The planet has shown itself itself to be an open system, not a collection of parts that can be studied in isolation, which has been what we have been doing through our specialized disciplines based on the world hypothesis of mechanism [Pepper, World Hypotheses, 1942].

    While our conceptualization remains wrong, we are going to get the wrong answers about the timing. The only climate scientists that have come close to conceptualizing the planet as an open system so far are Hansen, Lovelock and Trenberth because they are openly acknowledging that the system does not act linearly, ME

  38. Joan Savage says:

    Richard Brenne (#25 and #26)

    Thank you for your gracious answer! I’d be delighted to converse with experts, but with a caveat or two.

    My M.S. is in environmental science with professional experience in a cell biology lab and environmental education writing, but not specifically on meteorology, no way an expert on meteorology. Science journalist might be a better description; I do my homework and ask ‘outsider’ questions. This gets me in rather useful trouble at times, and I wouldn’t want you to regret an invitation.

  39. Mark says:

    “There were 362 tornadoes during last week’s outbreak, including a record-setting 312 in one 24-hour period. The National Oceanic and Atmospheric Administration said the largest previous number on record in one event occurred from April 3-4, 1974, with 148 tornadoes.”

    “NOAA said 340 people were killed from 8 a.m. Wednesday to 8 a.m. Thursday. It was the deadliest single day for tornadoes since the March 18, 1925, tornado outbreak that had 747 fatalities across seven states.”

  40. Colorado Bob says:

    From Wit’s link –

    Last month was the hottest April on record in England and Wales, Mr Knightley said.

    Rainfall was just 21 per cent of the expected levels, and the average temperature was the hottest since records began 353 years ago.

  41. Joan Savage says:

    Stu Ostro’s own words help a lot here: “The atmosphere was explosively unstable with summerlike heat and humidity, interacting with a classic wind shear setup as a strong jet stream and upper-level trough crashed overhead.”

    The “wind shear setup as a strong jet stream and upper-level trough..” part is worth highlighting for balance with our comments on temperature and humidity.

    In Jon Davies’ 1995 summary of the EHI Energy-Helicity Index
    he says,

    “For supercells to develop, wind factors (such as helicity) and thermodynamic factors (such as CAPE, or convective available potential energy) must combine to produce an environment that is favorable for the formation of rotating thunderstorms. Therefore, one important aspect of forecasting the likelihood of supercells (and by association, tornadoes) is estimating and assessing the combination of helicity and instability.”

    The role of climate change in directing the path of the jet stream is a huge topic, and the jet stream is definitely a component of the ‘other shoe’ in tornado formation.

  42. Dappledwater says:

    ME @ 38 – Re Dappledwater #31, has NZ ever had a tornado before?

    It’s a very rare occurrence here, and they are normally very short-lived and pitiful (thankfully) compared to the tornadoes that batter the US.

  43. catman306 says:

    Multi-dimensional thinking (two or more factors): able to understand that many factors can contribute to an event. Some will use advanced statistics and modeling to estimate the amount and characteristics of each contributing factor. Sometimes previously unknown factors will be discovered. Scientists, advertising and marketing people, and knowledgeable people in every field use these techniques and are usually multi-dimensional thinkers.

    One dimensional thinking looks for diametrically opposite contrasts of a single factor, e.g. left-right, right-wrong, up-down. Political, religious, and legal discussions and news reporting are often painted with a one-dimensional brush.

    No dimensional thinking takes whatever has been stated by an authority as gospel. Addicted TV viewers would be a great example of no dimensional thinkers.

    Knowing your audience’s thinking capabilities will help in any attempt to communicate with them.

    Unfortunately for us, climate and weather always involve many complex factors that can’t really be boiled down into one factor for consumption by non-scientists and those whose education ended without exposure to multi-dimensional concepts.

  44. Lewis C says:

    Catman – the simpler the audience the plainer the message – How about:

    “Disrespecting His Creation, with which we were entrusted with dominion, has terrible consequences here on earth, let alone in the hereafter.”



  45. Leland Palmer says:

    Perhaps the statistical evidence isn’t in yet for a climate change to tornado frequency link, but it does seem to me to make a consistent hypothesis. Certainly, increased water vapor in the atmosphere means increased heat of condensation, driving more convection, leading to stronger thunderstorms and so on.

    So, it’s a consistent enough hypothesis that we should prudently create public policy which reflects that increased risk.

    The increased number of extreme weather events we are seeing around the world in general is also indirect evidence of such a link between climate change and tornadoes.

    If I was to see this sort of consistent hypothesis in the lab, in the analytical chemistry methods I develop, I would change the method to avoid the risk if it was at all possible to do so.

  46. Matt says:

    It would be very helpful if there was a definitive and up-to-date list of the catastrophic weather events of recent years with links to official reports and analysis of the influence of climate change for each one.

    As this article mentions, some very important events such as the torrential rains in the Southeast weren’t covered in much depth by the media.

    I think we all have a tendency to live in the moment and can easily forget the magnitude of the recent winter storms, droughts, floods and storms.

    Climate Progress would be a great place for this database to be developed, considering all of the professional analysis that’s already been collected here.

  47. wili says:

    Matt at #49, good idea about a running log of extreme events with the approximate probability of their occurring without extra forcing of gw.

    Besides the floods, the Texas extreme drought and wildfires seem to have fallen out of the headlines, too.

    These, extreme floods and droughts, are exactly what is expected from gw, and they are devastating much of the country, yet we hear much more about royal weddings and political assassinations.

  48. Colorado Bob says:

    “Most systems change gradually most of the time, and tipping points are the exception,” Scheffer told LiveScience. “But they are a very interesting exception, because they usually imply radical change.”

    Warning signs

    One of the common warning signs of an impending tipping point is when a system takes longer to recover to equilibrium after it is disturbed. Most systems exist in temporarily stable states of equilibrium. If the system is perturbed by some force and pushed in a new direction, it usually moves back toward equilibrium quickly. But if the system is approaching a tipping point, it tends to take longer to recover its balance.

  49. Colorado Bob says:

    Poor John McCain , don’t you know he’s pissed, his face was in that room Sunday.

  50. Jay Banks says:

    It could be quite interesting to see the list of all up – to – date extreme weather events with all available data. We are facing the sudden weather changes that happen not only due to natural variability but also climate changes that had gradually developed for past few decades. Maybe we should focus more on protection of human lives and properties against future occurrences of this type.

  51. No worst blind than the one who does not want to see.

  52. Merrelyn Emery says:

    Thanks Dappledwater, ME

  53. Mike says:

    From the update: “So it is hypocritical of him to attack others for not constantly saying how much we need to improve housing for those in tornado alley. Obviously we do, and that’s a great thing for blogs that don’t focus on climate to write about.”

    I live along tornado alley. One proposal I have suggested to my Congressional representative is a program to help people in trailer homes move into energy efficient apartments or townhouses. These would also be safer during storms (and be a lower fire risk and be harder to break into). Under cap & trade, if that ever happens, utility companies could buy offsets by setting aside funds to finance this. My part of the country has a very high level of older trailer parks. A “cash for clunkers” type program would improve safety, create jobs and reduce GHG emissions.