As I explained earlier this week, questions related to any impact of human-driven global warming on tornadoes, while important, have almost no bearing on the challenge of reducing human vulnerability to these killer storms. The focus on the ground in Oklahoma, of course, will for years to come be on recovery and rebuilding ? hopefully with more attention across the region to developing policies and practices that cut losses the next time. (With this in mind, please read John Schwartz?s great feature, ?Why No Safe Room to Run To? Cost and Plains Culture.?)
The vulnerability is almost entirely the result of fast-paced, cost-cutting development patterns in tornado hot zones, and even if there were a greenhouse-tornado connection, actions that constrain greenhouse-gas emissions, while wise in the long run, would not have a substantial influence on climate patterns for decades because of inertia in the climate system.
Some climate scientists see compelling arguments for accumulating heat and added water vapor fueling the kinds of turbulent storms that spawn tornadoes. But a half century of observations in the United States show no change in tornado frequency and a declining frequency of strong tornadoes. [Update?| The meteorological conditions that shaped events in recent days are?nicely explored by Henry Fountain of The Times.]
Does any of this mean global warming is not a serious problem? No.
It just means assertions that all weird bad weather is, in essence, our fault are not grounded in science and, as a result, end up empowering those whose prime interest appears to to be sustaining the fossil fuel era as long as possible. I was glad to see the green blog Grist acknowledge as much.
On Tuesday, I sent the following query to a range of climate scientists and other researchers focused on extreme weather and climate change:
The climate community did a great service to the country in 2006 in putting out a joint statement [from some leading researchers] on the enormous human vulnerability in coastal zones to hurricanes ? setting aside questions about the role of greenhouse-driven warming in changing hurricane patterns?.
In this 2011 post I proposed that climate/weather/tornado experts do a similar statement for Tornado Alley.
I?d love to see a similar statement now from meteorologists, climatologists and other specialists studying trends in tornado zones. Any takers?
Before you dive in to the resulting discussion, it?s worth reading Andrew Freedman?s helpful Climate Central piece, ?Making Sense of the Moore Tornado in a Climate Context,? and a Daily Beast post by Josh Dzieza. The National Oceanic and Atmospheric Administration has posted a helpful new fact sheet, ?Tornadoes, Climate Variability, and Climate Change.? [Update|?Robert Kunzig of National Geographic has written?an excellent overview of tornadoes and greenhouse warming.]
Read on for the conversation on tornadoes and global warming, with some e-mail shorthand fixed.
First, I?m posting the comments that were focused on policy, then those focused on the details of the science:
Roger Pielke, Jr., professor of environmental studies, the University of Colorado:
People love to debate climate change, but I suspect that the community?s efforts are far better placed focusing attention on warnings and response. That is what will save lives and continue the really excellent job that has been done by NOAA and the National Weather Service. I?d much rather see a community statement highlighting the importance of NOAA/NWS funding!
There will always be fringe voices on all sides of the climate debate. With the basic facts related to tornadoes so widely appreciated (unlike perhaps drought, floods, hurricanes), I think that those who see climate change in every breeze are not particularly problematic or worthy of attention.
Here are some of those basic facts:
1. No long-term increase in tornadoes, especially the strongest ones.
2. A long-term decline in loss of life (the past year saw a record low total for more than a century).
3. No long-term increase in losses, hint of a decrease.
4. To date 2013 has been remarkably inactive.
5. The Moore tornado may have been the strongest one this year, bad luck had it track through a populated area (Bill Hooke brilliantly explained the issue here).
6. That said, climatology shows that Moore sits at the center of a statistical bullseye for tornado strikes for May 20th.
Kerry Emanuel, professor of atmospheric science, Massachusetts Institute of Technology (a signer of the 2006 statement):
I see the political problem with tornadoes as quite different from the hurricane problem we wrote about some years ago. To my knowledge, there are no massive subsidies to build in tornado regions, nor is insurance premium price fixing a big problem. Also, federal flood insurance is largely irrelevant to this problem. About the only thing in common is federal disaster relief, but it is hard to believe that people only build houses in huge swaths of tornado-susceptible territory because they believe they will be bailed out.
As you mention in your blog, the issues here revolve around such practical measures as safe rooms, and the role of government in mandating or subsidizing them. Perhaps one positive outcome of the latest horror story is that safe rooms in public buildings such as schools and hospitals will be mandated, given that they are apparently not all that expensive.
In my view, the data on tornadoes is so poor that it is difficult to say anything at all about observed trends, and the theoretical understanding of the relationship between severe thunderstorms in general (including hail storms) and climate is virtually non-existent. I regard this as a research failure of my profession and expect there will be a great deal more work on this in the near future. What little exists on the subject (e.g. the Trapp et al. paper from a few years ago) suggests that warming will increase the incidence of environments conducive to severe thunderstorms in the U.S. But this counts on climate models to get these factors right, and it may be premature to put much confidence in that.
Daniel Sutter, a professor of economics (focused on tornadoes), Troy University, offered the following thought after citing the Dot Earth comments of Kevin Simmons, his co-author on a recent book on tornadoes and society:
I would just add that the high cost per life saved through safe rooms which Kevin and I find in our research really indicates that tornado safety is about reducing and not eliminating risk. Safe rooms provide essentially absolute protection, but are expensive enough that many would likely judge them too expensive. We need to focus on ways to reasonably reduce risk. For instance, have engineers inspect schools and make sure the safest areas are indeed being used for shelter, or to see if there are relatively inexpensive designs that could strengthen interior hallways some.
I hate to say anything before I know for sure what the final story is from the Plaza Heights school, but the two schools yesterday appear to have provided pretty decent protection, especially since many homes around Briarwood school looked totally destroyed. Wind engineers have developed safe room designs which are great and engineering marvels, but we probably need designs that provide a good measure of safety at a portion of the price.
Also with regard to your previous post about flimsy homes, consider the contrast between how cars and houses are marketed. Cars are sold under brand names, and we have a dual system of federal regulation of designs for safety and auto makers designing cars that are safer than federal regulations require, with certification by the Insurance Institute for Highway Safety. Houses are mainly sold without brand names (I couldn?t tell you who built the house I own here in Alabama) with safety assurances coming through building codes. Many times we see that homes perform poorly in tornadoes or hurricanes, while during a commercial break on the Weather Channel last night there was a car ad touting the model?s crash test rating from the IIHS. If houses are indeed flimsy, there is probably a systematic reason for this.
Thomas Knutson, a research meteorologist at the Geophysical Fluid Dynamics Laboratory of the National Oceanic and Atmospheric Administration (a signer of the 2006 hurricane statement):
While the debate/discussion about possible climate change impacts (or not) on tornadoes is interesting, I thought that the helpful aspect of Kerry?s approach in 2006 was to set aside the climate change debate aspect and instead focus on something we could all agree on.
For example, in the tornado case, if some group of people who otherwise disagree about the climate change issue, could at least agree that it makes sense to have better physical designs of structures, have available shelters in mobile home parks, etc. in an effort to save live, would this be a useful statement to make? Maybe Bill [Hooke], Harold [Brooks] or others would have a good list of recommendations that make sense regardless of whether the tornado climate is stationary or not.
William H. Hooke, associate executive eirector and senior policy fellow, American Meteorological Society:
This has been a terrific discussion and Tom Knutson is getting us to a good spot.
Two points could be made in any such statement:
1. Nature is essentially playing that kids? game of ?Battleship? with us. Population growth and urban sprawl have transformed much of Tornado Alley from a rural area to a target-rich environment, as I described in a post two years ago.
2. It?s not enough to improve warnings. Those getting the warnings need real options for action. Evacuation is risky. Shelter-in-place in tornado shelters (used once in a blue moon if at all) has the obvious downside that unless maintained, they become a haven for snakes, vermin, insects? generally unpleasant destinations. Some owners of tornado shelters have had to lock them up because they?ve become ?attractive nuisances? in the legal jargon, serving as venues for pot parties, etc. Better option is the ?safe room? interior to the house that is used daily or frequently for other purposes.
Here?s the science-focused part of the discussion:
Kevin Trenberth, distinguished senior scientist, National Center for Atmospheric Research:
You do everyone a disservice to discount climate change the way you do. Of course tornadoes are very much a weather phenomenon. They come from certain thunderstorms, usually super-cell thunderstorms that are in a wind shear environment that promotes rotation. That environment is most common in spring across the US: when the storm track is just the right distance from the Gulf and other sources of moisture.
The main climate change connection is via the basic instability of the low level air that creates the convection and thunderstorms in the first place. Warmer and moister conditions are the key for unstable air. The oceans are warmer because of climate change.
The climate change effect is probably only a 5 to 10% effect in terms of the instability and subsequent rainfall, but it translates into up to a 33% effect in terms of damage. (It is highly nonlinear, for 10% it is 1.1 to the power of 3 = 1.33). So there is a chain of events and climate change mainly affects the first link: the basic buoyancy of the air is increased. Whether that translates into a super-cell storm and one with a tornado is largely chance weather.
Martin P. Hoerling, National Oceanic and Atmospheric Administration:
I am unaware, but interested to learn more, about the scientific evidence that supports the statement in Kevin?s e-mail. The context of the sentence is in regard to tornadoes, but perhaps the comment he makes pertains to something else?.it is unclear and confusing without the source reference:
Here is his statement he offers that I?m especially curious about:
?The climate change effect is probably only a 5 to 10% effect in terms of the instability and subsequent rainfall, but it translates into up to a 33% effect in terms of damage. (It is highly nonlinear, for 10% it is 1.1 to the power of 3 = 1.33).?
There are some additional points that Kevin raises which are interesting, but incomplete. He writes:
?The main climate change connection is via the basic instability of the low level air that creates the convection and thunderstorms in the first place. Warmer and moister conditions are the key for unstable air. The oceans are warmer because of climate change. ?
The environment conducive for severe tornadic storms requires many ingredients, not least of which is the vertical shear of the wind in the storms environment, especially in the region near the base of the cloud (Harold can give the specifics). A further potential climate change connection, which Kevin overlooks, is the impact of a warmer world on the strength of the prevailing winds, and their increase in strength with height. Indications from peer review (e.g., Trapp et la. 2007, PNAS) is that changes in shear will act in opposition to changes in stability, making an overall assessment of possible impacts of climate change on severe convection difficult at this time.
It should be obvious that if stability and moisture were the main connections to severe convection, then summer (rather than spring) would be the season of most severe weather over the U.S. The dynamical ingredients associated with wind shear, and the triggers for releasing the latent and sensible instabilities, are critical to the problem.
Trenberth, replying to Hoerling:
With respect to climate change, one has to ask what are the influences on the atmosphere either from changes in atmospheric composition etc. or the places where memory occurs of the accumulated effects: mainly the oceans. ?Yes wind shear, and weather systems etc. are important but they are largely internal to the atmosphere and dependent on the synoptic situation.
Even models that suggest a change in overall mean winds or wind shear at some point in the future may not be particularly relevant with respect to the weather and the synoptic situations. And those models don?t exactly simulate today?s conditions well anyway. We will still have spring. It may come a bit earlier in general (changing the timing of the tornado season) but I have no doubt it will come.
Tornadoes require thunderstorms and wind shear, which occur in spring, not summer. ?Indeed the location of the storm track relative to the Gulf and sources of moisture is pretty critical. If it is too far north then there is no link (last year), or too far south then the wind shear is lost. ?You should recognize the unique situation in the U.S. where more tornadoes occur than anywhere else in the world, and the geography, Rockies, Gulf etc. plus seasons and weather all come into play. ?It is trite to say ?what about summer!?
?There are many papers detailing changes in water vapor and precipitation (although the literature is confusing).? Here is an example that tries to cut through some confusion.
Trenberth, K. E., 2011: Changes in precipitation with climate change.?Climate Research,?47, 123-138, doi:10.3354/cr00953.?[PDF]
I have then presumed that an increase in intensity and a change in wind speed goes up as wind speed cubed with respect to power dissipation and damage potential.
Martin Hoerling, replying to Trenberth:
To the point of Trapp et al., they discuss more than the effect of changes in thermodynamic stability. To be sure, increased thermodynamic instability, increased moisture content in the atmosphere (2 factors that Kevin called out), and increased vertical wind shear within 5 kilometers above the ground create an environment more favorable for a tornado outbreak. In particular, tornadoes are more likely to occur when both low stability (reflected in high values of ?convective available potential energy? or ?CAPE?), and high shear are present.
Secondarily, the presence of an elevated mixed layer (reflected in moderate values of ?convective inhibition? or ?CIN?) can delay the onset of storms, such that when they occur, they do so more explosively and in the form of more long-lived, isolated supercells, which can spawn tornadoes. Trapp et al. suggest that the number of days during which meteorological conditions are favorable for severe storms may increase during latter decades of the 21st century, primarily due to increased instability, though they indicate that the projected decreases in vertical wind shear may oppose thermodynamic destabilization..
Harold Brooks, National Severe Storms Laboratory, Norman, Okla.:
I?m not sure what we?ll be able to come with, but some basic points.
1. Interannual variability is incredibly large. It will be very difficult to detect long-trend trends. In the last 3 years, we?ve set records for the most F1+ tornadoes (back to 1954) in a 12 consecutive month period and for the fewest F1+ tornadoes. I think there?s evidence to suggest that we have seen an increase in the variability of occurrence in the US.
2. Probability of occurrence is mostly driven by wind shear and intensity is almost completely independent of the thermodynamics. The observations are clear on that. As a result, expected changes in occurrence and intensity would be driven by wind shear changes. NOAA is doing some new work on this, but Brian Soden indicated to me that ~2/3 of the CMIP runs showed an increase in CAPE and a decrease in shear over the US.
3. There are more F1+ tornadoes in warm winter months and fewer in warm summer months. Given that there are more in the mean in the summer than winter, overall, if we take the warmest 10 Januarys, 10 Februarys, etc. back to 1954 and count the tornadoes in them and compare it to the coolest 10 Januarys, 10 Februarys, etc., there are ~20% fewer tornadoes in the collection of warm months.
11:41 a.m. | James B. Elsner, an atmospheric scientist at Florida State University studying extreme storms, sent this thought by e-mail:
As with hurricanes, I think frequency needs to be separated from intensity.
Climate change increases the available energy for tornadoes through a warmer and moister atmosphere. Wind shear decreases in the global mean, but this might be irrelevant locally when the jet stream dives southward like it did last weekend across the Plains.
I believe there is evidence that the strongest tornadoes are getting stronger. They are certainly getting longer and wider.
I was out chasing this weekend in the Plains with my graduate students and the interesting thing was the lack of a widespread outbreak in favor of a few mighty ones. Shear was amazing.
Source: http://dotearth.blogs.nytimes.com/2013/05/22/seeking-clarity-on-terrible-tornadoes-in-a-changing-climate/?partner=rss&emc=rss
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