“Likely …. the most prolific five-day period of tornado activity on record for so early in the year”
NBC: “It’s as if a huge chunk of the country has suffered a deep, deep scar.”
National Weather Service Warnings for Past Week
The unexpectedly fierce and fast tornado outbreak so early in the season has folks asking again about a possible link to climate change. Climatologist Dr. Kevin Trenberth emailed me that, because of climate change, “there is every expectation that the [tornado] season will move up in time. The warm winter in the US is perhaps an indicator of the nature of the changes to be expected.”
The former head of the Climate Analysis Section of the National Center for Atmospheric Research stands by his 2011 statement, “It is irresponsible not to mention climate change in stories that presume to say something about why all these storms and tornadoes are happening.” Below is some clarification of the context of that quote that he added. Trenberth also said:
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.
After April 2011 saw records set for most tornadoes in a month and in 24 hours, I examined the link in great detail here, looking at the data, the literature, and expert analysis. That piece concluded:
- 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, including deluges, droughts, and heat waves.
- Just because the tornado-warming link is more tenuous doesn’t mean that the subject of global warming should be avoided entirely when talking about tornadoes.
This post will run through the scientific literature along with some analyses from this year and last by leading experts.
First, though, some of the details on this week’s tornado outbreak.
This year’s unusually mild winter has led to ocean temperatures across the Gulf of Mexico that are approximately 1°C above average — among the top ten warmest values on record for this time of year, going back to the 1800s. (Averaged over the month of February, the highest sea surface temperatures on record in the Gulf between 20–30°N, 85–95°W occurred in 2002, when the waters were 1.34°C above average). Friday’s tornado outbreak was fueled, in part, by high instability created by unusually warm, moist air flowing north from the Gulf of Mexico due to the high water temperatures there. This exceptionally warm air set record high temperatures at 28 airports in Louisiana, Arkansas, Mississippi, Alabama, Tennessee, Kentucky, and Georgia the afternoon of the tornado outbreak (March 2.) Cold, dry air from Canada moved over the outbreak region at high altitudes. This created a highly unstable atmosphere — warm, low-density air rising in thunderstorm updrafts was able to accelerate rapidly upwards to the top of the lower atmosphere, since the surrounding air was cooler and denser at high altitudes. These vigorous updrafts needed some twisting motion to get them spinning and create tornadoes. Very strong twisting forces were present Friday over the tornado outbreak area, thanks to upper-level jet stream winds that blew in excess of 115 mph. These winds changed speed and direction sharply with height,imparting a shearing motion on the atmosphere (wind shear), causing the air to spin. High instability and a high wind shear are the two key ingredients for tornado formation.
Here’s more from Masters on the record-setting storms:
And yes we need to improve housing for those in tornado alley. 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.
BACKGROUND ON THE SCIENCE
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, former head of the Climate Analysis Section of the National Center for Atmospheric Research, explained here in 2010: “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.”
Munich Re, one of the world’s leading reinsurers, issued a news release in 2010, “large number of weather extremes as strong indication of climate change,” which noted:
Munich Re’s natural catastrophe database, the most comprehensive of its kind in the world, shows a marked increase in the number of weather-related events. For instance, globally there has been a more than threefold increase in loss-related floods since 1980 and more than double the number of windstorm natural catastrophes, with particularly heavy losses as a result of Atlantic hurricanes.
The rise in natural catastrophe losses is primarily due to socio-economic factors. In many countries, populations are rising, and more and more people moving into exposed areas. At the same time, greater prosperity is leading to higher property values. Nevertheless, it would seem that the only plausible explanation for the rise in weather-related catastrophes is climate change. The view that weather extremes are more frequent and intense due to global warming coincides with the current state of scientific knowledge as set out in the Fourth IPCC Assessment Report.
Here is their data:
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 recent remarkable outbreaks are connected to human-caused climate change — and they were indeed remarkable.
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 me last year:
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 last year to check his quote in ThinkProgress. And I again checked with him yesterday. He stands by the quote with the clarification he had added 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.
There are lots of posts from last year on this. Stu Ostro, Weather Channel Senior Meteorologist, wrote in his May 2011 post, “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.
Here is how meteorologist and former hurricane hunter Dr. Jeff Masters put it May 31, 2011:
In summary, this year’s incredibly violent tornado season is not part of a trend. It is either a fluke, the start of a new trend, or an early warning symptom that the climate is growing unstable and is transitioning to a new, higher energy state with the potential to create unprecedented weather and climate events. All are reasonable explanations, but we don’t have a long enough history of good tornado data to judge which is most likely to be correct.
Here is how Masters puts it today:
Last year’s tornado season was incredibly severe, and we are off to one of the worst early-season starts to tornado season on record now in 2012. However, it is too soon to ring the alarm bells on climate change being responsible for this. The tornado data base going back to 1950 doesn’t show an increasing trend in strong tornadoes in recent decades. While climate change could potentially lead to an increase in tornadoes, by increasing instability, it could also decrease them, by decreasing wind shear. I’d need to see a lot more bad tornado years before blaming climate change for the severe tornado seasons of the past two years. One thing that climate change may be doing, though, is shifting the season earlier in the year. The 5-day total of tornadoes from February 28 — March 3 will probably break the record of 131 set in 1999 for the largest tornado outbreak so early in the year. Warmer winters, and an earlier arrival of spring due to a warming climate, will allow tornado season to start earlier — and end earlier. This year’s early start to tornado season is consistent with what we would expect from a warming climate. I have a more extensive article on this subject that has just been published by Weatherwise magazine….
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:
A 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 directed us to this chart:
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.
The population hasn’t even acclimatized to the climate change we’ve had already — in part because the GOP and the fossil-fuel-funded disinformation campaign have obfuscated efforts to inform the public.
We’ve only warmed about a degree Fahrenheit in the past half-century. If we keep listening to the disinformers, we are on track to warm nearly 10 times that this century (see M.I.T. doubles its 2095 warming projection to 10°F — with 866 ppm and Arctic warming of 20°F). We ain’t seen nothing yet. Or, as one commenter put it:
“Mother nature is only warming up.”
Note: The NBC quote is by Tom Costello from the March 3, 2012 Evening News. Also, h/t Gail at Wit’s End
- Climate Story of the Year: Warming-Driven Drought and Extreme Weather Emerge as Key Threat to Global Food Security
- A New Record: 14 U.S. Billion-Dollar Weather Disasters in 2011
- NOAA: Climate change “largely irreversible for 1000 years,” with permanent Dust Bowls in Southwest and around the globe. This January 2009 PNAS paper finds “¦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
- Study: Global warming is driving increased frequency of extreme wet or dry summer weather in southeast, so droughts and deluges are likely to get worse