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Tornadoes and Climate Change: What Does the Science Say?

Science
Houses damaged by a tornado in Dayton, Ohio on May 28. SETH HERALD / AFP / Getty Images

By Zeke Hausfather

The U.S. has recently experienced one of its worst tornado outbreaks of the past decade, with more than 500 reported over 30 days. The number so far this year is also more than 200 above average.

This has raised the question of what role, if any, climate change may have played in this unusually intensive period of tornadoes. While some have suggested that climate change is driving the above-average numbers, the scientific community has pushed back on these claims.


Scientists have relatively low confidence in detecting a link between tornado activity and climate change. They cannot exclude the possibility of a link; rather, the science is so uncertain that they simply do not know at this point.

What is clear is that there is no observable increase in the number of strong tornadoes in the U.S. over the past few decades. At the same time, tornadoes have become more clustered, with outbreaks of multiple tornadoes becoming more common even as the overall number has remained unchanged. There is also evidence that tornado "power" has been increasing in recent years.

Some research has suggested that climate change will create conditions more favorable to the formation of severe thunderstorms and tornadoes, but such effects are not detectable in observations today.

Any role for climate change in affecting the conditions for tornado formation is still very much an open question and the subject of ongoing research by the scientific community.

Highly Uncertain Attribution

Climate change affects different extreme weather events in different ways. Some, such as increases in extreme heat events, reductions in extreme cold events, and increases in extreme precipitation events are easy to understand and attribute to a changing climate. Others, such as the severe convective storms that produce tornadoes, are much more difficult to unpick.

The figure below shows how well the effects of climate change on different extreme events are understood. It ranks each type of extreme event based on how well the effects of climate change are understood (the x-axis) and on the extent to which any individual event can be attributed to climate change (the y-axis).

Understanding and attribution of climate change impacts on extreme events, by event type.

Figure from the U.S. National Academy of Sciences report on the Attribution of Extreme Weather Events published in 2016.

According to this ranking, severe convective storms that produce tornadoes have both the least well understood link to climate change and the lowest confidence in attributing any individual storm (or tornado) to climate change.

This does not mean that there is definitively no climate link.

Dr. Marshall Shepherd, a professor at the University of Georgia and an author of the NAS report, explains in a recent column in Forbes:

"It is important to point out that just because an event is low on the scale, that doesn't mean there is no climate change influence; it simply means scientific evidence is not strong enough at this time to draw stronger conclusions."

As the NAS report points out, there is a much clearer climate link with extreme rainfall. Extreme rainfall has already increased over much of the central U.S., potentially contributing to ongoing devastating flooding in the region this year.

The 2018 Fourth National Climate Assessment has similar reservations about any links between climate change and tornadoes. It says:

"Observed and projected future increases in certain types of extreme weather, such as heavy rainfall and extreme heat, can be directly linked to a warmer world. Other types of extreme weather, such as tornadoes, hail, and thunderstorms, are also exhibiting changes that may be related to climate change, but scientific understanding is not yet detailed enough to confidently project the direction and magnitude of future change."

Some of the year-to-year variability in tornado numbers is influenced by El Niño and La Niña conditions. A 2017 paper found there are more U.S. tornadoes in La Niña years; however, the current large outbreak is during an El Niño year.

Other types of natural variability can affect tornado occurrence. For example, research has suggested that the "Madden-Julian oscillation," a periodic swing in temperature and moisture starting in the Indian Ocean, can have a large impact on tornado activity in the U.S. Based on this insight, scientists predicted in late April that there would be a high likelihood of tornadoes in late May.

U.S. tornado tracks by Fujito scale severity (F0-F5) from 1950-2016.

Image from usatornadoes.com.

While the overall number of reported tornadoes in the U.S. has doubled since the 1950s, this statistic is highly misleading. Until the 1990s, tornado records were mostly based on someone spotting a tornado and reporting it to the National Weather Service.

As most tornadoes are small and last only a few minutes, the number observed and reported will be considerably smaller than the true number that occurred. The increase in tornadoes over time is largely due to the advent of modern "Doppler" weather radar systems in the 1990s, which can detect weak tornadoes and those in sparsely populated areas that may previously have gone unreported.

If weak tornadoes are excluded, there is no detectable trend in tornadoes over the past century. The figure below, based on an analysis of reports in NOAA's Severe Weather Data Inventory by Carbon Brief, shows the total number of tornadoes in each year, excluding small F0 (or EF0) tornadoes that would likely have been underreported in the past.

Number of notable (F1+ or EF1+) tornadoes per year between 1950 and 2018. Data from NOAA's Severe Weather Data Inventory. Chart by Carbon Brief using Highcharts.

If only the strongest tornadoes are considered (F3-F5 or EF3-EF5), there is even weak evidence of a decline in numbers over the past few decades. However, experts warn against reading too much into an apparent decline in the number of severe tornadoes. They point out that the rating of strong tornadoes has not been consistent and that "early official records systematically rated tornadoes stronger" than those in the past three decades.

More Tornado Clusters

While there is little evidence of an increase in the number of tornadoes, there is evidence that the pattern of tornado occurrence has been changing. A 2014 study in Science found that there has been considerably more clustering of tornadoes in recent decades. In other words, there are more days in which multiple tornadoes occur, but fewer overall days with tornadoes.

The number of days each year with at least one tornado has declined in recent decades, as the chart below shows in black. At the same time, days with more than 30 tornadoes are becoming more frequent (grey).

Number of days with at least one F1+ tornado (black) and over 30 F1+ tornadoes (grey) between 1950 and 2014.

Figure 4 in Brooks et al 2014.

The authors suggest that this trend is robust, but do not have a good explanation as to why it is occurring. They cannot identify any reason why this behavior would be driven by observed climate changes, but at the same time say they cannot exclude climate change as a factor.

Other recent research suggests that overall tornado "power" has increased in recent years, once all other environmental variables are accounted for. A 2018 paper by Dr. James Elsner and colleagues found a clear upward trend in tornado power of 5.5% per year over the past few decades. However, they caution that "a majority of the trend is not attributable to changes in storm environments."

More Common Conditions for Tornadoes?

There is limited evidence that tornadoes have become more frequent in recent years. However, a number of climate modeling studies have suggested that conditions favoring the development of severe thunderstorms — and tornadoes — in the U.S. should become more common in the future.

As the Fourth National Climate Assessment reported:

Modeling studies consistently suggest that the frequency and intensity of severe thunderstorms in the U.S. could increase as climate changes, particularly over the U.S. Midwest and Southern Great Plains during spring. There is some indication that the atmosphere will become more conducive to severe thunderstorm formation and increased intensity, but confidence in the model projections is low. Similarly, there is only low confidence in observations that storms have already become stronger or more frequent. Much of the lack of confidence comes from the difficulty in both monitoring and modeling small-scale and short-lived phenomena.

A 2013 paper by Dr. Noah Diffenbaugh and colleagues examined how the conditions needed for severe thunderstorms and tornadoes to develop are projected to change in climate models.

Climate models are too coarse to model individual tornadoes. However, they show a strong increase in conditions favoring severe thunderstorms over the eastern U.S. during spring and autumn months, particularly once global warming exceeds 2°C above preindustrial levels.

Dr. Jennifer Francis at Woods Hole Research Center in Massachusetts has argued that changes in Arctic sea ice have made ridge patterns in the jet stream more common. In addition, she says that this configuration of the jet stream has played a large role in the current tornado outbreak.

Other researchers have been more skeptical of the role of changing Arctic conditions in current weather patterns and stress that this is still an area of vigorous scientific debate.

While scientists cannot exclude a role for climate change in changes in tornado activity, links between the two are still largely speculative, particularly for individual events such as the recent outbreak in the U.S. As Diffenbaugh recently told The New York Times:

"Tornadoes are the kind of extreme event where we have the least confidence in our ability to attribute the odds or characteristics of individual events to an influence of global warming."

Reposted with permission from our media associate Carbon Brief.

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Brazilians living in The Netherlands organized a demonstration in solidarity with rainforest protectors and against the president of Brazil, Jair Bolsonaro on Sept. 1 in The Hague, Netherlands. Romy Arroyo Fernandez / NurPhoto / Getty Images

By Tara Smith

Fires in the Brazilian Amazon have jumped 84 percent during President Jair Bolsonaro's first year in office and in July 2019 alone, an area of rainforest the size of Manhattan was lost every day. The Amazon fires may seem beyond human control, but they're not beyond human culpability.

Bolsonaro ran for president promising to "integrate the Amazon into the Brazilian economy". Once elected, he slashed the Brazilian environmental protection agency budget by 95 percent and relaxed safeguards for mining projects on indigenous lands. Farmers cited their support for Bolsonaro's approach as they set fires to clear rainforest for cattle grazing.

Bolsonaro's vandalism will be most painful for the indigenous people who call the Amazon home. But destruction of the world's largest rainforest may accelerate climate change and so cause further suffering worldwide. For that reason, Brazil's former environment minister, Marina Silva, called the Amazon fires a crime against humanity.

From a legal perspective, this might be a helpful way of prosecuting environmental destruction. Crimes against humanity are international crimes, like genocide and war crimes, which are considered to harm both the immediate victims and humanity as a whole. As such, all of humankind has an interest in their punishment and deterrence.

Historical Precedent

Crimes against humanity were first classified as an international crime during the Nuremberg trials that followed World War II. Two German Generals, Alfred Jodl and Lothar Rendulic, were charged with war crimes for implementing scorched earth policies in Finland and Norway. No one was charged with crimes against humanity for causing the unprecedented environmental damage that scarred the post-war landscapes though.

Our understanding of the Earth's ecology has matured since then, yet so has our capacity to pollute and destroy. It's now clear that the consequences of environmental destruction don't stop at national borders. All humanity is placed in jeopardy when burning rainforests flood the atmosphere with CO₂ and exacerbate climate change.

Holding someone like Bolsonaro to account for this by charging him with crimes against humanity would be a world first. If successful, it could set a precedent which might stimulate more aggressive legal action against environmental crimes. But do the Amazon fires fit the criteria?

Prosecuting crimes against humanity requires proof of widespread and systematic attacks against a civilian population. If a specific part of the global population is persecuted, this is an affront to the global conscience. In the same way, domestic crimes are an affront to the population of the state in which they occur.

When prosecuting prominent Nazis in Nuremberg, the US chief prosecutor, Robert Jackson, argued that crimes against humanity are committed by individuals, not abstract entities. Only by holding individuals accountable for their actions can widespread atrocities be deterred in future.

The International Criminal Court's Chief Prosecutor, Fatou Bensouda, has promised to apply the approach first developed in Nuremberg to prosecute individuals for international crimes that result in significant environmental damage. Her recommendations don't create new environmental crimes, such as "ecocide", which would punish severe environmental damage as a crime in itself. They do signal, however, a growing appreciation of the role that environmental damage plays in causing harm and suffering to people.

The International Criminal Court was asked in 2014 to open an investigation into allegations of land-grabbing by the Cambodian government. In Cambodia, large corporations and investment firms were being given prime agricultural land by the government, displacing up to 770,000 Cambodians from 4m hectares of land. Prosecuting these actions as crimes against humanity would be a positive first step towards holding individuals like Bolsonaro accountable.

But given the global consequences of the Amazon fires, could environmental destruction of this nature be legally considered a crime against all humanity? Defining it as such would be unprecedented. The same charge could apply to many politicians and business people. It's been argued that oil and gas executives who've funded disinformation about climate change for decades should be chief among them.

Charging individuals for environmental crimes against humanity could be an effective deterrent. But whether the law will develop in time to prosecute people like Bolsonaro is, as yet, uncertain. Until the International Criminal Court prosecutes individuals for crimes against humanity based on their environmental damage, holding individuals criminally accountable for climate change remains unlikely.

This story originally appeared in The Conversation. It is republished here as part of EcoWatch's partnership with Covering Climate Now, a global collaboration of more than 250 news outlets to strengthen coverage of the climate story.

Author, social activist and filmmaker Naomi Klein speaking on the one year anniversary of Hurricane Maria on Sept. 20, 2018. Erik McGregor / Pacific Press / LightRocket / Getty Images

By Natalie Hanman

Why are you publishing this book now?

I still feel that the way that we talk about climate change is too compartmentalised, too siloed from the other crises we face. A really strong theme running through the book is the links between it and the crisis of rising white supremacy, the various forms of nationalism and the fact that so many people are being forced from their homelands, and the war that is waged on our attention spans. These are intersecting and interconnecting crises and so the solutions have to be as well.

The book collects essays from the last decade, have you changed your mind about anything?

When I look back, I don't think I placed enough emphasis on the challenge climate change poses to the left. It's more obvious the way the climate crisis challenges a rightwing dominant worldview, and the cult of serious centrism that never wants to do anything big, that's always looking to split the difference. But this is also a challenge to a left worldview that is essentially only interested in redistributing the spoils of extractivism [the process of extracting natural resources from the earth] and not reckoning with the limits of endless consumption.

What's stopping the left doing this?

In a North American context, it's the greatest taboo of all to actually admit that there are going to be limits. You see that in the way Fox News has gone after the Green New Deal – they are coming after your hamburgers! It cuts to the heart of the American dream – every generation gets more than the last, there is always a new frontier to expand to, the whole idea of settler colonial nations like ours. When somebody comes along and says, actually, there are limits, we've got some tough decisions, we need to figure out how to manage what's left, we've got to share equitably – it is a psychic attack. And so the response [on the left] has been to avoid, and say no, no, we're not coming to take away your stuff, there are going to be all kinds of benefits. And there aregoing to be benefits: we'll have more livable cities, we'll have less polluted air, we'll spend less time stuck in traffic, we can design happier, richer lives in so many ways. But we are going to have to contract on the endless, disposable consumption side.

Do you feel encouraged by talk of the Green New Deal?

I feel a tremendous excitement and a sense of relief, that we are finally talking about solutions on the scale of the crisis we face. That we're not talking about a little carbon tax or a cap and trade scheme as a silver bullet. We're talking about transforming our economy. This system is failing the majority of people anyway, which is why we're in this period of such profound political destabilisation – that is giving us the Trumps and the Brexits, and all of these strongman leaders – so why don't we figure out how to change everything from bottom to top, and do it in a way that addresses all of these other crises at the same time? There is every chance we will miss the mark, but every fraction of a degree warming that we are able to hold off is a victory and every policy that we are able to win that makes our societies more humane, the more we will weather the inevitable shocks and storms to come without slipping into barbarism. Because what really terrifies me is what we are seeing at our borders in Europe and North America and Australia – I don't think it's coincidental that the settler colonial states and the countries that are the engines of that colonialism are at the forefront of this. We are seeing the beginnings of the era of climate barbarism. We saw it in Christchurch, we saw it in El Paso, where you have this marrying of white supremacist violence with vicious anti-immigrant racism.

That is one of the most chilling sections of your book: I think that's a link a lot of people haven't made.

This pattern has been clear for a while. White supremacy emerged not just because people felt like thinking up ideas that were going to get a lot of people killed but because it was useful to protect barbaric but highly profitable actions. The age of scientific racism begins alongside the transatlantic slave trade, it is a rationale for that brutality. If we are going to respond to climate change by fortressing our borders, then of course the theories that would justify that, that create these hierarchies of humanity, will come surging back. There have been signs of that for years, but it is getting harder to deny because you have killers who are screaming it from the rooftops.

One criticism you hear about the environment movement is that it is dominated by white people. How do you address that?

When you have a movement that is overwhelmingly representative of the most privileged sector of society then the approach is going to be much more fearful of change, because people who have a lot to lose tend to be more fearful of change, whereas people who have a lot to gain will tend to fight harder for it. That's the big benefit of having an approach to climate change that links it to those so called bread and butter issues: how are we going to get better paid jobs, affordable housing, a way for people to take care of their families?

I have had many conversations with environmentalists over the years where they seem really to believe that by linking fighting climate change with fighting poverty, or fighting for racial justice, it's going to make the fight harder. We have to get out of this "my crisis is bigger than your crisis: first we save the planet and then we fight poverty and racism, and violence against women". That doesn't work. That alienates the people who would fight hardest for change.

This debate has shifted a huge amount in the U.S. because of the leadership of the climate justice movement and because it is congresswomen of colour who are championing the Green New Deal. Alexandria Ocasio-Cortez, Ilhan Omar, Ayanna Pressley and Rashida Tlaibcome from communities that have gotten such a raw deal under the years of neoliberalism and longer, and are determined to represent, truly represent, the interests of those communities. They're not afraid of deep change because their communities desperately need it.

In the book, you write: "The hard truth is that the answer to the question 'What can I, as an individual, do to stop climate change?' is: nothing." Do you still believe that?

In terms of the carbon, the individual decisions that we make are not going to add up to anything like the kind of scale of change that we need. And I do believe that the fact that for so many people it's so much more comfortable to talk about our own personal consumption, than to talk about systemic change, is a product of neoliberalism, that we have been trained to see ourselves as consumers first. To me that's the benefit of bringing up these historical analogies, like the New Deal or the Marshall Plan – it brings our minds back to a time when we were able to think of change on that scale. Because we've been trained to think very small. It is incredibly significant that Greta Thunberg has turned her life into a living emergency.

Yes, she set sail for the UN climate summit in New York on a zero carbon yacht ...

Exactly. But this isn't about what Greta is doing as an individual. It's about what Greta is broadcasting in the choices that she makes as an activist, and I absolutely respect that. I think it's magnificent. She is using the power that she has to broadcast that this is an emergency, and trying to inspire politicians to treat it as an emergency. I don't think anybody is exempt from scrutinising their own decisions and behaviours but I think it is possible to overemphasise the individual choices. I have made a choice – and this has been true since I wrote No Logo, and I started getting these "what should I buy, where should I shop, what are the ethical clothes?" questions. My answer continues to be that I am not a lifestyle adviser, I am not anyone's shopping guru, and I make these decisions in my own life but I'm under no illusion that these decisions are going to make the difference.

Some people are choosing to go on birth strikes. What do you think about that?

I'm happy these discussions are coming into the public domain as opposed to being furtive issues we're afraid to talk about. It's been very isolating for people. It certainly was for me. One of the reasons I waited as long as I did to try and get pregnant, and I would say this to my partner all the time – what, you want to have a Mad Max water warrior fighting with their friends for food and water? It wasn't until I was part of the climate justice movement and I could see a path forward that I could even imagine having a kid. But I would never tell anybody how to answer this most intimate of questions. As a feminist who knows the brutal history of forced sterilisation and the ways in which women's bodies become battle zones when policymakers decide that they are going to try and control population, I think that the idea that there are regulatory solutions when it comes to whether or not to have kids is catastrophically ahistorical. We need to be struggling with our climate grief together and our climate fears together, through whatever decision we decide to make, but the discussion we need to have is how do we build a world so that those kids can have thriving, zero-carbon lives?

Over the summer, you encouraged people to read Richard Powers's novel, The Overstory. Why?

It's been incredibly important to me and I'm happy that so many people have written to me since. What Powers is writing about trees: that trees live in communities and are in communication, and plan and react together, and we've been completely wrong in the way we conceptualise them. It's the same conversation we're having about whether we are going to solve this as individuals or whether we are going to save the collective organism. It's also rare, in good fiction, to valorise activism, to treat it with real respect, failures and all, to acknowledge the heroism of the people who put their bodies on the line. I thought Powers did that in a really extraordinary way.

What are you views on what Extinction Rebellion has achieved?

One thing they have done so well is break us out of this classic campaign model we have been in for a long time, where you tell someone something scary, you ask them to click on something to do something about it, you skip out the whole phase where we need to grieve together and feel together and process what it is that we just saw. Because what I hear a lot from people is, ok, maybe those people back in the 1930s or 40s could organise neighbourhood by neighbourhood or workplace by workplace but we can't. We believe we've been so downgraded as a species that we are incapable of that. The only thing that is going to change that belief is getting face to face, in community, having experiences, off our screens, with one another on the streets and in nature, and winning some things and feeling that power.

You talk about stamina in the book. How do you keep going? Do you feel hopeful?

I have complicated feelings about the hope question. Not a day goes by that I don't have a moment of sheer panic, raw terror, complete conviction that we are doomed, and then I do pull myself out of it. I'm renewed by this new generation that is so determined, so forceful. I'm inspired by the willingness to engage in electoral politics, because my generation, when we were in our 20s and 30s, there was so much suspicion around getting our hands dirty with electoral politics that we lost a lot of opportunities. What gives me the most hope right now is that we've finally got the vision for what we want instead, or at least the first rough draft of it. This is the first time this has happened in my lifetime. And also, I did decide to have kids. I have a seven year old who is so completely obsessed and in love with the natural world. When I think about him, after we've spent an entire summer talking about the role of salmon in feeding the forests where he was born in British Columbia, and how they are linked to the health of the trees and the soil and the bears and the orcas and this entire magnificent ecosystem, and I think about what it would be like to have to tell him that there are no more salmon, it kills me. So that motivates me. And slays me.

This story was originally published by The Guardian, and is republished here as part of the Covering Climate Now partnership to strengthen the media's focus on the climate crisis.

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