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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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Guillain-Barre syndrome occurs when the body's own immune system attacks and injures the nerves outside of the spinal cord or brain – the peripheral nervous system. Niq Steele / Getty Images

By Sherry H-Y. Chou, Aarti Sarwal and Neha S. Dangayach

The patient in the case report (let's call him Tom) was 54 and in good health. For two days in May, he felt unwell and was too weak to get out of bed. When his family finally brought him to the hospital, doctors found that he had a fever and signs of a severe infection, or sepsis. He tested positive for SARS-CoV-2, the virus that causes COVID-19 infection. In addition to symptoms of COVID-19, he was also too weak to move his legs.

When a neurologist examined him, Tom was diagnosed with Guillain-Barre Syndrome, an autoimmune disease that causes abnormal sensation and weakness due to delays in sending signals through the nerves. Usually reversible, in severe cases it can cause prolonged paralysis involving breathing muscles, require ventilator support and sometimes leave permanent neurological deficits. Early recognition by expert neurologists is key to proper treatment.

We are neurologists specializing in intensive care and leading studies related to neurological complications from COVID-19. Given the occurrence of Guillain-Barre Syndrome in prior pandemics with other corona viruses like SARS and MERS, we are investigating a possible link between Guillain-Barre Syndrome and COVID-19 and tracking published reports to see if there is any link between Guillain-Barre Syndrome and COVID-19.

Some patients may not seek timely medical care for neurological symptoms like prolonged headache, vision loss and new muscle weakness due to fear of getting exposed to virus in the emergency setting. People need to know that medical facilities have taken full precautions to protect patients. Seeking timely medical evaluation for neurological symptoms can help treat many of these diseases.

What Is Guillain-Barre Syndrome?

Guillain-Barre syndrome occurs when the body's own immune system attacks and injures the nerves outside of the spinal cord or brain – the peripheral nervous system. Most commonly, the injury involves the protective sheath, or myelin, that wraps nerves and is essential to nerve function.

Without the myelin sheath, signals that go through a nerve are slowed or lost, which causes the nerve to malfunction.

To diagnose Guillain-Barre Syndrome, neurologists perform a detailed neurological exam. Due to the nerve injury, patients often may have loss of reflexes on examination. Doctors often need to perform a lumbar puncture, otherwise known as spinal tap, to sample spinal fluid and look for signs of inflammation and abnormal antibodies.

Studies have shown that giving patients an infusion of antibodies derived from donated blood or plasma exchange – a process that cleans patients' blood of harmful antibodies - can speed up recovery. A very small subset of patients may need these therapies long-term.

The majority of Guillain-Barre Syndrome patients improve within a few weeks and eventually can make a full recovery. However, some patients with Guillain-Barre Syndrome have lingering symptoms including weakness and abnormal sensations in arms and/or legs; rarely patients may be bedridden or disabled long-term.

Guillain-Barre Syndrome and Pandemics

As the COVID-19 pandemic sweeps across the globe, many neurologic specialists have been on the lookout for potentially serious nervous system complications such as Guillain-Barre Syndrome.

Though Guillain-Barre Syndrome is rare, it is well known to emerge following bacterial infections, such as Campylobacter jejuni, a common cause of food poisoning, and a multitude of viral infections including the flu virus, Zika virus and other coronaviruses.

Studies showed an increase in Guillain-Barre Syndrome cases following the 2009 H1N1 flu pandemic, suggesting a possible connection. The presumed cause for this link is that the body's own immune response to fight the infection turns on itself and attacks the peripheral nerves. This is called an "autoimmune" condition. When a pandemic affects as many people as our current COVID-19 crisis, even a rare complication can become a significant public health problem. That is especially true for one that causes neurological dysfunction where the recovery takes a long time and may be incomplete.

The first reports of Guillain-Barre Syndrome in COVID-19 pandemic originated from Italy, Spain and China, where the pandemic surged before the U.S. crisis.

Though there is clear clinical suspicion that COVID-19 can lead to Guillain-Barre Syndrome, many important questions remain. What are the chances that someone gets Guillain-Barre Syndrome during or following a COVID-19 infection? Does Guillain-Barre Syndrome happen more often in those who have been infected with COVID-19 compared to other types of infections, such as the flu?

The only way to get answers is through a prospective study where doctors perform systematic surveillance and collect data on a large group of patients. There are ongoing large research consortia hard at work to figure out answers to these questions.

Understanding the Association Between COVID-19 and Guillain-Barre Syndrome

While large research studies are underway, overall it appears that Guillain-Barre Syndrome is a rare but serious phenomenon possibly linked to COVID-19. Given that more than 10.7 million cases have been reported for COVID-19, there have been 10 reported cases of COVID-19 patients with Guillain-Barre Syndrome so far – only two reported cases in the U.S., five in Italy, two cases in Iran and one from Wuhan, China.

It is certainly possible that there are other cases that have not been reported. The Global Consortium Study of Neurological Dysfunctions in COVID-19 is actively underway to find out how often neurological problems like Guillain-Barre Syndrome is seen in hospitalized COVID-19 patients. Also, just because Guillain-Barre Syndrome occurs in a patient diagnosed with COVID-19, that does not imply that it was caused by the virus; this still may be a coincident occurrence. More research is needed to understand how the two events are related.

Due to the pandemic and infection-containment considerations, diagnostic tests, such as a nerve conduction study that used to be routine for patients with suspected Guillain-Barre Syndrome, are more difficult to do. In both U.S. cases, the initial diagnosis and treatment were all based on clinical examination by a neurological experts rather than any tests. Both patients survived but with significant residual weakness at the time these case reports came out, but that is not uncommon for Guillain-Barre Syndrome patients. The road to recovery may sometimes be long, but many patients can make a full recovery with time.

Though the reported cases of Guillain-Barre Syndrome so far all have severe symptoms, this is not uncommon in a pandemic situation where the less sick patients may stay home and not present for medical care for fear of being exposed to the virus. This, plus the limited COVID-19 testing capability across the U.S., may skew our current detection of Guillain-Barre Syndrome cases toward the sicker patients who have to go to a hospital. In general, the majority of Guillain-Barre Syndrome patients do recover, given enough time. We do not yet know whether this is true for COVID-19-related cases at this stage of the pandemic. We and colleagues around the world are working around the clock to find answers to these critical questions.

Sherry H-Y. Chou is an Associate Professor of Critical Care Medicine, Neurology, and Neurosurgery, University of Pittsburgh.

Aarti Sarwal is an Associate Professor, Neurology, Wake Forest University.

Neha S. Dangayach is an Assistant Professor of Neurology and Neurosurgery, Icahn School of Medicine at Mount Sinai.

Disclosure statement: Sherry H-Y. Chou receives funding from The University of Pittsburgh Clinical Translational Science Institute (CTSI), the National Institute of Health, and the University of Pittsburgh School of Medicine Dean's Faculty Advancement Award. Sherry H-Y. Chou is a member of Board of Directors for the Neurocritical Care Society. Neha S. Dangayach receives funding from the Bee Foundation, the Friedman Brain Institute, the Neurocritical Care Society, InCHIP-UConn Center for mHealth and Social Media Seed Grant. She is faculty for emcrit.org and for AiSinai. Aarti Sarwal does not work for, consult, own shares in or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.

Reposted with permission from The Conversation.


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