Tornadoes and Climate Change: What Does the Science Say?
By Zeke Hausfather
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.
"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.
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.
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."
Our thoughts are with everyone impacted by the devastating tornadoes across the U.S. this past week. 💚 https://t.co/iHmmXc8KbN— Greenpeace USA (@greenpeaceusa) May 28, 2019
Reposted with permission from our media associate Carbon Brief.
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By Katy Neusteter
The Biden-Harris transition team identified COVID-19, economic recovery, racial equity and climate change as its top priorities. Rivers are the through-line linking all of them. The fact is, healthy rivers can no longer be separated into the "nice-to-have" column of environmental progress. Rivers and streams provide more than 60 percent of our drinking water — and a clear path toward public health, a strong economy, a more just society and greater resilience to the impacts of the climate crisis.
Public Health<img lazy-loadable="true" data-runner-src="https://assets.rebelmouse.io/eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJpbWFnZSI6Imh0dHBzOi8vYXNzZXRzLnJibC5tcy8yNTUyNDY3MC9vcmlnaW4uanBnIiwiZXhwaXJlc19hdCI6MTY2MDkxMTkwNn0.pyP14Bg1WvcUvF_xUGgYVu8PS7Lu49Huzc3PXGvATi4/img.jpg?width=980" id="8e577" class="rm-shortcode" data-rm-shortcode-id="1efb3445f5c445e47d5937a72343c012" data-rm-shortcode-name="rebelmouse-image" data-width="3000" data-height="2302" />
Wild and Scenic Merced River, California. Bob Wick / BLM<p>Let's begin with COVID-19. More than <a href="https://www.nytimes.com/interactive/2020/us/coronavirus-us-cases.html?name=styln-coronavirus&region=TOP_BANNER&block=storyline_menu_recirc&action=click&pgtype=LegacyCollection&impression_id=2f508610-2a87-11eb-8622-4f6c038cbd1d&variant=1_Show" target="_blank">16 million Americans</a> have contracted the coronavirus and, tragically,<a href="https://www.nytimes.com/interactive/2020/us/coronavirus-us-cases.html?name=styln-coronavirus&region=TOP_BANNER&block=storyline_menu_recirc&action=click&pgtype=LegacyCollection&impression_id=2f508610-2a87-11eb-8622-4f6c038cbd1d&variant=1_Show" target="_blank"> more than</a> <a href="https://www.nytimes.com/interactive/2020/us/coronavirus-us-cases.html?name=styln-coronavirus&region=TOP_BANNER&block=storyline_menu_recirc&action=click&pgtype=LegacyCollection&impression_id=2f508610-2a87-11eb-8622-4f6c038cbd1d&variant=1_Show" target="_blank">300,000 have died</a> due to the pandemic. While health officials encourage hand-washing to contain the pandemic, at least <a href="https://closethewatergap.org/" target="_blank">2 million Americans</a> are currently living without running water, indoor plumbing or wastewater treatment. Meanwhile, <a href="https://www.theguardian.com/us-news/2020/jun/23/millions-of-americans-cant-afford-water-bills-rise" target="_blank">aging water infrastructure is growing increasingly costly for utilities to maintain</a>. That cost is passed along to consumers. The upshot? <a href="https://research.msu.edu/affordable-water-in-us-reaching-a-crisis/" target="_blank">More than 13 million</a> U.S. households regularly face unaffordable water bills — and, thus, the threat of water shutoffs. Without basic access to clean water, families and entire communities are at a higher risk of <a href="https://www.americanprogress.org/issues/green/news/2020/08/05/488705/bridging-water-access-gap-covid-19-relief/" target="_blank">contracting</a> and spreading COVID-19.</p><p>We have a moral duty to ensure that everyone has access to clean water to help prevent the spread of the coronavirus. Last spring, <a href="https://nymag.com/intelligencer/2020/03/coronavirus-stimulus-bill-explained-bailouts-unemployment-benefits.html" target="_blank">Congress appropriated more than $4 trillion</a> to jumpstart the economy and bring millions of unemployed Americans back to work. Additional federal assistance — desperately needed — will present a historic opportunity to improve our crumbling infrastructure, which has been <a href="https://www.theguardian.com/us-news/2020/jun/23/millions-of-americans-cant-afford-water-bills-rise" target="_blank" rel="noopener noreferrer">grossly underfunded for decades</a>.</p><p>A report by my organization, American Rivers, suggests that <a href="https://s3.amazonaws.com/american-rivers-website/wp-content/uploads/2020/07/09223525/ECONOMIC-ENGINES-Report-2020.pdf" target="_blank" rel="noopener noreferrer">Congress must invest at least $50 billion</a> "to address the urgent water infrastructure needs associated with COVID-19," including the rising cost of water. This initial boost would allow for the replacement and maintenance of sewers, stormwater infrastructure and water supply facilities.</p>
Economic Recovery<p>Investing in water infrastructure and healthy rivers also creates jobs. Consider, for example, that <a href="https://tinyurl.com/y9p6sgnk" target="_blank">every $1 million spent on water infrastructure in the United States generates more than 15 jobs</a> throughout the economy, according to a report by the Value of Water Campaign. Similarly, <a href="https://tinyurl.com/yyvd2ksp" target="_blank">every "$1 million invested in forest and watershed restoration contracting will generate between 15.7 and 23.8 jobs,</a> depending on the work type," states a working paper released by the Ecosystem Workforce Program, University of Oregon. Healthy rivers also spur tourism and recreation, which many communities rely on for their livelihoods. According to the findings by the Outdoor Industry Association, which have been shared in our report, "Americans participating in watersports and fishing spend over <a href="https://s3.amazonaws.com/american-rivers-website/wp-content/uploads/2020/06/30222425/Exec-summary-ECONOMIC-ENGINES-Report-June-30-2020.pdf" target="_blank">$174 billion</a> on gear and trip related expenses. And, the outdoor watersports and fishing economy supports over <a href="https://s3.amazonaws.com/american-rivers-website/wp-content/uploads/2020/06/30222425/Exec-summary-ECONOMIC-ENGINES-Report-June-30-2020.pdf" target="_blank">1.5 million jobs nationwide</a>."</p><p>After the 2008 financial crisis, Congress invested in infrastructure to put Americans back to work. The American Recovery and Reinvestment Act <a href="https://thehill.com/blogs/congress-blog/economy-a-budget/25941-clean-water-green-infrastructure-get-major-boost" target="_blank">of 2009 (ARRA) allocated $6 billion</a> for clean water and drinking water infrastructure to decrease unemployment and boost the economy. More specifically, <a href="https://www.conservationnw.org/news-updates/us-reps-push-for-millions-of-restoration-and-resilience-jobs/" target="_blank" rel="noopener noreferrer">an analysis of ARRA</a> "showed conservation investments generated 15 to 33 jobs per million dollars," and more than doubled the rate of return, according to a letter written in May 2020 by 79 members of Congress, seeking greater funding for restoration and resilience jobs.</p><p>Today, when considering how to create work for the <a href="https://www.bls.gov/news.release/pdf/empsit.pdf" target="_blank" rel="noopener noreferrer">10.7 million</a> people who are currently unemployed, Congress should review previous stimulus investments and build on their successes by embracing major investments in water infrastructure and watershed restoration.</p>
Racial Justice<p>American Rivers also recommends that Congress dedicate <a href="https://s3.amazonaws.com/american-rivers-website/wp-content/uploads/2020/07/09223525/ECONOMIC-ENGINES-Report-2020.pdf" target="_blank" rel="noopener noreferrer">$500 billion for rivers and clean water over the next 10 years</a> — not just for the benefit of our environment and economy, but also to begin to address the United States' history of deeply entrenched racial injustice.</p><p>The <a href="https://www.epa.gov/npdes/sanitary-sewer-overflows-ssos" target="_blank" rel="noopener noreferrer">23,000-75,000 sewer overflows</a> that occur each year release up to <a href="https://www.americanrivers.org/2020/05/fighting-for-rivers-means-fighting-for-justice/#:~:text=There%20are%20also%2023%2C000%20to%2075%2C000%20sanitary%20sewer,to%20do%20with%20the%20mission%20of%20American%20Rivers." target="_blank" rel="noopener noreferrer">10 billion gallons of toxic sewage</a> <em>every day</em> into rivers and streams. This disproportionately impacts communities of color, because, for generations, Black, Indigenous, Latinx and other people of color have been <a href="https://www.scientificamerican.com/article/flooding-disproportionately-harms-black-neighborhoods/" target="_blank" rel="noopener noreferrer">relegated</a> to live in flood-prone areas and in neighborhoods that have been intentionally burdened with a lack of development that degrades people's health and quality of life. In some communities of color, incessant flooding due to stormwater surges or <a href="https://www.ajc.com/opinion/opinion-partnering-to-better-manage-our-water/7WQ6SEAQP5E4LGQCEYY5DO334Y/" target="_blank" rel="noopener noreferrer">combined sewer overflows</a> has gone unmitigated for decades.</p><p>We have historically treated people as separate from rivers and water. We can't do that anymore. Every voice — particularly those of people most directly impacted — must have a loudspeaker and be included in decision-making at the highest levels.</p><p>Accordingly, the new administration must diligently invest in projects at the community level that will improve lives in our country's most marginalized communities. We also must go further to ensure that local leaders have a seat at the decision-making table. To this end, the Biden-Harris administration should restore <a href="https://www.epa.gov/cwa-401#:~:text=Section%20401%20Certification%20The%20Clean%20Water%20Act%20%28CWA%29,the%20United%20States.%20Learn%20more%20about%20401%20certification." target="_blank">Section 401 of the Clean Water Act</a>, which was undermined by the <a href="https://earthjustice.org/news/press/2020/tribes-and-environmental-groups-sue-trump-administration-to-preserve-clean-water-protections#:~:text=Under%20Section%20401%20of%20the%20Clean%20Water%20Act%2C,seeks%20to%20undermine%20that%20authority%20in%20several%20ways%3A" target="_blank">Trump administration's 2020 regulatory changes</a>. This provision gives states and tribes the authority to decide whether major development projects, such as hydropower and oil and gas projects, move forward.</p>
Climate Resilience<p>Of course, the menacing shadow looming over it all? Climate change. <a href="https://media.ifrc.org/ifrc/wp-content/uploads/2020/11/IFRC_wdr2020/IFRC_WDR_ExecutiveSummary_EN_Web.pdf" target="_blank">More than 100 climate-related catastrophes</a> have pummeled the Earth since the pandemic was declared last spring, including the blitzkrieg of megafires, superstorms and heat waves witnessed during the summer of 2020, directly impacting the lives of more than <a href="https://media.ifrc.org/ifrc/wp-content/uploads/2020/11/IFRC_wdr2020/IFRC_WDR_ExecutiveSummary_EN_Web.pdf" target="_blank">50 million people globally</a>.</p><p>Water and climate scientist Brad Udall often says, "<a href="https://www.youtube.com/watch?v=xQhpj5G0dME" target="_blank">Climate change is water change</a>." In other words, the most obvious and dire impacts of climate change are evidenced in profound changes to our rivers and water resources. You've likely seen it where you live: Floods are more damaging and frequent. Droughts are deeper and longer. Uncertainty is destabilizing industry and lives.</p><p>By galvanizing action for healthy rivers and managing our water resources more effectively, we can insure future generations against the consequences of climate change. First, we must safeguard rivers that are still healthy and free-flowing. Second, we must protect land and property against the ravages of flooding. And finally, we must promote policies and practical solutions that take the science of climate disruption into account when planning for increased flooding, water shortage and habitat disruption.</p><p>Imagine all that rivers do for us. Most of our towns and cities have a river running through them or flowing nearby. Rivers provide clean drinking water, irrigate crops that provide our food, power our homes and businesses, provide wildlife habitat, and are the lifeblood of the places where we enjoy and explore nature, and where we play and nourish our spirits. Healthy watersheds help <a href="https://news.un.org/en/story/2020/03/1059952" target="_blank" rel="noopener noreferrer">mitigate</a> climate change, absorbing and reducing the amount of carbon in the atmosphere. Healthy rivers and floodplains help communities adapt and build resilience in the face of climate change by improving flood protection and providing water supply and quality benefits. Rivers are the cornerstones of healthy, strong communities.</p><p>The more than <a href="https://archive.epa.gov/water/archive/web/html/index-17.html" target="_blank" rel="noopener noreferrer">3 million miles</a> of rivers and streams running across our country are a source of great strength and opportunity. When we invest in healthy rivers and clean water, we can improve our lives. When we invest in rivers, we create jobs and strengthen our economy. When we invest in rivers, we invest in our shared future.</p>
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