What causes a tornado?
Tornadoes are formed by a combination of atmospheric instability and wind shear. Instability occurs when warm, moist air is wedged under drier, cooler air aloft. This warm air rises, causing the intense updrafts and downdrafts seen in strong thunderstorms — the incubators of tornadoes. Wind shear refers to changes in wind direction and speed at different elevations in the atmosphere. The combination of instability and wind shear forms the circular air flow that generates a tornado.
Why are there so many in the U.S. Midwest?
The Central United States has an abundance of the ingredients necessary for tornado formation. During the spring, warm tropical air masses from the Gulf of Mexico collide with colder, drier air at higher altitudes to spawn intense thunderstorms. Thunderstorms also form into the summer and fall, as the region’s vast plains heat up air near the surface, causing atmospheric instability. These intense thunderstorms act as incubators for tornadoes. Tornadoes occur around the world but are most common in “Tornado Alley” of the Central United States.
Is there a link between climate change and tornadoes?
It is important to keep in mind that climate change has an impact on all weather events. The effect of the carbon dioxide in our atmosphere cannot be switched off – it influences our seemingly benign “everyday” or “normal” weather as well as the extreme events.
A specific link between tornadoes and climate change, however, is unclear. It is difficult to identify and diagnose trends in long-term records of tornadoes, since the population in many areas affected by tornadoes has grown (e.g., tornadoes in the early part of the 20th century may have occurred without anyone seeing them) and the technology used to observe tornadoes has improved (e.g., radars help us “see” tornadoes in ways that were not possible many decades ago).
How could climate change affect the frequency or intensity of tornadoes?
Researchers are working to better understand how the building blocks for tornadoes -- atmospheric instability and wind shear -- will respond to global warming. It is likely that a warmer, moister world would allow for more frequent instability. However, it is also likely that a warmer world would lessen chances for wind shear. Recent trends for these quantities in the Midwest during the spring are inconclusive. It is also possible that these changes could shift the timing of tornadoes or regions that are most likely to be hit.
Adding to the difficulty, tornadoes are too geographically small to be well simulated by climate models. Models can simulate some of the conditions that contribute to forming severe thunderstorms that often spawn tornadoes. Multiple studies (e.g., here and here) find the conditions that produce the most severe thunderstorms are likely to occur more often in the in a warmer world, even if the total number of thunderstorms decreases (because of fewer weak storms). However, this work does not conclusively tell us whether tornadoes should follow the same trend as their parent thunderstorms.
Extreme Weather Event Map: Click on any icon on the map above to see details on the extent of an extreme weather event between 1990 and 2012.
Extreme Weather Events Map
The events shown above are examples of four types of extreme weather that scientists say are becoming more frequent and intense because of climate change: extreme heat, drought, wildfires, and heavy precipitation. Individual events cannot be blamed on any single cause. However, the long-term trends in these types of events demonstrate that extreme weather risk is rising as a result of climate change. Each new event is an opportunity to better understand our vulnerabilities and ways to cope with these rising risks. Examined together, these events also can help us evaluate the benefits of actions and policies aimed at reducing the emissions of heat-trapping gases that are warming the planet and “juicing up the weather”.
Here are two C2ES papers taking a closer look at the trends shown in the map and how we can respond:
- Extreme Weather and Climate Change: Understanding the Link, Managing the Risk
This primer examines the link between extreme weather and climate change, presenting it in a risk management framework that can help to clarify and manage the rising risk of extreme weather and prepare for future vulnerabilities.
- A Climate of Extreme Weather Events
This background paper outlines the evidence showing the increasing risk and incidence of flooding, heat waves, wildfires and drought in the United States.
Scientific American Series on Extreme Weather, Climate Change, and the Risks We Face
Published in three parts in June 2011, this series in Scientific American provides firsthand accounts of record-breaking weather events, insights into their links to climate change, and what can be done to manage the growing risks. The articles were written by science journalist John Carey with support from C2ES.
- Part One - Storm Warnings: Extreme Weather Is a Product of Climate Change
- Part Two - Global Warming and the Science of Extreme Weather
- Part Three - Our Extreme Future: Predicting and Coping with a Changing Climate
Additional C2ES Resources
Find answers to some of the most frequently asked climate science questions and learn about the realities and misconceptions of climate change science.
Learn how Hurricane Sandy is a stark reminder of the rising risks of climate change.
See the latest on the 2012-2013 U.S. drought, its costs at home and its impacts on the rest of the world.
Keep up to date on current extreme weather events, climate change, and risk management.
Joseph Casola will speak on trends and projections.
President Obama’s forceful call for climate action in his inaugural address came after a year when climate change was barely whispered in the presidential campaign but its effects were loud and clear here in the United States and around the world.
As Hurricane Sandy moves out of the region, people in affected areas are beginning to take stock of the damage. Flooding in parts of New Jersey and New York from the storm surge hit record levels. The 13.8-foot surge measured at Battery Park in Lower Manhattan surpassed the all-time record of 11.2 feet set in 1821, flooding the New York subway system and two major commuter tunnels. Along the Eastern Seaboard, an estimated 7.5 million people lost power. Farther inland, blizzard conditions dropped as much as 2 feet of snow as Sandy crashed into arctic air over the Midwest. While early estimates indicate direct damages from the hurricane may be as much as $20 billion, the total economic losses, including losses in consumer and business spending, could be more than twice that amount.
A number of climate change-related factors may well have intensified the storm's impact: higher ocean temperatures, higher sea levels, and an atmospheric traffic jam that may be related to Arctic melting. Hurricane Sandy is also a clear reminder of how vulnerable our homes and infrastructure already are to extreme weather — and this risk is growing.
Hurricane Sandy is a stark reminder of the rising risks of climate change. A number of warming-related factors may well have intensified the storm's impact. Higher ocean temperatures contributed to heavier rainfall. Higher sea levels produced stronger storm surges. New research suggests that Arctic melting may be increasing the risk of the kind of atmospheric traffic jam that drove Sandy inland. While no single weather event can be said to have been directly caused by climate change, our weather now is the product of our changing climate, as increased warming raises the probability of extreme weather events. In highlighting our vulnerabilities to extreme weather, Hurricane Sandy underscores two imperatives: We need to reduce the risks of climate change by reducing our carbon emissions, and we must strengthen our defenses against future impacts that it may be too late to avoid.
Warming increases the amount of moisture in the atmosphere. Globally, the resulting increases in heavy precipitation are well documented. Like Hurricane Irene last year, Sandy carried an unusual amount of moisture, which increased the risk of very heavy precipitation within its path. Much of the warming from climate change occurs in the ocean, and this year’s sea surface temperatures are well above normal. September saw the second highest ocean temperatures on record globally. Sandy spent significant time over uncommonly warm sea surface temperatures – 5 degrees above normal -- boosting the amount of moisture available to rain down on the Northeast U.S.
Sea Level Rise
Recent studies have identified the Northeast U.S. as a “hotspot” of accelerated sea level rise. Over the past 30 years, sea levels in the region have risen four times faster than the global average, increasing the risk of storm surges and flooding. During a storm surge, a matter of inches can mean the difference between safety and extensive flooding. Hurricane Sandy’s storm surge was exacerbated by both the warming-driven sea-level rise and the timing of the lunar cycle. Sandy occurred during the astronomical high tide, which is 2 to 3 inches above a normal high tide. Global sea level has already increased by 4 inches since 1950, creating the equivalent of a fulltime astronomical high tide. In New York, a record storm surge 13 feet above mean low water level flooded parts of lower Manhattan and poured into subway tunnels.
Atlantic Traffic Jam
Hurricane Sandy encountered a “traffic jam” in the North Atlantic, known as a “block” to meteorologists. This block did not allow Sandy to track out to sea like most northeast storms. Meanwhile, a storm associated with some very cold air over the Midwest also ran into this Atlantic traffic jam, resulting in an unusual “hybrid” storm. Recent research has shown that these blocking events and fall cold outbreaks are related to sea ice loss in the Arctic. In short, open water in the arctic helps break down the barrier between high- and mid-latitude weather, which increases the risk of cold outbreaks and blocking events. Hurricane Sandy seems to have tracked into the middle of one of these unusual meanders in the jet stream. While this is an evolving field of research and these conditions could have occurred in the absence of climate change, there is growing evidence that climate change is increasing the risk of extreme atmospheric arrangements.
Hurricane Sandy provides important lessons about our vulnerability to the kinds of extreme weather likely to become more common in a warming world. Our levees, sea walls and other infrastructure were built to cope with the extreme weather risks of the 20th century. Sandy is offering an opportunity to see where we fall short in preparations for the 21st century.
Statement from Elliot Diringer
Executive Vice President, Center for Climate and Energy Solutions
Oct. 29, 2012
Hurricane Sandy is a stark reminder of the rising risks of climate change. While climate change didn’t cause the hurricane, a number of warming-related factors may well be intensifying its impact.
Higher ocean temperatures, in this case 5 degrees above normal, contribute to heavier rainfall. Higher sea level means stronger storm surges. And new research suggests that Arctic melting may be increasing the risk of the kind of atmospheric traffic jam that is driving Sandy inland.
But whatever’s behind it, Sandy clearly highlights our vulnerabilities to extreme weather. We’ve loaded the dice and events we once thought of as rare are becoming more common.
At a minimum, this is another foretaste of what we face in a warming world. It tells us two things: We’d better do all we can to reduce the risks by reducing our carbon emissions, and we’d better strengthen our defenses against future impacts that it’s already too late to avoid.
To get in touch with a C2ES science expert, contact Laura Rehrmann at firstname.lastname@example.org or 703-774-5480.
The Center for Climate and Energy Solutions (C2ES) is an independent nonprofit, nonpartisan organization promoting strong policy and action to address the twin challenges of energy and climate change. Launched in November 2011, C2ES is the successor to the Pew Center on Global Climate Change.
I recently replied to ta question on the National Journal blog, "How is the absence of discussion about global warming going to affect our ability to do something about it?"
You can read more on the original blog post and other responses at the National Journal.
Here is my response:
A report released this week by two senior members of Congress notes that the unusual number of extreme weather events in 2012 has cost the country billions of dollars and that the unusual frequency of these events is consistent with what scientists have predicted from climate change.
The staff report, “Going to Extremes: Climate Change and the Increasing Risk of Weather Disasters” is from the offices of Reps. Edward Markey (D-MA) and Henry Waxman (D-CA), the prime movers behind the last attempt at significant climate legislation. It cites information from a variety of sources, including NOAA, the news media and the private sector to show how rising weather risk costs real money.
Their report comes a week after Congress headed home for the elections having accomplished very little to address climate change. Nearly half the bills introduced by the current Congress would block or hinder climate action, though none of these have been enacted into law.