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It’s windy out there. High winds are ripping through Michigan today, and a first-ever December tornado hit Minnesota last night.
This follows last weekend’s devastating tornadoes, which hit five states and left at least 74 dead in Kentucky.
All of this has many wondering: Is climate change influencing tornado behavior?
To find out, we spoke to Dr. Todd W. Moore, an Associate Professor & Chair of the Department of Geosciences at Fort Hays State University in Kansas.
How tornado outbreaks occur
Moore describes the conditions for tornadoes to occur as the right mix of “ingredients.”
“Severe thunderstorms and tornadoes occur when the atmosphere is unstable (meaning air tends to rise), humid (moisture, water vapor, releases energy into thunderstorms as it rises and condenses into liquid water (aka clouds)), and when wind shear is present (change in wind speed and/or direction with altitude),” Moore said. “The concurrence of these ingredients is essential for tornado outbreaks.
When these ingredients come together, in any place and at any time, there is a potential for severe thunderstorms and tornadoes, Moore said. It just so happens that these ingredients come together most often between the Rocky and the Appalachian Mountains, which is why we see greater risk for tornadoes in this region than in any other area in the entire world.
Is climate change causing more tornado outbreaks?
“The evidence between climate change and tornadoes is building,” Moore told Michigan Climate News. “We cannot say at this point that climate change caused a particular outbreak. But what we can do is say that there is a link between severe weather outbreaks and tornado outbreaks and climate change, because the evidence is building across three different areas.”
Moore references three types of evidence: reasoning, modeling studies, and observation studies.
In terms of basic reasoning, it makes logical sense that climate change would be producing more tornado outbreaks.
That’s because as the planet warms, the lower atmosphere is warming most. According to Moore, humidity (moisture) increases as the temperature rises because of more evaporation from the surface into the atmosphere. Warming and humidifying the lower atmosphere means that two of the ingredients for severe thunderstorms and tornadoes — instability and moisture — are increasing.
Wind shear is more uncertain because the jet stream is weakening due to climate change. But even if wind shear decreases, some modeling studies show that the increases in instability and moisture will offset any decrease in wind shear, and conditions will generally be more favorable for severe thunderstorms.
And some scientists hypothesize that the jet stream is becoming wavier due to a decrease in temperature differential between the poles and the equator. That could introduce more wind shear into an already more humid and warm environment – setting the stage for tornado outbreaks.
“When the jetstream dips down and provides that last ingredient – the shear – in the warmer and more humid atmosphere [more conducive to tornado formation], then the atmosphere is essentially more efficiently producing tornadoes.”
Modeling studies support this reasoning. “A recent study, for example, shows that atmospheric conditions will become more suitable for severe thunderstorms as the planet warms, and that this is driven mostly by substantial increases in instability,” Moore said.
But what do the data say? Are we seeing more tornado outbreaks?
“We are already seeing changes in tornado activity in the United States,” Moore said. “Tornadoes are occurring on fewer days of the year, but outbreaks with many tornadoes are occurring more often.”
Tornadoes are also concentrating more in space — and where on the continent they are most likely to occur is also changing. Moisture and instability have decreased in portions of the Great Plains but increased in parts of the Southeast and Midwest. These studies suggest that climate variability contributes to the changes we see in tornado activity.
“Regionally, we are seeing decreased tornado activity in portions of the Great Plains and increases farther east in the Southeast and portions of the Midwest,” Moore said. “Some of the biggest tornado outbreaks have occurred in the Southeast and Midwest, so as a larger percentage of annual tornadoes occurs in outbreaks, we might expect to continue to see this ‘eastward shift.’”
These eastern, late-season tornadoes are more likely to be nocturnal. That makes them more deadly, because they happen when people are asleep.
“Nighttime tornadoes tend to occur in the wintertime and the fall, a little bit farther east, and are more dangerous,” Moore said.
Do Michiganders have to worry about more tornado outbreaks?
Michigan has far fewer tornadoes every year than other states in the south and Great Plains.
Most tornadoes in Michigan occur in summer, but the number of summer tornadoes in the Midwest, including the southern Great Lakes area, is decreasing. However, the number of (potentially more deadly) tornadoes in fall and winter is on the rise in the Midwest.
Between 1950 and 2021, Michigan saw more than a thousand tornadoes with 244 fatalities and more than $1 billion in property damage. Here are some of the worst ones.
And the top 10 counties for tornado activity in Michigan are as follows:
For now, the tornado outlook in Michigan appears stable. While recent years have seen a lull in tornado activity in Michigan, that trend is reduced somewhat when outlier data from the 1970s is removed, Moore said.
The number of days per year with tornadoes in Michigan has decreased (see top graph below), and the average number of tornadoes per day has stayed relatively stable but has become more variable, especially after 1990 (see bottom graph).
Courtesy Todd Moore
In the broader Midwest and Great Lakes, the number of tornadoes per year has also been relatively stable. Still, there is a notable decline in the number of days per year with tornadoes and an increase in the average number of tornadoes per day (on days when tornadoes do occur). That could mean more tornado outbreaks.
Tornadoes also seem to appear later in the year further northward than is usual, Moore said. One example is yesterday’s projected prediction for tornado probability, which is not commonly seen so far north this time of year.
Tornado Outlook. Dec. 15,, 2021. Source: National Oceanic and Atmospheric Administration.
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