When it comes to climate change and tornadoes, mysteries remain

The United States experiences more tornadoes than any other country

Scientists are increasingly able to link global temperature rise to heatwaves, droughts, flash flooding and hurricanes.

But figuring out what impact a hotter world will have on tornadoes is more complicated.

The United States is uniquely suited to tornadoes and experiences more than any other country - an average of 800 each year. Most occur in the middle of the country over an area of the Great Plains dubbed “Tornado Alley”.

Thunderstorms spawn tornadoes when key ingredients collide: warm, moist air near the ground; cool, dry air above; and changes in wind speed and/or direction, known as wind shear.

How the climate crisis is impacting those different conditions is not completely understood, said Harold Brooks, a senior research scientist at the National Oceanic and Atmospheric Administration’s National Severe Storms Laboratory in Norman, Oklahoma.

“It doesn’t mean that there’s not a connection, it just means that it’s a lot harder to pick out than some other things might be,” he told The Independent.

For example, the fast-warming polar regions are decreasing average wind shear, reducing one condition that’s necessary for a tornado to form.

“Some things are favorable, some things aren’t,” Mr Brooks said.

Still, there are a few “relatively robust” tornado observations possibly linked to climate change, he added.

There has been a decrease in the number of days per year with tornadoes - but an increase in the number of days with multiple or “clusters” of tornadoes over the last 50 years.

Dr Brooks and Vittorio Gensini, a meteorology professor at Northern Illinois University and a longtime tornado expert, also discovered that more tornadoes have been forming further east since the late Seventies.

“The number of tornadoes has increased in the Mid-South around Memphis, Tennessee, and the surrounding area of a couple of hundred miles,” Dr Brooks said. “In the same time period, there’s been a decrease in tornadoes over the High Plains from the Texas Panhandle through Western Kansas.”

Dr Brooks said that it would be “really tempting” to connect this finding to climate change given that average temperatures have risen more quickly since the late 1970s.

Yet, tornadoes are fickle beasts. He pointed to exhaustive research on the history of tornadoes by meteorologist Thomas Grazulis who was commissioned by the US government’s Nuclear Regulatory Commission in the late Seventies to do a risk assessment.

Mr Grazulis traveled the country gathering research and ended up collating evidence of 60,000 tornadoes in the US, dating back to the 1600s.

As Dr Brooks explained, that authoritative tornado database provides important historical context. It reveals that over a 15-year period from the early 1920s to mid-1930s, the strongest tornadoes occurred in the Mid-South, with the peak in Mississippi. In later periods, the strongest tornadoes occcurred in the High Plains.

While Dr Brooks said that his “gut” instinct was that planetary warming was causing more tornadoes further east, he only felt confident to say that with “55 per cent certainty”.

Still, new evidence is emerging all the time. A study, published in the Bulletin of the American Meteorological Society in January, found that the number of deadly supercell storms will probably increase and shift east in the US as the world gets hotter.

Supercells -- intense, long-lived thunderstorms defined by a rotating updraft -- are responsible for the most damaging hail and deadly tornadoes.

In March a supercell struck Mississippi and Alabama, killing 26 people and leaving a trail of destruction through at least half a dozen towns.

The study predicts a 6.6 per cent increase in supercells in the US, and a 25.8 per cent rise in the strongest storms under moderate levels of future warming - which the planet is currently on pace to overshoot.