US tornadoes: Is climate change to blame?

Several US states have been hit by a devastating series of tornadoes, with an expected death toll of more than 100.

These are extremely rare outside the spring and summer, in the US, but this December there has been a record number of tornado warnings.

So is climate change causing more frequent and stronger tornadoes?

When warm moist air is trapped by cooler air, it causes thunder clouds to form.

The warm air rises, creating an updraft.

And if there are also strong winds moving in different directions, the air column starts to rotate.

Increasing amounts of warm air are drawn in, speeding up the wind spiral, which then extends out of the bottom of the thunder clouds.

And once this touches the ground, it is a tornado.

There is more warm moist air in the hotter months.

And a cluster of tornadoes of this size and power in December is extremely unusual in the US.

Far more tornadoes have been recorded in the past 20 years than the previous 20 - but some of this is due to improved tracking.

As data-collecting methods have improved, less severe tornadoes have been recorded more consistently.

"To an untrained eye, it may look like we are having more of these events happening - but in reality what is happening is we have much better tools for identifying relatively weaker tornadoes," Dr Jana Houser, professor of meteorology, at the University of Ohio, says.

But clusters of tornadoes - when six or more start within six hours of each other - are becoming more likely.

No single weather event can be put down to climate change alone.

The increasing amount of tornado clusters "clearly implies that the patterns of the atmosphere have changed", meteorologist Harold Brooks, at the US National Oceanic and Atmospheric Administration (NOAA), says.

"That may be related to climate change - but we cannot make a full conclusion," he adds.

Warmer temperatures make extreme weather events more likely and potentially more destructive.

But "there is no scientific consensus" on tornadoes, Mr Brooks says.

"It is not like the consensus on the increase in heatwaves or heavy rain at all."

Wind speed and energy in the atmosphere are two of the main drivers of tornadoes - but there is a lack of definitive evidence on how they may change with a warming climate.

At a briefing following the tornadoes, US President Joe Biden said he would "be asking the Environmental Protection Agency and others to take a look at [the role of climate change].

"The fact is that we know everything is more intense when the climate is warming," he added.

But other conditions, which may lessen the likelihood of tornadoes, also exist.

The most devastating of this weekend's tornadoes has become known as the Quad-State - cutting a swathe through Arkansas, Missouri, Tennessee and Kentucky.

It is not the wind speeds, though, that were record-breaking but its size.

It was reportedly three-quarters of a mile (1.2km) wide and travelled for more than 220 miles - considerably further than the distance from London to Paris.

The most frequent tornadoes have traditionally been in a strip through the central US known as Tornado Alley.

This includes areas of Texas, Louisiana, Arkansas, Oklahoma, Kansas, South Dakota, Iowa and Nebraska.

Mr Brooks says: "Tornado Alley is not well defined, so that is hard to say - if there is a definite shift.

"We have seen an increase in tornadoes of about 10% over 40 years in the mid-South region and a decrease in the Texas, Kansas sort of region.

"It has shifted - but it is not a huge shift."