The deadly tornadoes that pummeled six states on Friday, killing more than 80 people and leaving dozens missing, came at the end of a year of compounding extreme weather events, from heat waves and hurricanes to flooding and wildfires.
Scientists have been able to draw links between a warming planet and hurricanes, heat waves and droughts, attributing the likelihood that climate change played a role in individual isolated events. The same can’t be said for tornadoes yet.
“This is the hardest phenomenon to connect to climate change,” said Michael Tippett, an associate professor of applied physics and mathematics at Columbia University who studies extreme weather and climate.
Even as scientists are discovering trends around tornadoes and their behavior, it remains unclear the role that climate change plays. “For a lot of our questions about climate change and tornadoes, the answer is we don’t know,” said Harold Brooks, a senior research scientist at the National Oceanic and Atmospheric Administration’s National Severe Storms Laboratory. “We don’t see evidence for changes in average annual occurrence or intensity over the last 40 to 60 years.”
What causes a tornado?
Tornadoes form inside large rotating thunderstorms and the ingredients have to be just right. Tornadoes occur when there is a perfect mix of temperature, moisture profile and wind profile.
When the air is unstable, cold air is pushed over warmer humid air, creating an updraft as the warm air rises. When a wind’s speed or direction changes over a short distance, the air inside the clouds can start to spin. If the air column begins spinning vertically and rotates near the ground, it can intensify the friction on Earth’s surface, accelerating the air inward, forming a tornado.
How are they measured?
Like hurricanes and earthquakes, tornadoes are rated on a scale. The Enhanced Fujita, or EF, scale, runs from 0 to 5.
The tornado that traveled across Northeast Arkansas, Tennessee and western Kentucky over the weekend was estimated to be three-quarters of a mile wide with wind speeds that peaked between 158 and 206 miles per hour, giving it a EF rank of at least 3.
Because it’s challenging to measure the winds in a tornado directly, surveyors usually evaluate tornadoes by their level of damage to different structures.
For instance, they may look to see if the damage is limited to missing roof shingles or whether entire sections of roofs or walls are missing. Based on the level of damage, scientists then reverse-engineer the wind speeds and assign a tornado a rating on the scale.
Have tornadoes changed over the years?
Researchers say that in recent years tornadoes seem to be occurring in greater “clusters,” and that the region known as tornado alley in the Great Plains, where most tornadoes occur, appears to be shifting eastward. The overall number of tornadoes annually is holding steady around 1,200.
Tornadoes in the United States in December are unusual. They typically occur in the spring. Friday’s tornadoes may have occurred because the wind shear was high (it tends to peak in the winter) and the weather was warmer than normal. This year, the region has experienced an uncharacteristically warm December, and temperatures in Arkansas and Kansas on Friday were in the 70s and 80s.
Is climate change the cause?
The ingredients that give rise to tornadoes include warm, moist air at ground level; cool dry air higher up; and wind shear, which is the change in wind speed or direction. Each of these factors may be affected differently by climate change.
As the planet warms and the climate changes, “we don’t think they are all going to go in the same direction,” said Dr. Brooks of NOAA. For instance, overall temperature and humidity, which provide energy in the air, may rise with a warming climate, but wind shear may not.
“If there is not enough shear to make something rotate, it doesn’t matter how strong the energy is.” he said. “If there is all kind of wind shear, but you don’t have a storm, you won’t get a tornado, either.”
Although we know that climate change may be playing a role in making some storms more powerful, the complexity of tornadoes means that it is hard to extend that connection with certainty, especially for an individual event.
Scale is everything
A tornado’s relatively small size also makes it harder to model, the primary tool that scientists use when attributing extreme weather events to climate change. “We are working at such small scales that the model you would use to do the attribution studies just can’t capture the phenomenon,” Dr. Brooks said.
A shorter, spottier, historical record
The tornado record is still sparse compared with other types of events. One possible reason is that tornadoes are relatively local weather events. Tornado records have largely been based on someone seeing a tornado and reporting it to the National Weather Service. This means that smaller tornadoes that occur in rural areas and do not cause property damage or injury may not be reported.
“We are pretty sure we know how many hurricanes make landfall in the United States each year,” Dr. Brooks said. “With tornadoes, we may not know how many occurred yesterday and last night.”
Another kind of link
A 2015 paper found that La Niña conditions, like those we are experiencing now and which will most likely prolong the Western drought, are more favorable for tornado activity. Dr. Tippett, one of the study’s authors, said that the observable relationship between the two was modest, though a “little less caveated” than the relationship between tornadoes and climate change.
Still, Dr. Tippett said that, based on all the evidence, computer modeling showed that the environmental conditions favorable to tornadoes might increase in the future. “Our confidence is low, but the evidence points to the same direction.”