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 behaviour, 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. WHAT
CAUSES A TORNADO? Tornadoes
form inside large rotating thunderstorms. They 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 travelled 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 mph, 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. HAVE
TORNADOES CHANGED? 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. 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 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. 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,” Brooks said. A
SHORTER, SPOTTIER, 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 or unseen tornadoes may
not be reported. 
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