Tornadoes and man-made climate change - a perfect storm?
- 09 Mar 2012, 13:45
- Verity Payne
A spate of tornadoes hit the US Midwest on Monday, leaving
dozens dead across several states. As tornado season gets going
in the country with the fiercest debate around climate change, it
must be about time for the media to ask the inevitable question -
is there a causal link between a changing climate and tornados?
It's a perennial. Take last year, for example, which saw a
record-breaking spate of tornadoes across the southern US during April,
closely followed by the sixth most deadly US tornado on record
hitting Joplin, Missouri, in May.
media and blogosphere were buzzing with discussion about
potential links between tornadoes and man-made climate change.
At the time the Climate Science Rapid Response Team (CSRRT)
useful summary of expert current understanding into the links
between tornadoes and climate change. They explained that although
US tornado activity has increased over recent decades, the increase
in numbers seems to be confined to the least powerful tornadoes.
And of course this trend could be a result of changes in the way
tornadoes are measured or recorded in recent years.
It's not clear if a warmer climate means more or less
Tornadoes need two things to form: warm moist air, and high
'wind shear' which causes the air rotate. A key 2007 study used
climate models to project that whilst the number of warm, moist
days is likely to increase with man-made climate change, wind shear
will probably decrease. So at the moment it's not clear if a warmer
climate means more or less tornado activity, as CSRRT concluded and
we reported here.
This year one quite
re-posted Reuters article -
Scientists see rise in tornado-creating conditions - has caused
a little confusion.
The article is itself a little unclear about where scientific
opinion lies, and you could coming away with the impression that
scientists agree man-made climate change will cause more, or
bigger, tornadoes in the future.
Much of the article is fine - it starts by pointing out that the
US experienced a warm winter which has led to the relatively early
outbreak of tornado activity, and continues:
"Whether climate change will also affect
the frequency or severity of tornadoes, however, remains very much
an open question, and one that has received surprisingly little
The piece notes that the factors which cause tornadoes are
likely to be affected in different ways, as we noted earlier:
"The scientific challenge is this: the
two conditions necessary to spawn a twister are expected to be
affected in opposite ways. A warmer climate will likely boost the
intensity of thunderstorms but could dampen wind shear, the
increase of wind speed at higher altitudes, researchers say."
So far, so good. But later on the article wants to set up some
scientific disagreement, so it turns to the 'alternative view':
"Other scientists are not so sure, and
they see a surge in tornadoes last year as ominous."
'Other scientists' here means Kevin Trenberth, head of the
Climate Analysis Section at the US National Center for Atmospheric
Research (NCAR), who has previously
stated: "It is irresponsible not to mention climate change in
stories that presume to say something about why all these storms
and tornadoes are happening."
The rest of the article features Trenberth heavily. This seems
fine as the views of a prominent NCAR scientist are worth
featuring, in our view. But rather than rely on the views of just
one scientist for the argument that tornadoes and climate change
are linked, the piece intersperses carefully selected quotes from
the 2007 paper which seem to back Trenbeth's views up.
But the 2007 paper was authored by, amongst others, tornado
specialists Jeff Trapp of Purdue University and Harold Brooks of
the US National Severe Storms Laboratory.
It seems strange that the article uses quotes from their paper
to support Trenbeth's views rather than turn to these scientists
themselves, who actually give a rather more cautious view
For example, Trapp says in an
abc news report this week:
"Our biggest question is: how has, or
will human activity affect tornadoes? [...] That's the thing that
we haven't been able to unravel yet."
And co-author Brooks is very reticent to link tornadoes to
man-made climate change yet, as he
explained to Chris Mooney from Desmogblog last year:
"Brooks also thinks global warming is
likely to impact many weather phenomena-increasing the risk of heat
waves, for instance, and stronger precipitation events.
'But it doesn't necessarily mean that every bad weather event is
going to get worse,' Brooks continues, and when it comes to
tornadoes, 'I get really worried when people oversell the case.'
After all, if we're wrong and we go through a series of quiet
tornado years in the coming years, it will be just another weapon
with which to attack those who want climate action."
The media's need to set up a clear disagreement often simply
doesn't work for these kinds of science stories, where the reality
tends not to be so clear cut. For the moment, on tornadoes at
least, the general view from the scientific field seems to be: 'we