New study offers different take on the future of tropical cyclones
- 08 Jul 2013, 20:31
- Freya Roberts
The best evidence scientists have at the moment
suggests tropical cyclones may become more intense under climate
change, but are unlikely to increase in number.
But a new study just published in the
Proceedings of the National Academy of Sciences challenges the
status quo, suggesting tropical cyclones will become more intense,
and occur more frequently.
We take a closer look at the
study from Professor Kerry Emanuel and investigate
how his modelling predictions fit with other research in the
More, heavier storms
Tropical cyclones are large storm systems with
spinning winds, fuelled as warm moist air rises and releases
energy. They bring heavy rainfall and high winds, and drive up sea
levels creating storm surges. This handy video
from the Met Office explains more.
In order to find out how tropical cyclones might
be affected by climate change, Emanuel's
new study used a 'downscaling' approach - taking
data from global climate models and feeding it into smaller, more
specialised climate models.
Six global scale climate models were used to
produce a broad picture about what earth's climate would be like
under high levels of greenhouse gas emissions. From this,
information about air and sea temperatures, wind patterns and
atmospheric moisture, was used to simulate where and when tropical
cyclones might occur in the future.
The results suggest that the number of tropical
cyclones could exceed 100 per year by about 2070, compared to an
average of 90 per year at the moment. Tropical cyclones could get
more intense too, if the modelling is right.
The total amount of energy tropical cyclones
release is expected to increase by 45 per cent over the course of
the 21st century. Some of that energy would be spent by the extra
10 or so tropical cyclones per year, but half of it would be
released by intense storms getting even stronger - meaning higher
winds, taller storm surges and greater economic costs.
These findings - that both the intensity and
frequency of tropical cyclones could increase - are unusual. Most
research suggests that while tropical cyclones could become more
intense under climate change, the number of tropical cyclones is
likely to stay roughly the same, or decrease.
The Intergovernmental Panel on Climate Change
(IPCC) made this point clearly in its extensive 2012
report on extreme weather events (p.158). It
"It is likely that the
global frequency of tropical cyclones will either decrease or
remain essentially unchanged. An increase in mean tropical cyclone
maximum wind speed is likely, although increases may not occur in
all tropical regions."
Other studies since the IPCC's report have
reached similar conclusions.
One found that the number tropical cyclones
in the North Atlantic would change little over the 21st century,
another study using similar techniques found
the number would decrease. A third study due to be published soon*
used a slightly different modelling technique, but found there
would be little significant change in the global number of tropical
cyclones over the 21st century.
There are a number of reasons why the
conclusions of Emanuel's
new paper may differ from other studies. Not all
studies assume future emissions will pan out in the same way, and
not all studies focus on the global picture.
On top of that, not all research on tropical
cyclones and climate change uses the 'downscaling' approach. Some
studies instead use the global climate models to try and predict
how tropical cyclones will change in the future. But these models
are thought to underestimate the frequency of tropical cyclones,
and struggle to accurately simulate how intense those storms will
Prof. Emanuel explained to Carbon
"General Circulation Models
are really too coarse to produce good representations of tropical
cyclones, though they often produce something resembling such
Why so different?
Given the differences in methods, not all
studies can be compared like for like. But Emanuel admits it is
surprising that the results of his study differ so much from others
which also use the downscaling technique.
Carbon Brief asked Prof. Emanuel why his
modelling suggested the frequency of tropical storms would increase
where other studies haven't. He told us:
"We are not yet sure why
the technique applied to the current generation of global climate
models shows an increase in activity, though I suspect it has more
to do with projected decreases in manmade aerosols than with
increasing carbon dioxide."
Research has recently
suggested that emissions of small particles
called aerosols might have damped down tropical storm activity in
recent decades. It's expected that fewer of these particles will be
released into the atmosphere in the future, so storm activity could
rise again. At the moment though this is a relatively new area of
science, and not something this study looks at in any
For now, Emanuel's study appears to disagree
with the body of scientific evidence when it comes to the frequency
of tropical cyclones under climate change. A leaked draft of the
IPCC's next major report on climate science suggest mosts studies
have not moved on from the majority view that cyclone frequency
will not increase.
But on at least one thing, and it's an important
thing, this study and the rest of the research agrees. The
strongest tropical cyclones are likely to get even more intense
under climate change - and these are the storms which are most
costly in economic terms, and for human lives.
*Camargo, S. (2013) Global and regional aspects of
tropical cyclone activity in the CMIP5 models. Journal of
Emanuel, K. (2013) Downscaling CMIP5 climate
models shows increased tropical cyclone activity over the 21st
century. Proceedings of the National Academy of Sciences. DOI: