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Carbon Brief Staff

08.07.2013 | 8:31pm
ScienceNew study offers different take on the future of tropical cyclones
SCIENCE | July 8. 2013. 20:31
New study offers different take on the future of tropical cyclones

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 field.

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.

An outlier?

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 says:

“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, while 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 be.

Prof. Emanuel explained to Carbon Brief:

“General Circulation Models are really too coarse to produce good representations of tropical cyclones, though they often produce something resembling such storms.”

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 detail.

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 Climate

Emanuel, K. (2013) Downscaling CMIP5 climate models shows increased tropical cyclone activity over the 21st century. Proceedings of the National Academy of Sciences. DOI: 10.1073/pnas.1301293110

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