COP21: How global warming is leading to bigger and deadlier storms?

Leading US meteorologist, Alexander MacDonald, explains to France 24 the effects of warming oceans on hurricanes, cyclones, typhoons

Dire warnings about devastating hurricanes are a hot topic at the COP21 climate talks in Paris. FRANCE 24 asked a leading meteorologist to explain how global warming is leading to bigger and deadlier storms.
UN Secretary-General Ban Ki-moon on Wednesday announced a slew of initiatives, along with investment pledges of up to $1 billion, to better protect "hundreds of millions of people" who are the worst hit by "extreme climate".
The new projects unveiled at the COP21 conference in Paris are meant to better prepare vulnerable communities for natural disasters and help them rebuild after being hit by hurricanes or floods. Another key component of the initiative is funding to develop early warning systems for over 50 poor countries and small island states.
It is often repeated that warmer ocean waters are leading to bigger and more frequent storms. But how does this phenomenon actually work, and is it true the world is doomed to increasingly severe hurricanes in the future?
FRANCE 24 asked Alexander MacDonald – Director of the Earth Systems Research Lab, a United States-government weather and climate laboratory in the state of Colorado, and the president of American Meteorologal Society – to explain.
FRANCE 24: Why do we say warmer ocean waters lead to bigger hurricanes?
Alexander MacDonald: There was a famous Frenchman, Sadi Carnot, who showed that if you want to build an engine it depended on having warm air and cold air, and the difference in temperature between those two could tell you how strong you could make your engine. In the past decades, really wonderful work by a scientist by the name of Kerry Emanuel showed that basically a hurricane or a typhoon is a heat engine that gets its energy from the difference of temperature between the warm ocean and the stratosphere. About 15 kilometres above the surface of the earth it's very cold, minus 80 degrees Celsius. The amount of energy available in a heat engine, which a hurricane is, depends on the temperature difference. The bigger the temperature difference between the warm and the cold part, the bigger the energy. And there is something else. A tropical storm gets so strong that it starts to stir up the water, bringing cold water up from below and sort of commiting suicide. But if there's really warm water all the way down to 200 and 300 metres deep in the ocean, it doesn't commit suicide. So the warmer and deeper the warm water, the more the storm can just keep growing and growing.
FRANCE 24: So is it true the Earth is seeing bigger and more frequent hurricanes, and is the trend likely to continue?
There is a lot of discussion of the effects of warming oceans on hurricanes, cyclones, typhoons. All of these are different names for the same thing, which is a tropical storm. A lot of the research shows that perhaps the frequency of tropical storms may not change that much. However we do know that warmer water and deeper water enhances the more severe tropical storms. And therefore some studies indicate that later in this century there will more of these category 3, 4 and 5 tropical storms, in other words, hurricanes and typhoons that are tremendously destructive. I think that we are already seeing some tremendously powerful storms. We just recently saw a hurricane off of Mexico, the deepest [lowest pressure] storm in the history of the Western Hemisphere, we had Typhoon Haiyan a couple years ago come into the Philippines. It was extremely destructive.
FRANCE 24: Are some areas of the globe likely to experience worse storms than others?
Geographically I think we don't really know. There was one study that showed that the Atlantic is likely to have stronger winds up above as global temperatures change, which would make it harder for the storms to grow. That means we wouldn't have as many storms in the future. But the Pacific is going the other direction, it seems there would be more Pacific storms. But these are all preliminary studies.
FRANCE 24: Are you satisfied with the advances in predicting storm paths that allow populations to prepare for them?
I am more than satisfied with what is happening. When I started out as a young weather forecaster in the United States Air Force our forecasts had almost no skill. This was 40 years ago. If you take our hurricane track forecasts, how accurate they are three days from landfall, the average error 40 years ago was about 600 kilometers. Our three-day predictions now are down in the area of 120 kilometres, so we have improved by a factor of four in these 40 years. So I am really proud as a researcher who has helped develop these greatly improved, complicated global weather models that deliver the kind of forecasts that people need to plan and to stay safe.
FRANCE 24: Can the scientific community continue to improve early warning systems, or has it more or less hit a ceiling?
We are a long way from a ceiling. If you take our skill in forecasting, it has been going up steadily at almost the same rate for four decades, and I bet we can do it another four decades.
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