Hurricanes and Climate Change – What We Know For Certain

Hurricanes are bad news. Not only do they bring high winds strong enough to do significant damage and threaten lives, but they bring storm surges which temporarily and locally raise sea levels and inundate low lying shorelines.

What’s more they dump rain. Lots of rain. Rain by the foot; by the meter.

Hurricanes go by different names in different parts of the world (typhoons, cyclones) and so technically are lumped under the name “tropical cyclones.”

Are tropical cyclones getting worse in a warming world? It depends by what you mean by worse:

  • Are they getting bigger and stronger? Yes they appear to be.
  • Are they more damaging? Yes they appear to be.
  • Do they extend into more places than they used to? Yes they appear to.
  • Are they more frequent? Not so sure about this one, some forecasts are for reduced frequency.

But even the most authoritative body in the world on the topic of climate change, the IPCC, still finds it difficult be certain.

Part of the challenge is that tropical cyclones don’t happen frequently enough to build a big database rapidly. It wasn’t until after the mid twentieth century that recordkeeping became consistent and widespread enough to be able to compare storms quantitatively.

Another challenge is how complicated tropical cyclones are. They only occur over warm oceans and we know that the oceans are getting warmer. But as a cyclone gets stronger, the theory goes, it should kick up larger and larger waves out in the open ocean, and the larger waves might tend to mix cooler seawater from deeper below the surface to act as a kind of break on tropical cyclone growth.

At the same time, the warming oceans and the warmer air above them also mean that a greater volume of water will have evaporated up into the building storm. As it comes down as rain the fact that it is freshwater means that there could be a layer of less salty water at the ocean surface. Freshwater is less dense that saltwater and this difference could tend to reduce the mixing up of cooler subsurface water, putting a break on the break.

One thing that kills a cyclone dead is wind shear. For tropical storms to grow the overall airmass in which they form can’t be shifting along horizontal lines. Wind shear tends to blow their heads off and in a warming world there may well be more wind shear which would tend to break more tropical storms before they really got going.

Scientists also think that tiny particles in the air, something they call aerosols, are further mucking up the ability to see the effects of a warming world on tropical cyclones. Aerosols might be pollution from smokestacks but it could also be dust from deserts and anything else you can think of. Some of these might be making the atmosphere warmer, others making it cooler. As the heat of climate change builds though there is some expectation that the confusion caused by aerosols in figuring out cyclones will become less of a factor and negative side effects will dominate.

Lacking centuries-long accumulations of direct observation and measurement of cyclones, scientists have looked for signs of past cyclones in the traces they leave. Near shore lakes that regularly get inundated show traces of past storms going back thousands of years in their sediments. Stalagmites in some caves also show variation in isotopes based on flooding that could be associated with past tropical cyclones. These types of techniques are attempting to allow a better picture of how tropical cyclones behaved deeper into the past.

What we do know is that one of the main drivers of tropical cyclones is warmer oceans. We also know that we’re getting warmer oceans, both in terms of sea surface temperatures and also in terms of how high in latitude waters warm enough to spawn tropical storms extend.

This means not only more spawning area but also that if tropical cyclones do make landfall the extensions of coastline at risk are greater and extend further from the equator.

If that wasn’t enough bad news, the changes in the extent of the tropical zones come with a disruption in weather patterns further north that may be blocking cyclones and slowing down their progress. This means when they do hit land they hang around longer without moving on and do damage for longer while they’re there.

In terms of predicting tropical storms, higher surface water temperatures seem to make things more difficult. The conditions that might make a storm seem to click over then accelerate into a storm more quickly so forecasters have less time to evaluate their growing risk.

Warmer waters are coupled with warmer air. The two of them together mean that there is an increase in evaporation and ultimately rainfall. As the world continues to warm, the Intergovernmental Panel on Climate Change in it’s 2019 Special Report on the Ocean and Cryosphere in a Changing Climate predicts that tropical cyclones will increase the rain they bring.

The increasingly warmer sea surface temperatures also cause the IPCC in that report to predict that, of the tropical storms that do rise to the intensity of tropical cyclones, a greater number of them will continue to grow into major cyclones of category 4 and 5.

Even if tropical cyclones were not to get bigger and more intense, the IPCC has long since established conclusively that the oceans are rising due to climate change. In addition to the icecaps and glaciers melting, simply the fact that warmer water takes up greater volume means the oceans have risen and will continue to do so. When a tropical cyclone brings a storm surge to a sea level that’s higher anyway, even if the cyclone isn’t any bigger or stronger than historical averages, it’s bound to do more damage.

On the other hand, that 2019 IPCC report actually cautiously suggests that although tropical cyclones are likely to get bigger and badder, there may well be fewer of them.

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The confidence and caution with which the IPCC makes all these predictions is really a function of the robust consensus approach they take to writing their reports. In order for the IPCC to make a claim they have had to extensively consult all the published scientific papers on the subject; assessed what they all mean with the assistance of hundreds, or even thousands, of experts around the world; reviewed their findings with virtually all those experts and achieved formal consensus; then reviewed the result with representatives from virtually every nation on earth. It’s an exhausting, impossible sounding process. But it really does happen. And the result are IPCC reports that really represent a world consensus.

For tropical cyclones the science is still building and the scientists aren’t always all on the same page. So the predictions made about tropical cyclones in their oceans and cryosphere report are couched in terms of their degrees of confidence. The prediction that storm surges are going to get bigger than ever is made with “very high confidence.” They actually calibrate this statement to mean that they think they’d be right about this about nine out of ten times.

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The predictions that tropical cyclones will more often reach category 4 and 5, that they will carry 7% more rain for every degree rise in temperature are made with “medium confidence” which is to say they think they think it could happen but there’s about an equal chance they might be wrong. You can imagine that among participating scientists there would be differing views with some being very sure while others still want to see more evidence before they commit.

The cautious suggestion that the overall number of tropical cyclones might actually decrease is phrased as “low confidence (low agreement, medium evidence).” The calibration of this phrase is that they think there is about a two in ten chance the prediction could be right. Here they’ve also revealed that the scientific experts can’t really come to agreement yet, likely because although there is some evidence, there needs to be more.

studies continue – still expect decline in frequency but increase in strength, range, rain & slowing tracking

For more info on the IPCC — in a digestible form — check out the book Because IPCC.

It’s a short, 33 illustrated pages, an upbeat, entertaining story that explains the history and science of the IPCC. The scene is 100 years in the future when the world has “solved” climate change and people are looking back, inspired by the dedication, rigor and achievements of the scientists of today.

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