Advertisement

Column: Where are the hurricanes?

 
Published July 15, 2016

The United States coastline has been calm so far this hurricane season, just as it has been over the last decade. Since 2005, the year of Hurricanes Katrina, Rita and Wilma, the country has been in a hurricane "drought," with no major hurricane (Category 3 or above, meaning winds above 110 miles per hour) making landfall. The nation's most hurricane-prone regions, the Southeast and Gulf Coasts, have been eerily quiet.

Even so, climate scientists like me believe that human-induced climate change will strengthen hurricanes and lead to worse disasters. We know that significant global warming, over a degree and a half Fahrenheit, has already occurred since preindustrial days. So where, you might ask, are the powerful hurricanes?

They're coming, if we don't take more aggressive action to slow climate change.

What we have seen recently is consistent with our scientific understanding of hurricanes and climate. That knowledge is far from perfect, but the prediction of stronger hurricanes is not contradicted by the data thus far.

The best science doesn't, in fact, predict that the future will hold more hurricanes; most of our best models predict there may be fewer. But these predictions of changes in the number of hurricanes are quite uncertain, in part because they are connected to a more basic problem: Why does the number of tropical cyclones average about 90 per year, and not more or fewer?

We don't really know.

But when it comes to the strength of hurricanes, we have a pretty good comprehension of the physical science of how hurricane intensity is controlled by the large-scale climate. In a paper last week in the journal Science, several colleagues and I assess the state of this understanding and what it implies about the recent past, present and future of hurricanes.

As the climate warms, the physics says hurricanes should get stronger, because the tropical ocean surface heats up more than the atmosphere above it, increasing the temperature differential on which storms feed. The best computer models also predict stronger storms, so we have separate but consistent lines of evidence. Even if the number of hurricanes decreases somewhat, the overall increase in intensity may well mean that there are more of the strongest storms. And the very strongest storms of the future will probably exceed any of the past in their intensities.

While several groups of scientists who have done statistical analyses of data on all storms for the last few decades have found significant increases in the numbers of the strongest ones, Categories 4 and 5, it is also true that those results are not consistent across all studies.

It turns out that human influence on storm intensity is more complicated than we had thought. Human activities have not just increased atmospheric concentrations of greenhouse gases, but also concentrations of aerosols — tiny liquid or solid particles from cars, industrial smokestacks and fires. These particles tend to cool the climate by absorbing and reflecting sunlight, though they haven't been enough to prevent significant global warming over the last century.

Spend your days with Hayes

Spend your days with Hayes

Subscribe to our free Stephinitely newsletter

Columnist Stephanie Hayes will share thoughts, feelings and funny business with you every Monday.

You’re all signed up!

Want more of our free, weekly newsletters in your inbox? Let’s get started.

Explore all your options

But aerosol cooling appears to be disproportionately effective in reducing hurricane intensity, and climate models suggest that, because of the aerosols, hurricane intensity globally should not have increased much yet, despite warming from greenhouse gases.

But it isn't likely to stay that way.

Global aerosol concentrations appear to have reached something of a plateau, thanks to air quality regulations in the United States and Europe. While increases in aerosol emissions in Asia have offset the decline elsewhere, this pollution is unlikely to keep pace with rising greenhouse gas emissions. Moreover, aerosols tend to wash out after a few weeks, while carbon dioxide stays in the atmosphere for a very long time.

Barring global emissions reductions much larger than anything agreed on in the Paris climate accord (or the development of carbon-capture technology on a significant scale), greenhouse gas concentrations will continue to increase. And without the compensating effect of increasing aerosol emissions, the warming of the future, more than the warming of the recent past, will strengthen the most powerful and destructive storms that the planet can produce.

Adam Sobel, an atmospheric scientist at Columbia University, is the author of "Storm Surge: Hurricane Sandy, Our Changing Climate and Extreme Weather of the Past and Future."

© 2016 New York Times