News•August 15, 2016

Aerosols May Be Masking Trend in Stronger Hurricanes

Over the course of more than a decade of research, scientists studying how the steady warming of the planet might impact hurricanes have become fairly certain that such storms will become more intense as the Earth continues to heat up. But they haven’t been able to consistently detect such a trend, despite the clear temperature rise over the past century.

Hurricane Patricia, one of the strongest hurricanes on record, as it neared the coast of Mexico in October 2015.
Credit: NASA

That could be because tiny particles called aerosols are having the opposite effect, a new study in a recent issue of the journal Science suggests. But that won’t be the case forever, as the influence of warming will eventually win out and the increase in hurricane intensity will become clear, the study authors say.

Reaching Their Potential

Hurricanes feed off warm ocean waters – the heat released by the evaporation of that water fuels the convection that powers storms. The atmosphere must also be unstable enough for convection to occur, which means temperatures higher in the atmosphere must be cooler than those below.

Every hurricane has a maximum intensity it could theoretically achieve given the temperature difference between the ocean surface and the upper atmosphere, a measure called potential intensity. (Other factors like dry air and wind shear can keep it from reaching that maximum.)

As greenhouse gases like carbon dioxide trap increasing amounts of heat, the bulk of that heat is absorbed by the oceans, and so the ocean warms faster than the atmosphere. That means that as the world warms, the potential intensity of all hurricanes should increase. And because potential intensity seems to track with actual hurricane intensities, this suggests “that the strongest future storms will exceed the strength of any in the past,” the study authors wrote.

Earth has already seen a temperature rise of nearly 2°F (1°C) since the beginning of the 20th century. Based on that amount of warming, both theory and climate models suggest that the trend in increasing hurricane intensity should already be apparent, if fairly small. But no such trend has been reliably detected on a global scale.

As was first suggested in a 2006 study focused on the Atlantic Ocean basin, the reason could be the competing influence of aerosols. These particles absorb and reflect sunlight back to space.

And as it turns out, aerosols seem to have a bigger impact on limiting hurricane potential intensity than they do on dampening global surface temperatures. That is likely because they act on a different wavelength of light than greenhouse gases, but the exact reasons are unclear, study author Adam Sobel, director of Columbia University's Initiative on Extreme Weather and Climate, said.

Effectively, aerosols could be counteracting the influence of warming, and masking the small uptick in intensity we otherwise would expect to see by now in the data.

The one ocean basin where indisputable evidence of an increase in intensity has been found is in the Atlantic, where the U.S. Clean Air Act and European efforts have substantially reduced aerosol amounts since the 1970s. Whether that or natural variations in hurricane activity are behind the trend is hotly debated by scientists, though.

Greenhouse Gases Will Win Out

Complicating efforts to better understand the historical trends is the relatively short time scale of reliable hurricane records.

While the past few decades saw increasing aerosol pollution from developing economies like China and India, aerosols globally have seemed to plateau.

The combination of efforts to further curtail aerosol emissions (because of the harmful public health impacts they can have) and ever-rising greenhouse gas emissions mean that aerosols won’t mask the upward trend in intensity forever.

That is particularly true if nothing is done to curb greenhouse gas emissions soon, and because carbon dioxide stays in the atmosphere far longer than aerosols, which wash out in a matter of days or weeks.

Exactly when the trend will become apparent isn’t certain, given the all the unknowns scientists are still trying to figure out.

“My guess would be that we will see trends in intensity pretty clearly by end of the century, but it's just a guess,” Sobel said in an email.