Over the past 40 years, typhoons that strike East and Southeast Asia have become stronger — and presumably will only continue gathering strength, thanks to climate change. The likely cause is warming ocean waters near the coasts, according to new research.

The study, published today in the journal Nature Geoscience, falls short of pointing to climate change as the main culprit, but it warns that typhoons hitting eastern China, Taiwan, Korea, and Japan will likely be even stronger — and deadlier — in the future. That’s because ocean surface water is projected to become warmer in the years ahead.

"If you have warming coastal water, it means that typhoons can get a little extra jolt just before they make landfall," says Kerry Emanuel, a professor of meteorology at the Massachusetts Institute of Technology, who wasn’t involved in the study but provided some data to the researchers, "and that's obviously not good news."

"That’s obviously not good news"

Typhoons are storms that form over the ocean in tropical areas, with winds of at least 74 miles per hour. When they make landfall, they can become deadly and destructive. The most recent typhoon to hit the shores, called Lionrock, hit Japan last week and killed at least nine people.

Warming ocean waters intensify typhoons because they provide more heat — therefore, more energy — to the storm. To help illustrate the problem, imagine getting out of the shower; you feel chilly because the water is evaporating from your skin, carrying heat with it. That heat energy that’s leaving your body doesn’t disappear, it gets added to the air. The same happens with storms. "The fuel that powers the [storm] is an enormous transfer of heat from the ocean to the atmosphere when you have strong winds blowing across the surface," Emanuel says.

The researchers looked at two different data sets to calculate the intensity of the tropical storms from 1977 to today: the Joint Typhoon Warning Center (JTWC), managed by the US Navy and Air Force, and the Japan Meteorological Agency (JMA). They found that typhoons that strike East and Southeast Asia have intensified by 12 to 14 percent. They also found that the number of category 4 and 5 typhoons — those with wind speeds between 130 mph and 157 mph or higher — increased to around seven per year today, from less than five in late 1970s.

The researchers also looked at how the ocean water temperatures in the northwest Pacific changed between 1977 and 2013. They found that the ocean waters off the coasts of East and Southeast Asia, where typhoons got stronger, became a lot warmer. In the open ocean, instead, where temperatures haven’t increased that much, the intensity of typhoons hasn’t changed significantly. That means that the stronger storms are probably due to the warming waters, which are known to funnel energy into tropical storms.

Is it human-made climate change? We can't really tell

Instead of looking at global trends, the study focuses on specific typhoons that make landfall in highly populated areas and usually make severe damage. "It’s the first time that anybody has really looked in a concrete way at such a regional trend," Emanuel says. But the study also falls short of saying why exactly typhoons are getting stronger: is it human-made climate change, or just variations that happen naturally on our planet? With only about 40 years worth of data, we can’t really tell.

"The caveat with the study is the data they’re dealing with," says Suzana Camargo, a research professor at the Lamont-Doherty Earth Observatory at Columbia University who wasn’t involved in the study, "but it’s a problem there’s no way around."

Before 1977, the JMA didn’t provide wind measurements of typhoons, says Wei Mei, an assistant professor at the University of North Carolina at Chapel Hill and the lead study author. And not a lot of satellite measurements were done before the 1970s, Mei says.

"Typhoons can cause very severe damage in human society."

Some researchers point to the fact that the typhoon data provided by the JTWC and JMA is inconsistent. The JMA, for example, calculates the storm’s wind averages over 10 minutes, while the JTWC calculates it over one minute. The researchers had to adjust the JMA data to make up for the discrepancy, which of course leaves room for error. "There is some uncertainty in the results," Christina Patricola, an associate research atmospheric scientist at Texas A&M University wrote in an email, "because those estimates rely on the validity of the assumptions behind them."

But Emanuel, the meteorology professor at MIT, thinks the authors did a good job at cross checking the JTWC and JMA data with other sources, like satellite measurements and their own modeling. "I think it’s pretty strong," Emanuel says.

What remains to be determined is the million-dollar question: are the stronger typhoons a result to human-made climate change? Even if we don’t have the answer to that question yet, we know that climate models project ocean waters off the coast of China and Southeast Asia to continue to warm up. So that means that stronger and deadlier storms are likely to occur in the future. "Typhoons can cause very severe damage in human society," Mei says. "An important factor in determining the damage is intensity and also the size of the storm."