Hurricanes have become stronger worldwide during the past four decades, an analysis of observational data shows, supporting what theory and computer models have long suggested: climate change is making these storms more intense and destructive.

The analysis, of satellite images dating to 1979, shows that warming has increased the likelihood of a hurricane developing into a major one of Category 3 or higher, with sustained winds greater than 110 miles an hour, by about 8 percent a decade.

“The trend is there and it is real,” said James P. Kossin, a researcher with the National Oceanic and Atmospheric Administration and lead author of the study, published Monday in Proceedings of the National Academy of Sciences. “There’s this remarkable building of this body of evidence that we’re making these storms more deleterious.”

Kerry Emanuel, a hurricane expert at the Massachusetts Institute of Technology who was not involved in the study, said the findings were “much in line with what’s expected.”

“When you see things going up all over the globe like that, the ducks are kind of in order,” he said.

Physics suggests that as the world warms, hurricanes and other tropical cyclones should get stronger, because warmer water provides more of the energy that fuels these storms. And climate simulations have long showed an increase in stronger hurricanes as warming continues.

But confirming that through observations has been problematic, because of the relatively small number of hurricanes every year and the difficulty of obtaining data on their wind speeds and other characteristics. Even in the United States, storms that do not potentially threaten populations are measured less than others.

“We’re doing collectively a bad job of measuring tropical cyclones around the world,” Dr. Emanuel said. “We’ve all believed we should see more intense hurricanes. But it’s very very tricky to find it in the data.”

Dr. Kossin and his colleagues got around the limitations by using satellite images of storms worldwide and using computers to interpret them with a long-accepted pattern-matching algorithm, or set of instructions. They had done this before, in a study published in 2013, but that analysis only included imagery from 1982 to 2009 and the findings, while similar, were not statistically significant.

In the new study the researchers extended the data set by 11 years, using imagery from 1979 to 2017.

“The first time through we found trends but they hadn’t risen to the level of confidence that we would require,” Dr. Kossin said. The findings of the new study are statistically significant.

“This is saying, OK now, the historical observations are also in agreement” with the theory and models, he added.

The study looked at tropical storms worldwide because that provided a lot more data than looking at those in just one region. And every region has natural variability or other factors that can affect storm intensity and make it more difficult to tease out the effects of warming.

“When you look at the picture globally, it tends to wash away that regional variability,” Dr. Kossin said. “The trend rises above the noise.”

The North Atlantic has seen increased hurricane activity in recent decades, by a measure that combines intensity with other characteristics like duration and frequency of storms. On Thursday, NOAA will issue its forecast of activity for this season, which officially runs from June 1 to November 30. Forecasts by other organizations have suggested that this year may be an active one.

But the North Atlantic is one region where climate change may be overshadowed by other factors, Dr. Emanuel said.

“We do see clear signals and strong trends in the North Atlantic,” he said. “The problem is we can’t uniquely attribute that to greenhouse gases.”

Some scientists say that long-term natural variability in sea surface temperatures, on a time scale of decades, has played the major role in affecting North Atlantic storm activity. Others say that mandated reductions in sulfur emissions from fossil-fuel burning over the past few decades may be more important, by affecting ocean temperatures through a series of atmospheric connections.

Whatever the main factors are, the study suggests that climate change will play a long-term role in increasing the strength of storms in the North Atlantic and elsewhere, Dr. Kossin said. Planning for how to mitigate the impact of major storms must take this into account.

“From a short time scale, these trends are not going to change the risk landscape,” Dr. Kossin said. But over the long term, he said, “the risk landscape could change, and in a bad way, not in a good way.”

source: nytimes.com

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