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Tropical Cyclone Amphan is approaching the coasts of India and Bangladesh on May 19.
enlarge Tropical Cyclone Amphan is approaching the coasts of India and Bangladesh on May 19.

Science is relatively simple if you can compare a model prediction with data and assume that the first accurately reflects our theories and the second accurately what happens in nature. But for some reason, the data doesn’t always live up to the task of representing the natural world well. That’s part of what makes the relationship between climate change and tropical cyclones something that requires a little explanation.

Trend spots

From climate model projections (and basic physics), some things are pretty well established: sea level rise exacerbates storm surge damage, warmer temperatures increase the atmosphere’s ability to trap water vapor (and dump it as rain), and warmer temperatures increase the energy available to tropical cyclones. to make it stronger. Sea level rise is, of course, simple enough to measure, but checking tropical cyclone records for trends in intensity is much more complicated.

This is partly because tropical cyclones in different ocean basins can be affected by natural variations in regional atmospheric patterns and ocean temperatures. But the bigger problem is that our methods of collecting data on tropical cyclones have changed and improved over time. If a trend appears in the data, is it real or is it just an artifact of the advent of expanded satellite coverage? (A trend that may be expanded by improvements in satellite resolution or instrumentation.)

For these reasons, the expectation that the strongest tropical cyclones would become more frequent was difficult to verify against the data. In 2013, a group of researchers attempted to overcome this limitation by producing a consistent apple-to-apple dataset for the satellite era, which was from 1982 to 2009 at the time. This included an automated algorithm to identify and quantify storms and resample the data at a common resolution.

That study found a trend toward stronger storms worldwide, but fell short of the usual standard for statistical significance. The power of the trend matched predictions, but a 28-year record wasn’t long enough to expect that trend to weather the noise of natural variability.

More time, more data

In a new study, the researchers extended their approach from 1979 to 2017 and about 4,000 tropical cyclones. And the global analysis is doing now show a statistically significant trend towards stronger tropical cyclones. For example, if you compare the first half of that 39-year period to the second half, the proportion of storms entering Category 3 or higher has increased by about 8 percent per decade.

This is the global fraction of tropical cyclones belonging to category 3-5.

This is the global fraction of tropical cyclones belonging to category 3-5.

Somewhat surprisingly, the increase stops at Category 5. However, this could simply be a function of resampling all the data at 5 miles (5 km) resolution – stronger storms tend to have smaller eyes and so are more difficult to distinguish at this resolution. The data does show a trend towards smaller eye size, so that makes sense.

However, things get messier when you go to individual ocean basins. The greatest effect is seen here with hurricanes in the North Atlantic. Typhoons in the western North Pacific, on the other hand, don’t actually show any change.

Why the differences? Dividing the dataset into smaller chunks increases the prevalence of noise, just as looking at a shorter span of time would. And global warming isn’t the only factor at play. Regional variability that more or less cancels out on a global scale has an impact when you zoom in. Volcanic eruptions and even greenhouse gas emissions can also have effects that vary from place to place, as shown, for example, in a study published earlier this month.

Specifically for the western North Pacific, the researchers note that the poleward migration of storm tracks there may also contribute to the lack of trend in intensity. That migration has in fact grown into an area where conditions are less than ideal for typhoons.

“Ultimately,” the researchers write, “there are many factors that contribute to the characteristics and observed changes in [tropical cyclone] intensity, and this work makes no attempt to formally unravel all of these factors.” But, they say, “the consistency of the trends identified here with expectations based on physical insight and simulations of global warming increases confidence that [tropical cyclones] have become significantly stronger, and that there is likely a human fingerprint on this increase.”

PNAS, 2020. DOI: 10.1073/pnas.1920849117 (About DOIs).

By akfire1

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