Tropical cyclones becoming fewer as climate warms

Tropical cyclones have become less frequent because of climate change, according to a new analysis.

The research, which was published in Nature Climate Change, suggests an overall decline in tropical cyclones globally occurred during the 20^th Century and that trend is continuing.

But the researchers emphasise that this doesn’t mean we can lower our cyclone guard in the 21^st Century – fewer cyclones today can still cause more damage.

It’s difficult to examine the history of extreme weather events like cyclones before the 1960s, because human records are incomplete. (From the 1960s, satellite data has made things much easier.)

The researchers addressed this by using the Twentieth Century Reanalysis dataset. The Twentieth Century Reanalysis project, spearheaded by the US National Oceanic and Atmospheric Administration, uses modelling and records to reconstruct a dataset of global weather conditions in fine detail, all the way back to 1836.

Using this proxy, the researchers found that there had been an overall 13% decline in the number of cyclones forming during the 20^th century. Cyclones decreased in intensity in nearly every place they occur on the globe.

“The only exception to this trend is the North Atlantic basin, where the number of tropical cyclones has increased over recent decades,” says lead author Dr Savin Chand, a researcher at Federation University.

“This may be because the basin is recovering from a decline in tropical cyclone numbers due to human-related aerosol emissions in the late 20th Century. The number of annual storms is still, however, lower than in pre-industrial times.”

The researchers theorise that increased temperatures have made conditions more hostile to cyclone formation. There are a few atmospheric reasons that may account for this, including drying conditions in the middle of the troposphere, and increasing wind shear.

“It may be good news that fewer cyclones are forming because of anthropogenic global warming. It should be noted, however, that frequency is only one aspect controlling the risks associated with tropical cyclones,” says Chand.

“Geographical distributions of cyclones are shifting, and tropical cyclones have been getting more intense in recent decades.

“In some parts of the world, they are also moving closer to coastal areas where populations and developments are growing.”

Other research has suggested we can expect higher rainfall from the cyclones that do form.

“However, as these factors were not assessed in our study, no direct conclusions on the overall changes in risk can be derived,” says Chand.

As modelling improves, the researchers expect to be able to find more long-term trends on tropical cyclone intensity and other risks.