Extreme weather events to expand into more populated regions

Climate change will expand the range of tropical cyclones, making millions more people vulnerable to these devastating storms, a new study says.

At present, these cyclones - or hurricanes as they are also known - are mainly confined to the tropical regions north and south of the equator.

But researchers say that rising temperatures will allow these weather events to form in the mid-latitudes.

This area includes cities such as New York, Beijing, Boston and Tokyo.

The study has been published in the journal Nature Geoscience.

The scientists involved say their work shows by the end of this century, cyclones will likely occur over a wider range than they have for three million years.

When subtropical storm Alpha made landfall in Portugal in September 2020, the relatively small scale of damage caused by the cyclone made few headlines.

But for scientists this was quite a momentous event.

"We hadn't observed this before," said Dr Joshua Studholme, a physicist from Yale University.

"You had a traditional kind of mid-latitude storm, that sort of decayed, and in its decay, the right conditions for a tropical cyclone to form occurred, and that hadn't happened to Portugal before."

Dr Studholme is the lead author of this new study, which projects that a warming climate will see the formation of more of these types of storms in the mid-latitudes, where most of the world's population lives, and where most economic activity takes place.

He explained that as the world gets hotter, the difference in temperature between the equator and polar regions will decline, and this will impact the flow of the jet streams.

Normally, these high-altitude rivers of air act as a kind of border guard for hurricanes, keeping them closer to the equator.

"As the climate warms, that sort of jet stream activity that happens in the middle latitude, will weaken and in extreme cases split, allowing this sort of cyclone formation to occur."

The question of the impact of human induced climate change on hurricanes has been contentious in the past, but recent research suggests that the connections are becoming clearer.

Last August, the Intergovernmental Panel on Climate Change published the first part of its sixth assessment report, dealing with the science of a warming climate.

In relation to hurricanes and tropical cyclones, the authors said they had "high confidence" that the evidence of human influence has strengthened.

"The proportion of intense tropical cyclones, average peak tropical cyclone wind speeds, and peak wind speeds of the most intense tropical cyclones will increase on the global scale with increasing global warming," the IPCC said.

The new research published on Wednesday makes use of multiple strands of evidence to show that tropical cyclones in future are likely to occur over a wider range than previously thought.

"What we've done is make explicit the links between the physics going on within storms themselves and the dynamics of the atmosphere at the planetary scale," said Dr Studholme.

"This is a hard problem because this physics isn't well simulated in numerical models run on modern computers."

The likely expansion of these storms poses a significant danger to the world, especially when the other impacts of warming come into play.

"Tropical cyclones in the mid-latitude band could experience other changes such as slower motion and heavier rainfall," said Dr Gan Zhang, previously an atmospheric scientist at Princeton University and NOAA who wasn't involved in the new paper.

"These tropical cyclone changes, plus pronounced coastal sea level rise might compound potential societal impacts."

Dr Zhang cautioned that the sensitivity of tropical cyclones to warming has a high level of uncertainty but he said the risk from these storms could still increase even with moderate levels of warming.

Certainly, the authors argue that this course is not set in stone and that dramatic reductions in carbon emissions over the next decade particularly, could alter the outcome.

"The control over this is the temperature gradient between the tropics and the poles, and that's very tightly linked to overall climate change," said Dr Studholme.

"By end of this century, the difference in that gradient between a high emission scenario and a low emission scenario is dramatic. That can be very significant in terms of how these hurricanes play out."