Could a tropical cyclone hit Sydney? What climate change science does — and doesn't — say about future storms

Tropical Cyclone Alfred was no normal cyclone.

Firstly, it moved the wrong way. Most cyclones that form in the Coral Sea move east then out to sea by the time they get as far south as Brisbane. This one began on that trajectory, then turned around.

It also threatened to make landfall further south than usual. The last cyclone to get that close to Brisbane was Tropical Cyclone Wanda, which hit just north of the Sunshine Coast in 1974.

Alfred also moved at a slower pace than your average cyclone, generating more damaging waves and washing away more sand from beaches in the process.

As global temperatures climb, scientists say some of the things that made Alfred unique could be a taste of the future.

So, could climate change one day send a cyclone as far south as Sydney? And will such destructive storms become stronger or more frequent?

Let's look at what we do and don't know about the future of cyclones in a warmer world.

Tropical Cyclone Alfred moved down the eastern coast of Australia, before turning west towards south-east Queensland. (ABC News)

Cyclones will become more intense

One thing all climate scientists agree on is that tropical cyclones will probably become more intense in the future.

To explain why, it might help to look at some of the ingredients you need to make a cyclone in the first place:

• Warm water (generally 26.5 degrees Celsius or more)
• What's called "low vertical wind shear" (that means pretty even wind speeds at different heights of the atmosphere — if winds increase too rapidly with height, that can break up the circular motion of a cyclone while it forms)
• High humidity throughout the atmosphere

As global temperatures build, some of those ingredients are intensified — essentially creating more fuel for tropical cyclones.

Strong winds pictured at Airlie Beach during Cyclone Debbie in 2017. (AAP: Dan Peled)

One of those ingredients is warmer water, says Mark Howden, the director of the Institute for Climate, Energy and Disaster Solutions at The Australian National University.

"Because of increased ocean temperatures, those cyclones that do occur are likely to be stronger than they would otherwise be," Professor Howden says.

"That's because there's more energy in the ocean, and so those cyclones, when they do form … become much stronger, with higher wind speeds."

It's not just about the warmer water. A warmer atmosphere also helps add fuel to cyclones, says Kimberley Reid, a climate scientist with the ARC Centre of Excellence for the Weather of the 21st Century at the University of Melbourne.

"As the atmosphere warms, it can hold more moisture, so that means when it rains, it will likely rain harder," she explains.

On top of that, Dr Reid says, condensation (the process of water vapour turning into a liquid, which is what happens when clouds form and rain falls) is "exothermic" — meaning it adds even more energy to the atmosphere.

So a wetter atmosphere actually makes cyclones both windier and rainier.

That's bad news because even small increases in wind speed mean big increases in damage. Wind's power — and potential to cause damage — is equivalent to the cube of its speed. That means if wind speed doubles, the damage it causes can increase eightfold.

Rising sea levels and higher rainfall mean we can also expect bigger storm surges and more coastal erosion from tropical cyclones, says Hamish Ramsay, a principal research scientist at CSIRO who specialises in tropical cyclones.

He also points out that this is not a new area of research: Even back in 1987 scientists found increasing CO2 in the atmosphere could make hurricanes more destructive.

"Climate scientists have been studying the impact of climate change on tropical cyclones for decades now," Dr Ramsay says.

Cyclones could become less frequent

There is possibly some good news: we might see fewer tropical cyclones in the future. But it's far from certain.

"There actually is a reasonable amount of dispute in the scientific community about how many cyclones we're likely to get under climate change," Professor Howden says.

We have already seen a declining number of tropical cyclones around Australia. This has been documented by satellite data dating back to the early 1980s, and also seen in other less reliable data from the beginning of the century onwards, Dr Ramsay says.

The scientific debate is about whether this trend is likely to continue.

Rising global sea levels mean we can expect bigger storm surges from cyclones. (Supplied: Stuart McPhail Photography)

Some projections say cyclones will become less frequent, but stronger. Others say they will become more frequent and stronger.

"I think the key issue is we really need longer data sets to be able to make confident statements, and we're dealing with something that happens a few times a year, so we're not confident yet that these trends are necessarily going to continue," Dr Reid says.

Despite the uncertainty over possible future frequency, Professor Howden stresses: "There is very little debate that, of the cyclones that occur, they're likely to be considerably stronger than they have been in the past, and have all those other characteristics: higher peak wind speeds, more intense rainfall, more opportunities for rapid intensification."

Conversely, even though tropical cyclones could become less frequent, the cyclone season could become longer. It comes down to that key ingredient again: warm water.

"With average sea surface temperatures going up, the threshold over which the temperatures go up in the summer period gets exceeded earlier on," Professor Howden says.

"So the cyclone season extends, just like our fire season has extended here in Australia."

Cyclones could move further south

Remember how a cyclone needs a bunch of ingredients, including warm water — roughly 26.5C or higher? Well, as the climate warms, water further and further from the equator reaches that temperature.

Professor Howden says to date, some estimates indicate the area where cyclones can form could grow by about 60 kilometres each decade.

"So if you're looking over a 40-year period, you've got an extra 240km of Australia's coastline which could be affected by cyclones, that used not to be affected by cyclones," he said.

We've already seen this happening: research published in 2014 found the locations where cyclones reach their peak intensity have shifted further north in the northern hemisphere, and further south in the southern hemisphere. 

But Dr Ramsay says the scientific community is less certain about whether that trend will continue into the future, and that more research is needed.

"It doesn't mean … that it won't happen. It just means that we are still investigating," he says.

Experts say it's "highly unlikely" a cyclone could make it as far south as Sydney. (ABC News: Taryn Southcombe)

So if there's a chance tropical cyclones could move further south, does that mean one could hit Sydney? Experts say it's highly unlikely to happen any time soon.

There are two big reasons for that. Firstly, waters off the coast of Sydney are a bit too cool for tropical cyclones. And even if the water was warm enough, that far south the strong atmospheric winds (or "high vertical wind shear") are the kind that tear cyclones apart.

"It's pretty unlikely you'll see a full blown tropical cyclone hitting Sydney," Dr Reid says.

"However, it's pretty normal to see, say, an ex-tropical cyclone, and they can still do quite a bit of damage. They can still bring quite a lot of rainfall. So even though it's not officially a tropical cyclone, you can still get strong weather systems like a tropical cyclone hitting Sydney."

Climate change is 'embedded' in all cyclones

Given some of these projections are uncertain, a few people have asked the ABC why they should give a damn about what climate science tells us about cyclones in the first place.

Experts such as Dr Reid say it's about being prepared for the future, even if that future is uncertain.

"There are a range of possibilities. They [tropical cyclones] may decrease in frequency. They could be more intense," she says.

"I guess it's a matter of individuals' and governments' appetite for risk. Do you want to spend the money to build the infrastructure that is more resilient to a tropical cyclone, even if you might not get an intense cyclone? Or would you want to take the risk of not doing that?

"The thing about climate change scenarios is it's all probability. No-one can see the future. We're just making our best possible guess based on our understanding of physics and the planet."

Even though cyclones can reach as far south as Brisbane (pictured in 1974), homes do not need to be built to cyclone standards. (Supplied: John Oxley Library, State Library of Queensland)

Another question that's emerged is whether Cyclone Alfred was caused by climate change. Some scientists, such as Professor Howden, believe this is the wrong way to frame the issue.

"What I try to emphasise is that every cyclone that we see now across the globe has climate change embedded in it, and that's because sea surface temperatures have gone up everywhere," he says.

"So it's not a question of … 'Is this cyclone caused by climate change?' It's a question of, 'How much stronger is this cyclone because of climate change?'"