Australia Weather News

You've probably heard people blame the devastating impacts of cyclones, hurricanes, or typhoons on climate change.

On the other hand there are those that dismiss any suggestion of a link, often pointing out that extreme weather is part of nature and has nothing to do with us.

While most scientific predictions agree that cyclones will become more intense as the climate warms, it's been very difficult to isolate the impact of climate change on storm severity to date, because there are so many interacting factors.

But new research, published in Nature today shows that there is a relationship between existing climate change — the world has warmed by 1 degree Celsius, on average, since 1900 — and storm severity.

Researchers from Berkeley in California have analysed the intensity of three major US storms: hurricanes Katrina, Irma and Maria.

They created computer-generated simulations of each storm, based on the known atmospheric and ocean temperature conditions that influenced each of these weather events.

They fine-tuned the parameters of their models until they consistently mirrored the intensity and track of the actual hurricanes.

Once they were confident that their models accurately replicated the storms as they occurred in 2005 for Katrina, and September 2017 for both Irma and Maria, they were able to adjust the temperature variables to test their influence.

When they compared the simulations using pre-industrial temperatures with the simulations based on the actual conditions, they found that there was little difference in the wind speeds of the three hurricanes, but that rainfall was higher, according to researcher Christina Patricola.

"For those three storms, the effect of climate change so far produced an increase in rainfall in the order of 5-10 per cent," Dr Patricola said.

"So it's suggesting that we're already starting to see some influence of climate change on tropical cyclones."

For Hurricane Katrina, they found that rainfall was increased by between 4 and 9 per cent, 6 per cent for Hurricane Irma and Hurricane Maria's rainfall was increased by 9 per cent.

Flooding, rather than high winds often causes the most damage from cyclones, and can lead to other issues like water-borne diseases in the aftermath.

Worst-case scenarios produce a third more rain, strong winds

The researchers also ran simulations using predicted future temperatures under different climate change scenarios as outlined by the Intergovernmental Panel on Climate Change (IPCC).

The four scenarios they tested, known as Representative Concentration Pathways (RCPs), correspond to average temperature increases of 4.9C, 3.0C, 2.4C and 1.5C by 2100.

Under each of those RCPs, the intensity of the storms were increased significantly, according to Dr Patricola.

"In the future scenarios, it didn't matter which RCP it was, we saw that the model was giving a strong indication of increases in wind speed and increases in rainfall," she said.

Under the worst-case climate change scenario, rainfall was increased by up to 30 per cent in some regions and wind speeds were 54 kilometres per hour faster than present-day peaks.

Although their research was specific to those three storms, their findings are consistent with existing hypotheses regarding the impact of climate change on tropical cyclones.

And it is likely that warming is also already having an affect on cyclones in Australia, according to tropical storm expert Liz Ritchie-Tyo from UNSW.

"The warmer the temperature the more water is in the atmosphere, so the more water that will be precipitated out," Associate Professor Ritchie-Tyo, who wasn't involved in the study, said.

"For every degree of warming you get about a 7 per cent increase in atmospheric water vapour."

As well as increasing rainfall, Dr Ritchie-Tyo said that there is evidence that shifting atmospheric circulation is causing cyclones to slow down.

"Under climate change, that mid-latitude temperature gradient is shifting towards the poles," she said.

Slower-travelling cyclones have more time to dump more rainfall in coastal cities.

City skylines create drag on storm systems

But climate isn't the only factor affecting storm intensity.

Research also published in Nature today has shown that urbanisation significantly contributed to the amount of rainfall dumped on Houston, Texas, during Hurricane Harvey in 2017.

Over 1,300 millimetres of rain fell on the Houston region during that storm.

According to lead researcher Gabriele Villarini, from the University of Iowa, the "roughness" of the city — as in the buildings and infrastructure — created drag on the storm system, causing it to slow down, resulting in more rain over the city area.

"When you have a rougher surface, in this case the city … as the air masses approach the city it tends to slow down a little bit," Associate Professor Villarini said.

"Because of the increase in the drag, there is an increase in the lower level convergence, there is an increase in the upper level divergence and also an increase in the vertical velocities.

"These are the key ingredients for heavy precipitation."

Their findings were based on computer simulations of Hurricane Harvey.

In one set of simulations, they modelled the rainfall over Houston, and compared that to the rainfall when they replaced Houston's skyline with cropland.

As much as 200 millimetres more rain fell in some regions under the city simulations.

As well as getting a higher amount of rainfall, the cities infrastructure influenced how water behaved once it hit the ground.

"We expected the city to exacerbate flooding, everything else being equal, because of the presence of impervious surfaces which reduce infiltration of the rainfall into the ground," Dr Villarini said.

"So from a physical perspective it did what we would have expected — it increased the magnitude of the flood peaks."