Globally the tropical cyclones have become intense over the last 40 years, with a large number of devastating storms than before generating more often, with which the theory that warming oceans would drive more dangerous cyclones have been further confirmed according to a study.
The destructive cyclones, also known as hurricanes or typhoons, are on the clear rise according to the analysis of satellite records from 1979 to 2017 that deliver continuing winds in excess of about 185km/h.
Across the two ocean basins, the Southern Indian Ocean and the Southern Pacific Ocean, where cyclones form, Australia sits. At that location, the study also identified rising trends of more destructive storms.
The finding was in line with the predictions of the climate model and the knowledge that tropical storms get more energy from increasing ocean temperatures.
Dr Hamish Ramsay, a senior research scientist at CSIRO who studies cyclones, said: “This study confirms what the climate models have been predicting for some time – that the proportion of the most intense storms will increase as the climate warms.”
The climate scientists have predicted since long that global heating would deliver stronger cyclones. This trend was statistically significant; however, it has been challenging to identify in part because of the natural swings in the climate masking changes taking place all over the world.
The study was carried out by scientists at the US government’s National Oceanic and Atmospheric Administration and published in the leading journal, the Proceedings of the National Academy of Sciences.
Instead of finding a cause for the increase in more dangerous cyclones, the scientists said the trends were consistent with the understanding of physics and modeling, and the finding “increases confidence that [tropical cyclones] have become substantially stronger, and that there is a likely human fingerprint on this increase.”
There was a 15% rise in the probability of a cyclone reaching wind speeds greater than 185km/h over the 39 years studied.
The study said a previous study of the same data was for a shorter period from 1982 to 2009. Although positive trends had been found, they were statistically not at significant levels.
The study also looked at changes in cyclone intensity region-wise while looking at the number of cyclones forming globally. Both of the Southern Indian Ocean and the Southern Pacific Ocean showed an increase in the number of more intense storms. However, the trends in each region were not as robust due to the smaller number of cyclones.
“[The study] suggests that the climate change signal in the data is potentially already emerging and this is something that climate scientists have been saying for some time,” Ramsay said. “We may be at a point now where we are starting to evidence from observational data that supports what the models have been telling us.”
According to Ramsay, besides the increase in the wind speeds in cyclones, they would also be delivering more rainfall due to warming oceans. However, it is not sure whether the numbers of all categories of cyclones would rise or fall under climate change.
Since 1982 there had been a downward trend in the number of all tropical cyclones in the Australian region, as per Australia’s 2018 State of the Climate Report. However, it failed to see any trend in the intensity of cyclones.
Previous research has found that when cyclones form, they tend to move more slowly while delivering more rain.
Dr Greg Holland, a senior scientist emeritus at the National Center for Atmospheric Research in Colorado, has studied cyclones for about 40 years.
The Melbourne-based scientist said while there were legitimate arguments over the minute details of trends and intensity of tropical cyclones: “The work all points in the same direction – the proportion of the most intense cyclones is going up.”
He said: “There is nobody saying the trend will go the other way. The physics has been well set out for 30 or 40 years. If you get a warmer ocean then the intensities of cyclones goes up. That’s a 5% or 10% increase in maximum winds for every 1C of warming in the ocean. The world is warming and it’s because we have put more greenhouse gases into the atmosphere.
“For the Australian region – which is the eastern Indian and southern west Pacific – it means we now have the potential for there to be more intense tropical cyclones coming ashore and doing more damage. “The chances of us getting an intense cyclone have gone up already and they will go up in the future.”
Holland said evidence was also there that the ocean region of Australia where intense cyclones could strike was also expanding. The observed movement was only about 150km, he said, however, Brisbane would soon be “in the cyclone zone.”
Prof Steven Sherwood, of the UNSW Climate Change Research Centre, said the study was important “because it shows that the upward trends first reported about a decade ago in cyclone activity have been sustained, and have now gone on long enough that it is no longer possible for them to be a random natural variation.”
He said the study found that 30% of cyclones in the 1980s were “major” compared with 40% of cyclones now when averaged globally.
“There is of course nothing surprising about this — we’ve just reached the point of ‘non-deniability,’” he said.
“The implication for Australia is that our risk exposure to strong cyclones will almost certainly continue to increase as long as global temperatures increase.
“It also appears that storms are encroaching farther away from the equator, although this is harder to confirm observationally. If this is true, it means that the Queensland southern coast in particular may be at growing risk.”
