SCIENCE

Australian Cyclone Activity Hits Record Low Levels

Climate change may explain the recent drop, scientists say

January 29, 2014

Tropical cyclone Yasi — Australia has a long record of devastating tropical cyclones, such as Yasi, which made landfall in Queensland in February 2011. But a new study finds such storms to be on the decline. NASA image courtesy Jeff Schmaltz, MODIS Rapid Response Team at NASA GSFC

In Queensland, on the northeast coast of Australia, residents have been warned that a tropical low is forming off the coast this week and could quickly develop into a cyclone, to be named Dylan. The storm would be the first to hit this part of the country in more than two years, and it brings the potential for damaging winds, heavy rainfall and flooding from higher than normal tides.

Tropical cyclones—what are called hurricanes in the North Atlantic—are not unusual in Australia, but they used to be a lot more common, finds a study published today by Nature. The frequency of these storms has reached an unprecedented low level not seen in the past 550 to 1,500 years, say researchers led by Jordahna Haig of James Cook University in Cairns, Australia. And, the researchers note, they can’t rule out climate change as the cause for the drop in activity.

Scientists and the public have been interested in how hurricane and tropical cyclone activity might change in the future because these can be incredibly devastating storms. Hurricanes such as Sandy, Katrina and Andrew have left their mark on the United States and images of their destruction are embedded in national memory. And Australia has experienced its own share of storm destruction. Cyclone Mahina, which struck Queensland in 1899, for instance, holds the world storm surge record at 48 feet.

But studying these storms isn’t easy. There aren’t that many hurricanes and tropical cyclones each year, and good records don’t go back all that far. Instrumental records of the storms cover less than 50 years, and this observational record isn’t great for storms that happened before 1990. Researchers need some sort of marker of storm activity if they are to see how activity has changed over time. Several years ago, though, scientists discovered that stalagmites in caves held just such a record.

Tropical cyclones produce rain with water molecules that hold a higher amount of a lighter form of oxygen, the isotope oxygen-16, compared to the average precipitation from monsoons. In general, lighter water molecules evaporate more easily to form storm clouds. But cyclones have a propertly distinct from monsoons: while cyclones travel over seawater, they cannibalize the water they precipitate—water molecules with lighter oxygen are continually re-evaporated by storm clouds as the cyclone progresses, making it isotopically different from normal rain.

When a cyclone dumps its rain on top of a cave, this light water soaks into the ground and starts to drip into the cave. The oxygen from the water becomes incorporated into the calcium carbonate that forms stalagmites.

In Australia, stalagmites grow with alternating dark and light bands, representing the wet and dry seasons, respectively. This means that a stalagmite can record the yearly change in hurricane activity, similar to how an ice core might hold a record of past changes in the composition of the atmosphere. By measuring the ratio of oxygen-16 to heavier oxygen-18 within the wet-season bands, scientists gain a window into the cyclone activity of a specific year in the past. Higher levels of oxygen-16 indicate a year of stronger cyclone activity.

For the new study, researchers analyzed two cylindrical stalagmites—one each from the states of Queensland and Western Australia, because storms can come from either the Pacific or Indian oceans. They then built a record of cyclone activity for the past 700 years in Queensland and 1,500 years in Western Australia. The west coast of Australia is more prone to tropical cyclones than the east coast, but the stalagmite record revealed that the frequency of cyclones in that area has dropped off in recent years. Storm activity in that region since 1970 has not been that low for the past 1,460 years. The east coast is also at a historical low, the analysis showed, reaching levels not seen for the past 550 years.

“The Australian region seems to be experiencing the most pronounced phase of tropical cyclone inactivity for the past 550–1,500 years,” the researchers write. “The dramatic reductions in activity since the industrial revolution suggest that climate change cannot be ruled out as a causative factor.”

These results are in line with climate models that have predicted Australia will experience fewer storms because of climate change. Those same models, however, also say that the cyclones that do hit the continent will likely be of greater intensity, which means that they’ll have a greater potential for destruction.

How climate change might alter the tracks of future hurricanes and tropical cyclones varies by region. In the North Atlantic, for instance, researchers last year found that warmer air should push storms away from the East Coast, lessening the chance for something like Sandy to hit. But the constant across predictions is that the energy added to storms by anthropogenic climate change should lead to more intense storms. That could mean more rainfall, stronger winds and higher storm surges, all of which would bring more devastation to any of the coastal areas worldwide that are unlucky enough to be struck by a storm.