Extreme storm events such as super typhoon Haiyan, which wreaked havoc in the Philippines on Friday, are more likely in the future as the build-up of greenhouse gases warms the planet, scientists say.
Winds from typhoon Haiyan were estimated to have been 314km/h or higher when the monster storm made landfall on the Philippine island of Samar. That speed, if confirmed, would make it the strongest storm on record, exceeding hurricane Camille, which hit Mississippi in the US in 1969, according to US meteorologist Jeff Masters' WunderBlog.
Australian scientists say gauging the intensity of the storm – which included a tsunami-like storm surge and heavy rainfall – would be difficult because of limited information emanating from the storm-battered region. The death toll from the city of Tacloban alone may exceed 10,000 people, local authorities say.
Professor Will Steffen, a researcher at the ANU and member of the Climate Council, said scientists understand how a hotter, moister climate is already affecting storms such as Haiyan.
“Once [cyclones] do form, they get most of their energy from the surface waters of the ocean,” Professor Steffen said. “We know sea-surface temperatures are warming pretty much around the planet, so that's a pretty direct influence of climate change on the nature of the storm.”
Data compiled from the US National Oceanic and Atmospheric Administration shows sea temperatures were about 0.5 to 1 degree above normal in the waters to the east of the Philippines as Haiyan began forming. The waters cooled in the storm's wake, an indication of how the storm sucked up energy.
Typhoons – or tropical cyclones as they are known in Australia, and hurricanes in the US – require sea-surface temperatures of at least 26.5 degrees to form, according to the Bureau of Meteorology. The low-pressure systems can persist over lower sea-surface temperatures once they get going.
Kevin Walsh, an associate professor at the University of Melbourne and an expert in tropical meteorology, said warmer sea-surface temperatures are only one factor in determining the ferocity of a cyclonic event. The key is the temperature difference between those seas and the tops of the storms, high in the troposphere.
While data on temperatures at sea levels well known, gathering the information at levels up to about 20 kilometres above the surface has been more difficult until recent times, with weather balloons and other devices now more common.
That temperature differential in cyclones, though, is expected to widen as storm heights push higher in the atmosphere, Dr Walsh said.
“In the future, you’re talking about the difference of the sea-surface temperature and the temperature of the troposphere height – that increases even though the upper troposphere warms in a warming world.”
A lack of data on tropical cyclones has made it difficult for scientists to determine whether global warming is already affecting major storms, with only North Atlantic data on hurricanes considered robust enough over decades to identify trends.
Research, though, indicates that while there may be fewer tropical cyclones in the future, their intensity will increase.
A peer-reviewed report in Nature Geoscience published in 2010 by a team including Australia's Dr John McBride, found that models indicate a shift towards stronger storms in the future.
“Existing modelling studies ... consistently project decreases in the globally averaged frequency of tropical cyclones, by 6 to 34 per cent,” the report said. “Balanced against this, higher-resolution modelling studies typically project substantial increases in the frequency of the most intense cyclones, and increases of the order of 20 per cent in the precipitation rate within 100 kilometres of the storm centre.”
Since the atmosphere can hold 4 to 8 per cent more water per degree of warming, scientists say rain events have the potential to get more extreme, including cyclones.
The risk is for intense rainfall and flooding when they hit land and drop their water as rainfall increases with global warming, Professor Steffen said. “They're packing more punch in terms of the wind and also the water vapour or rain that they are carrying.”
Much of the devastation in typhoons comes with the related storm surge. Since climate change is already raising sea levels, the risk for severe inundation is also increasing, Dr Steffen said.