Days of severe pollution in Australia's biggest cities will worsen in coming decades as a warming climate triggers more intense temperature inversion events, exacerbating health issues, according to new research.
With more than 3000 premature deaths a year in Australia already linked to urban air pollution, worsening low-level air quality could increase the toll, said Jason Evans, a professor at the ARC Centre of Excellence for Climate Extremes and a co-author of the report published recently in the Climate Dynamics journal.
Inversion events reverse normal conditions, with cool air near the surface trapped beneath warmer air. The resulting lack of mixing allows pollutants, dust and pollen to build up, potentially harming health.
The new study examined data for the 1990-2009 period from nine weather sites. They ranged from Brisbane along the eastern seaboard to Adelaide, and took in inland cities such as Canberra.
It then applied regional climate models based on a business-as-usual carbon emissions trajectory to project results for 2020-39 and 2060-2079.
Significant changes in the intensity of inversions – based on the increasing differential between the temperature at the top of the warm layer compared with the air near the land – were detected in the latter period at all of the nine locations.
"Even though the overall number of inversions didn't change, we saw a substantial reduction in weak inversions and a marked increase in stronger inversion layers," Professor Evans said.
For Sydney, the increasing strength of daytime inversions was about 46 per cent for the 2060-79 period, compared with 1990-2009.
For Melbourne, the increase was as about 53 per cent, Brisbane 64 per cent, Adelaide 69 percent and Canberra about 80 per cent, according to the paper.
"With more than 80 per cent of Australia's population living [in the region studied] and large increases in population projected, the impact of more intense air pollution events in the future could be substantial," the paper said.
The models indicated only small changes in the duration of inversions, with those in the south becoming longer and those in the north shorter.
Since inversions typically develop during periods of calm winds, clear skies and long nights, the worst events will likely occur during the winter months.
Inversion conditions may occur on Monday in Sydney when overnight temperatures were tipped to drop to as low as 4 degrees at Observatory Hill, potentially the lowest July reading in 11 years.
Winds are expected to be light, with fog also settling in over places such as Richmond, on the city's north-west.
Paramatta North and Richmond were two sites to report poor air quality on Sunday morning, as did Armidale and Gunnedah, according to the Office of Environment and Heritage.
As inversion events usually take place during daylight hours, health effects will be amplified because "that's when people are out and about and most exposed", Professor Evans told Fairfax Media.
One role climate change could play is that with warming conditions, cities in Australia – and elsewhere – are expected to cop more rainfall but during fewer events. That would leave longer periods of relatively clear skies, conducive to inversion events, he said.
Climate models also project a decline of wind strength over land, Professor Evans said.
While the paper did not examine sources of pollution, one of the biggest for cities such as Sydney and Melbourne can be smoke from hazard-reduction burning.
Fire researchers say the window for conducting such burn-offs has been narrowing, particularly in the spring. That makes it more likely authorities will step up prescribed burning during periods of light winds and dry conditions – precisely the conditions favouring inversion events.
Peter Hannam is Environment Editor at The Sydney Morning Herald. He covers broad environmental issues ranging from climate change to renewable energy for Fairfax Media.
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