The last two major bushfires in Victoria, Australia, were perhaps linked to lower than normal sea-surface temperatures in the eastern Indian Ocean.
The Ash Wednesday bushfires in February 1983 and the Black Saturday bushfires in February of this year were preceded by months of very dry conditions. Those dry conditions were partly caused by cooler ocean sea-surface temperatures in the eastern Indian Ocean, which contributed to a substantial reduction in spring-time rainfall over the south-east of Australia, say researchers with the Wealth from Oceans National Research Flagship, an arm of the Commonwealth Scientific and Industrial Research Organisation(CSIRO).
The see-sawing nature of sea-surface temperatures in the east and western Indian Ocean is commonly referred to as the Indian Ocean Dipole. When the dipole is in a positive phase sea-water off the Sumatra-Java coast, northwest of Australia, tends to be cooler than normal, leading to a reduction in the rain-bearing systems that normally extend to Victoria during spring.
According to CSIRO's Dr Wenju Cai, the recent bushfires in Victoria occurred during a protracted drought made worse by an unprecedented three consecutive positive Indian Ocean Dipole events from 2006 to 2008.
"The sequence of these dipole events were captured by Argo measurements, which use robotic floats that spend most of their life drifting below the ocean surface," Dr Cai says.
"Another study examining temperature records of the past 100 years shows that the frequency of positive Indian Ocean Dipoles in the past three decades is much higher than over the previous 70 years. This trend is consistent with climate change experiments from the IPCC Fourth Assessment Report, published in 2007, which projects a mean warming pattern across the Indian Ocean reminiscent of a positive dipole pattern."
The research, co-funded by the Department of Climate Change, will be presented at the GREENHOUSE 2009 Conference being held in Perth from 23-26 March 2009.
Scientists believe more bushfires generated by rising temperatures and lower rainfall will lead to lower air quality over a greater number of days in Australia, particularly in the south-east.
In a presentation to the GREENHOUSE 2009 conference in Perth today, CSIRO's Dr Mick Meyer said measurements taken at the Bayside Air Quality Station in Melbourne's south-east during the 2003 and 2006 bushfires provided a guide to what could be experienced as the climate changes.
"While air quality standards generally appear to be improving, a future issue will be quality during times of wildfire for which there is little control and during autumn and winter prescribed burning when there is more control," Dr Meyer said.
Dr Meyer, from the Centre for Australian Weather and Climate Research, said the 2003 bushfires burnt an area of 1.1 million hectares and the 2006 fires 1.3 million hectares. This compares with approximately 450 000 hectares burnt during the February 2009 fires in Victoria.
From December 2006 to February 2007 bushfires in the Great Divide burned for 69 days. On several occasions, thick smoke haze was transported to the Melbourne CBD and particulate matter concentrations at several Environmental Protection Agency Victoria air quality monitoring sites peaked at four times the National Environment Protection Measure 24-hour standard.
Analysis of the measurements showed:
High concentrations of fine particles between 0.1 and 0.5 ĩm, diameter, largely composed of non-volatile organic material. Particles of this size are easily respired and can cause significant health impacts.
High concentrations of carbon monoxide and nitrogen dioxide.
Elevated concentrations of ozone.
The fingerprint is distinctly different from industrial and vehicular pollution sources.
Dr Meyer said that under a changing climate the frequency of bushfires, the duration of the bushfire season and the severity of bushfires are expected to change. Current projections for south-eastern Australia suggest an increase in the frequency of very high and extreme fire days and that periods suitable for prescribed burning would move towards winter.
In addition to the health impacts of increased fire intensity, duration and frequency, biomass burning also results in the emission of significant quantities of trace gases and aerosols to the atmosphere, and these subsequently can influence cloud processes.
"They also reduce visibility, influence atmospheric photochemistry and can be inhaled into the deepest parts of the lungs impacting on human health," Dr Meyer said
Savanna forest burning in northern Australia accounts for the majority of carbon emissions from burning, and 90-95 per cent of this is from wildfires. These savannah fires ccontribute about eight per cent of global carbon emissions from vegetation fires.