This article is adapted from Links between global warming and NSW bush fires, an article submitted by astone to the website of the Australian Research Council Centre of Excellence for Climate System Science. The full article, with references, is available there.
Bushfires are a natural phenomenon in Australia and a specific fire event as seen in the Blue Mountains in the last week is not caused by global warming.
The questions are more around:
• Is global warming increasing the risk of bushfires?
• Did global warming make the recent fires more likely?
• Is there a global warming link to the fires in the Blue Mountains?
Global warming does increase the amount of fuel. Elevated CO2 acts as a fertiliser and increases net primary productivity. So far as anyone knows, this effect is not sustainable over the long term due to limits imposed by nutrients. The fertilisation effect works in a farmer’s greenhouse where CO2 is elevated because the farmer also waters and fertilises the crops. We do not water or fertilise native bush.
There is evidence that the Earth’s biosphere is sucking up more CO2 now than in the past, but failing to keep up with the increase in human emissions (Raupach et al., 2008). Whichever way you consider the system, increased CO2, if absorbed into the biosphere, must increase the amount of fuel. Therefore, increased CO2, linked to increased activity by vegetation, has increased fuel loads. This need not lead to more fires, but it means that if a fire occurs then there is more fuel to burn.
While there has been low rainfall over the last 3 and 6 months this is not particularly unusual and it is not possible to prove that the low rainfall has any global warming link. That is, the low recent rainfall cannot be shown to be linked to global warming. This does not mean it is not. It might be, but it might not be, and we cannot say anything more definitive than that. The short story is that global warming explains, to an important degree, observations of warming over Australia and global warming increases the risks of very warm summers and very warm winters a great deal.
There is a reasonable link between how dry the growing trees are and global warming which we cannot currently put numbers on, but at the very least means no one can say no link exists. So, it is very likely that as a consequence of the unusually hot summer, and the unusually warm winter, the landscape was unusually dry and a natural feedback that cools was switched off. Therefore, the question is, was the last summer and winter unusually warm because of global warming because if it was, then there is a legitimate link between global warming, warmth, and dryness. The short story is that global warming explains, to an important degree, observations of warming over Australia, and global warming increases the risks of very warm summers and very warm winters a great deal.
The risk for fire is linked to temperature, humidity, wind and dryness. One way to link these together is to use the McArthur Forest Fire Danger Index (FFDI). Clarke et al. (2012) analysed observations from high quality stations from the Bureau of Meteorology and calculated the FFDI from 1973 to 2010. There is an upward trend overall in FFDI. Most individual stations show an upward trend. No stations show a downward trend. In short, the risk of fire, as measured by FFDI, has increased since 1973 over NSW.
Their results suggested a doubling in risk of extreme bushfire risk by 2050. In short, so far as I know, no study has ever found that fire risk will reduce, or stay the same, in the future; every study points to increasing risks.
Outstanding risk minimisation by emergency services, changes in individual preparedness, improved weather forecasting by the Bureau of Meteorology, better building codes, new fire fighting technologies and a considerable and sustained effort by professional and volunteer fire fighters have all helped reduce the vulnerability of settlements in the Blue Mountains to fire. However, if we continue to drive climate through emissions of greenhouse gases, the risk of fire and associated losses will continue to grow.
Failure to accept a link between global warming and fire risk means not reducing the climate linked risk. It therefore leaves all management of the risk to NSW state agencies and mitigation by Rural Fire Service employees and volunteers. So, the NSW Premier Barry O’Farrell was right in noting a link between climate change and bush fire risk.