The New South Wales fires have sparked a hot debate on the question whether these fires are linked to climate change. Prime Minister Tony Abbott joined in too stating, “These fires are certainly not a function of climate change - they’re just a function of life in Australia”.
With first-hand experience in fire fighting, the PM can talk with some authority about bushfires. But does the science support the PM’s assertion about the fires and climate change or his high confidence in that assessment?
The present fire activity in eastern NSW is extreme and consistent with what we know about climate-fire relations and the below-average rainfall and above-average temperatures of the last six months. However, whether the current fires in NSW are related to climate change is unknown.
This relationship will remain uncertain until we know to what extent the anomalous climate conditions of 2013 in Australia are within the natural variability of the region or an expression of global environmental changes.
The last report of the Intergovernmental Panel on Climate Change (IPCC) repeatedly states that anomalous seasonal conditions and changes in the frequency and intensity of extreme weather events are likely in future. But it emphasises that prediction of future conditions are inherently uncertain, while making no specific predictions about Australian bushfires in particular.
Consistent with that uncertainty, the IPCC uses a well-defined scale to express the likelihood of their science-based assessments. The IPCC likelihood scale goes from “exceptionally unlikely” for probabilities of 0-1% to “virtually certain” for probabilities of 99-100%.
The term “certainly” is not included in their terminology. By suggesting certainty where it doesn’t exist, the PM appears disconnected from the relevant science.
So what’s the science behind the PM’s second assertion, about fires being just part of life in Australia? Though basically true, some nuance is needed for a productive discussion. Fire is an integral aspect of most Australian environments, but there are important variations in the frequency, season, sizes, intensity and impacts of burning.
Basic biophysical principles cause these fire properties to be related, such that certain combinations (such as frequent, low-intensity fire in winter or infrequent, high-intensity fire in summer) occur over large areas while others are rare (such as frequent, high-intensity fire). In fire research and management those combinations of fire properties are often referred to as “fire regimes”.
Though regional differences in Australian fire regimes have been known for many decades, a continental classification and mapping was published only recently. How continental and global fire regimes are related to climate remains incompletely understood and poorly quantified, with associated uncertainty about potential changes under projected future climates.
We know that bushfires can only occur where and when the following four limiting factors are overcome:
There must be at least some ground cover of plant material or fuel.
Fuel must be dry enough to burn.
There must be an ignition.
Weather conditions must be favourable for the fire to propagate across the landscape.
The four limiting factors are like switches in an electrical circuit: a fire requires all four switches to be “on”. Across Australia there is important variation in the frequency with which the four switches are “on” or “off” producing a rich pattern of fire regimes.
In many of our arid environments fuel material is scarce and patchy for much of the time but can be abundant enough after above-average rainfall to support large fires. On the other hand, in our more humid climates of the southeast, forests and woodlands produce large quantities of fuel material but that fuel will be too wet to burn for much of the time. It is very difficult to assess exactly when and where forest fuels are dry enough to burn.
We do know that large forest fires like the present ones in the Blue Mountains tend to occur when dry fuel areas become widespread and spatially connected.
The four-switch model provides useful guidance for evaluating effects of climate change on fire. It captures the key mechanisms underlying the diversity of fire regimes and, importantly, clarifies that fire results from processes operating at disparate time scales.
The distribution and species composition of forests and woodlands in Australia reflects long-term climate conditions and related fire regimes. This may change in response to altered future climate and elevated atmospheric CO2. But such changes will be relatively slow (probably over decades), though potentially reinforced by altered fire regimes.
At seasonal or annual time scales, the critical switch for fire occurrence in forests in the southeast is related to fuel dryness. Hence, changes in the frequency, intensity, timing or duration of dry weather conditions can potentially affect fire activity in this environment. Once the fuel dryness switch is flicked “on”, fire activity becomes a function of the occurrence of ignition sources and hot windy weather conditions.
In conclusion, we can be virtually certain that fire activity in Australia will respond if climate change were to affect one or more of the mentioned switches. We can also be virtually certain that altered fire regimes will have potentially far-reaching implications for life in Australia.
One of the few certainties here is that we need to invest in a strong science base to guide us forward in case the future turns out different from what it used to be.