A great deal has been said about the recent New South Wales bush fires and whether there is a link between these bush fires and global warming. An attempt to explain what is and is not known is provided here.
First, some context setting. Bush fires have occurred in Australia for a long time. There is a history of fire in Australia exceeding 400,000 years with high variability in fire frequency associated with natural climate variability (Kershaw et al. 2002). A substantial increase in fire frequency occurred about 38,000 years ago. This was probably related to human activity given that there is little evidence for a coincident change in climate. A second peak in fire occurrence was associated with European settlement in 1788 (Kershaw et al. 2002). Currently, around 5% of the Australian land surface is burned annually consuming approximately 10% of the net primary productivity of the continent (Pittock 2003).
So, fire is 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. I am not aware of anyone who says it is. The questions are more around whether global warming is increasing the risk of bush fires, did global warming make the recent fires more likely and therefore whether there is a global warming link to the fires in the Blue Mountains.
The risk of bush fire is driven by the amount and dryness of fuel, ambient weather and ignitions (Archibald et al., 2009). That is, you need fuel, you need hot and dry conditions and you need an ignition source. I’ll deal with what we know for each of these in turn.
Does global warming affect ignition sources?
There is some limited evidence that global warming will increase the frequency of lightening. Price and Rind (1994) suggest that there will be an increase of about 5–6% in global lightning frequencies for every 1°C global warming. However, there is absolutely no evidence that the recent NSW fires were triggered by lightening and so this will not be explored further here.
Does global warming affect the amount of fuel?
Global warming does increase the amount of fuel. Elevated CO2 acts as a fertilizer and increases net primary productivity. Before anyone misinterprets this, so far as anyone knows this effect is not sustainable over the long term due to limits imposed by nutrients. This fertilization effect works in a farmer’s greenhouse where CO2 is elevated because the farmer also waters and fertilizes the crops. We do not water or fertilize native bush.
Satellite observations analyzed by Donohue et al. (2013) reveal a greening of the globe over recent decades. They attribute this in some regions to the CO2 fertilization effect. Donahue et al. (2009) reported a greening of the Australian continent as a whole although there is a strong regional variability. This greening is associated with many factors including changes in rainfall, changes in temperature, CO2 fertilization etc. There is also 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).
When vegetation takes up CO2 it fixes it as carbon in leaves, woody matter, roots etc. It is therefore inevitable if the biosphere is taking up more CO2, or greening, then it fixes more carbon. If it fixes more carbon in woody matter this is fuel for fire. If vegetation uses the additional carbon for leaves then this becomes fuel later when the leaves drop. Whichever way you consider the system, increased CO2, if absorbed into the biosphere, must increase the amount of fuel.
It is therefore the case that elevated 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.
Does global warming affect the dryness of fuel
There are several ways that fuel can become dry or drier.
(1) First, rainfall can change. There are observed trends in rainfall over NSW. Over the last 3 months, NSW has been unusually dry.
NSW has also been unusually dry over the last 6 months.
Rainfall is extremely variable over NSW. While these maps point to 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.
(2) Fuel can be dry because of increased evaporation. Evaporation is complex requiring energy (sun light), a gradient of moisture (the air needs to be more dry than the surface) and wind. Observations of these things are not as good as observations of temperature or rainfall and so far as I know there are no trends in these things that can be linked to global warming over NSW. There might be trends, they might be linked to global warming, but they might not be and we cannot say anything definitive so far as I know.
However, it is not only dead leaves, dead branches that provide fuel. The growing vegetation also catches fire if it is dry enough. 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.
Trees suck up water from the soil when they are growing. Below is the 30 year trend in growing season length. Red dots mean the growing season is getting longer.
The longer growing season means:
- (a) more carbon is fixed and than leads to more fuel (see above) and
- (b) more moisture is sucked up by the growing vegetation and transpired into the atmosphere.
This inevitably dries the soil. The very warm conditions through autumn and spring in 2013 likely enabled the vegetation over the Blue Mountains to sustain a higher transpiration rate than would otherwise have occurred. Over winter, therefore, moisture was removed from the soil.
When we then got some very hot days in spring the usual response by the vegetation would be to transpire actively. Transpiration is a process of evaporation. For each kilogram of water evaporated, some 2,500,000 Joules of energy is taken away (which is why you cool when you sweat). If there is no soil moisture, vegetation cannot access water and if there is no water it cannot transpire and it will heat up (which is why you must drink water on hot days of course). It is very likely that in the recent fires, rather than having large areas of transpiring vegetation helping cool, helping add moisture in to the air and increasing humidity the vegetation was water limited. Consequently, the vegetation was much hotter, with lower moisture content and therefore much more likely to burn.
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.
Does global warming affect ambient weather?
I will focus on temperature here because it is central to whether global warming contributed to the fires in NSW. The past 12 months have been the hottest in Australia for more than a hundred years. Temperatures averaged across Australia between September 2012 and August 2013 were hotter than any year since good records began in 1910. Lewis and Karoly (2013) have shown that human-caused climate change substantially increased – by more than 5 times – the likelihood of the very hot 2013 Australian summer. Lewis and Karoly (2013) have also analysed the past 12-month’s temperatures across Australia. They showed that human influences on the climate very likely increased by more than one hundred times the chances of the record 12-month Australia temperature. They noted that these recent record temperatures were notable because they occurred at a time when El Niño-Southern Oscillation (ENSO) conditions were neutral (neither El Niño nor La Niña occurring), which typically produces normal, rather than warm, temperatures across Australia.
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.
Changes in Fire Risk
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). Clarke et al. (2012) analysed observations from high quality stations from the Bureau of Meteorology and calculated the FFDI from 1973 to 2010). These are the trends in the 90th percentile of FFDI at those stations.
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.
Changes in Fire Impacts
The impact of something is basically risk multiplied by vulnerability. Bush fire risk can in part be linked to global warming but how that risk leads to impacts is highly dependent on vulnerability. Crompton et al. (2010) discusses many of these things.
My understanding is that there is no trend in losses associated with bush fires once you normalize by population, economic value etc (McAneney et al., 2009). Outstanding risk minimization 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, increased settlement in the mountains has increased the vulnerability and the overall impact of these things is not within my expertise to comment on. I refer the reader to Crompton et al. (2010).
However, just because losses do not show a trend at this time cannot be used to infer there is not trend in risk and cannot be used to infer that losses will not increase in the future as risk increases further under global warming. Risk might be mitigated by planning, vulnerability minimization etc but if we continue to drive climate through emissions of greenhouse gases the risk of fire will continue to grow. That is clear in all the science literature that has considered this issue that I am aware of including early studies on the effects of climate change using global climate model (GCM) simulations that found an increase in bushfire weather under increased CO2 (Beer and Williams 1995; Williams et al. 2001; Cary and Banks 1999; Cary 2002). Lucas et al. (2007) found that by 2050, average annual FFDI is projected to increase by up to 30%, while the uppermost values of the index are projected to increase by up to 300%, depending on the location and whether the lower or upper IPCC projection was used. Bradstock et al. (2009) predicted a 20-84% increase in potential large (≥1000ha) fire ignition days in the Blue Mountains and Central Coast regions of NSW by 2050. Pitman et al. (2007) found a 25% increase in fire risk by 2050. Hasson et al. (2009) investigated the impact of climate change on fire weather using a synoptic feature characteristic of some of the most extreme fire events in southeast Australia in the last 50 years. Their results suggested a doubling in risk of extreme bushfire risk by 2050. In short, so far as I know, no study for NSW has found that fire risk will reduce, or stay the same, in the future; every study points to an increase in risk.
If you know you are at risk, whether it is you personally, your family, your business you assess the scale of that risk and respond. You might be willing to accept the risk and carry on. In the case of bush fire risk, global warming is increasing the risk. Off-setting this risk are the (quite phenomenal) strategic planning by NSW state agencies and mitigation by Rural Fire Service employees and volunteers. 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. More profoundly, he was right in noting that the devil is what to do about it in an environment like the Blue Mountains which has developed over 200 years, seen significant increases in population, is densely vegetated and has limited accessibility.
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