So Global Warming will Not cause more Bush-Fires in Australia?
Perhaps not — but fire danger will increase
If Australia’s terrible bush fires of 2019/2020 were the result of just one degree of global warming, what on Earth will happen with three degrees?
Last Australian summer (2019–2020) a lot people were drawing a fairly direct link between the extraordinary bush fires, and global warming. Some were drawing a link direct from the fires to the coal-mining policies of the Australian government. It seemed reasonable to think that, even if this outburst was somehow just more example of extreme Australian weather, then it was certainly a vision of what we could expect much more of in the future.
Or was it?
One of the core predictions of global warming is that ‘dry will get drier and wet will get wetter’. Basically this is because more energy in the system will drive an enhanced hydrological cycle. Dry regions will see more of their moisture evaporated, and wet areas will get this as more intense dumps of rainfall. One might then assume that increasing heat will mean more droughts and more forest fires. But here is where things get complicated.
I was recently pointed (Thanks to TheMuttonFlap blog) in the direction of Michael Roderick (Research School of Earth Sciences, ANU), and specifically this YouTube video (part of the Research School of Earth Sciences Seminar Series), ‘Drought, Bushfires and Climate Change: The Hot Topic’:
Roderick’s talk focusses on the 2017–2019 drought, which leads in to the 2019–2020 fires, and finally discusses “whether there is a reasonable expectation for more drought and bushfires with warming”. Right up front (3:45) he states the question that is on many people’s minds, and which I paraphrased at the start of this post:
“If the 2019/2020 Black Summer is the result from 1 degree warming, what will it be like when we have 3 degrees of warming?” and adds the kicker: “Will it be three times worse?”
Roderick has no issue with south-eastern Australia having become significantly warmer over the last century and more. Having said that, he notes that “We all know that Australia is a land of drought, flood and fire”.
He points out there “remains” (an interesting choice of word) a “widespread perception that climate change caused the fires” and notes that the great man himself, Sir David Attenborough, gave his seal of approval in this interview:
Attenborough said: “As I speak, south-east Australia is on fire. Why? Because the temperatures of the Earth are increasing …. And to say, ‘Oh it’s nothing to do with the climate’, is palpably nonsense.”
Now, while the climate change misinformation mob are desperate to tell us that the fires weren’t started by climate change (maybe by someone with a match, for example), Roderick is into something well beyond that nonsense. Still, to him, an expert in the field of evaporation, “the role of climate change” in the fires was not obvious.
Roderick points out that “bad fires in SE Australia are often preceded by drought”. There is little doubt about that, but has the region actually become drier? In other words, has there been a trend of increasing dryness, as the area warmed?
Rainfall, especially in Australia, is notoriously variable from year to year. This makes it “noisy” data, and any trends are very sensitive to the time period analysed. Roderick points out that in terms of annual rainfall, Australian Bureau of Meteorology data show that over the period 1970–2019, eastern Australia apparently dried. But looking at the much longer period, 1900–2019, there has actually been a slow increase (9:38).
These rainfall data were then analysed for trends in more detail by Anna Ukkola (a research fellow at ANU, along with colleagues, including Roderick) and published as Ukkola et al. (2019). The probably surprising result is that most of Australia has become wetter — at least in terms of annual rainfall (SW Australia, including Perth, is clearly getting drier).
Looked at on a seasonal basis, most of Australia is getting more summer rain — a situation that favours plant growth (and not river flow). As Roderick puts it (12:27) “Most parts of Australia are wetting-up, and they are wetting-up because they are getting more summer rain”. But, in southeastern Australia, this has come with increased variability. While 2010–2016 was “exceptionally wet”, 2017–2019 was “the driest on record”. In fact, “the distinguishing feature of that (2017–2019) drought, was actually three dry winters in a row” and very little spring rainfall in 2019.
Just how remarkable this was is emphasised by the fact that Australia has had even drier years, but these have always been followed by a wet year (great news for farmers). This is roughly a one in a thousand year occurrence. “Exceptionally rare” — but statistically not impossible. The statistics show that over 1900–2019, the rainfall data are a random, and stationary, times series. As he emphasises (14:08) “Things like droughts and floods and fires, are about variability. Primarily”.
However, as he says, it’s not just about dry years, but wet ones too. That near-unprecedented three-year dry period was preceded by an exceptionally wet several year period where there was much plant growth. In other words, the wet years of 2010–2016 were terrific for plant growth, which then dried over 2017–2019, converting it to abundant dry fuel. Roderick calls the eventual result “No real surprise”.
“This is all about variability” (20:20). An exceptionally wet period followed by an exceptionally dry period gave rise to a “perfect storm”. But “in terms of climate change, it’s not obvious what the link is” (22:34).
It’s a fact that drought years tend to be warmer years, and vice-versa. But at this point Roderick then (23:53) introduces an issue that might surprise many of us:
“So you often hear, and I’ve heard it so many times in the press, the warm temperatures during the drought increase evaporation and that is why the soils, rivers and dams are dry.” He points out, this is the wrong concept.
The physics of evaporation mean that its actually the drought that’s making it warmer. And while many of us might assume that hotter conditions drive evaporation up, Roderick quotes a standard environmental physics text-book (Monteith and Unsworth, various years), that “The temperature dependence of evaporation is negligible”. Instead, it’s “mostly about radiation, humidity, wind, and vegetation”.
Unfortunately these are not variables that are nicely available in national climate databases. In lieu of that, there are some long ‘pan evaporation’ records, as a measure of the ‘thirst of the air’. These are, as the term suggests, daily records of the amount of water in a standard sized ‘pan’ set on (or in) the ground. Roderick emphasises that these are not measuring evaporation as such — but that ‘thirst of the air’, or the ‘Evaporative Demand’. And he should know — it’s his research specialty, on which he has published several academic papers (e.g. Roderick and Farquhar, 2002; Roderick et al., 2007).
There is some variability in Pan Evaporation across Australia — but importantly, there is no correlation with air temperature. In fact, some areas with the most warming, show the biggest decrease in the thirst of the air. And here is where things get weird — over much of the planet, Evaporative Demand has shown a clear decrease — despite global warming. Roderick et al. (2009) point to declining wind speed as the main reason.
Historical records for SE Australia show that Evaporative Demand is high during drought years — but that this shows no trend towards increasing. In fact, it seems to show a small decrease. From this conclusion, Roderick offers that (33:22) “If climate change has had an effect, we would have to say that it’s reduced the thirst of the air — if you subscribe to the view, that that’s what’s caused the pan evaporation to drop”.
“The drought may be associated with climate change indirectly, for example by a change in the circulation, an anomalous change in the circulation — and that’s a more complex argument which I won’t get into here.”
At this point (35:04), Roderick asks, and answers, three questions:
“Q. Has the ongoing accumulation of greenhouse gases in the atmosphere caused more drought in NSW [i.e southeastern Australia]?”
“A. No, not unless it is related to the increase in rainfall variability.”
“Q. Will the ongoing accumulation of greenhouse gases in the atmosphere cause more drought?”
“A. Not necessarily. (But to be prudent, we should plan for that possibility).”
The third question returns to the issue of fire:
“Q. Will the ongoing accumulation of greenhouse gases in the atmosphere lead to more fire?”
Roderick answers in the affirmative – but sees the increase in fire as an indirect consequence of climate change. Basically, vegetation productivity will increase — and therefore “Fuel is going to accumulate faster”.
In detail, vegetation productivity is a partly a function of Water Use Efficiency, which depends on Atmospheric CO2 and the Vapor Pressure Deficit (VPD). Along with increasing CO2, the VPD increases as a response to the increase in temperature. The increase in CO2 is beneficial to vegetation, the increase in VPD is not. By AD 2050, Roderick calculates that the VPD “deficit should go up about 14%”, but the increase in CO2 by then will be enough to more than counter-act its negative effect.
This is essentially the phenomenon of so-called ‘global greening’, whereby a higher CO2 atmosphere leads to more plant growth — although for places like Australia, a more apt description might be ‘fueling’.
But this is curious, because Roderick said that the VPD will increase by 14% by 2050 , because of the increase in temperature— but the VPD is another expression of …. the ‘thirst of the atmosphere’, which according to his Pan Evaporation work, is decreasing, and has nothing to do with temperature.
This is confirmed in Farquhar and Roderick (2007), where they wrote: “Although green-house warming has presumably caused the increase in average surface temperatures, it cannot have been causing a large increase in vapour pressure deficit over land — that would have increased pan evaporation.”
Pan Evaporation is an actual measurement of the thirst, while the VPD is an estimation of what it ought to be, based on some basic climatic variables. But here’s the nub of the problem – it’s difficult to have it both ways. If you’ve accepted that ‘thirst of the atmosphere’ based on Pan Evaporation is declining, independent of global warming, you can’t happily accept that ‘Thirst of the atmosphere’ based on VPD is increasing because global warmth is going up.
Pan Evaporation is a complex phenomena, or rather, results from the complex interplay of several phenomena. The fact that it definitely decreased in many parts of the world, when it was theoretically expected to increase, gave rise to the idea of an ‘evaporation paradox’ (Brutsaert and Parlange, 1998). Some research since then (e.g. suggests that the cause of the ‘paradox’ lies in local trends of factors such as wind speed and cloudiness/sunshine duration (Yan et al., 2019) overriding the effects of increasing temperature. Importantly, one study concluded that, as global warming proceeds, the temperature effect will assert itself, pan evaporation will increase, and the paradox will cease to be (Wang et al., 2017). But in any case, when it comes to the future of fire danger in Australia (at least), the pan evaporation saga is basically a red herring.
The gist of Roderick’s talk then is that major bushfires in Australia follow periods of extreme wet (fuel builds up), and extreme dry (fuel dries out)— but that because these periods are inherently extremely variable in Australia, they have no clear correlation with global warming, and therefore there is no indication that global warming will result in “more” fire.
Despite the extreme variability, a signal from global warming may yet become apparent. For example, those wet and dry periods are driven by oscillations of warmer and colder water in the Indian, Pacific and Southern Oceans (El Nino/La Nina, Southern Oscillation, and Indian Ocean Dipole), and it does seem likely that they are responding to global warming (Cai et al., 2009; Thompson et al., 2011).
There is also an expanding southern edge of the Hadley Cell (the outer edges of the Hadley Cells basically control the world’s desert areas), which is likely a response to global warming (Post et al., 2014). Roderick is aware of such global-warming mediated atmospheric circulation changes, but avoided them in his talk: “Drought may be associated with climate change indirectly, for example by a change in circulation, and that’s a more complex argument which I won’t get into here” [34:50].
Also, perhaps for good measure, the end of 2019 saw a ‘Sudden Stratospheric Warming’ event thrown into the mix. It caused some wild weather, although it probably has nothing to do with global warming.
But the critical point is — that on top of that extreme moisture variability, there are superimposed trends to do with warmth, such as an increasingly long fire season, which are definitely responding to global warming — and which are markedly increasing ‘fire weather’ (e.g. Clarke et al., 2012; Clarke and Evans, 2019; Bradstock, 2010, 2020) and summarised nicely in a Scientific American post:
With uncanny accuracy, the ‘Garnau Climate Change Review’ of 2011 proffered that longer and general more intense fire seasons “should be directly observable by 2020”.
These warmth factors are incorporated within indices such as the ‘McArthur Forest Fire Danger Index’ — which has increased over much of Australia, particularly in the south-east, from 1978–2017 (CSIRO/BOM State of the Climate Report, 2018). The simple fact is that there are other phenomena besides the underlying runs of wet and dry years which drive the intensity and extent of bushfires. Temperature, trending upwards in response to global warming, remains important. A recent attempt to attribute the fires (van Oldenborgh et al., 2020) had this to say:
“the probability of a Fire Weather Index this high [as in 2019/20] has increased by at least 30% since 1900 as a result of anthropogenic climate change… the real increase could be much higher. … Projected into the future,…a Fire Weather Index at the 2019/20 level would be at least four times more likely with a 2 C temperature rise, compared with 1900… this is likely an underestimate.”
While Roderick may be (arguably perhaps) correct that global warming will not result in “more” fires, it’s important not to get the wrong message out of the whole argument: — the heat-related trends, irrespective of basic rainfall and evaporation, mean that when fires do break out, they are more likely to expand more quickly, burn more intensely, cover larger areas and be harder to control. Fire danger is definitely going to get worse.
I’ll leave the (nearly) last words to Dr Sarah Perkins Kirkpatrick, a senior research associate in the Climate Change Research Centre at UNSW and specialises in climate extremes. With respect to bushfires, Perkins Kirkpatrick said:
“As the climate warms even more, what’s going to happen then? Sure, climate variability will have a role, it always has a role — but if it’s two or three degrees hotter, not just one degree hotter, how bad can it really get?” (these were the also the final words in potholer54’s YouTube video ‘The cause of Australia’s bushfires — what the SCIENCE says’):
It’s a rhetorical question from Perkins Kirkpatrick of course, but with the clear implication is that things are likely get a lot worse…..
References
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Acknowledgments
Thanks to TheMuttonFlap
