by Mark Goldstone
On October 21st UN climate chief Christiana Figueres made the unusual statement that the bushfire crisis in NSW is “absolutely” linked to climate change.
Figueres prefaced her comments by noting that the World Meteorological Organisation had not yet established a definitive link between climate change and these specific fires, but went on to say that fires of this nature were clearly more and more in our future unless we take rigorous action. My question: Is there really such a simplistic link between climate change and fires of this nature? That is, as things get hotter will the dryer conditions lead to forest fires?
First up, I am not a meteorologist per se, but I do have a PhD in Air Pollution and some nearly thirty years of recording air quality and observing the meteorlogical phenomena that affect air quality. To be an air quality scientist, with any predictive capability, you have to know what weather is likely to be doing at any typical time of the year.
I thought I would write this summary of current conditions so that people could understand the potential complexity of what may be going on. I need to state clearly that this is my own interpretation and does not necessarily represent the views of companies and societies that I am associated with.
The attached figure shows the current surface pressure map for Australia. Credit to the Bureau of Meteorlogy.
The southern portion Australian meteorology is characterised by a progression of high and lows travelling from west to east, which can clearly be seen in this synoptic chart. In winter the highs are interspersed with lows and as summer progresses the lows are replaced with troughs. This normal sequence of highs and troughs means that strong northerlies travelling from the tropics bring high temperatures and increase the risk of fire.
As we progress towards summer, they tend to transport monsoonal storm cells down the east coast bring summer rain. In spring, however these storm cells may not form, and instead the trough driven northerly winds bring hot dry conditions. What I am writing is supported by the Australian Bureau of Meteorology Data. A quick look at the following map confirms the general climate belts of the Continent. The area of where fires are currently occurring is almost entirely within the yellow area described as having wet summers and low winter rainfall.
Paradoxically, a late onset of the summer cycle increases the risk of extreme fire weather in NSW. As spring advances the rate of evaporation increases leading to the drying of undergrowth. However, if this is not accompanied by the onset of summer rain then this can cause the weather scenario we are seeing right now.
So why would this year be different from any other? Recent seasons have brought significant summer rain to the Eastern Seaboard, and this may have led to an increase in the fuel load. Alternatively, or perhaps additionally, this season the progression from winter to summer seems to be slightly later than normal and as described above, this does not necessarily mean that the Eastern Seaboard is cooler – deep troughs caused by higher northerly penetration of lows means potentially higher winds carrying heat from the north, but as yet without the rain.
It is reasonable, though paradoxical, to conclude that the current fire weather in NSW is not caused by climate warming, but may be caused by the opposite: a climate condition that may lead to late onset of summer rains. Unfortunately this is likely to mean that fire weather has a higher probability of remaining longer into spring, and the later onset of summer rains may mean that may be reduced summer rain leading to drought conditions on the East Coast.
JC comment: This post was submitted via email. It is certainly a timely topic for us to consider. As with all guest posts, keep your comments relevant and civil.