Tuesday, February 4, 2020

More trouble with Australian bushfire statistics.

I wrote last month about a fallacy that was entering the discussion of extent of bushfires in Australia. The very bad forest fires that we have been having have burned about 10 million hectares in SE Australia. That is a huge amount historically, and is an area larger than Hungary or Portugal (or Indiana). It is quite large proportion of our temperate forest, where many people live, and overlaps with a lot of agriculture.

Wikipedia keeps a list of past bushfires in which areas are quoted, and has often been used as a source. I noted that their list caused confusion, because along with forest fires they had included, for random reasons, just a few years with extraordinarily high totals, which are not otherwise known as bad bushfire years. One in particular was 1974/5, with a total of 117 million ha. Inferences have been drawn from this. However, the comparisons are spurious, because Wiki included in that year the huge areas that are regularly burnt in the northern savanna scrublands. Although that was an unusually large year, figures of 20-30 million hectares reported are common. I emphasised that it was important to separate these fires, which have little adverse impact, from the hugely destructive temperate forest fires in statistics for comparison.

Unfortunately the situation has got worse, and I think all Australian bushfire area statistics should now be regarded with scepticism. The Wikipedia list has got totally out of hand. I entered discussion there, pointing out that 1969 had an area burnt in the Northern Territory equal to that of 1974. This was intended as a reductio ad absurdum, but all it achieved was that that total was included too, making another year to be quoted for its extreme fires. And 1974/5 is a total mess, with not only the huge total but also the separate state totals, and then a collection of NSW fires.

But it is now worse again, in that the 2019/20 total has been boosted to 18.6 million ha, by inclusion of this year's NT fires (6.8 million ha). That totally corrupts the statistics. 6.8 was actually a low number for NT; next year it could easily be more than 20 million ha, and we will be told that 2019/20 was nothing in comparison.

And now I find that it is not just Wiki but the Australian Gov't Dept of Home Affairs which has put out numbers using similar arithmetic. I don't know why DHA is publishing bushfire stats at all, but I am very suspicious, because its minister is Peter Dutton, the conservative enforcer in the Cabinet, and I don't think he has the accurate appreciation of bushfire severity as a prime aim. I think the prospects of Wiki and similar sources getting it right are now bad, and the statistics are best ignored.

Reliability of savanna fire statistics

I think the savanna stats should be kept separate, because they represent very low impact fires, where there is no real effort at suppression. But I also think the numbers are very dubious. They are taken from a worldwide database compiled by Giglio et al (paper here) and based on satellite MODIS observations. As far as I can tell from the paper, they mostly rely on monitoring the actual flames, rather than deciding on the burnt appearance afterwards. So the translation of that into area burnt involves some guesswork, and then there is the question of what burnt even means where the vegetation is sparse. (See comment below by WT)

And sparse it often is. At WUWT a post displayed this map of the 2001 fire distribution in Australia:

By contrast here is a Wiki map of the named desert regions

As you can see, the area burnt includes a lot of desert, including the Great Sandy Desert, Tanami Desert, Gibson Desert. Whether these could really be said to have meaningfully burned, there is no useful comparison of a hectare of such desert with one of temperate forest in NSW.

Update: I found a survey paper on savanna fires in Australia here.


  1. Hi Nick, the Giglio dataset isn't based on obs of "flames" (or active fires). It's built up from the 500-m MCD64A1 burned area product, which detects date of burn by looking at temporal change of surface reflectance (e.g. vegetation) at each pixel.

  2. I think my main takeaway from all this is that none of the quoted measures (area, $ impact, lives lost, buildings burnt etc) have any credible relationship from one year to the next, and so the great majority of the comparisons are essentially meaningless except for emotional impact and hyperbole.