As last summer’s horrific bushfires raged, so too did debate about what caused them. Despite the prolonged drought and ever worsening climate change, some people sought to blame the fires largely on arson.
Fire authorities rejected the arson claims, saying most fires were thought to be caused by lightning.
We dug into open data resources to learn more about the causes of last summer’s bushfires in Victoria, and further test the arson claim. Our analysis suggests 82% of the fires can be attributed to lightning, 14% to accidents and 1% to burning off. Only 4% can be attributed to arson.
What we did
We started with hotspots data taken from the Himawari-8 satellite, which shows heat source locations over time and space, in almost real time. We omitted hotspots unlikely to be bushfires, and used a type of data mining called “spatiotemporal clustering” – where time dimension is introduced to geographic data – to estimate ignition time and location.
We supplemented this with data from other sources: temperature, moisture, rainfall, wind, sun exposure, fuel load, as well as distance to camp sites, roads and Country Fire Authority (CFA) stations.
Victoria’s Department of Environment, Land, Water and Planning (DELWP) holds historical data on bushfire ignition from 2000 to the 2018-19 summer. The forensic research required to determine fire cause is laborious, and remotely sensed data from satellites may be useful and more immediate.
By training our model on the historical data, we can more immediately predict causes of last summer’s fires detected from satellite data. (Note: even though we were analysing events in the past, we use the term “predict” because authorities have not released official data.)
DELWP’s data attributes 41% of fires to lightning, 17% to arson, 34% to accidents and 7% to hazard reduction or back burning which escaped containment lines (which our analysis refers to as burning off).
To make predictions for the 2019-20 bushfires, we needed an accurate model for causes in the historical data. We trained the model to predict one of four causes – lightning, accident, arson, burning off – using a machine learning algorithm.
The model performed well on the historical data: 75% overall accuracy, 90% accurate on lightning, 78% for accidents, and 54% for arson (which was mostly confused with accident, as would make sense).
The most important contributors to distinguishing between lightning and arson (or accident) ignition were distance to CFA stations, roads and camp sites, and average wind speed.
As might be expected, smaller distances to CFA stations, roads and camp sites, and higher than average winds, meant the fire was most likely the result of arson or accident. In the case of longer distances, where bush would have been largely inaccessible to the public, lightning was predicted to be the cause.
What we found
Our model predicted that 82% of Victoria’s fires in the summer of 2019-2020 were due to lightning. Most fires were located in densely vegetated areas inaccessible by road – similar to the historical locations. (The percentage is double that in the historical data, though, probably because the satellite hotspot data can see fire ignitions in locations inaccessible to fire experts).
All fires in February 2020 were predicted to be due to lightning. Accident and arson were commonly predicted causes in March, and early in the season. Reassuringly, ignition due to burning off was predicted primarily in October 2019, prior to the fire restrictions.
Quicker fire ignition information
Our analysis used open-data and open-source software, and could be applied to fires elsewhere in Australia.
This analysis shows how we can quickly predict causes of bushfires, using satellite data combined with other information. It could reduce the work of fire forensics teams, and provide more complete fire ignition data in future.
This analysis is based on thesis research by Monash University Honours student Weihao Li. She was supervised by the author, and former Principal Inventive Scientist at AT&T Labs Research, Emily Dodwell. The Australian Centre of Excellence for Mathematical and Statistical Frontiers supported Emily’s travel to Australia to start this project. The full analysis is available here.
Every year Tasmania is hit by thousands of lightning strikes, which harmlessly hit wet ground. But a huge swathe of the state is now burning as a result of “dry lightning” strikes.
Dry lightning occurs when a storm forms from high temperatures or along a weather front (as usual) but, unlike normal thunderstorms, the rain evaporates before it reaches the ground, so lightning strikes dry vegetation and sparks bushfires.
Dangerous, large fires occur when dry lightning strikes in very dry environments that are full of fuel ready to burn. Cold fronts in Tasmania, which often carry fire-extinguishing rain, have recently been dry, making these fires worse. The fronts draw in strong hot, dry northerly winds, fanning the flames.
Research has found that as climate change creates a drier Tasmania landscape, dry lightning – and therefore these kinds of fires – are likely to increase.
History and detection in Tasmania
Lightning has always started fires across Tasmania. Fire scars and other paleo evidence across Tasmania show large fires are a natural process in some places. However, frequent large, intense fires were rare. Now such fires are being fought almost every year.
Contrary to anecdotal belief, our recent preliminary work suggests that lightning activity has not increased over recent decades. So why do fires started by lightning appear to be increasing?
As temperatures rise, evaporation rates are increasing, but current rainfall rates are about the same. In combination this means the Tasmanian landscape is drying. The landscape is more often primed, waiting for an ignition source such as a dry-lightning strike. In such conditions, it only takes one.
When dry lighting strikes
Lightning struck just such a landscape in late December 2018, starting the Gell River bushfire in southwest Tasmania. This uncontrollable fire burnt about 20,000 hectares in the first half of January and is still burning. These large fires deplete the state’s resources, fatigue our volunteer and professional fire fighters and can have disastrous effects on natural systems.
With no significant rain falling over Tasmania since mid-December, the island is breaking dry spell records and thousands of dry lightning events have occurred. On January 15 alone over 2,000 lightning strikes sparked more than 60 bushfires.
Most of these were controlled rapidly, a credit to Tasmania’s emergency responders. One of the worst-hit areas was the Tasmanian Wilderness World Heritage Area, where many bushfires continue to burn in inaccessible locations.
This is putting some of Tasmania’s most pristine and valuable places in danger of being lost. The state stands to lose its most remarkable old-growth forests, like Mount Anne, which is home to some of the world’s largest King Billy Pines, a species endemic to Tasmania.
Increasing dry area
Ongoing climate change is making dry spells longer and more frequent, increasing the fire-prone area of Tasmania. Almost the whole state is becoming vulnerable to dry lightning.
Some regions of the west coast of Tasmania used to have very little to no risk of bushfires as they were always damp. However, this is no longer the case, resulting in species coming under threat.
Unlike most of Australia’s vegetation, many of Tasmania’s alpine and subalpine species evolved in the absence of fire and therefore do not recover after being burnt. Endemic species like Pencil Pine, Huon Pine and Deciduous Beech may be wiped out by one fire.
So what does the future hold? Using data from Climate Futures for Tasmania, we can peek into the future. Our models indicate that climate change is highly likely to result in profound changes to the fire climate of Tasmania, especially in the west.
Climate change already playing a role
With a warming climate, the rain-producing low-pressure systems are moving south and many storms that used to hit Tasmania are drifting south, leaving the island drier. This, combined with increasing evaporation rates, result in rapid drying of some areas. Areas that historically rarely experienced fire will become increasingly prone to burn. The drying trend is projected to be particularly profound throughout western Tasmania.
By the end of the century, summer conditions are projected to last eight weeks longer. This drying means that lightning events (and therefore dry lightning) will become an ever-increasing threat and the impact of these events will become more significant.
Higher levels of dryness will mean when bushfires occur the potential for these to burn into the rainforest, peat soils and alpine areas will be significantly increased.
How far away was that lightning?
These changes are already happening and will get progressively worse throughout the 21st century. Climate change is no longer a threat of the future: we are experiencing it now.
Nick Earl, Postdoctoral associate, School of Earth Sciences, University of Melbourne; Peter Love, Atmospheric Physicist, University of Tasmania; Rebecca Harris, Climate Research Fellow, University of Tasmania, and Tomas Remenyi, Climate Research Fellow, Climate Futures Group, Antarctic Climate and Ecosystems CRC, University of Tasmania