Bushfires & climate change

Everyone (well almost everyone!) said the 2019/20 summer bushfires in eastern Australia were “unprecedented”. But what made them so bad? Here’s a quick summary.

How bushfires start

There has been much misinformation about this. Contrary to some claims, most of the 2019/20 bushfires were not started by arsonists.

Fire experts can often determine the way a fire started from its location, the weather at the time, the number and location of ignition points and the direction and pattern of burning.

Previous assessment

Various estimates were given in the past for the causes of fires.

  • The ABC gave the figures: 13% were deliberate, 37% were suspicious and 35% were accidental (burning off, equipment, cigarettes, etc).
  • This source gives slightly higher estimates for arson, but points out a reason why the figures can be confusing. There are far more grass fires than full-on forest fires, and these are more commonly lit by arsonists or by accident and are generally close to settled areas. But deliberately lit fires less commonly develop into major bushfires.
  • The Australian Government’s Geoscience Australia suggests human and natural sources each contribute to about half of all bushfires.

But the 2019/20 fires were very different.

Re-assessment for 2019/20

The 2019/20 fires were ‘unprecedented’ in many ways, including the fact that the majority were started by lighting strikes.

Only a small number, leading to less than 1% of the land burnt, were considered by CSIRO and ABC News to be deliberately lit.

Causes of 2019/20 bushfires

Four main factors influence the occurrence and severity of bushfires:

  • Weather
  • Vegetation, the fuel for the fire
  • The terrain
  • The cause of initial ignition


 ‘Bushfire weather’ is a combination of strong winds, low humidity and high temperatures – in the preceding days to dry out the vegetation and at the time of the fire to allow the fire to develop from initial ignition.

Mean temperature averaged over Australia (BoM). 2019 was the hottest year on record.

Southern and eastern Australia experienced bushfire weather in 2019/20. Record low rainfall and record high temperatures led to increased frequency of fire weather days:

  • 2019 was the driest year since records began in many locations on the north coast, and generally in the driest 10% of years on the south coast;
  • humidity was low when the fires were raging.
  • 2019 was Australia’s warmest year, with the annual mean temperature 1.5 °C above average. Temperatures over 40°C were experienced on many of the fire-grounds.
  • Winds were often strong and unpredictable.

The hot dry weather of 2019 meant that the fire season started earlier than usual and prevented a significant amount of hazard reduction burning, all of which increased the likelihood of fires.

Map showing rainfall for 2019 (BoM). Most of Australia was exceptionally dry.
Pyro-cumulonimbus clouds

When fires become as large and hot as occurred in the 2019/20 summer, there is an enormous updraft of hot air. The small amount of water vapour in the air condenses and forms a thundercloud (a pyro cumulonimbus cloud).

The atmospheric instability can generate static electricity in the cloud that causes lightning strikes, which can start new fires some distance away from the active fire. And the air is so hot that the moisture in the cloud evaporates before it reaches the ground as rain, hence the term “dry lightning”.

The enormous Gospers Mountain fire started from a storm in October 2019 in which more than 19,000 lightning strikes were recorded.


Vegetation includes living trees, shrubs and grasses, but also dead branches and leaves on the ground. If vegetation is plentiful and dry, fire risk is greater

Southeastern Australia had experienced hotter and drier than normal weather for two years prior to the very hot and dry 2019. As a result, the densely forested areas carried large amounts of dry fuel. Even gullies with a small watercourse (which are normally damp and may slow the rate of fire spreading) would have been drier than usual. University researchers calculated that the moisture conditions in coastal NSW were much drier than normal.

Experts say that hazard reduction burning is less useful in the mega fires. Nevertheless, the longer fire season in 2019 (noted above) reduced the amount of hazard reduction possible, further contributing to fuel load.


Many of the major 2019/20 fires began in rugged bushland along the Great Dividing Range. The remote locations make it more difficult to notice and attack fires in time to prevent them getting established, and the steep country can speed fires up as they burn up steep hills.


As already noted, these mega fires generally began in remote country with lightning strikes. (Arsonists generally don’t travel kilometres into thick forest.)


Thus all the conditions were “right” for disastrous and ferocious fires – many lightning strikes into remote and dry bushland on hot dry windy days.

The Forest Fire Danger Index (FFDI) is an annual accumulation of all the daily fire risks shown on fire rating signs all over Australia.

The 2019 annual accumulated FFDI was the highest on record.

Unprecedented outcomes

The 2019/20 summer fires were unprecedented in their destructive effects (figures taken mostly from the Bushfire Royal Commission report).

  • 240,000 square kilometres were burnt – much of it temperate forest (some previous fires burnt larger areas, but mainly grassland, with less catastrophic results);
  • the Gospers Mountain fire was the largest forest fire recorded in Australia since European settlement;
  • more than 3,000 homes were destroyed;
  • 33 people died directly because of the fires and it is estimated that more than 400 people died and 4,500 treated at hospitals because of the smoke;
  • more than a billion animals were killed and billions more displaced;
  • estimated cost of the fires was more than $10 billion.

Climate change connection

It isn’t hard to see that climate change is a major factor in the disastrous fires. Global warming has led to lower rainfall and hotter temperatures in southeast Australia, and a longer fire season, which have made worse three of the four key factors – weather, vegetation and ignition.

The Australian Government’s ‘Bushfire Royal Commission’ recognised this when it said:

“Climate change has already increased the frequency and intensity of extreme weather and climate systems that influence natural hazards. ….. Floods and bushfires are expected to become more frequent and intense.

The 2019-2020 bushfire season demonstrated that bushfire behaviour is becoming more extreme and less predictable. Catastrophic fire conditions may become more common, rendering traditional bushfire prediction models and firefighting techniques less effective.”

Action is needed

Australia needs its government to act decisively to avert even worse disasters by reducing emissions much faster than currently planned.

Australia has so many natural advantages to produce renewable energy, we should be leading the world in addressing climate change.

Further reading

Main photo: Backburning by RFS to contain a fire at Glenmore Park near Sydney. Quarrie Photography. Fire rating sign. CSIRO.