This is a great question, but one that science can’t fully answer yet. There are several ingredients necessary for fire, which means it is often difficult to say that a particular fire was caused only by climate change. For bushfires to occur, there needs to be:

• fuel to burn, which is typically leaf litter and twigs, grasses, shrubs and trees;
• the fuel needs to be dry enough to catch on fire;
• an ignition source, such as a camp fire that somebody failed to put out, lightning, or a downed powerline, and
• fire weather: typically hot, dry, windy weather that causes a fire to spread quickly. 

Because all these ingredients are required for a bushfire, it is difficult to say how many fires are specifically caused by climate change. We do know, however, that climate change is affecting each of these ingredients. Climate change is leading to changes in plant growth (fuel) and causing fuels to dry out more quickly because the air is hotter and drier. Climate change seems be leading to more ignitions from dry lightning (lightning that occurs when it is not raining). Climate change is certainly causing more dangerous fire weather (hot, windy weather) and fire seasons that start earlier in the year and finish later.

So, although science cannot yet say how many fires are caused by climate change, we can be confident that climate change is leading to more large forest fires around the world.

Like making dinner, bushfires require some key ingredients. These are: (1) fuel, (2) fuel dryness, (3) ignitions and (4) fire weather.

Fuel means plants that can be burned. This includes grasses and shrubs, leaf litter and twigs, tree trunks, and really any other plant material.

For fires to occur, the fuel must be dry enough to catch on fire. If the plant material is still growing and therefore has water inside of it, it is more difficult for it to catch on fire. Likewise, if the fuel is moist because it has recently rained, then it is unlikely to catch on fire. This is one of the reasons bushfires are more common during summer, when it is warmer and dryer.

For a fire to occur, there also needs to be an ignition source. Ignitions are often accidentally caused by humans. For example, there have been ignitions caused by a people driving over a dry paddock and their hot exhaust pipe ignited long dry grass. There have also been examples when people failed to put out their camp fire. These human-caused ignitions are something that we humans have control over, and thankfully we seem to be getting better at managing them. There are also natural sources of ignitions, like dry lighting, which is lightning that occurs when it is not raining. Dry lightning unfortunately seems to be getting more common.

The final key ingredient of a bushfire is fire weather. Fire weather describes the type of weather that would cause a bushfire to start spreading. Bad fire weather consists of hot, dry, and windy weather occurring at the same time. When it is hot and windy, the fire can quickly go from a small fire to a raging bushfire. These are the days when fire bans are in full effect, which is why it’s so important for us to make sure we don’t provide any new ignitions on days when fires can spread easily.

We cannot say that climate change was entirely responsible for the Black Summer Bushfires of 2019/20, but we can certainly say that it played a very important role.

2019 was Australia’s hottest and driest year on record. The combination of hot and dry weather leads to high fire risk because it produces fuel (plant material) that is exceptionally dry and easy to ignite, and it also produces extreme fire weather (hot, windy) that makes it difficult to fight the fires.

These extreme climatic conditions in 2019/20 were strongly influenced by climate change. However, the fires were also influenced by climate events that occur naturally from time to time. An example of this is that the fires occurred during an El Nino event, which is a natural climate pattern that leads to hotter and dryer weather in Australia. The effects of climate change, like hotter weather, made the natural El Nino event much more extreme.

To summarise, the Black Summer Bushfires occurred during the hottest and driest year on record in Australia, which is largely caused by climate change. Even though there were other naturally occurring cyclical climate events involved (like El Nino), the fires would have been more manageable without human-caused climate change.