Causes of extreme fire weather in Australia

Key points

• The weather has a crucial influence on the severity of fires. Hot, dry conditions mean that once a fire has started, whether by human or natural causes, it can burn more intensely and spread faster. Warmer temperatures cause more evaporation which dries the vegetation faster.
• Over recent decades, the weather in much of Australia has shown a trend of increasing fire danger.
• The recent high temperatures and drought in Australia arose from weather patterns associated with natural climate variability, but can be expected to have been hotter because of human-induced climate change.
• Ongoing climate change is projected to increase fire weather in Australia and elsewhere.

Large-scale fires have been burning across parts of Australia since September last year. An estimated 10 million hectares have burned so far, destroying homes and causing deaths of people and vast numbers of animals.

In some dry savannah ecosystems, fire is a natural and important part of maintaining the fire-adapted vegetation. However, this can become a problem when the natural cycle is altered, for example by shifting the climate envelope into a warmer world.

We take a brief look here at the weather and climate drivers that have contributed to the recent extreme heat and drought over Australia in the context of a changing climate.

Influences on fire: ignition, fuel and weather conditions

Fires depend on a combination of available fuel, fire weather conditions, and ignition. Vegetation provides the fuel for fires, and both the amount of vegetation and its level of dryness are important. Moist, live vegetation burns less readily whereas dry, dead vegetation burns more easily. Fires can be ignited naturally by lightning, but are also started by people, either accidentally or deliberately. Drier fuel is more likely to catch fire and allow the fire to spread rapidly.

The weather therefore plays a critical role. Periods of low rainfall leave the fuel dry, and this can be exacerbated by low humidity and high temperatures which dry the fuel faster through evaporation. High winds cause fires to spread faster.

In Australia, weather conditions are routinely categorised by the McArthur Forest Fire Danger Index (FFDI)[1]^,[2]. This represents the level of risk of a fire spreading and becoming severe once lit, by whatever cause, if sufficient fuel is available (Table 1). A similar index categorises the danger of grass fires.

Table1: McArthur FFDI scale of fire danger.

*Catastrophic refers to fires that spread so quickly that they present a threat to life and safety

Fire weather is increasing

Australia has seen an increase in extreme heat events and the severity of drought, according to the CSIRO and Australian Bureau of Meteorology’s State of the Climate 2018 report[3]. This has led to an increase in fire weather over much of Australia. The FFDI has increased in many parts of the country over the decades from 1978-2017, particularly in the south east (Figure 1).

Figure 1: Source- CSIRO/BOM State of the Climate Report 2018³

a) Annual sum of daily FFDI, 1978-2017. Positive trends, shown in the yellow to red colours, show an increasing length and intensity of the fire weather season.

b)  Average Area of the number of days with FFDI greater than 25 (very high fire danger) in Victoria in spring for the years starting in July (1978–2017). Although there is considerable interannual variability in the index, there is also a clear trend in more recent decades towards a greater number of very high fire weather days in spring.

The 2019/2020 summer: the role of natural climate variability

The recent conditions in Australia have largely been driven by the pattern of Sea Surface Temperatures (SST) in both the western equatorial Pacific and Indian oceans, and a sudden stratospheric warming. These events are now occurring on top of a background of continued warming, which is contributing to increases in extremes.

A phenomenon called the Indian Ocean Dipole (IOD) is one of the drivers of Australian climatic variability, affecting temperature and rainfall patterns. The IOD is defined by differences in west-east ocean surface temperatures across the Indian Ocean, which develop in the austral winter (JJA) and peak in the austral spring (SON), and last for up to 6 months. When in a positive phase, ocean-atmosphere interactions in the Indian Ocean lead to cooler water in the east and warmer than normal water in the west. This results in the suppression of rainfall in southern parts of Australia, and together with higher than normal temperatures, adds to drought conditions.

ENSO, through its El Niño (warm) and La Niña (cool) SST extremes in the equatorial Pacific, drives Australian drought and flooding extremes respectively. ENSO events can take different forms, generally they occur every 2-7 years and last up to a year or so, although some ‘protracted’ episodes can last for a number of years[4]. One of the main characteristics of ‘protracted’ El Niño episodes are warm SST anomalies in the western equatorial Pacific Ocean, which leads to enhanced atmospheric convection in that region, and the generation of a teleconnection which suppresses rainfall across eastern Australia. This is what has been seen since April-May 2018.

There was also a rare sudden stratospheric warming in the months before the heatwave[5]. Although it did not quite make the criterion for a full event, it resulted in a highly disturbed stratospheric polar vortex over Antarctica. 

This dramatic fluctuation in the stratosphere is known to temporarily alleviate the ozone hole over Antarctica, but it also alters the weather in the lower atmosphere by affecting the southern hemisphere jet stream. This results in drier conditions over Australia and probably exacerbated the Australian fires. Sudden stratospheric warming events occur every couple of years in the northern hemisphere, but are very rare in the southern hemisphere with only one full blown occurrence in the current observational record which occurred in September 2002.

Contributions of anthropogenic climate change

The world has already warmed by over 1°C globally since the preindustrial era[6]. This warming is changing weather patterns globally. The IPCC 5^th Assessment report concluded that there are now more hot days globally, and more heat waves being experienced as a result of climate change[7]. Events such as a positive IOD now have the potential to reach more extreme temperatures on top of this background warming. A recent review of the scientific literature highlighted an increase in fire weather across the world, with the fire season now being 20% longer on average[8]. Clear contributions of human-caused climate change to this have been identified over 22% of the global land surface, including southern Europe, Scandinavia, Amazonia, Canada and the western USA.

Globally, the area burned has decreased over recent decades, due to increased agriculture, urbanisation and land management practices[9]. Nevertheless, the increase in fire weather means that when fires do start, they are more likely to burn more severely and be harder to control.

In Australia, previously-published scientific literature has not yet found the impact of human-induced climate change on fire weather to be emerging from natural climate variability. Studies are already underway using the most up-to-date data on the recent events. Nevertheless, with temperature being a key factor in fire weather, it can be expected that the warming climate will be starting to contribute to increased fire danger, and that this influence will continue to increase and become apparent in the coming decades if global warming continues.

Implications of future climate change for fire in Australia

Work from the Met Office suggests that as the world warms, some areas may become more exposed to higher temperatures and to higher fire danger[10]^,[11]. At 2°C of warming, models calculate that more areas globally would be at risk of higher fire danger for longer periods compared to today. In south-western and eastern Australia, where many of the recent severe fires have occurred, 2°C is projected to lead to an additional 20 to 30 days per year on average with the Forest Fire Danger Index at “very high” or above (Figure 2a). In Western Australia, there would be up to 7 more days per year at “extreme” FFDI or above (Figure 2b). The area at risk of “extreme” fire danger in Australia could increase by around 20 million hectares at 2˚C compared to present day. This is equivalent to an area nearly the size of the UK or the state of Victoria.

Figure 2: Average additional number of days per year (compared to present day) for which fire danger is (a)“very high” or over and (b) “extreme” or over on the McArthur FFDI scale, at 2˚C global warming

Research is underway to quantify the change in likelihood of the recent extreme fire weather in Australia occurring as a result of climate change. It is important to understand the relative impact of climate change, ocean temperature anomalies and the sudden stratospheric warming which all altered this year’s weather patterns and temperatures over Australia, and the extent to which these effects have influenced the large-scale fires. But as climate change continues to bring increasing temperatures, we can already expect the risk of heatwaves and other extreme weather events to increase in the future.

References