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31 January 2020 14 min read

Dr Michael Grose (pictured) is a Senior Research Scientist in the CSIRO Climate Science Centre.

ECOS: The science is clear about climate change and the role of greenhouse gases from human activity in driving global climate change, but how does climate change impact extreme weather events in Australia?

Dr Michael Grose (MG): As well as a warming of the global average temperature, human activity is affecting all aspects of the climate system and changes are not uniform around the world. Climate change affects rainfall, weather patterns, storms, evaporation, ocean currents, the distribution of snow and ice, and much more. Changes to atmospheric circulation patterns mean that some areas are getting drier while others are getting wetter, and in some cases, seasons are shifting or changing. Climate change is likely affecting the so-called drivers of climate variability, such as the El Niño Southern Oscillation as well. Along with the effects on the climate, increasing carbon dioxide concentrations are having direct effects such as ocean acidification and the fertilization of plants. For any particular region, these various climate processes come together and interact in complex ways, including feedbacks that can amplify or dampen the initial change. These interactions and feedbacks affect the intensity, frequency and duration of climate variability and extremes in a changing climate, and therefore the physical impacts of these changes.

Some changes to climate extremes driven by climate change are relatively simple to understand and recognise in our everyday experience. For example, just increasing the average means that we get more hot years, hot months and hot days including unprecedented heat extremes without any change to climate variability. This means that more heat records have been breaking than cold. This can be seen in Australia’s observed climate record. Australia’s temperature has risen by just over 1°C since reliable records started in 1910, and 2019 was Australia’s hottest year.  Similarly, oceans have warmed significantly leading to an increase in the frequency, duration and intensity of marine heatwaves. Changes to the dynamics of the weather that bring heat, or factors that amplify heat extremes such as land-atmosphere feedbacks, can then enhance or offset the increase expected from changes to the average temperature.

Aside from heat, flow-on effects to other climate phenomena can be less obvious and sometimes more complex. Southern Australia has been getting drier and this is consistent with a change in circulation and weather patterns expected under climate change. Last year was Australia’s driest year on record since 1900, with very dry conditions in virtually all of southern and eastern Australia. Despite a drying average climate in much of southern Australia, when rainfall does occur, the warmer climate means it is more likely to fall in more concentrated downpours. On top of this, these extreme events are becoming more extreme. This is already happening and can be seen in the observed record in recent decades.

The Australian 2019/20 bushfire season started way back in August 2019, before our winter had officially finished. Does climate change science provide any insights into why this is the case?

As this handy and authoritative summary from the NESP Earth System and Climate Change hub explains, fire seasons in southern and eastern Australia have been getting longer, mainly through an earlier spring start to the season. Last year showed that the southern fire season can in fact begin in winter.

The fire season is largely determined by the prevalence of fire weather conditions, conditions that are described by indices such as the McArthur Forest Fire Danger Index (FFDI). This is the information we’re accustomed to seeing on the iconic fire danger signs by the roadside, with values from low-moderate (0-11), through high (12-24), very high (25-49), severe (50-74), extreme (75-99) to catastrophic (100+). The FFDI combines air temperature, relative humidity, wind and soil dryness. Long-term changes to these variables, and the weather systems that affect them, have largely driven the long-term shifts in the fire season that we are experiencing across Australia. The northern Australian fire regime is quite different from the south, with relatively few fires in the wet season, but annual large-scale savannah burning in the dry season from April to October. Of course, fire weather isn’t the whole story regarding fire danger.

Australian Fire Danger Warning System, based on the McArthur Forest Fire Danger Index.

Yes, researchers talk about the four factors that contribute to bushfires: the weather conditions that allow fires to spread; something to light a fire (the ignition); how dry the fuel is and the nature of what is available to burn. How does climate change impact on these four factors?

That’s right, it’s important to think about all the factors that create the total bushfire risk and assess if and how climate change may be affecting each one of them and how they all come together. The science of teasing this apart, and determining the influence of the changing climate is called event attribution.

First looking at the weather that allows fire to spread once ignited, which FFDI mainly covers, we see some clear trends in many places in Australia. Not only has there been an earlier start to the season, but also greater accumulated fire weather risk over the year and more days of high, extreme or catastrophic danger ratings. If you add up the FFDI values for every day over a year, you get what’s called the ‘annual accumulated FFDI’. Last year saw the highest annual accumulated FFDI on record. This underlying long-term trend of increasing fire danger over time is projected to continue under climate change, with more days of high danger categories, but of course with large ongoing variability from year to year.

If we then look at ignition, fires are ignited by people, accidental or not, and naturally - mostly from lightning strikes. Lightning accompanied by relatively little rainfall, known as ‘dry- lightning’, is a major cause of bushfires that can burn large areas of land in Australia. The NESP Earth System and Climate Change hub thunderstorms and climate change summary shows that there is some evidence for an increase in dry lightning occurrence and a projected increase into the future; although the evidence is still unclear and so the jury is still out as to whether these trends can be taken with confidence. With more careful measurements and research, we may get insight into this phenomenon.

Dryness of the ‘fuel’ (e.g. vegetation) is another important ingredient for fire danger. Dryness of fine fuels in the recent past (like grass and twigs) is captured in the FFDI, however, the effect of heat and rainfall in the months leading up to the fire season can also be very important. This is especially true for fuel types such as wet forest that only become flammable after prolonged dryness. Rainfall in parts of southern Australia has decreased in recent decades – especially the so-called cool season rainfall (from April to October), and this trend is projected to continue, especially in spring which is important for setting up the fire season. Our changing climate means fuel dryness is a growing issue in southern Australia.

An event attribution study of the 2015/16 spring/summer in Tasmania found that it was hotter and drier than it would have been without climate change. This set up the landscape for the fires in early 2016. A recent study examined the factors behind the fire weather and fuel dryness observed in the Queensland fires of 2018. The circulation pattern and preceding dryness and heat were important factors, and climate change made the heat more likely than would be expected with climate variability alone.

Finally, looking at fuel type, structure and condition, there are interactions between climate and fuel both in terms of how the fuel, such as leaf litter, builds up, and in our opportunity to manage the fuel. Hazard reduction burns need to be done safely, both in terms of the fire and the smoke they produce, and the lengthened fire season restricts our opportunity to safely do this. Also, the climate is crucial in determining the overall landscape over time – we even talk about ‘bioclimatic zones’ from wet forest to grassland. Changes to temperature, rainfall, wind and extremes can all cause a change to the landscape. Another factor is carbon dioxide fertilisation – a higher concentration of carbon dioxide in the air means that plants can photosynthesise more readily and use water more efficiently since they don’t have to open their stomata (similar to our pores) as widely. Changes in the climate and carbon dioxide fertilisation results in changes to fuel condition, the size of the fuel (e.g. stem thickness) and even the type of the vegetation. The fire regime itself then affects the fuel structure, forming a feedback. For example, many eucalypts such as Mountain Ash require fire to germinate but can’t grow to maturity if it burns too often, so if fire comes too often or too seldom the species can’t survive and the forest may change to a different type.

We know climate change is causing temperatures to rise, which is affecting our bioclimatic zones. We also know carbon dioxide fertilisation is occurring, which will change and potentially increase plant growth and biomass production. Climate change is also driving a reduction in rainfall in parts of southern Australia. These and other climate change effects are likely to have important effects on our vegetation, soil and fuel moisture and landscapes over time, and this is the topic of a whole branch of study called fire ecology.

Dry landscape burning. The North Black range fire just west of Braidwood, east of Canberra. Picture by CSIRO

So, in terms of the climate leading up to a fire season – a few factors are clearly very important. How did Australia’s climate last year, in 2019, precondition the landscape for fire season we are now having?

While factors such as fuel management and human ignitions are important to consider, the climate variability and change was clearly a huge player in setting up the fires. As outlined in the Bureau of Meteorology’s annual climate statement, it was Australia’s hottest and driest year since records began. Dry conditions persisted through the whole year, and across the majority of southern and eastern Australia. Similarly, the record heat resulted in the national annual average temperature being 1.52 °C above the 1961-1990 average. Importantly, the heat was persistent in space and time. December daytime maximum temperature was an extraordinary 4.15 °C above the 1961- 1990 average.

The record heat and dry was a result of long-term climate trends coming together with swings of variability. There was a swing of variability towards hotter and drier, but the conditions would not have been possible without the longer-term trends. For example, if we remove the long-term trend in mean annual temperature, the temperature anomaly in 2019 goes from 1.52 °C to just 0.6 °C; a hot year but nowhere near what we experienced. We are confident that most of the warming trend since the 1950s is due to human induced climate change.

This fire season has had devastating impacts over much of the nation. Climate change appears to be a major factor in the conditions – but what does the science say about connecting this particular fire season, or even an individual fire, to climate change?

Sure, it’s complex but we can say something. As I mentioned earlier, we need to carefully examine all the interacting drivers of any event and carefully look at any possible effect of climate change on each driver as well as on the total risk. We can then estimate what we would have experienced in a world without human induced climate change and compare this to the world as we experience it now. This allows us to look at what we call the ‘fraction of attributable risk’ due to climate change.

To do this well, we need three main ingredients: good quality and long observational records, a good understanding of the drivers of the type of event, and the ability to simulate the event in climate models. The ideas, issues and methods are outlined in this article, and various examples from around the world each year are described here. Some changes can be confidently attributed, including an increase in heat, reduction in sea ice cover, some heavy rainfall events, and marine heatwaves. Others are less clear, such as multi-year droughts, due to limitations in our observed record, understanding of the events and their simulation in models. Overall fire danger is also complex, so we need to do careful analysis and research to understand the contributions of many factors.

Ok, so the science is complex in this space, but clearly climate change is playing a role.

That’s right, there are many complexities here, but this shouldn’t distract us from some very clear and confident findings – a hotter and drier southern Australia means greater fire danger. Imagine you are tending to a garden. How you manage the garden is very important – your watering, weeding, planting and fertilizing all matter – but the climate is also central to the garden. You just can’t grow paw paws on Macquarie Island, and you simply can’t grow Pinot Noir grapes in Cairns. Fire risk is a bit similar - our management makes a difference, but the climate is a fundamental determinant of the fire risk we face, and Australia’s climate is changing as seen in our observed temperature, rainfall record and the long-term trends (from 1950) in FFDI. This is what the scientific evidence indicates, as noted by the Australian Academy of Science

Michael, is there a positive feedback loop with carbon emissions from the current fires contributing to accelerated climate change?

It is true that trees, which absorb carbon through photosynthesis, also release carbon back to the atmosphere when they burn. Some of this carbon is re-captured by trees and plants that grow back after fires. This Australian fire season has seen a significant flux of carbon back to the atmosphere, which will likely be measurable as a distinct event in the atmospheric carbon dioxide record – we are expecting to see a signal at Cape Grim Baseline Air Pollution Station in upcoming measurements. Scientists are currently working to quantify the total amount of carbon emissions from fires and consider how to respond.

Scientists are also exploring whether the significant losses in SE forests this season may lead to a long-term net gain of CO2 in the atmosphere because the usual regrowth and sequestration may not be enough to offset the huge GHG emissions this season.

Our scientists are also analysing long-term trends in the burnt area of Australia to also better understand those factors contributing to changing nature of fire risk.

Can we ask you about the ocean’s role here? We’ve heard about natural drivers of our climate and weather systems like the El Niño Southern Oscillation and the Indian Ocean Dipole. Have these played a part in our bushfire season?

Yes definitely. The ocean and atmosphere work as a closely coupled system to determine our weather and climate, with ice and the land surface also interacting. In particular, the ocean-atmosphere oscillations close to Australia impart a big influence on our seasonal climate, and the most well-known is the El Niño Southern Oscillation (ENSO). Late last year some of the drivers of our seasonal climate swung in favour of hot and dry conditions, with a strong positive Indian Ocean Dipole (IOD) event occurring.

Another important driver of southern Australia’s weather and climate is the Southern Annular Mode (SAM), and this was in a negative phase for parts of spring.

Are they likely to change in the future under climate change?

Yes, this is likely. Climate change affects all aspects of the earth system, and a change in the nature or effect of these oscillations appears inevitable. The exact effect is still a topic for ongoing research, and is a question that requires further observations, process-understanding and model development to answer with confidence. However, ground-breaking work done in Australia, by Dr. Wenju Cai’s team at CSIRO and through the Centre for Southern Hemisphere Oceans Research is contributing to our understanding. More extreme El Niño, La Niña and IOD events, and a change in the impact these events have on Australia have been shown in the latest research.

One last question, what research is being done to look further at the question on the link between climate change and bushfires in Australia?

There is a broad range of research needed to better understand fires, their climate drivers, and their impacts – including fundamental climate science, forecasting and projection science, fire weather science, fire ecology, biodiversity research, human-systems research, complex systems science and more. From the climate side, Australia, through the work of CSIRO in collaboration with the Bureau of Meteorology, is contributing towards the measurement of climate information in the atmosphere, ocean and land, understanding of climate variability and change, the modelling of the climate and producing projections of Australia’s future climate. There is also a lot to learn from indigenous fire management practices developed over tens of thousands of years by Australia’s first peoples.

Overall, there are many important research questions in all aspects of fire risk: fire weather and its drivers, fire ignitions, fuel dryness, fuel type and climate aspects of fuel management. Developments in our knowledge and understanding of these questions will allow us to better plan for and manage risks now and in the future.

Michael Grose is the Lead Chief Investigator in the National Environmental Science Program’s (NESP) Earth Systems and Climate Change (ESCC) Hub’s Project 2.6: Regional climate projections science, information and services. The NESP ESCC Hub provides funding for this research.

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