A changing climate could challenge our understanding of bushfires as ‘carbon neutral’ events

By Sophie Schmidt 26 February 2020 8 min read

AROUND the country, predominantly in the south and south-east of Australia, many communities are beginning recovery from a severely damaging bushfire season. So far, lives have been lost, and homes and vulnerable wildlife and wilderness have perished. Millions of city-dwellers have also been exposed to hazardous smoke over a prolonged period, and many sacred Indigenous sites have been destroyed.

The bushfires have also seen a significant flux of greenhouse gases released into the atmosphere.

Scientists typically consider bushfires in the long run to be carbon-neutral events. When vegetation burns, greenhouse gases—primarily carbon dioxide (CO₂)—are released through the process of combustion. Vegetation eventually recovers though, to a point at which it can re-absorb the CO₂ emitted, closing the feedback loop. This process might take years or up to a century, depending on what has burned and how much.

In the case of Australia’s recent bushfires, drought has exacerbated dry soil conditions over a long period, and some scientists are casting doubt over the ability of vegetation, acting as a ‘sink’ for carbon, to fully recover to the point of 'neutrality’.

Early observations at Cape Grim

CSIRO scientists at our Cape Grim Baseline Air Pollution Station in Tasmania repeatedly observed bushfire emissions first-hand during late December and January as smoke plumes passed directly over the station. According to Cape Grim Lead Scientist Paul Krummel, this caused large, striking spikes in our measurements of trace gases like carbon monoxide, carbon dioxide and methane, as well as particles and black carbon.

Krummel’s team will be closely studying background air measurements over the coming months to see if there is any perturbation to baseline atmospheric levels.

“We think we can already see a perturbation in the baseline levels of CO₂ at Cape Grim, but it will take some months – until May, or June this year - to fully quantify in the record,” says Krummel.

Satellites are key to understanding bushfire emissions

Meanwhile, CSIRO senior principal scientist Dr Pep Canadell has been working to calculate the greenhouse gases emitted by the bushfires.

He’s joined up with other CSIRO scientists and the Global Fire Emissions Database, a consortium of research institutions including NASA, to produce a preliminary analysis. Their methodology includes using satellite imagery to understand the amount of area recently burnt and from there, they can estimate the amount of CO₂ produced in total.

“Satellites are key to mapping the geographic distribution of the thousands of fires occurring each year in Australia, with records now extending back to 1980s,” says Canadell.

“This allows us to understand exactly the type of vegetation that burns which in turn is critical to estimate greenhouse emissions from the fires.”

The full scale of the fires isn’t known yet as the fire season is not over, but the latest estimates produced from NASA satellite imagery are of 18.6 million hectares of burnt area across Australia – more than the size of a small country such as Syria or Cambodia.

“Our preliminary analyses for 2019 tell us fire emissions from the southeast last year were more than the entire annual Australian CO₂ emissions inventory from the combustion of fossil fuels and land clearing,” says Canadell.

“We’ve basically doubled the nation’s carbon footprint for 2019, and counting emissions from 2020’s bushfires, we may have even tripled it.”

“We also know that fire emissions in Australia last year were higher than any previous year since satellite-based records began.”

How bushfires are included in global reporting

Canadell is also an executive director of the Global Carbon Project, which produces an annual report of the entire global budget of anthropogenic and natural fluxes of greenhouse gas emissions. Last year’s Global Carbon Budget report calculated that global fossil emissions reached yet another record high of 36.8 billion tonnes.

The Global Carbon Budget looks at the ability of ocean or land sinks to sequester CO₂, and carefully accounts for uncertainties in each source and sink. This methodology allows for tracking of global emissions progress towards the targets of the Paris Agreement.

The Budget also includes emissions from human-induced fires such as those with the intent to clear land for other uses, such as agriculture or pastures. These types of emissions are classed as ‘anthropogenic’ and counted in the ‘land use change carbon flux’ category: the CO₂ is presumed lost to the atmosphere because the vegetation will not be allowed to regrow.

Last year’s Budget, for example, already reported higher than average emissions from fires in Brazil and southeast Asia in this category.

So how will Australia’s bushfire emissions be managed in the global reporting for the Paris Agreement? According to Canadell, they’ll be excluded - given that they were not lit for the purpose of land use change. The emissions are considered to be neutral in the long-term: any lost carbon will get sapped up with regrowth that sequesters CO₂.

This assumption of neutrality, however, relies on a steady climate on a timescale over many years to centuries.

A changing climate affects neutrality

Many types of eucalypt forests in the southeast of Australia quickly begin to regrow from branches, trunks and burls soon after fire and once rain has fallen.

However, fire weather is increasing in severity and length, and forests are beginning to experience more fire activity than in the past.

According to the State of the Climate 2018 report, the annual most extreme 10 per cent of fire weather days has increased in recent decades across many regions of Australia, especially in southern and eastern Australia.

We’re already seeing changes in fire weather, particularly with more extreme heat days, more variable rainfall, and decreasing rainfall in south east Australia during the cooler months, priming those forests for bigger fire seasons.

With these changing conditions, will the idea of carbon neutrality still fully apply?

“Under climate change, we can expect to see an increasing frequency and severity of bushfires in Australia, which means increasing levels of degradation, and permanent transfers of carbon to the atmosphere,” says Canadell.

“In the case of the recent Australian bushfires, this means that we may not be able to get all of the emitted carbon back. Fire-sensitive ecosystems that burn in increasing frequency, such as alpine and temperate rainforests, again, will unlikely be able to recover to their pre-fire levels.”

“In addition, soon after the fires, some regions received extreme amounts of rainfall. This washed off ash, black carbon and soil sediments, containing more nutrients and carbon. Satellite imagery shows us that this top soil was transported down the rivers and into the coastal ocean.”

If not land sinks, where?

Just like land sinks, ocean sinks play a critical role in the carbon cycle, acting as a sponge to absorb CO₂, pulling in about a quarter of each year’s CO₂ emissions to the atmosphere that are released by the burning of fossil fuels and extreme events like bushfires. The ocean has been accumulating CO₂ for many decades.

The CO₂ absorbed by the oceans reacts, causing an increase in the acidity level of surface waters. Many organisms such as shellfish and corals are sensitive to ocean acidification, and this is expected to impact biodiversity and some economically important aquaculture fisheries. The combined effects of ocean acidification and ocean warming are major concerns for the future health and sustainability of marine ecosystems.

According to CSIRO’s Bronte Tilbrook, a senior principal research scientist, an increase in CO₂ in the atmosphere as a result of the bushfires may drive a slightly greater ocean uptake, hastening acidification.

“There is likely to be a slight increase in ocean uptake of carbon dioxide, but I doubt this is measurable in terms of the carbon budget for the atmosphere or through direct ocean measurements,” says Tilbrook.

Drawing a link between any ocean measurements and the bushfires will be like reading “a small signal on top of a large background”.

Tilbrook also says it’s "optimistic" to assume that the ocean will negate the emissions released by the bushfires through sequestration.

“While the oceans are the long-term sink for CO₂ emissions, the rate of emissions to the atmosphere is vastly greater than the rate at which the oceans can take up the emissions, so the atmospheric concentration of CO₂ is rising,” says Tilbrook.

Understanding how the ocean uptake is changing is critical to Tilbrook’s research.

“We're only just beginning to understand the complexities of the physical, chemical and biological system that drives the ocean carbon cycle and ocean acidification,” says Tilbrook.

"If we’re to understand what our climate and marine ecosystems will look like in the coming decades, we need to understand the ocean’s role in the global carbon cycle,” says Tilbrook.