How Australia’s land can drink up carbon

Australia is a big, wide country. It sprawls continental, a massive landscape of burnished rock and dusty grasslands, vivid rainforests and agricultural plains – a miraculous but struggling biodiversity hotspot. It’s also one of the world’s more significant contributors to climate change and, of the so-called ‘developed’ nations, one of the most reticent to abandon its attachment to the fossil-fuel sector.

All that land, all that coal, all that CO₂. But what to do with it all?

This week, the Intergovernmental Panel on Climate Change (IPCC) published the third part of its sixth assessment report, a hefty set of documents years in the making.

While climate change news tends to trend towards the pessimistic, the report finds that the world’s emissions could be halved by 2030 – necessary to have any hope of limiting warming below 1.5°C – provided we rapidly cut our emissions and engage in serious carbon sequestration efforts.

While climate change news tends to trend towards the pessimistic, the report finds that the world’s emissions could be halved by 2030

That’s a big could, because fossil fuel use is winding down at too slow a pace, and the fuel crisis spurred on by the Russian invasion of Ukraine has forced several nations to temporarily soften their climate policy.

The report notes that carbon drawdown – sucking carbon dioxide out of the atmosphere and sequestering it in soil, vegetation or even rock – will be a cornerstone of reaching net zero, because we can’t fully eliminate all of our CO₂ emissions.

In Australia, carbon drawdown has additional benefits. Where you plant native trees, revegetate over-grazed land or restore a forest to its natural state, plant, insect and animal life will surely follow.

In a timely fashion the Victorian government this week announced it would restore an area five times the size of Melbourne through a $31 million promise for private land conservation, under its $77 million BushBank scheme.

So, can Australia’s prodigious land-mass help compensate for its historic contributions to climate change? What are these methods? And do they work?

Farting cows and open fields: how can farmland be the frontman in carbon drawdown?

Agriculture is at once part of the problem and a key to the solution. According to Annette Cowie, a research scientist in climate policy at the NSW Government’s Department of Primary Industries, and a co-author of the IPCC report, agriculture contributes around a fifth of global emissions, and the global food system as a whole contributes about a third.

Farming requires clearing land, reducing biodiversity, running machinery and, often, raising livestock that burp methane, another potent and aggressive greenhouse gas. On the other hand, we need to feed a growing population, and if the past two years have shown us anything it’s that food security begins at home.

But there are ways that agriculture can help remove CO₂ from the atmosphere and sequester it, permanently or semi-permanently, in plant matter and soils. The report finds that tree planting and revegetation, soil carbon, and some more emerging methods like biochar are among the most promising methods.

“Many of the options have co-benefits in enhancing sustainability of our agricultural systems,” says Cowie, who was the lead author on chapter 12 of the report. “We all know how good it is to put compost into the soil, it enhances the fertility of the soil, its nutrient holding capacity, its water holding capacity.”

Done right, land-use methods of carbon storage could in theory be a win-win for Aussie farmers. And to some extent, these methods are already being used.

Under the Federal Government’s Emissions Reduction Fund, Australian farmers can voluntarily claim Australian Carbon Credit Units (ACCUs) for tonnes of CO₂ abated through carbon sequestration methods.

But how well developed are these methods? And how can we actually measure them?

Carbon drawdown: methods and mysteries

The first and most widely used carbon drawdown method in Australia is revegetation, whether by planting ground cover or trees.

“Most of the offsets we’re seeing in Australia are really around that at the moment,” says John O’Brien, a partner in energy transition and decarbonisation at Deloitte. “Particularly where the land has been cleared historically, and you’re putting it back to its original state.”

Australian farmers can be paid in exchange for carbon credits for planting trees in their field margins, or for revegetating previously barren land. Companies, abroad or at home, can purchase these credits to offset their emissions.

But these schemes are optional and, for the moment says O’Brien, the carbon price isn’t high enough to produce a strong incentive for more landowners to engage. But that could all change as companies race to meet their net-zero targets under tightening pressure.

“When the carbon price is $300 a tonne, that’s actually going to put all sorts of interesting pressures on land use,” he says. “If you’re a farmer, you’ll go, ‘I’m not going to grow stuff anymore, I’m going to grow carbon and I’m going to make a fortune.”

“If you’re a farmer, you’ll go, ‘I’m not going to grow stuff anymore, I’m going to grow carbon and I’m going to make a fortune.”

John O’Brien, Deloitte.

Aside from the intrinsic issues with the concept of carbon offsets as an emissions reduction approach, how effective is tree planting as a carbon-drawdown strategy? It all depends on the type of vegetation, how robust the plantation scheme is, and the growth-rate of the tree.

“Trees grow quickly at first, and then they slow down and reach a maximum point where you don’t get a lot of additional annual carbon,” explains Brett Bryan, Alfred Deakin Professor of Global Change, Environment and Society at Deakin University. So, you can’t claim carbon credits on those trees after a certain amount of years, but it’s important that any planted trees are retained.

But Australia is a land that burns; how can we be sure our planted trees will retain that carbon for a long, illustrious lifetime?

“The land will burn from time to time, but most times it won’t be a complete burn and the trees will bounce back,” says Bryan. Which means all that carbon lost in a burn can hopefully be regained within a few years.

Bryan explains it’s also important to be conscious of which trees you plant: not all plantings are created equal.

“So, plant trees in the landscape, but not just any trees,” he says. “We could plant monocultures of blue gums as far as the eye can see, or perhaps pine trees for softwood timber, or we could actually restore native habitats and ecosystems, native plant and animal communities that are local to the area.”

In 2015, while working at CSIRO, Bryan co-authored a report about the potential for carbon sequestration in Australia. I ask him how far he thinks we’ve come, seven years on.

“I don’t think we’ve come a long way in terms of getting trees in the ground,” he says. But it’s not necessarily because of a lack of concern. Instead, Bryan says that Australian farmers, many of whom have struggled through successive years of drought and flooding, have to consider the trade-offs.

“Something we need to understand here is that there’s not only co-benefits [to tree planting] in terms of biodiversity, there can be trade-offs,” he explains. “If we’re reforesting agricultural land, that has a cost to farmers, the ongoing cost of foregone production. It can also mean less water in our catchments, because fast-growing trees use much more water than shallow-rooted crops and pastures.

“So, it’s not always a win-win situation.”

The carbon in the soil

Then, there’s the somewhat vague concept of ‘soil carbon’. Soil carbon is considered one of the cornerstones of carbon sequestration, and we know that carbon can be effectively stored in the soil almost in perpetuity – if it’s in the right form.

But according to Justin Borevitz, an expert in plant biology and soil sciences at the Australian National University, soil carbon is actually a poorly understood and deeply complex nexus of different carbon states.

“Soil carbon is not one thing, it’s probably hundreds of things,” he explains. But for the sake of ease, he divides it into four main categories.

The first is dissolved organic carbon (DOC) – “basically leaf juice, that nobody’s really counting”.

This type of carbon actually makes up possibly half of all the carbon that’s sucked out of the air by the plant, travels through the roots and then makes it back into the air within a short timeframe.

“The other lightweight type of soil carbon that people like to cheat and count is called particulate organic matter (POM),” he says. That’s leaf particle matter of two millimetres or less. It can make up a huge part of the carbon content of soil, but it vanishes quickly.

“If you do your soil test at the end of the season, when there’s a lot of leaf biomass, you’re going to get a nice big carbon hit,” Borevitz says. “And if you come back after winter, when it decomposes, that will be gone.”

Those two types of soil carbon are not tradeable on any carbon market.

Then there’s the heavier carbon. The first type is called mineral associated organic matter (MAOM) – fine particles within the soil that have been there possibly for thousands or tens of thousands of years, attached to minerals. And then even heavier is aggregate carbon, what Borevitz calls “the muck that sticks to the bottom”.

“That’s as tough as wood.”

This all raises a complicated question. If the two heavier, longer-lasting types of soil carbon take potentially hundreds or thousands of years to form, but the lighter types of soil carbon are ephemeral and easily released back into the atmosphere, what does soil carbon sequestration actually mean, in practice? How can we do it here and now?

It’s actually quite difficult to measure and quantify soil carbon, which makes understanding how much carbon we’re actually sequestering in the soil rather difficult.

“We’re actively looking at processes to build mineral associated organic matter, increasing aggregate formation. The right microbes are probably part of that, and the right rocks,” Borevitz says.

Moreover, it’s actually quite difficult to measure and quantify soil carbon, which makes understanding how much carbon we’re actually sequestering in the soil rather difficult.

According to Borevitz, we used to measure soil carbon with modelling; now we do it with more approved measurement methods. But he says there’s flaws in both systems.

“The truth is today, it’s the Wild West. You’ve got modelling, which was a bit too conservative, you’ve got measurement, which is too expensive and noisy. There are solutions to that scientifically, but they’re not quite ready yet.”

So, the science of soil carbon is still being drawn out through research. Most importantly, according to Borevitz, despite the uncertainty, the long timeframes required to actually build soil carbon mean we need to start now, alongside doing everything else in our power to limit CO₂ emissions.

“Soil carbon is not a quick fix, it’s a solution for the long term,” he says. “And we need it for other reasons, just to underlie our food security, and to give us flood control and drought relief.”

Emerging methods

Then, there’s emerging methods. Biochar, in particular, is seen by the IPCC as a potentially valuable carbon sequestration method.

To make biochar, you burn organic matter in an oxygen-free environment to produce a type of charcoal that’s rich in carbon and can endure in soil for thousands of years. When spread on the soil, it can also condition it. It thus kills two birds with one stone. The biomass from dead plant matter doesn’t decay, and the CO₂ stored within becomes locked-in rather than leaking back into the atmosphere. Not only that, it can make agricultural land more productive.

Another, similar method is known as bioenergy with carbon capture and storage (CCS). You take the organic plant matter, burn it in a power station, capture the CO₂ released and store it deep underground, in a stable liquid state.

“So, there are many ways we can introduce carbon dioxide removal into our agricultural systems that are actually positive, that provide co-benefits for productivity and the environment,” says Cowie.