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How much carbon can trees absorb?

Australia’s agriculture and forestry - land-based abatement - can make a valuable contribution to lowering Australia’s greenhouse emissions. The scale of contribution has been widely discussed. But the…

There are limits to the amount of carbon dioxide plantations can absorb. David Clarke

Australia’s agriculture and forestry - land-based abatement - can make a valuable contribution to lowering Australia’s greenhouse emissions. The scale of contribution has been widely discussed. But the dynamics of land-based abatement are as important as trying to forecast the ultimate abatement opportunity.

Understanding the rate at which offsets from the land can be created - for example knowing the number of hectares which can physically be planted with trees per year, or the number of fields which farmers will switch to conservation tillage - will help in judging when and to what extent sectors of the economy will have to contribute if we are to meet national targets. The time till saturation of some of the land-based abatement opportunities (such as when forests are fully grown) will determine when emissions reductions in other sectors need to be achieved.

To figure out the level of abatement likely to be realised, and the time frames of delivery, a range of practical factors must be considered. These factors include:

  • land owner practices (for example how able are farmers to access the machinery for no-till farming)

  • knowledge and preferences (for example whether farmers want to grow trees rather than raise animals, or whether local indigenous groups are willing and able to engage in fire management)

  • competing options for land use (for instance whether carbon farming is competitive with traditional farming, or whether farmers see carbon markets as too risky compared with known commodity markets)

  • logistical constraints in new forest plantings (can we get enough seedlings and people into remote areas to get large-scale plantations established)

  • the extent to which the wider policy environment provides incentives for carbon offsets (recognising that without substantial price incentives farmers and landholders will be motivated primarily to undertake carbon farming for its cobenefits, such as production increase, diversified income stream, on-farm amenity, and better livelihoods outcomes).

I have suggested previously that within 40 years, and with sustained effort, agriculture could abate up to 20% of our year 2000 national net emissions (abatement of approximately 116 Mt CO₂-e/yr). To achieve this requires a lead time so we can develop practice change and new technologies.

All the while Australia’s emissions are projected to increase from the current 580 Mt CO₂-e/yr to 1000 Mt CO₂-e/yr in 2050. By this time, the Mt CO₂-e/yr abatement contribution from agriculture will only be 11% of total emissions.

The Department of Climate Change and Treasury have both published trajectories of the likely rate at which abatement opportunities will be generated under the Carbon Farming Initiative (CFI). Total abatement is estimated in the range 5-15 Mt CO₂-e/yr.

The table below revisits these numbers, focussing less on the outcome under CFI and more on the technical and social barriers to adoption. It takes a less constrained view of upper levels of abatement. The conclusions are similar: land-based abatement will make a small to modest contribution up to 2020 with growing contribution to 2050.

Reforestation and increase in rangelands carbon stocks (these make up most of the native vegetation listed above) dominate the potential abatement out to 2050. The uptake rate of these opportunities is highly dependent on economic factors such as opportunity cost, carbon price and discount rates, and the rate at which these new forests sequester carbon and generate income. These dependencies make forecasting fraught with uncertainty.

In carbon forestry, small adjustments to the factors can change the area of opportunity for profitable carbon farming from negligible to very large. The area of profitable opportunity is finely balanced by the way discount rates compound establishment costs, and by the way the stream of revenue from carbon offsets compares with the variable income stream from year-to-year commodity prices.

The estimate of the upper limit in 2020 of carbon forestry and, to lesser extent, rangelands carbon sequestration is dominated by the rate at which we can create abatement. There are also technological limitations in measurement which will affect what we might consider as verifiable abatement. Longer term estimates have considerable economic and social uncertainty that decrease confidence in estimates.

Carbon forestry and rangelands carbon sequestration, along with soil carbon, dominate agricultural abatement in the longer term. But they all only offer abatement for a limited period of time. Forest or rangeland growth slows and carbon in the system eventually reaches a steady-state.

Forests will cease to put on additional material and growth will be directed at renewing existing material at which point no net abatement occurs - this could be 40 to 100 years after establishment. Soils will gradually come into a new balance with changed management practice. The longer term contribution from agriculture is confined to reducing emissions (32 Mt CO₂-e/yr abatement in the table above) which will only comprise a small part of the project 880 Mt CO₂-e/yr abatement target forecast by Treasury.

In the longer term, carbon sequestered in biomass from the new carbon forests we create could be converted to a more stable form such as biochar or could be used as a substitute for fossil fuel in bio-energy production.

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6 Comments sorted by

  1. Donncha Redmond

    Software Developer

    If forests are managed for productive uses, e.g: trees are felled and timber used for building, couldn't they go on providing abatement for ever?

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    1. Michael Battaglia

      Theme Leader, Greenhouse Gas Abatement and Carbon Storage in Land Use Systems at CSIRO

      In reply to Donncha Redmond

      Forest products, including paper, all have an average sevice life (or half-life). This has been calculated for many products. As you suggest utilising wood products would provide additional abatement (though you would have to reduce the average stocks in your new forests since they would on average then hold less carbon), and this effect might be amplified if the wood substituted for other products that had a high embodied energy cost. An issue is how such considerations are handled in international treaties that impact on how carbon markets operate in Australia. Under the existing Kyoto protocol and 'chop and cop it' formula operates - that is when forests are felled the carbon is assumed to be immediately liberated from wood products

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  2. Tim Scanlon

    Author and Scientist

    Mike, I appreciate your figures, but my concerns come from the recent publications that have been distributed by DAFF. As one farmer group I work with referred to it "We did this all back in the '90s, didn't work. We need new stuff not the wheel being reinvented." This was in reference to the proposed investigations into biochar, tree farming (like oil mallees), conservation areas, etc. This was all being referred to as carbon farming and a way to make money. This is just false.

    I am still rather…

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  3. Malcolm Robinson

    Geologist

    Michael. Do you have any comment on the research that suggests desert soils absorb large amounts of CO2, possibly as much as forests. If true then, given Australia's high percentage of arid land, our net emissions may even be negative. All that calcrete just below the surface in inland Australia has to come from somewhere!
    http://www.ecostudies.org/press/Schlesinger_Science_13_June_2008.pdf
    http://mgg.tongji.edu.cn/space/shouye/files/2011/11/China-looks-to-balance-its-carbon-books-science-news201111.pdf

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    1. Michael Battaglia

      Theme Leader, Greenhouse Gas Abatement and Carbon Storage in Land Use Systems at CSIRO

      In reply to Malcolm Robinson

      Not really my area but I suppose the issue is to ask has this always been going on - in which case it is part of the normal carbon cycle, and it is just that we haven't been accounting for it. The problem is the increase in emissions relative to the rate that systems are sequestering carbon.

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  4. wilma western

    logged in via email @bigpond.com

    The overall aim of this article seems to be to warn that farm or plantation- based sequestration of carbon is no panacea and is in fact considerably limited in its ability to reduce Australia's emissions , even though the statement "the time till saturation of some land-based abatement opportunities... will determine when emissions reductions in other sectors need to be achieved" is downright confusing. Surely the message is if you believe urgent action is needed to reduce ghg emissions you need…

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