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How much carbon can trees absorb?

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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