Soil Carbon sequestration limited in mitigating fossil fuel emissions say scientists

Submitted by takver on Sun, 02/06/2013 - 11:28pm

Rebuilding soils through carbon sequestration and mitigating fossil fuel emissions sounds like a win-win solution all around. A reason the Liberal and National Parties in Australia adopted it as a major part of their 2010 Direct Action climate change policy. But a new international study by Australian and UK scientists said soil carbon programs while important, have many limitations, and provide too much false hope in mitigating emissions from fossil fuels.

"The capacity of terrestrial ecosystems to store carbon is finite and the current sequestration potential primarily reflects depletion due to past land use. Avoiding emissions from land carbon stocks and refilling depleted stocks reduces atmospheric CO2 concentration, but the maximum amount of this reduction is equivalent to only a small fraction of potential fossil fuel emissions." (Untangling the confusion around land carbon science and climate change mitigation policy, Brendan Mackey et al, 2013)

About 40 per cent of the world's agricultural soil is degraded or severely degraded. Up to 70 per cent of topsoil is already gone through erosion. This results in loss of natural fertility and loss of water storage capacity. Increasingly water stress will impact our populations and our agriculture. By 2030 it is estimated about half the world's population will suffer severe water stress.

Soil carbon sequestration is achieved using a range of methods in 3 categories including: land use changes, such as the conversion of crop land to forests; management practices, such as minimum tillage and changing the periodicity of crop rotation; and the addition of external sources of carbon to the land, such as biochar. But the capacity of soils to sequester carbon is finite. As soils reach their maximum capacity, soil carbon sequestration will slow and eventually plateau say the scientists.

This new study - Untangling the confusion around land carbon science and climate change mitigation policy (abstract) - emphasised that protecting natural forests avoids emissions that would otherwise result from logging and land clearing while also conserving biodiversity. Restoring degraded ecosystems or planting new forests helps store some of the carbon dioxide that was emitted from past land use activities say the scientists.

"While protecting and restoring natural forests is part of the solution, the reality is that for all practical purposes fossil fuel CO2 emissions are irreversible," said study lead author Professor Brendan Mackey, from Griffith University Climate Change Response Program. "There is a danger in believing that land carbon sinks can solve the problem of atmospheric carbon emissions because this legitimises the ongoing use of fossil fuels", Professor Mackey said.

State Governments proceeding with land clearing and logging of native forests

This is in sharp contrast to actions by State Liberal and National Party Governments in power in New South Wales, Queensland and Victoria. Scientists have recently heavily criticised new land clearing laws and regulations in Queenlsand.

The Victorian Government has been sharply criticised for continued Logging of native forests which is driving species to extinction such as Leadbeater's Possum. Victoria has some of the World's most carbon dense forests.

But clear-fell logging in Victoria's central highlands continues, and the subsequent regrowth increases bushfire risk, and enhancing the risk of landscape traps changing the ecosystems of a landscape irreversibly.

Re-afforestation and protecting mature and regrowth native forests for their capacity to store carbon and conserving biodiversity should, if anything, be encouraged, stress the scientists. "These land management actions should be rewarded as they are an important part of the solution," Professor Mackay said.

"However, no amount of reafforestation or growing of new trees will ultimately off-set continuing CO2 emissions due to environmental constraints on plant growth and the large amounts of remaining fossil fuel reserves." said Professor Mackay.

Soil carbon targets in Liberal National Direct Action Plan impossible to meet

There is bipartisan commitment by the Government and opposition parties to reduce Australia's carbon emissions by 5 per cent below 2000 levels by 2020. To reach that target requires taking over 700 million tonnes of carbon pollution out of the atmosphere between now and 2020. To reach the 2050 objective of 80 per cent below 2000 levels requires reductions of over 17 billion tonnes in CO2 or equivalent emissions.

The Australian Government has encouraged and invested heavily in research and development of soil carbon sequestration under the Government's Carbon Farming Initiative (CFI). The first stage was developing methodologies for soil carbon farming and reliable measurement techniques. Carbon farming and accurate measurements of soil sequestration are still in the early stages of development.

The Liberal National Parties 2010 climate policy aims to achieve 85 million tonnes of abatement from soil carbon at a cost of around $8 a tonne. This is about 60 per cent of emissions reduction under the Direct Action Plan. In March 2011 Opposition Climate Change spokesperson Greg Hunt told Lateline that "A million hectares at a 150 tonnes of CO2 equivalent per hectare is the figure that we're talking about, but that's the intensive number."

In a report on Lateline broadcast 18th April 2013 the Opposition Direct Action Plan for soil carbon targets was questioned based upon a CSIRO report which showed "In the parts of the national soil carbon program that studied soil carbon changes over time - most showed soil carbon changes that were within the range (0.3 - 2.0 tonnes of CO2-equivalent per hectare per year) or lower."

If you achieved the top end of that carbon sequestration range, then 75 million hectares would be needed to abate 150 tonnes of CO2 equivalent. At the bottom of the range using the 0.3 per cent figure, then 500 million hectares, or two thirds of the land mass of Australia would be needed, according to the CSIRO research.

At a Senate estimates hearing on May 27, officials from the Department of Industry, Innovation, Climate Change, Science, Research and Tertiary Education said the Carbon Farming Initiative was expected to drive less than 4 million tonnes of annual abatement on grazing lands and croplands by 2020. Around 3 million tonnes a year could come from re-vegetation activities on grazing land and about 0.7 million tonnes a year from cropland management.

A University of Western Australia study showed that the economic cost of abatement for carbon farming would amount to about $80 per tonne, far exceeding the Coalition Parties estimate of $8 per tonne for soil carbon farming.

The abstract for this study concludes:

"...the cost effectiveness of alternative land-use and land-management practices that can increase soil carbon sequestration is analysed by integrating biophysical modelling of carbon sequestration with wholefarm economic modelling. Results suggest that, for a case study model of a crop-livestock farm in the Western Australian wheatbelt, sequestering higher levels of soil carbon by changing rotations (to include longer pasture phases) incur considerable opportunity costs. Under current commodity prices, farmers would forego more than $80 in profit for every additional tonne of CO2-e stored in soil, depending on their adoption of crop residue retention practices. This is much higher than the initial carbon price of $23 t-1 in Australia's recently legislated carbon tax. This analysis does not incorporate the possibility that greenhouse gas emissions may increase as a result of including longer pasture phases. Accounting for emissions may substantially reduce the potential for net carbon sequestration at low carbon prices."

In other worlds, we would need a carbon price of at least $80 per tonne to make active soil carbon farming an economic proposition for most broadacre farmers in Australia.

"There are a lot of opportunities to increase soil carbon but pretty much most of those are categorised as conservation practices and those conservation practices won't be eligible for carbon credits under additionality", said Associate Professor Kragt from the University of Western Australia.

An analysis of the Viability of the Coalition's Direct Action Plan by Tim Lubcke, a research officer from Monash University, said that:

"while sequestration is of value, to rely upon it in the order of which DAP appears to is unlikely to be viable, especially by 2020, and is more than likely to become very expensive as the scale is adjusted over time or adjusted to increasingly depend upon tree plantation. Combined with the initiative to fund industrial transition to low carbon alternatives, DAP will cost the taxpayer, either through additional taxes directly or through the loss of public services that currently exist. This is contrary to the claims made by the Coalition."

If soil sequestration doesn't meet the Coalition's rosy targets, then re-afforestation programs would be implemented according to Greg Hunt. The problem with this is the large amount of land required for re-afforestation. Groundwater and runoff to fill dams and rivers would also be affected as young growing forests results in uptake of more water, with less available for drinking and irrigation. Young forests also increase fire vulnerability as growing forests are far more vulnerable to fire and burn at greater intensity adding substantially to fire management risks and costs to rural communities.

Professors Rod Keenan, Peter Grace and Snow Barlow in an article on Don't bet it all on the carbon farming on The Conversation website said:

"Before we can undertake large-scale revegetation we need enough land, enough seedlings, and enough labour to plant and manage these areas. It cost more than $4 billion to establish one million hectares of new forest plantations under Managed Investment Schemes between 1995 to 2009. At its peak, the planting rate was 140,000 hectares per year. At that rate, it would take 980 years to plant the 147 million hectares that CSIRO recently suggested was potentially available for carbon forestry."

While re-afforestation will increase carbon locked away in biomass, rising carbon dioxide in the atmosphere will also speed carbon dioxide loss from forest soils according to a University of Indiana research study. This adds another vicious Global Warming Feedback loop say researchers. Carbon stored in soils rather than woody biomass in forests is more desirable as it is seen as more stable over longer periods of time.

Increased atmospheric CO2 is also likely to increase soil emissions of other powerful greenhouse gases like methane and Nitrous Oxide, partly negating any gains through soil carbon farming increasing the terrestrial carbon sink. The 2011 study by Kees Jan van Groenigen et al in Nature - Increased soil emissions of potent greenhouse gases under increased atmospheric CO2 (abstract) - concludes:

"Because enhanced greenhouse-gas emissions add to the radiative forcing of terrestrial ecosystems, these emissions are expected to negate at least 16.6 per cent of the climate change mitigation potential previously predicted from an increase in the terrestrial carbon sink under increased atmospheric CO2 concentrations4. Our results therefore suggest that the capacity of land ecosystems to slow climate warming has been overestimated."

Maintaining biodiversity is integral to managing soils

Soil carbon farming and sequestration programs are important for the long term health and improvement of our soils, which we depend upon for our food security and that ultimately supports our civilization. Managing soils is about moving carbon out of the atmosphere rejuvenating biological productivity. Maintaining biodiversity is integral to managing soils.

Unfortunately for our agricultural land, short term business drivers and commodification of food at prices driven by wholesalers and distributors often results in farming practices that result in cheaper food in the short term at the expense of degrading the biological production resources: the health of the soil and species biodiversity. The supermarkets driving the consumer price of milk down to $1 a litre, with reduced returns to dairy farmers, highlights this issue.

This 20 minute video by Tony Lovell highlights some basic maths, and some of the huge numbers involved when dealing with climate change, and the role of soils in the carbon cycle.

Tony Lovell in the above video from TEDx Dubbo in August 2011 puts forward the following process flow in a "Hierarchy of Ecosystem Functionality", when we start reducing biodiversity such as in monoculture farming. Soils are intimately linked with species biodiversity, nutrient cycling, rainfall and water retention, weather and climate.

  • Reduction in Biodiversity
  • Reduction in Biomass (plant cover)
  • Reduction in Photosynthesis
  • Reduction in Carbon uptake and manufacture of oxygen
  • Reduction in accumulation of organic matter
  • Disruption of nutrient cycling
  • Reduction of fertility
  • Reduction of Infiltration and Retention of Rainfall
  • Changes in soil moisture
  • Changes in relative humidity
  • Changes in weather
  • Changes in Climate

By increasing the biodiversity complexity the flow can be reversed the other way which will have a positive impact on ecosystem functionality and climate.

There are finite limits to sequestering carbon in soils. While we work stopping soil degradation and improving soil carbon farming practices we are improving the biological processes that act as carbon pumps taking carbon out of the atmosphere and sequestering it in biomass or soil.

But these actions by themselves are simply not enough. We also need to drastically cut our industrial greenhouse gas emissions.

"Unfortunately there is no option but to cut fossil fuel emissions deeply as about a third of the CO2 stays in the atmosphere for 2 to 20 millennia." concluded Professor Mackay.

Sources

Keywords: 
climate change
forests
carbon sink
soil
Geography: 
New South Wales
Queensland
Victoria
Western Australia
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