348. Soil carbon is a highly flawed climate policy, Part 3
I conclude my critical look at whether soil-carbon sequestration should be a priority of climate policy. Apart from challenges with additionality, which I covered last week in PD347, there are a number of other issues that hinder the success of a soil-carbon sequestration policy.
In PD346, I quoted Michael Crawford, who is CEO of the CRC for High Performance Soils. He observed that some of the main methods being advocated to increase soil carbon (e.g. no-till and stubble retention) are already practiced by the majority of farmers, so the opportunities for further gains in those areas are low. At least in Australia, much of the low-hanging fruit has already been picked (more so in some states than others).
The option of converting crop to perennial pasture would increase soil carbon, but it is not a practical option (even if it was economically viable) because it would also increase emissions from livestock. (There is a comment that reinforces this point by Peter Thorburn in response to PD346.) Even if the land is originally in a less productive pasture, a switch to more productive perennial pasture would probably see increased stock numbers and therefore an increase in methane emissions (which might or might not fully offset the increase in soil carbon).
The permanence of carbon sequestration is another concern. In a previous policy phase, the Australian Government found that requiring soil carbon to be maintained for 100 years was so unattractive to farmers that they would not participate. In response, the Government reduced the requirement to 25 years, which is a short time frame in the context of climate change. After that time, farmers who have been paid to sequester carbon will be free to switch to a more profitable land use without penalty even if that results in the release of all the newly stored carbon. If we want the carbon to stay in the soil, we’ll have to pay the farmers again. To account for that, the Australian Government discounts the number of tonnes of carbon sequestered by 20%. That discount was calculated at a time when the discount rate used by the Government was 7%. I don’t know what discount rate is currently being used, but with interest rates close to zero, it must be much less than 7%. For example, if it was 2%, the appropriate discount for 25-year permanence would be over 50%. Unfortunately, this is extremely difficult to fix because the 20% discount is specified in the legislation. Changing it would require the issue to go back to the parliament, and that’s highly unlikely.
A problem that might or might not be fixable is the high cost of monitoring and measuring soil carbon. These costs eat up much of the potential benefits of the program. Time will tell if efforts to substantially reduce these costs are successful. A potential concern is that less costly measurements might be less accurate. Reduced accuracy would also eat into the benefits of the program. Weighing up the trade-off between the accuracy and cost of this information would require a complex and sophisticated analysis. It should not be assumed that cheaper is better.
Finally, I come back to the fact that increasing soil-carbon sequestration on a piece of farm land is a one-time event. Once a new farming system is adopted, soil carbon increases for about 20-30 years and then stops increasing. The same piece of land does not continue to offset new emissions indefinitely. This means that, at best, sequestration should be looked on as buying a bit of time while we get a system for reducing emissions in place. It is not a long-term solution.
Overall, it is extraordinarily difficult (probably impossible) to succeed with a policy of paying farmers to increase the sequestration of carbon in agricultural soils. By “succeed” I mean that the policy actually results in the sequestration of substantial amounts of carbon that would not otherwise have been sequestered, at a reasonable cost. And even if it could be made to work, it could never be a long-term solution that addresses our ongoing emissions. For those reasons, soil carbon policy is a distraction that risks diverting the Government’s attention, energy and resources away from other approaches to climate policy that could actually work.
David, I think you need to make a distinction between the use of a monetary/intergenerational discount rate and the rate used to discount physical carbon, something I have been arguing for in a number of papers. Discounting of physical flows originates with Ciriacy-Wantrup at UC Berkeley in the 1950s or 60s. In the case of carbon, the choice of a discount rate is a policy variable related to the urgency to address climate change. Clearly, if the globe is ‘fried’ in 12 years, any carbon fluxes after that time are worthless. So, the Australian government’s 20% rate is indicative of great urgency as carbon sequestration today is highly valued, but that occurring later is worth less and less, and declines to almost zero after 2050 (as expected).
Thanks Kees. That’s a good point. In my comments, I wasn’t factoring in the potential that sequestration that occurs in the more distant future has less physical effect on the impacts of climate change.
By the way, Kees, I’d like to acknowledge the influence of your earlier writings about sequestration on my thinking.
David I have just been made aware of your website (by Derek Byerlee) and now am a subscriber. I support what you are saying fully on soil C sequestration, but as a late comer, I may have missed another issue I see as limiting C sequestration in cropping soils. I refer to the research, much done in the last decade at CSIRO Canberra by Clive Kirkby and Ph D student Elizabeth Coonan, showing the C sequestered as soil humus (as most is) carries a nutrient opportunity cost of N, P and S in the ratio C:N:P:S of 1000:80:20:14 (my book p 430 free on web http://aciar.gov.au/publication/mn158 but Kirkby and Coonan have published much more since then). Briefly 1 ton of C sequestered as humus locks up $100+ of nutrients which must be supplied from somewhere, although legumes can help with the N. It is reversing the process by which we mined the native SOC of most soils for their nutrients to feed crops. Many soil “health” advocates seem to prefer to overlook this!
Thanks Tony. Welcome! Yes, I do mention that issue in Part 1 of the series. I learned about it from John Passioura. He was presumably referring to this work by Kirkby and Coonan.
Another problem with soil carbon being a “one-time event”, is that increased soil carbon concentrations can stimulate emissions of nitrous oxide* from the soil in cropped land. These emissions occur more-or-less all the time. Nitrous oxide has about 300 times the GHG effect as carbon dioxide, so a small increase in these emissions can offset the benefits of sequestered carbon. This plays out through time – as carbon sequestration slows, the higher rate of nitrous oxide emissions keep happening so the net GHG abatement reduces through time. It may even get to the point where GHG emissions are higher under practices that increase soil carbon. For some examples see Fig 4 of Palmer et al (2017).
* Nitrous oxide emissions from the soil are mainly driven by nitrogen fertilizer applications in cropped lands. Higher soil carbon creates better conditions for fertilizer to be converted into the gas.
Palmer et al (2017). Management practices likely to provide greenhouse gas abatement in grain farms in New South Wales, Australia. Crop and Pasture Science 68, 390–400 http://dx.doi.org/10.1071/CP17026
Thanks Peter. An important example of “leakage”. I was considering including a different example of leakage in my piece but left it out for brevity. Your example is even more potent (and easier to explain) than the one I was considering.
Hi David
I’m just wondering whether you took the opportunity to provide input/participate in the Federal Government’s expert workshops on soil carbon sequestration methods and/or provided any advice?
Hi Rachel, I didn’t know that was happening. It doesn’t sound like quite the right forum. Also, I guess I’m somewhat pessimistic about the potential for sound public policy to be adopted in this area. Politicians state and federal have very publicly jumped on the soil carbon train and the chance of derailing the train seems remote. There is, however, a long game to be played, so I’ll keep putting my case when I’m aware of opportunities.