Monday 7 September 2009

Global Warming Bleg: Update

My Bleg from last week has generated two different possibilities for why we should be taxing methane emissions from cows. BK Drinkwater and John Small (who have both weighed in with their own posts here and here, respectively) have posited the fertiliser theory: If the fertiliser used on the land grazed by cows contains carbon taken from the earth, then the methane emissions from a steady-state stock of cows will produce a continued increase in the stock of GHGs in the atmosphere (measured in CO2 equivalents).

John suggests that the source of the net carbon would be urea, although it is not clear from a quick reading of the Wikipedia article on it if the CO2 used to create the urea is a net addition of has already been created as a by-product of existing processes. In any event, as John points out, the policy implication would be to tax the use of urea.

The other possibility, suggested by PaulL in the comments is that the concern about methane is that the stock of GHGs in the atmosphere affects the flow of temperature increases, at least until temperature catches up. Paul suggests that this provides a reason for taxing a one-off increase in methane levels every year for a century.

The trouble with this line of reasoning, as I see it, is that the same argument would apply in spades to on-going emissions. That is, if methane is twenty times as powerful a GHG as CO2 for the same amount of carbon, my logic suggests that the tax on methane emissions that are part of a natural cycle (assuming no net additions from fertiliser) should be way less than 20 times the tax on CO2 emissions.


  1. I think the same logic does apply to ongoing emissions.

    Methane emitted in a particular year would logically attract more tax than CO2:
    - allowance for the greater greenhouse effect during the half life of 7 years
    - allowance for the CO2 impact for the remaining years

    Methane emitted next year has the same effect - it needs to be taxed for the effect that it has in the first few years, and for the ongoing effect for the remainder of the hundred years.

    I'm not sure why we'd distinguish between greenhouse gases from a natural cycle and those not. I don't believe that the source of the gas has any impact on the warming it causes. If we choose to emit lots of CO2 from burning fossil fuels (arguably also a natural cycle, just one with a much longer time frame, so I'll let that go) and reduce our emissions from pasture land, it has the same impact.

    My understanding is that the majority of the methane is emitted by bacteria in the cow's stomach. There is a symbiotic relationship in which the bacteria help the cow to digest. If alternate or additional bacteria could be found that reduced the methane production, then that would reduce our warming. The tax provides the incentive to find those bacteria and feed them to our herd. Depending on how exactly the bacteria work, arguably it might improve the efficiency of the cow - more meat/milk from the same amount of grass input.

  2. Paul,

    Assume for now that the carbon in the methane only comes from the carbon in the grass which is replenished from CO2 in the atsmosphere.

    Now imagine two emissions of the same amount of carbon, one from cow methane, one from burning fossil fuels. At first, the methane has 20 times the GHG effect as the fossil-fuel CO2, suggesting a tax at 20 times the rate. But then, over time, the methane breaks down into CO2, so the aggregate effect of the methane over time is less than 20 times that of the fossil-fuel-derived CO2. Further, the pasture that was eaten to create the methane inside the cow has to be replenished by drawing CO2 from the atmosphere, and so the methane emission has generated a partial offset that is not true of the fossil-fuel emission.

    In steady-state, the last effect should exactly cancel the emissions. There is an interesting question about whether on-going taxes would be a good way to create an incentive to change the stock of cows, as opposed to one-off taxes and subsidies for the change at the time they occur, but that is a separate issue.

  3. 1. Yes, the total taxation on methane should be less than 20 times that on CO2, presuming the taxation seeks to internalise the costs over more than 7 years (I'd guess we're trying to internalise about 100 years of costs). But more than the same amount. I can't be bothered doing the maths, but I'd guess 20 times for the first 10 years, same for the remaining 90 years, which gives me an average of 3 times the rate for CO2. If we're discounting future warming then that multiple would increase.

    2. What discount rate do we use on future warming? I understand that the US are using a substantial discount rate in calculating the costs of their cap and trade scheme, presumably we should also discount future warming somehow?

    3. We assume steady state, but there isn't evidence to prove that. Some people believe pasture farming slowly depletes the soil of humus (essentially carbon), so it is no different than burning fossil fuels. Others believe that pasture farming is actually sequestering carbon in the soil. I suspect the answer depends on exactly how you do it, and our current taxation instruments do not have the ability to discern which. And you'd need a huge bureaucracy to work out which for any given farmer.

    4. When we talk about steady state (which remains unproven), what would that same pastureland do in the absence of the cows? Would the grass die and release it's carbon into the atmosphere? Would the carbon go into the soil - so steady state in the absence of the cows would involve that pasture sequestering carbon? Would it release any methane? I'm just wondering why we're so interested in steady state as if that is answering the question somehow. It seems there are multiple possible equilibria here, so the fact that we have equilibrium isn't telling us much? Am I failing to understand?

  4. FWIW, I have a new post up here. I think I'm successful in constructing "the scientific argument for a tax on ruminants", at least in the sense of giving a plausible scientific reason that takes the carbon-cycle into account.

    Whether or not you buy the argument will probably depend on whether you think there are better ways of achieving methane reductions.

  5. Paul,

    Re your point 4 and final sentence. We are on the same page. The kinds of possibilities you raise here with regard to what would be the effect of not having cows is exactly the sort of thing I was seeking in my bleg: What is it about the biochemistry of cows methane production that makes them a net contributor to global warming?

  6. Seamus: I think the argument on that point goes like this:

    In steady-state, there's an equilibrium between CO2 and CH4. In equilibrium, CH4 levels are proportional to the "CH4-churn" (the magnitude of the CH4-flow through the atmosphere->oxidization->photosynthesis->cow->atmosphere cycle).

    A change in the number of cows results in a change in the magnitude of churn, and, by proportionality, a change in the level of atmospheric methane: the result is a shift of the CO2/CH4 equilibrium.

    CH4 has a stronger warming effect than CO2 in the following sense: carbon-atom for carbon-atom, a delta change in CH4 levels creates a greater delta change in radiative forcing—the measure of how far GHGs take global temperatures away from "natural" levels (defined at some pre-industrial equilibrium)—than does an equivalent delta change in CO2 levels.

    So in the event of a shift of the CO2/CH4 equilibrium, the change in "warming" is dominated by the effect of the shift in CH4 levels.

    This argument is independent of the precise nature of a cow's biochemistry: all that matters is that a cow is a pump (maybe relay is the better metaphor) that boosts the level of methane flowing through the cycle.

  7. Seamus,

    My view, in the absence of any actual qualifications in this area, is that:
    1. Without the cows there would be little or no methane, so that portion is definitely additive over a steady state with no cows

    2. I suspect that most farming slowly depletes the soil (without addition of inorganic fertilizer), so it isn't really a steady state at all

    3. If there were no cows and the land lay fallow, then I believe the pasture would sequester carbon - the grass would grow, die and form humus in the soil. So if we're comparing farming cows to fallow land, then farming cows has higher emissions of both CO2 and CH4, and therefore causes warming.

    4. We're completely ignoring reality here. If we weren't farming cows on that land we'd farm something else. That something else, unless it were trees, would probably also be a net CO2 emitter.

    5. We're also ignoring that stopping farming cows in NZ doesn't mean people don't eat meat, it just means that the meat we used to export is now exported from Brazil or Argentina. Likely with higher carbon emissions.

    6. Finally, we're ignoring that variability in the farming itself. I suspect that farming that plows in every year has a different carbon profile than organic farming, for example. Which direction, I don't know. Arguably growing a feed crop, plowing in the waste and thereby sequestering that carbon, then growing another crop, would slowly build up the soil. Conversely, ploughing often leads to soils washing or blowing away.

    Overall, given the offset that will occur elsewhere in the world, I'd say it's a waste of time attempting to apply a tax to agriculture unless that tax is sufficiently targeted that it discriminates between farmers that are emitting carbon and those that are sequestering it. The administration cost in that would be huge, and I doubt that anyone thinks it could be done reliably given our current state of scientific knowledge about the matter.