Thursday 3 September 2009

Global Warming Bleg

Does any faithful reader of Offsetting Behvaiour know enough physics to answer a question for me? I don't understand why NZ should be trying to reduce its methane emissions from cows as part of an anti-global-warming strategy.

This post is not about whether global warming is real and man-made, whether the benefits of reduced carbon emissions outweigh the benefits, or what role a small country like New Zealand should take as part of a global strategy. It is simply about the science of methane.

I am puzzled about why we should worry about methane emissions, given that they result from a circular process whereby carbon in grass is converted into methane by cows, but then carbon is reabsorbed from the atmosphere to re-grow the grass.

I understand that methane is considered to be twenty-times more potent than carbon dioxide as a greenhouse gas. (A nice primer on the science of this is here: HT Marginal Revolution.)

But I also have read that methane quickly oxides into carbon dioxide, with a half-life of only seven years. Yes, increasing our number of cows would increase the steady-state levels of methane in the atmosphere, but once our cow numbers are in steady state, won't the on-going net flow of greenhouse gas emission from that stock of cows be zero?


  1. I'm not quite a physicist, so this is to be taken with grain of salt.

    The concern is that many farms use "fossil fuel" fertilizers: i.e. stuff taken out of the ground. Carbon from these fertilizers turns into grass, and once eaten by animals, some of it turns into methane. This methane is "new" to the carbon cycle, since it had previously been sequestered underground.

    This begs the question: why tax animals and not "fossil-fuel" fertilizer? I've got no good answer to that.

  2. I like this explanation, in the sense that it is a theoretically consistent reason that our cows would have a carbon footprint in steady-state.

    The trouble is, as you say, that this suggests taxing fertiliser use, not cow emissions. Furthermore, my memory from high-school science and geography is an important source of New Zealand's advantage in agrigculture is that we don't have need for nitrogen fertilisers (something to do with clover), just for superphosphate. And a quick Wikipedia search confirmed that while Carbon is present in urea (a typical nitrogen fertiliser), it is not present in super phosphate.

    I may have the science or NZ geography all wrong, of course. Please put me straight.

  3. Hmm... you've got me there. I'll ask around in the morning.

    While I'm at in, I'll ask for other reasons taxing cows might make sense. I'm personally not sold on the idea—for the same reason you posted—, but I'm non-expert.

  4. I don't think the fertilizer is the problem - or certainly I've never heard anyone say that it is.

    I'll first say that looking for logic in the pronouncements of the global warming crew is not necessarily a useful spend of time - the fact that people are concerned about methane doesn't necessarily indicate that there is any science behind it.

    Leaving that aside, I see some potential flaws in your short discussion above:
    1. Yes, carbon may be part of a natural cycle, but it doesn't follow that we shouldn't be concerned. If, in the absence of the cows, that carbon wasn't released into the atmosphere, then it is material that the cows are there. Think about it another way - imagine an empty field, in steady state with nature. Imagine we go out once a year and plough up that field, and that through some as yet undiscovered phenomena some of the carbon leaks into the atmosphere as greenhouse gas. Even if I go out and do that every year once a year, that doesn't imply I'm in a steady state. I may be gradually depleting the carbon in the soil and pushing it into the atmosphere. It's a made up example, but natural cycle does not necessarily imply steady state. Arguably the cows are moving more carbon into the atmosphere than would occur with undeveloped farmland (although I've never seen undeveloped farmland - that wouldn't really be a farm - so I suspect that we're failing to take into account the alternate uses of that land. I guess we could plant trees.....)

    2. On cows and methane, I think you may be mixing carbon and methane as interchangeable. Methane isn't more potent because it has more carbon, it is more potent as it absorbs heat better. So an amount of carbon tied up in methane causes more warming than the same amount of carbon tied up in CO2. Yes, the methane will break down to CO2, but as you say with a half life of 7 years. So, as long as the cows exist the carbon that is going into the atmosphere will be in greater proportion of methane:CO2 than it otherwise would have been, and the warming is greater than it would otherwise have been.

    3. Whether we reach steady state probably relates to whether the breakdown of methane is truly a half life - the amount broken down is proportional to the amount in the atmosphere - or whether the breakdown process has some sort of fixed capacity per year. In other words, if we are making more of it than can break down, we'll get a build up. If it breaks down all on its own, then your thesis of no excess buildup could be correct.

    4. I believe the warming isn't modelled as an instant thing. There is a lag of up to 100 years for the full warming to occur - so if our average temperature is 22 degrees, and we double our CO2 today, then next year we might be 22.1, the year after 22.2, the next year 22.3 etc. So the warming is actually a factor of both the amount of greenhouse gas, and the length of time they are present. Again, your steady state thesis is incorrect, as we continue to warm over time, and that warming would be avoided by dropping the greenhouse gas levels.

    Anyway, no physics background here, so they're just the thoughts that come to mind as to how it might work.

  5. For what it's worth, I've written my own post here: it seems that an increase in the cattle muster only results in a one-off bump in atmospheric methane levels.

    The policy-consequence would be: (1) measure the bump; (2) determine if it falls within acceptable parameters; (3) if not, then tax cows until the muster falls enough for the bump to fall within acceptable parameters.

  6. @PaulL: Seamus's point, I believe, wasn't that methane isn't a problem; rather, it's that methane isn't an accumulating problem. So with reference to your points 2&3, if doubling our herd of cows increases our methane emissions by x tons, then we don't see x tons * # years of increase; instead, we see a level shift in the amount of methane that's a multiple of x but not a large multiple of x and not one that's increasing over time if the size of the herd then is constant.

    I'll defer to the chemists and physicists if somebody out there knows what Seamus and BK have wrong, if anything. I kinda remember balancing equations from high school chemistry, but that's as much as I ever did.

  7. Crampton,

    OK, understand that.

    My point is that a one off lift in methane levels doesn't correspond to a one off increase in temperature. According to the model, a one off increase in methane levels would lead to a year on year increase for 100 years. And therefore, logically, it makes sense to charge for that methane level every year, as every year it is causing warming.

  8. OK, I see where you're coming from. I think we're operating on different time-scales.

    All that 100 years worth of year-on-year warming, added together, give "aggregate warming" caused by cow-methane: this is a one-off, although it takes a while to equilibrate.

  9. Seamus: a physicist (well, astrophysicist, but that's close enough...) friend left a comment at my blog:

    "I think that you are correct, that the quantity of methane in the atmosphere is going to be proportional to the amount we pump into it through whatever means. Similarly for CO2.

    "I believe though that the concern is not that the methane and CO2 levels are going to dramatically run away. The concern is that the environment hasn't actually reached equilibrium with these "new" levels of greenhouse gases, and that the global temperature will increase something like 3-10 degrees (model dependent, and also I just made those numbers up but it's something of that order) over the next so many hundred years, even if emissions are kept constant.

    "So the idea is that by reducing methane/carbon emissions, we are "unbumping" the CO2/methane in the atmosphere, and reducing the long-term effects beyond the concentration of the gases."

    This reflects PaulL's argument, and makes sense. (I had put it out of my own mind because I assumed once a level was "accounted for", it wasn't a problem... this teaches me the following of being a mathematician and not a scientist.)

    Sometime tomorrow I'll get my paws on some data and estimate the actual warming cause by NZ cows in the long run. Right now, I'm already running late...

  10. FWIW, it doesn't mean I particularly agree that we need to go out and do something about it. I'm not a big believer in positive feedbacks - it just seems to me that if our environment was balanced so precariously that any move away from equilibrium would accelerate, that we almost certainly wouldn't have had such a stable climate for so long, and with such different atmospheric levels of greenhouse gases.

    The empirical evidence suggests to me that there is some level of negative feedback that is driving us towards a climate similar to that we have now.

    Some (inexpensive) reduction in emissions would be good, as would a move away from the most obvious sources of emissions. But I think we should accept some level of warming as being inevitable.

  11. SOME species of Australian birds are shrinking and the trend will likely continue because of global warming, a scientist said.

    Janet Gardner, an Australian National University biologist, led a team of scientists who measured museum specimens to plot the decline in size of eight species of Australian birds over the past century.

    The research, published last week in the British journal Proceedings of the Royal Society B: Biological Sciences, found the birds in Australia’s southeast had become between 2 per cent to 4 per cent smaller.

    Over the same century, Australia’s average daily temperature rose 1.3 degrees Fahrenheit (0.7 deg C), with the sharpest increase since the 1950s.

  12. Bhuvan - your comment doesn't explain what it is about temperature change that leads to birds becoming smaller. I can understand that there is a correlation, but I'm not understanding the mechanism for causation.

  13. ...would surely be of some help, however it's a little difficult to search the site for this specific issue seeing as 'cows' and 'methane' tend to feature in every 5th post. It has been discussed before.