Because of the difference in nature between methane and carbon dioxide we should cease expressing the climate effect of methane in carbon dioxide equivalents. This has important implications for policy as well as for the assessment of different strategies for minimizing the climate effect of production or lifestyles. Culling all cows may sound like a great proposition if we use the conventional metrics but is actually a rather futile effort to curb climate change.
If you put ice in your drink it will cool off rapidly, but if it is hot in the air it will soon be warm again. That is perhaps the best way to describe the effect on the climate of a one-time reduction of methane emissions. It would give a rapid effect but after some decades it would hardly be noticeable. The same apply for a one-time increase in methane emissions, it leads to immediate warming, but the long term effect is small. This is very different from carbon dioxide, of which we hardly can measure the short term effects of a decrease or increase in emissions, but the effect will last for thousand years. How can we then measure their climate effect with the same unit?
The most common way of comparing the greenhouse gas effect of different gases is to express them as carbon dioxide equivalents, i.e. how much carbon dioxide corresponds to a pulse (a one-time) emission of the gas in question. The most common way of expressing this is by the unit GWP-100, which express the cumulative forcing over hundred years. For methane (CH4) the GWP-100 value is 28, i.e. a pulse of methane emission of 1 kg corresponds to a pulse of 28 kg carbon dioxide emission. But we could equally use other figures as shown in the table below (from the IPCC Synthesis report 2014). For example the GTP-100 measures the actual temperature change after 100 years. With that measurement a pulse of 1 kg methane corresponds only to 4 kg of carbon dioxide. The actual effect on the temperature is probably more in line with what most people expect of the comparisons between greenhouse gases.
But neither the GWP nor the GTP can properly reflect the difference between short lived greenhouse gases such as methane and long-lived carbon dioxide. In the article New use of global warming potentials to compare cumulative and short-lived climate pollutants in Nature Climate Change Myles Allen and colleagues demonstrate how the calculations for expressing methane in carbon dioxide equivalents hides a lot of information. For short-lived greenhouse gases the comparison with carbon dioxide based on a pulse of emissions of both gases gives a reasonably correct result only in a time span of a few decades. In the longer term, the more correct comparison is between a pulse of carbon dioxide and an constant rate of methane emissions. [ed. note: the preceding sentence was corrected 14/12/17.] Or as expressed in the article
“The notion of ‘CO2-equivalent’ pulse emissions of cumulative and short-lived climate pollutants [SLCP, among which methane is the most important one, my comment] will always be ambiguous because they act to warm the climate system in fundamentally different ways. To date, this ambiguity may have had only a limited impact, not least because emission reductions have so far been relatively unambitious.”
… and
“GWP100 can be used in the traditional way, comparing pulse emissions of different greenhouse gases, to specify how mitigation of both short-lived and cumulative climate pollutants may reduce the rate and magnitude of climate change over the next 20-40 years, but only over that time. To achieve a balance between sources and sinks of greenhouse gases in the very long term, net emissions of cumulative pollutants such as CO2 need to be reduced to zero, while emissions of SLCPs simply need to be stabilised.“
The long term effect of a pulse emission of methane is thus very small after 100 years. What really matters in the long term is that CO2 emissions are cut dramatically. On the other hand, in the very short term, the effect of decrease in methane emissions is huge. It can be tempting for politicians or others who want to show quick results to focus on reduction of methane emissions and ”buy time” for the needed cuts in carbon dioxide emissions, or simply neglect it as the buck is passed to other generations.
A recent article in Energies by researchers from Chalmers University in Gothenburg makes the case that a culling of all domestic ruminants (they express it as a shift in diet to a non-ruminant diet, but that clearly assumes that all of them are slaughtered) would “buy us” time to delay the necessary transition of the energy system. But this one-time radical decrease of methane emissions doesn’t really take away the need for a fundamental change of the energy system, it just allows for a few years more of continued carbon dioxide emissions. After that we still need to cut them as much as without any cut in methane emissions.
The argument from the researchers why culling cattle is better than reducing carbon dioxide emissions is purely economic, it is simply cheaper in their analysis (which also assumes a 5% discount rate…). It is not clear how they calculated the loss of income for hundreds of million of people and the loss of animal traction and transport for even more people, or how the loss of all the 187 million cows in India (12 % of all cows in the world) would even be possible. But I assume this is totally irrelevant for the researchers and their economic calculations. After all, in the big scheme, a rich European’s trip to a tropical holiday resort has a higher “value” than the yearly income of an Indian farmer.
There is a case for a reduction in methane emissions to avoid that the climate reaches certain ”tipping points”, such as the melting of the permafrost. However, that argument only holds if carbon emissions are simultaneously reduced, thus not for the scenario where methane reduction is used to buy time as in the cull the cow scenario above. A cut in fossil fuels will also give a direct reduction of methane emissions as fossil fuel extraction is the biggest contribution to anthropogenic methane emissions. In addition, the climate effect of methane emissions from fossil fuels is higher than the methane from cattle, because the carbon in fossil methane adds to the long term carbon dioxide increase of the atmosphere while the carbon in the methane from ruminants is part of a biological cycle, and will be taken up by the grass eaten by the cows etc.
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So much for the big picture, what about the emissions and grazing cattle? Do the more than hundred million people who primarily get their livelihoods from pastoralism or extensive grazing and their cattle, destroy the climate?
If we see a ruminant and the pasture that feeds it, this is a balanced system where there is no increase in methane emissions. The methane emissions are just the same every year. As we seen above that will have had a small effect on the climate, meaning they have contributed to global warming. But equally important is that the methane emissions from these animals don’t cause a continued warming, the annual emissions are balanced by equally high breakdown of earlier emission. A lifecycle analysis, however, will treat this as a new system and each animal as an addition to emissions, i.e. a pulse emission in the GWP-100 metric, and therefore it will concluded that the cattle and its products cause huge emissions. This demonstrates that the results of a lifecycle assessment is determined by its assumptions and the metrics used, and why lifecycle assessments are not a good tool for use when assessing complicated biological systems.
But haven’t the number of ruminants increased rapidly? And haven’t huge areas been converted to pastures lately?
The number of cattle and buffaloes have indeed increased by 50% since 1961 (the human population grew by 135% in the same period). Most of these additional cattle are fed with cultivated feedstuffs (silage, grains, corn, soybeans) either in US style feedlots for beef or in modern dairy production, i.e. they don’t graze. We have all heard the stories about massive tracts of land, often rainforest being converted to pasture. And it is true on a regional level, but globally there are also other trends where massive tracts of pasture is converted to forest or arable land. Most rich countries experience that pastures are converted to forests, for example in Sweden around 80 percent of all pastures have been abandoned the last 100 years. The global area of grasslands even shrank since 2000. Between 1961 and 2014 grasslands only increased with roughly 8 % globally.
Assuming that these lands were grazed by methane emitting ruminants already 1961, the net increase in methane emissions caused by grazing ruminants represent a very minor contribution to climate change, and most of the warming caused by these animals occurred long time ago. This doesn’t account for the laughing gas emissions (N2O) which is another climate gas. But it also doesn’t consider the sequestration of carbon in grassland soils. Both these merit their own articles and are subject to huge scientific arguments.
To continue keeping these animals, keeping the pasture grazed, doesn’t contribute anything to further climate change. Culling them would – possibly — result in a one-time small reduction of global warming, hardly noticeable for future generations* If it is worth it, is a question of values and of which food system we favour, and not a strategy for tackling climate change.
*If the land were converted to arable farming it would emit more carbon which would cause more global warming.
