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 [corrected 13 nov] methane emissions. 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 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.
*
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 with 50% since 1961 (the human
population grew with 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 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.
