Why methane from cattle warms the climate differently than CO2 from
fossil fuels
July 07, 2020
Methane is a potent greenhouse gas
with a warming potential more than 28 times that of carbon dioxide
(CO2). But when it comes to livestock and climate change, there are
many other characteristics that set biogenic methane (methane from
cattle) apart from CO2. Here are an important four:
- It stays in our atmosphere for
about 12 years
- It’s derived from atmospheric
carbon, such as CO2
- It’s part of the biogenic carbon
cycle
- It eventually returns to the
atmosphere as CO2, making it recycled carbon
It should be noted that
methane from fossil fuels doesn’t have all the same
characteristics as biogenic methane – that is methane from ruminant
animals such as cattle, or wetlands. Aside from its short life span,
fossil methane shares more traits with CO2 from fossil fuels in how it
warms our planet, since it’s not derived from atmospheric carbon (it’s
pulled from the earth) and is new to the atmosphere. It’s worth noting
that methane emissions from fossil fuel extraction have been
severely underestimated. We should certainly strive to
reduce methane from all sources.
Methane stays in our atmosphere
for 12 years
Methane has a relatively short life
of
12 years compared to the hundreds or even thousands of years that
CO2 hangs around. After about 12 years, 80 to 89 percent of methane is
removed by oxidation with
tropical hydroxyl radicals (OH), a process referred to as hydroxyl
oxidation. As a result of its short lifespan, methane is only
significantly warming our atmosphere for those 12 years, which is why
it is considered a short-lived climate pollutant (SLCP).
Its short lifespan is further
relevant in regard to warming, because it means that as methane is
being emitted it is also being destroyed in the atmosphere,
making it a
flow gas.
This illustrates that methane’s
warming impact isn’t determined by how much is being emitted – since
it’s destroyed relatively quickly – but by how much more or less
methane is being emitted over a period of time. This is a change in
the rate of emission.
What is notable about methane, is
that it’s possible the amount being emitted can equal the amount being
destroyed. For example, if a herd of cattle emits the same amount of
methane over 12 years, they are contributing to warming for those 12
years. But afterward the same amount being emitted is the same that is
being destroyed through oxidation, and thus warming is neutral.
Schematic illustration of how global mean
temperatures respond to different emissions trends in carbon dioxide
(CO2) and methane (CH4). Source: Briefing paper, “Climate
metrics under ambitious mitigation”.
It should be pointed out that
additional methane outside of that equilibrium – such as before
reaching it or adding more after – warms at 28 times that of CO2 over
100 years, making it important we do not increase methane emissions.
But a really intriguing aspect of
biogenic methane, is that if we are able to reduce it, such as with
dairy digesters, then we can create a cooling effect since there
is more methane being destroyed than emitted. These warming and
cooling situations are considered in a new climate change matrix
called GWP*, which better quantifies the warming effects of
short-lived climate pollutants such as methane.
Methane is created from
atmospheric CO2
The critical difference between
biogenic methane and a fossil fuel greenhouse gas, is that methane
from sources like cattle begin as CO2 that is already in the
atmosphere. Gases that result from fossil fuel production begin deep
in the earth, where they’ve been stored for millions of years, away
from the atmosphere.
So how does CO2 become methane?
Meet the biogenic carbon cycle
The cyclical nature of biogenic
carbon starts with plants. Think back to your grade school years –
what do plants need to grow?
Water, sunlight and CO2.
As part of the
biogenic carbon cycle, plants absorb carbon dioxide, and through
the process of photosynthesis, they harness the energy of the sun to
produce carbohydrates such as cellulose. Indigestible by humans,
cellulose is a key feed ingredient for cattle and other ruminant
animals. They are able to break it down in their rumens, taking the
carbon that makes up the cellulose they consume and emitting a portion
as methane, which is CH4 (note the carbon molecule). After about 12
years, the methane is converted into carbon dioxide through hydroxyl
oxidation. That carbon is the same carbon that was in the air prior to
being consumed by an animal. It is recycled carbon.
A quick note: while both biogenic
and
methane from fossil fuels are chemically identical, the resulting
CO2 from oxidation has a different warming impact. The biogenic carbon
from cattle and wetlands is returned to the atmosphere as that is
where it started, while fossil carbon is brand new atmospheric carbon,
and hence, new warming.
What does this difference mean?
The difference between biogenic
methane and CO2 is significant when we talk about warming, which is
ultimately what we care about when discussing greenhouse gases. The
current standard for determining how greenhouse gases warm the planet,
which is GWP100, doesn’t reflect the
differing characteristics of methane and other short-lived climate
pollutants from CO2 and long-lived climate pollutants.
If we really want to find climate
solutions, then we need to accurately understand how various
greenhouse gases actually warm the planet, because we may be missing
opportunities to reduce global warming because we misunderstand the
roles different greenhouse gases play in climate change. This isn’t to
negate the value of GWP100, because it does a good job of representing
CO2 and other long-lived climate pollutants, but it’s more productive
to look at short-lived climate pollutants in a better way – in other
words, having the right tool for the right job.
Check out this
tweet from Dave Frame, who is one of the authors of GWP*:
Dr.
Frame points out that our efforts to reduce biogenic methane are
important, but they shouldn’t distract us from the more critical need
of finding ways to lower the CO2 emissions that arise from the burning
of fossil fuels. In other words, if we ignore what’s happening with
CO2 and fossil fuels, we’re all but guaranteed to end up with a warmer
climate. That’s because the effects of a reduction of biogenic methane
would be short lived as the emissions would balance out as mentioned
above. On the other hand, CO2 would continue to build up in the
atmosphere and warm the planet increasingly.
Overall, it is worthwhile to reduce
biogenic methane emissions from animal agriculture, as it can buy time
for the global community to develop solutions that stop climate
change. But we must consider how methane and other greenhouse gases
actually warm the planet if we want to have long-lasting effects,
otherwise we may nonetheless end up with a warmer planet.
Green Play Ammonia™, Yielder® NFuel Energy.
Spokane, Washington. 99212
www.exactrix.com
509 995 1879 cell, Pacific.
Nathan1@greenplayammonia.com
exactrix@exactrix.com
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