Why does

Published: July 17, 2019

Tagged with:

  • Overview
  • Agricultural methane
  • Climate change
  • Methane emissions reduction
  • Methane heat trapping

Methane emitted into the atmosphere decays a lot faster than other greenhouse gases, but while it is there, each molecule has a potent warming effect.

Why does methane matter?


Duration: 3:21

So how exactly is methane produced, and how does it contribute to climate change? There are a lot of different stories out there, but this is how it actually works.

Methane comes from several places, including wetlands, landfills, forest fires, agriculture and fossil fuel extraction. But here in New Zealand, the largest proportion by far is belched out by livestock.

[What, all livestock?]

[Not us, surely!]

It’s perfectly natural. Microbes in the fore-stomach of ruminant animals, like cows and sheep, break down the pasture the animals have eaten and produce methane. And, well, it has to come out somehow.

Great … but it breaks down though, right?

Well, it’s true that methane released into the atmosphere breaks down much faster than other greenhouse gases like carbon dioxide. But while it exists, it has a big impact. Tonne for tonne, methane is actually many times more effective at absorbing heat than carbon dioxide.

An emission of methane will mostly disappear from the atmosphere within 50 years. But while it’s up there, every molecule traps lots of heat. An equivalent emission of carbon dioxide traps less heat, but stays around much, much longer. Thousands of years in fact.

We’ll show you what it looks like. Let’s say filling up the board with symbols is equivalent to a certain temperature increase.

I’m emitting carbon dioxide into the atmosphere every year. Because CO2 takes so long to break down, every emission adds to the warming caused by previous emissions. The amount of CO2 increases over time, and the effects become bigger and bigger.

Okay. I’m emitting methane into the atmosphere. Each emission makes a big contribution to warming, but fortunately we’re not emitting nearly as much methane as we are carbon dioxide.

So as we do this, Gavin is going to rub out the methane symbols to show they break down at a much faster rate in the atmosphere, and I’m going to continue emitting at the same rate.

So if we keep emitting the same amount of methane, the amount in the atmosphere levels out, because the new emissions by and large just replace the previous emissions that have now disappeared.

[So, what’s the problem?]

[New burps replace the warming caused by old ones.]

But, we’ve increased our methane emissions a lot over the past century. This has pushed temperatures up already, and globally they’re still going up.

If we keep emitting methane at the current rate, that will keep the atmosphere a lot warmer than it used to be.

So, at a minimum, if we want to stop additional warming from methane we need to reduce the amount of emissions.

They don’t have to be stopped completely, but the more that they’re reduced, the less warming is caused and the better for the climate. How far could we go?

[I don’t mind a change of diet!]

Climate change is a complex challenge. But we’ve already achieved heaps thanks to some clever Kiwi innovation and farmers becoming more and more efficient. But there is more that we can do. New technology will play its part, but there’s more that you can do now. Find out what in our next video.

 Produced by the New Zealand Agricultural Greenhouse Gas Research Centre. Funded by the New Zealand Government

Note that nitrous oxide (N2O), another important agricultural greenhouse gas, is both potent and long-lived. However, it accounts for a much smaller proportion of New Zealand’s annual gross emissions than carbon dioxide or methane. Information about nitrous oxide’s sources and behaviour in the atmosphere, as well as options for reducing emissions, will be added to this site in the near future. Get notified by email when updates happen.

Where does methane come from?

Methane has several sources, including wetlands, landfills, forest fires, agriculture and fossil fuel extraction. In New Zealand, the largest proportion by far is belched out by livestock.

Ruminants such as cows, sheep, deer and goats have four-chambered stomachs, enabling them to readily break down and extract energy and nutrients from fibrous plants like grass. Microbes in the rumen break down complex carbohydrates into simpler molecules – a process known as enteric fermentation. Some of these microbes produce methane, which the animal then burps out.

The amount of methane produced by an individual ruminant animal is directly linked to how much it eats. The average dairy animal produces approximately 82kg of methane per year, the average deer approximately 22kg per year, and the average sheep approximately 12kg per year.

You can see how methane from livestock contributes to climate change in this diagram [PDF, 169 KB] by the Ministry for the Environment.

For more on methane and its impact in the atmosphere:

Is methane really an issue?

In short, yes. Scientists estimate that methane emissions are responsible for almost 40% of the total warming effect generated by human activities so far.

Carbon dioxide and nitrous oxide are long-lived gases, staying in the atmosphere for centuries in the case of nitrous oxide and millennia in the case of carbon dioxide. Therefore, every unit emitted today increases their concentration in the atmosphere and adds to the warming caused by past emissions. By comparison, methane is a relatively short-lived gas – most of a methane emission disappears within about 50 years. So new methane emissions today replace, rather than add to, previous methane emissions that have already disappeared.

However, while methane is up there, it is much more effective at trapping heat than carbon dioxide. Averaged over 100 years, one tonne of methane causes about 30 times the warming as one tonne of carbon dioxide. Some of the heat trapped by methane causes other changes in the global climate system as well, resulting in warming that extends beyond methane’s relatively short lifetime in the atmosphere.

Because methane doesn’t accumulate in the atmosphere as carbon dioxide and nitrous oxide do, methane emissions don’t need to reduce to zero to stabilise the climate. But ongoing methane emissions keep Earth a lot warmer than it would be otherwise. So, the more we can reduce methane emissions, the more we will reduce our contribution to climate change.

Addressing all greenhouse gas emissions is critical for keeping warming within the international target of well below 2°C (ideally 1.5°C) above pre-industrial times. This target reflects comprehensive scientific evidence that the risks related to climate change increase the more Earth warms up. It is no longer possible to stop warming immediately, but every half degree matters.

More information on the impacts and risks related to global climate change are in the 2018 report by the Intergovernmental Panel on Climate Change on global warming of 1.5°C.

For more:

What influences how much methane an animal produces?

Feed intake is the main influence on how much methane a ruminant animal produces.

Cattle grazingApproximately 21 grams of methane are produced per kilogram of dry matter eaten. This varies only slightly across the typical feeds in New Zealand’s pastoral systems: fresh pasture (rye grass and clover), pasture silage and maize silage.

However, some feeds can result in lower emissions.

  • Forage rape produces about 30% less methane per kilogram of dry matter eaten than pasture.
  • Fodder beet reduces methane emissions if it makes up a very high fraction of the diet (which can be difficult to manage).
  • Diets rich in grains also result in lower methane emissions per kilogram of dry matter, but such intensive feeding systems are not common in New Zealand.

Additives such as lipids, monensin, essential oils and garlic can reduce emissions in some circumstances, but the effect is small and variable, and these approaches tend to be costly and difficult to implement in New Zealand’s grazing-based farm systems.

For more information on what can be done now, at the farm level, to influence greenhouse gas emissions see here.

The Government has invested significantly, in partnership with industry, in research to discover and develop new ways to reduce emissions. For an overview of current investment streams, see here. For more information on this research programme and some promising future options, see here.

How do livestock affect the carbon cycle?

Many people ask whether methane is part of a ‘carbon neutral cycle’; carbon in and carbon out meaning that it doesn’t need to be reduced. Unfortunately, it’s not as simple as that.

Grazing livestock eat plants which grow by taking carbon dioxide out of the atmosphere and storing some of it as carbon. A small amount of this plant carbon is converted to methane when it is digested by ruminants while most of it is lost as carbon dioxide. Some of the carbon is lost in dung and urine and this produces methane (and nitrous oxide). Some of the plant carbon ends up in milk and meat but when these products are eaten this carbon is released as carbon dioxide.

Methane eventually decays back into carbon dioxide after around 12 years. But while it is in the atmosphere, it makes a significant contribution to the overall warming effect because it is much more effective at absorbing heat radiation than carbon dioxide. This means that while the cycle is still ‘carbon neutral’, it is not greenhouse gas or warming-neutral.

For more, including some helpful diagrams, see here.

Published: July 17, 2019