Why isn’t grass in, methane out, carbon neutral?

Looking at New Zealand’s rich pastures, it might appear as if carbon’s journey from the atmosphere, through a cow, and back again is a picture of clean, green, carbon neutrality. It seems simple enough: grass pulls carbon dioxide, or CO2, from the atmosphere to grow, cows eat the grass, excreting some of the carbon, breathing and burping a lot of it out, and leaving the rest in their milk and flesh to head back to the atmosphere when it’s consumed or decays.

The same amount of carbon has cycled from the atmosphere and back again. So without any extra being added, it’s carbon neutral, right? It would be if all carbons behaved the same way. But, when it comes to their effect on the greenhouse, they don’t.

In the process of digesting grass, a ruminant animal like a cow or a sheep produces the gas methane, or CH4, in its stomach, which it then belches into the atmosphere. The carbon molecules making up the methane might be the same ones originally pulled out of the atmosphere by the grass, but in this new form they’re anything but neutral.

Methane is a flammable, natural gas which the average cow produces enough of to power a car for 4 kilometres every day. But that’s not methane’s problem in the atmosphere. Weight for weight, methane is at least 21 times more potent as a greenhouse gas than CO2. And it’s the CO2 equivalent warming effect of a gas which is measured when it comes to carbon neutrality.

Methane acts as a blanket in the atmosphere, absorbing infrared rays as they radiate off the earth and bouncing some of them back down to re-heat it. In that way it traps heat like all greenhouse gases, including CO2, water vapour, and nitrous oxide (N2O). But methane is more efficient than CO2 at absorbing infrared rays. On top of that it spends an average of nine years in the atmosphere before it is oxidised back into CO2.

All of which is why methane has become such a significant issue for New Zealand.

The sheer numbers of cows and sheep in relation to humans here means we have an emissions profile which is unique in the developed world. Agriculture is the source of 47 percent of all our greenhouse gas emissions (on CO2 equivalent basis). As signatories to the United Nations Framework Convention on Climate Change, we are obliged to try to reduce our emissions. And while other countries, where agriculture may only be responsible for 5 to 10 percent of emissions, can choose to ignore it, that’s not so easy for us.

That’s the dichotomy decision-makers and scientists face today: how do we both sustain and protect our most important industry while also contributing to a global effort to stem greenhouse gas emissions?

There are still those who argue livestock-generated methane is not the problem it’s portrayed as. Critics often point to the ability of well-managed pasture to sequester, or trap, carbon in its grasses and soils.

But, while it’s true that pasture acts as a carbon sink in the same way forestry does, the sheer potency of methane as a greenhouse gas, including the fact it stays that way for nine years, outweighs the ability of pasture to trap carbon as CO2.

Not that naturally occurring greenhouse gases don’t play a vital role. Without them warming the Earth’s surface it would be 33 degrees cooler on average and uninhabitable. But anthropogenic – or human activity generated – sources of the gas are now responsible for about 1.5 times the amount produced by natural means. Livestock alone account for 15 percent of all global methane emissions. And, as the world slowly warms, it’s likely that all human-caused greenhouse gas emissions are at least partly to blame.

CO2 remains by far the most important anthropogenic greenhouse gas, responsible for about 63 percent of the warming over the last 200 years.That’s due largely to human activities, including the burning of fossil fuels which frees previously locked-up carbon, causing the natural cycle to get out of balance.

But methane is the second most important greenhouse gas, and it too is on the rise.

NIWA’s research shows the influence agriculture has had on atmospheric methane levels over the centuries. The institute has been measuring CO2 in the atmosphere since 1972, and methane since 1989. We know from taking samples of air trapped in polar ice that methane levels have grown by 150 percent since organised animal farming began in the early 1700s. By comparison, CO2 levels have grown by 35 percent. Methane levels have continued to rise over the decades, growing fastest through the 1960s, '70s and '80s, before slowing down in the 1990s, levelling off in the early 2000s, and increasing again from 2007.

This levelling has caused some to suggest that animals are not the problem. But with livestock accounting for only 15 percent of emissions, it’s more likely that the sources of the other 85 percent – like wetlands, which are susceptible to floods and drought – are responsible for the fluctuation.

In New Zealand, where cattle numbers have increased dramatically, and individual cows now produce 30 percent more milk than they did in 1990, there is little doubt that agricultural methane emissions will have increased too, even though sheep numbers have fallen.

What remains for New Zealand and the rest of the world is to work out what to do about it.

Options range from breeding animals that produce less methane to investigating how changing the way we feed and farm them might produce less greenhouse gases.

But any benefits from introducing new farming practices like dry-stalling could be outweighed by associated increases in other greenhouse emissions, such as N2O that results from applying fertiliser to cropland for animal feed and CO2 produced by burning fossil fuels in the harvesting and distribution of that feed.

And with any change new environmental and political dilemmas emerge. How would we deal with effluent disposal from grain-fed animals for example? What about the inevitable dilemma over how far – or if – to proceed with genetic engineering?

There is a quiet optimism in the scientific community that a way to reduce agricultural methane emissions will be found. But if any real solution is on the horizon it’s likely to be a closely kept secret. With global demand for dairy and beef increasing, and pressure mounting on the farming industry to mitigate their emissions, any solution would be very valuable.

Cow in the Wairarapa. [Dave Allen]
Diagram showing the movement of carbon in pastoral agriculture. Carbon flows reflect a near-steady state (units:t(C)/ha/yr). Flows to or from the atmosphere are CO2 except where indicated. Source: Tony Parsons, AgResearch, based on Parsons & Chapman (2000).