Off the grass

We need to talk about methane (7 minute read)

High on a hill, in the suburb of Newlands, is one of my favourite areas for walking my dog, Waihinahina Park. It is one of the largest off-leash dog exercise areas in Wellington, with a huge expanse of grass. There’s plenty of space to get away from other dogs, if your dog is like mine and not always friendly. Sure, the ground is a little swampy – to be fair, it’s actually very swampy – but I have gumboots and my dog likes to splash in the giant puddles.

Scattered across this park are a number of strange, red pipes, rising up from the ground and ending in a small cap. These mysterious pipes give a clue to the origin of this park, and indicate why, despite its lovely position overlooking the harbour, the park will remain a dog exercise area and not be turned into housing. Until the mid-1990s, Waihinahina Park was the Horokiwi landfill.

Donna beside gas vent at Waihinahina Park

The purpose of the pipes is to allow the escape of something called “landfill gas”, which is mostly a mixture of methane and carbon dioxide, with trace amounts of other gases. Landfill gas is caused by the decomposition of organic material, such as food and garden waste, inside landfills. And it is a significant contributor to climate change.

You’ve probably heard that plants, as they grow, extract carbon dioxide from the air (this is something I’ve talked about before, when I wrote about deforestation). Carbon dioxide from the air is used to make the molecules from which all living things are made. When living things die, those molecules break down, releasing carbon dioxide again. This process is known as the carbon cycle.

However, when living things break down in the absence of oxygen, something different happens. Instead of releasing carbon dioxide, methane is released. The inside of a landfill lacks oxygen, and so it releases methane, which can seep through spaces in the ground and escape into the atmosphere, or it can build up in particular areas, creating pockets of gas. Neither situation is good.

The most immediate problem with landfill gas is that methane is flammable. Landfills, both active and old sites, have actually experienced explosions caused by the build-up of landfill gas, but, fortunately, this doesn’t seem to be a common problem. I’ve found instances of this in England, the USA, Denmark and Mexico, among others, although I don’t know of any examples where this has happened in New Zealand. I assume that the risk of gas build-up is the reason for the vents at Waihinahina Park.

Allowing landfill gas to escape into the atmosphere, however, still creates a problem. As I mentioned, the two main components of landfill gas are methane and carbon dioxide. These are both greenhouse gases, so they contribute to climate change. But here is where things get a bit complicated. When the carbon cycle is in balance, it doesn’t contribute to an increase in carbon dioxide in the atmosphere – carbon dioxide is absorbed and carbon dioxide is released. So, what’s the problem?

The problem is methane – the gas that food and garden waste break down into without oxygen. Even though a methane molecule and a carbon dioxide molecule have exactly the same amount of carbon, methane makes a bigger contribution to climate change. Methane is at least 100 times better at trapping heat in the atmosphere than carbon dioxide. On the other hand, methane doesn’t stay as methane forever. In contact with ozone, it forms carbon dioxide and water, and on average it only floats around in the atmosphere for around 10 years.

In order to understand the impacts of different greenhouse gases, scientists calculate a figure called the Global Warming Potential. This figure takes into account how effectively the different gases trap heat in the atmosphere, and how long they last. Over a period of 20 years, methane has a Global Warming Potential around 80 times that of carbon dioxide. However, because it is short-lived, if you do the calculations over a longer period such as 100 years, methane’s Global Warming Potential drops to around 25 times that of carbon dioxide. Most often, the figure you hear quoted is for 100 years, but we need to get climate change under control much faster than that, so it’s better to focus on the 20 year figure. And that means we need to get serious about methane emissions.

There’s a small glimmer of good news in this story, so I’ll pause and share that with you. At Wellington’s Southern Landfill, there is a small electricity generation plant. This plant captures and burns methane from the landfill and feeds the energy produced back into the local electrical network. The plant’s operators say that they will produce enough electricity each year to power more than 1200 homes. That may not seem like a lot, but it is still worth celebrating.

So, that’s the good news. Unfortunately, it goes downhill from here, because landfills aren’t the only source of methane. They’re not even the largest source of methane, not by a long way.

Methane accounts for around 20-30% of global warming (the exact numbers vary depending on which time period you consider and which source you look at). Around a third of these emissions are natural, mostly from decomposition in wetlands. Another third comes from the fossil fuel industry, for example, from coal mining. Coal contains trapped methane, which is released when the coal is mined (the disastrous explosion at Pike River was caused by the ignition of methane released from the coal). Most of the remaining third of the methane in the atmosphere comes from agriculture, and most of that agricultural methane comes from livestock, in particular cattle and sheep.

In New Zealand, the picture is different. Methane makes up 44% of our contribution to global warming, about the same as our carbon dioxide emissions. However, that figure is based on a Global Warming Potential calculated over a period of 100 years, which is perhaps misleading when we are trying to get our emissions to “net zero[1]” by 2050. Remember, if we calculate our methane emissions over a 20 year time scale, as some scientists suggest we should, methane is not 25 times worse for the atmosphere than carbon dioxide – it’s 80 times worse. After all, 2050 is not much more than 20 years away.

So, where does New Zealand’s methane come from? More than 80% comes from cattle and sheep agriculture.

But why are cattle and sheep such a problem? The reason has to do with the food they have evolved to eat. Cattle and sheep eat grass and other plant material that humans and most other animals don’t digest very well. In fact, cattle and sheep don’t digest this material well either, instead they have microbes in a compartment of their stomach, called the rumen, which help with the digestion. Animals with a rumen, known as ruminants, aren’t born with microbes in their rumen, and so at first they can’t digest plant material. However, in their first couple of months of life they acquire the necessary microbes from contact with other animals and the environment.

The inside of the rumen is an environment which lacks oxygen. And so, just as with waste in a landfill, the breakdown of plant material inside a rumen produces methane. This methane has to go somewhere, so the ruminants expel the methane when they burp.

This leaves New Zealand in an uncomfortable position. We are deeply dependent on agriculture, and particularly cattle and sheep – dairy, beef and lamb are important sources of export income. If we are serious about tackling climate change, we have to do something about cattle and sheep. The problem is, what? Do we need to turn away from an industry which has been central to our economy for so long? Or are there other solutions?

There is research underway, in New Zealand and overseas, which is looking at ways to reduce the amount of methane produced by ruminants. For example, a study in the USA found that supplementing the diet of cattle with small amounts of seaweed reduced the amount of methane they produced, without affecting their overall health. New Zealand researchers are looking at potential solutions based on breeding, forage type and even vaccines against the specific microbes which produce the most methane.

It’s unlikely that there is a single answer to the problem of ruminants and methane. Scientists may find ways to reduce the amount of methane that the animals produce, but it’s hard to escape the conclusion that we also need to reduce the numbers of cattle and sheep we have in New Zealand. We can look at other sources, such as landfills, but that will be almost meaningless unless we tackle our biggest contributor.

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[1] Net zero means that our total carbon emitted is no more than the amount we are removing from the atmosphere, for example by restoring and protecting our peat wetlands.