While the burning of coal, oil and gas is the dominant source of the greenhouse gases in our atmosphere and so the dominant cause of the worsening impacts of climate change, there are several other sources. Unfortunately, there is considerable confusion about the Australian agricultural sector’s relative impact on the climate compared to Australian fossil fuel consumption.

Some of this confusion derives from incorrect or openly misleading statements about the relative impacts of the sectors (the fossil fuel and agricultural sectors). However, comparing these sectors is a genuinely complex topic. This article is intended to reduce some of this extraordinary complexity to a level that is easier to understand.

We hope you’re sitting comfortably. This is a long one, but the issues are complex and important.

Agriculture’s greenhouse gas emissions: cows and grazing land

In Australia, ‘agriculture’ contributes around 13% of our greenhouse gas emissions each year. By weight, about half of the agricultural sector’s emissions – or 42% – are methane. Most of this is the methane produced by cows and other livestock due to the fermentation of plant matter in their stomachs. Smaller volumes of emissions come from other sources such as fertiliser applied to vegetable crops and wastes, including manure and decaying vegetable matter.

Most of the greenhouse gas emissions produced by agriculture are methane, and most of this methane has its origins with livestock, such as cattle.

Another separate source of emissions related to agriculture is land clearing for pastures and grazing land. While the climate impact of land clearing is partly offset by land restoration activity elsewhere and management of Australia’s forests, land clearing for agriculture nonetheless contributes significantly to Australia’s total emissions. Australia has repeatedly been identified as a global hotspot for land-clearing, and much of this has occurred to facilitate the growth of the agricultural sector.

In recent years, agriculture and associated land clearing released around 115 million tonnes of greenhouse gas (measured as carbon dioxide equivalent) into the atmosphere per year, according to conventional analyses. In comparison, burning fossil fuels for energy released around 400 million tonnes of greenhouse gas into the atmosphere, measured using the same approach. In some ways, these conventional analyses can be misleading, and tend to overstate the real effect of agricultural emissions.

Carbon dioxide and methane: the tortoise and the hare

The majority of agriculture’s emissions are methane. Yet, you might notice that the emissions above are quoted as ‘tonnes of carbon dioxide equivalent’. Like the methane that escapes burping cows, the term ‘tonnes of carbon dioxide equivalent’ is a mouthful, but it bears discussing for a moment.

‘Carbon dioxide equivalent’ (sometimes written as ‘CO₂-eq’ or ‘CO₂e’) is a way of referring to all greenhouse gases (carbon dioxide, methane, nitrous oxide) in common terms by reducing them to a single metric that can be discussed more simply. But there’s a problem: methane and carbon dioxide are both greenhouse gases that contribute to climate change, they each have different properties, potencies and lifetimes.

You might already know that methane is a more potent greenhouse gas than carbon dioxide, which is certainly true. You might even have heard any number of different comparisons between carbon dioxide and methane. Depending on what is considered and the time horizon used, on simple analyses, methane is commonly referred to as being anywhere from 28 to 100 times more powerful than carbon dioxide. This simple metric means that 1 tonne of methane released into the atmosphere will cause a comparable amount of heating to between 28 and 100 tonnes of carbon dioxide over a given period.

In the short-term, releasing methane into the atmosphere is far more harmful than releasing carbon dioxide.

This simple metric is justified and useful. It is designed for policy-makers and for instances where you need to find a basis for comparing emissions without firing up a climate model. It is also a metric that we at the Climate Council rely on, but it is also an intentional simplification of a very complex topic. There are other meaningful differences between greenhouse gases and highlighting these paints a very different picture. Simple metrics are valid and useful, but it’s vital to keep in mind what is lost in the process of trying to compare two fundamentally different things.

When it comes to the difference between methane and carbon dioxide, the most important aspect that is missed by the simple metrics is full consideration of the gases’ different lifespans in the atmosphere.

Methane is a live-fast, die-young gas. Once released into the atmosphere, methane traps heat much more efficiently than carbon dioxide, but only over the course of around a decade. After this point, it is broken down into carbon dioxide and water, among other things, in a complex set of chemical reactions. Once methane becomes carbon dioxide is remains stable in the atmosphere until it is drawn down.

Carbon dioxide, on the other hand, is a ‘slow and steady’, tortoise-type greenhouse gas. It has a lower potential to heat the atmosphere, but it is a relatively stable gas. This means that unless it is drawn out of the atmosphere through a natural or human process, it continues to heat the atmosphere more-or-less indefinitely. It doesn’t get broken down in the same way that methane is.

A simple analogy to water tanks can help to think through the implications of this.

Adding carbon dioxide to the active biosphere is a bit like adding water to a sealed water tank. Just like adding water to the tank raises the water level in the tank, so too does adding carbon dioxide to the biosphere. Unless it is removed by some additional means, adding more of both water and carbon dioxide over time results in more being present. This is pretty simple.

Adding methane works differently, it’s a bit more like a tank with an old-slow-to-start, automatic pump attached. This pump tries to remove water at the same rate that it’s added, but takes a long time to start. In the short term, adding more to the tank lifts the level in the tank, but in time – once the pump kicks in – as much is removed as is added. If the rate that water is added remains stable, eventually the amount in the tank will stabilise at a higher level. The breakdown of methane in the atmosphere works in a similar way. Changing the rate that methane is added causes a short-term shift in the amount of methane present, but if the same rate is sustained, then after about a decade the total amount in the atmosphere stabilises as additions are balanced by removals.

Things get more complicated after hundreds or thousands of years, but this holds true for meaningful timescales related to climate policy.

This is shown in the infographic below.

The dynamic impacts of an emission of methane or carbon dioxide on the atmosphere are fundamentally different. This is of vital importance to understanding the system that we live within and the impact of our greenhouse gas emissions. It is also the reason why some in the scientific community advocate powerfully that a different approach to comparing greenhouse gases be taken. But again, we note that this does not impact the output of climate models. Climate models bypass the issue by fully modelling the impact of different gases, without relying on simpleifiedmetrics.

With one complication out of the way, it is also important to note that there are others. A further complication is added when another feature of methane is considered: its source.

Jurassic farts and fossil methane: why the past is better left buried

The sources of methane can be broadly split into two groups: biogenic and fossil sources. Chemically, these are largely identical – both are made up of one carbon atom and four hydrogen atoms – but their origins mean that they have a different overall impact on the climate.

Biogenic methane

Methane generated by ‘life’ is called biogenic methane. One notable source of biogenic methane is ruminants, such as sheep and cattle.

Biogenic methane is any methane created by things alive today and things that have very recently died. That could be from cows, landfills, or microbes in stagnant ponds. This is a major contributor to the agricultural sector’s climate impact. The creation of biogenic methane is intricately linked to the drawdown of carbon dioxide by photosynthesis.

Fossil methane

Methane generated by burning fossil fuels is termed ‘fossil methane’ and its impact on our climate is far more destructive.

Fossil methane is made from carbon that has been stored underground for millions of years far from the surface and the global atmosphere. Virtually all gas burned for energy today is fossil methane, and the production of all fossil fuels involves releasing significant amounts of methane. Releasing this methane involves re-introducing old carbon to the active biosphere that had long ago been removed from the system.

Quite apart from the short-term impact of adding methane to the atmosphere, emitting fossil methane eventually adds new carbon dioxide to the atmosphere as well. Emitting agricultural methane, on the other hand, involves cycling carbon dioxide through the climate system. This means that the release of methane from fossil fuels has a larger overall impact on the climate than methane from agriculture, waste, or other biogenic sources. When assessed using conventional methods that don’t fully account for methane’s shorter lifespan, the distinction between sources is noteworthy, but relatively minor. However, the additional burden of carbon dioxide from fossil fuels becomes increasingly important if your perspective shifts to account for the flow of methane through the climate system.

If we pull the various threads together, it becomes clear that the overall impact of the Australian agriculture sector is very significantly lower than the overall impact of burning coal, oil and gas. This is because:

• On the raw numbers – after using conventional approaches to determining the ‘carbon dioxide equivalent’ greenhouse gas emissions of both sectors – the consumption of coal, oil and gas produces well over three times more greenhouse gas per year than agriculture, even after factoring in land clearing.
• The influence of methane on climate change is more complicated than the simplified way that is normally discussed. A greater proportion of agriculture’s effect on the climate comes from methane than it does in the production and use of coal, oil, and gas. There are good reasons to consider the live-fast-die-young nature of methane in the atmosphere when making the comparison between the sectors. This doesn’t negate the effect of methane, but nonetheless qualifies it.
• While most of the emissions from agriculture are methane, and most of the emissions from burning fossil fuels are carbon dioxide, the production and use of fossil fuels for energy produces significant quantities of methane in Australia – around half as much as the agricultural sector. Fossil-derived methane has a greater impact on the energy system than biogenic methane, the kind produced by agriculture.

None of this means that agriculture isn’t an important part of the climate change problem, or an area that is ripe for finding solutions.

Fixing historical damage of yesteryear’s cows and avoiding the worst in the future

Methane emissions from all sources have risen very, very significantly since the birth of industrial agriculture and consequently, so has the amount of methane in the atmosphere. While methane emissions from agriculture in Australia are substantially lower today than they were in the mid-1970s – and this means that Australia’s agriculture sector has undone some of its climate damage – there is unrealised potential to further lower the impact of Australia’s agriculture sector.

Australia’s agricultural emissions have fallen slightly since the 1970s, which means that we’ve undone some historical damage, given the way atmospheric methane operates. There is still plenty to do, however.

As well, what is true in Australia is not necessarily true worldwide. Globally, methane emissions from all sources are increasing, and methane concentrations in the atmosphere are not just growing, but accelerating.

A recent report of the Climate and Clean Air Coalition and the United Nations Environment Programme found that some of the easiest climate change mitigation wins come from taking meaningful and urgent steps to reduce methane emissions. By acting fast to reduce sources of methane, we can buy extra time in the global carbon budget. As shown in our recent report Aim High, Go Fast: Why emissions need to plummet this decade, there is a desperate need to tackle many problems at once, and we need all hands on deck.

But among the many unrealised mitigation opportunities we have at our disposal, it’s vital to remember one clear thing. If we fail to act on getting emissions from coal, oil and gas down to as close to zero as possible, then those quick short-term wins from agriculture will amount to very little for Australian lives, livelihoods and the places we love.