What questions were asked?
Dr Andy Reisinger of the New Zealand Agricultural Greenhouse Gas Research Centre (NZAGGRC) was commissioned to undertake modelling to answer the following questions:
- If methane emissions from livestock were held steady at current levels, or follow business-as-usual projections, what would be their future contribution to warming?
- What reduction in methane emissions from livestock would be needed to achieve no additional contribution to warming from livestock methane above the current level?
To do this, Dr Reisinger needed first to estimate New Zealand’s historical emissions of methane from livestock. This is because the current level of methane-induced warming is also affected by past emissions. Those estimates were then combined with data from New Zealand’s greenhouse gas inventory and emission projections. This information was fed into a widely-used, relatively simple climate model, which simulated the warming effect resulting from the scenarios outlined above.
These questions were designed to clarify the links between methane emissions, concentrations and global temperature resulting from livestock production in New Zealand. Livestock methane accounts for around 85 per cent of New Zealand’s annual methane emissions.
The intention of the first question was to help us understand how much additional warming would be caused if methane emissions continue at or around current levels. Given methane’s short life in the atmosphere, there has been conjecture that this is negligible.
The second question was designed to estimate a trajectory for livestock methane that would generate no additional contribution to warming. A baseline year of 2016 was chosen because it is the latest year for which greenhouse gas data is available and is where New Zealand is starting from today.
New Zealand might, of course, consider reducing its warming contribution from livestock methane below this level in the context of its Paris Agreement commitments. That, however, is a matter for policy making; it cannot be answered by science alone.
A quick recap on methane and the carbon cycle
Greenhouse gases have different chemical and physical properties. The contribution they make to global warming depends on their atmospheric concentrations, lifetimes and decay products.
Carbon dioxide and methane are part of a cycling of carbon on a planetary scale. The carbon cycle is made up of vast reservoirs of carbon that exist in various places – the atmosphere, soil and vegetation, the oceans and fossil fuels – as well as continuous exchanges, or ‘fluxes’, between these reservoirs. Some of these exchanges operate on very short timeframes, while others are much slower.
Prior to industrialisation, carbon dioxide levels in the atmosphere remained fairly constant over the past few thousand years. Human activity has dramatically increased the release of carbon from land and fossil fuel sources. Processes operating to remove carbon from the atmosphere have not kept up with emissions.
By injecting large quantities of carbon dioxide into the atmosphere, human activities today will continue to perturb the carbon cycle for thousands of years. Describing how long any given emission of carbon dioxide remains in the atmosphere is not easy because it is involved in so many aspects of the carbon cycle. Individual molecules of carbon dioxide are continually being exchanged between the atmosphere, oceans and living things.
But it is still possible to calculate the net effect of adding carbon dioxide to the atmosphere. Carbon dioxide removal is fast initially, as the rate of carbon dioxide absorbed by oceans and living organisms exceeds the rate of re-release to the atmosphere. The remaining carbon dioxide is removed from the atmosphere at a diminishing rate while continuing to cause warming over millennia.
Methane comes from a range of biological processes including the decay of organic material in wetlands and belching from ruminant animals. The biological methane cycle can be considered a loop in the larger carbon cycle, where carbon dioxide from the atmosphere is absorbed by growing plants, converted into methane by microbes, released into the atmosphere, and then oxidised back into carbon dioxide.
The lifetime of methane in the atmosphere is simpler to explain than carbon dioxide. The amount left is reduced by two thirds roughly every 12 years. Almost all of this is broken down through chemical reactions with hydroxyl radicals in the lower atmosphere.
The warming effect of methane
Methane amplifies the warming associated with the carbon cycle because it is a more potent greenhouse gas than carbon dioxide. Most of the warming caused by methane occurs during the first few decades. One tonne of biological methane traps approximately 33 times more heat than a tonne of carbon dioxide over a 100-year period. However, carbon dioxide causes sustained warming for thousands of years.
The timing of any warming and its strength will depend on the amounts of the two gases in the atmosphere – and all the other greenhouse gases. There is nothing simple about atmospheric chemistry.
The warming caused by methane includes direct warming from methane itself, as well as indirect warming from by-products of methane breakdown. These indirect effects account for about one third of methane’s total warming impact. The most significant of these indirect effects are the production of stratospheric water vapour and tropospheric ozone – both powerful greenhouse gases in their own right.
Some methane is emitted to the atmosphere during the extraction of fossil fuels. The warming caused by this methane includes a small additional warming effect from the carbon dioxide produced when fossil methane is oxidised in the atmosphere. This is because the carbon in fossil methane comes from coal, oil and gas deposits that have been buried deep underground for millions of years, so additional carbon is added to the atmosphere when fossil methane oxidises.
This extra warming does not occur with the decay of biological methane, because the carbon is part of the fast biological carbon cycle. Atmospheric carbon dioxide is absorbed by plants, converted into biological methane by microbes, then emitted and converted back into atmospheric carbon dioxide. Hence, the warming caused by this recycled carbon dioxide is not counted towards the warming potential of biological methane.
Although methane emissions are relatively short-lived, some of the warming they cause continues long after the emissions themselves have decayed. This is for two reasons.
First, there is significant inertia in the climate system and this means there is a long lag time between changes in methane emission levels and their full impact on global temperature. Figure 1 shows this in idealised terms using a simple sketch. A constant flow of methane emissions results in a constant methane concentration after around 50 years, but its impact on temperature continues to increase for several centuries. Three hundred years after a constant flow of methane emission has started, the warming effect is more than twice as high as it is after 50 years.
Figure 1: Idealised atmospheric concentration and warming resulting from a constant rate of methane emissions over a 300-year period.
Secondly, higher concentrations and warming effects of methane and other greenhouse gases perturb the global carbon cycle. A warmer climate causes any carbon dioxide already in the atmosphere to remain there for longer, and more carbon dioxide to be released from oceans and the biosphere. These changes amplify and prolong the warming caused directly by the emission of methane (or any other greenhouse gas). While there is uncertainty regarding the magnitude of these feedbacks, there is robust evidence that they result in additional warming, and that this warming can be significant over time.
The lifetimes and potencies of greenhouse gases are not fixed; they respond to the constantly changing background composition of the atmosphere. For instance, the amount of warming a greenhouse gas causes depends on how much of that gas is already in the atmosphere. As a result, emissions of methane gradually become less potent as its concentration in the atmosphere increases, and vice versa.
Estimates of the lifetimes and the amount of warming different greenhouse gases cause are published by the Intergovernmental Panel on Climate Change (IPCC). These are regularly updated as scientific understanding develops.
What the modelling tells us
The key findings from the modelling were as follows. First, if New Zealand’s emissions of livestock methane were held steady at 2016 levels, then within about ten years the amount of methane in the atmosphere from that source would level off. However, the warming effect of that methane would continue to increase, at a gradually declining rate, for more than a century. In the year 2050, holding New Zealand’s livestock methane steady at 2016 levels would cause additional warming of 10-20 per cent above current levels. This warming would increase to 25-40 per cent by 2100.
Secondly, if New Zealand wished to ensure that methane from livestock caused no additional contribution to warming beyond the current level, emissions would need to be reduced by at least 10-22 per cent below 2016 levels by 2050, and 20-27 per cent by 2100.
Further reductions would then be required beyond 2100 to maintain this stable contribution to warming – although the rate of reduction required would get smaller and smaller over time. This is why it is best to think about reducing these emissions as a trajectory.
The 22 per cent level in 2050 reflects a scenario in which other countries take strong action and meet the Paris Agreement goals. The 10 per cent level reflects a scenario in which other countries take some action, but not enough to achieve the Paris Agreement goals.
Figure two shows estimated trajectories of livestock methane emissions from New Zealand that would result in no additional contribution to warming above current levels. The trajectories make different assumptions about global greenhouse gas concentrations, depending on the level of collective action to reduce emissions globally. ‘More global action’ (RCP 2.6) is a scenario that would limit the increase in global average temperatures to less than 2°C relative to pre-industrial levels. ‘Less global action’ (RCP 4.5) assumes global action, but not enough to limit warming to under 2°C, as a best estimate.
Figure 2: Trajectories for methane from livestock that would result in no additional contribution to warming above current levels.
This modelling tells us that if other countries take strong and rapid action to meet the ‘well-below 2°C’ goal, New Zealand’s emissions of methane from livestock would need to be reduced by about 22 per cent to avoid additional warming. This is because the background concentrations of methane will be lower if other countries take strong action, so the methane emitted by New Zealand causes more warming.
If, on the other hand, we assume that other countries take some action on climate change, but not enough to achieve the well-below 2°C goal, New Zealand’s emissions of livestock methane would need to be reduced by about 10 per cent to achieve the same temperature objective.