The role of methane
We are often asked about the role of methane in climate change, and how much methane needs to reduce to help us meet our climate targets.
The science of different greenhouse gases can be challening to understand. However, understanding at a basic level what role different gases play, and how different metrics and science interact as New Zealand works to meet its emissions reduction targets is important. Below we take a look at the role of methane in climate change. When looking at ‘how much’ and ‘by when’ methane needs to reduce science can help answer some of this, but only gets us so far before value judgements need to come into play.
For a more indepth discussion on this, you can watch this video of the Environment and Primary Production Select Committees at Parliament hearing from a panel of scientists on the role of methane.
Methane is a short-lived greenhouse gas, that contributes to warming our climate. It is known as a ‘trace gas’, which means it’s only around in small concentrations – but it has a large impact on climate change.
Most methane emissions in Aotearoa are ‘biogenic’ in origin – which means they are produced from living things and processes. Of our methane emissions, 91% of biogenic methane comes from ruminant animals in agriculture, and 9% comes from organic waste breaking down in landfill.
Non-biogenic methane emissions are also leaked during oil and gas extraction. While this proportion is small in Aotearoa, it is more significant for other countries. This is important to consider when broad international statements are made about reducing methane emissions, such as the pledge Aotearoa signed up to at COP26 to reduce global emissions by 30%.
Methane behaves differently in our atmosphere compared to carbon dioxide. While CO2 (carbon dioxide) and N2O (nitrous oxide) stay in the atmosphere for hundreds of years, and are considered ‘long-lived’ gases, methane only stays in the atmosphere for a couple of decades and is considered to be a ‘short-lived’ gas. The targets we have in Aotearoa reflect the different nature of these gases.
To stabilise global temperatures, we need to significantly reduce emissions of greenhouse gases (GHGs) into our atmosphere. To limit warming to 1.5°C above pre-industrial levels, ‘long-lived’ gas emissions need to reach net zero by 2050 – and potentially hit negative figures in the second half of the century. Methane emissions still need to reduce and stabilise at some level – the question is, by how much?
Metrics are often used to simplify how we compare the climate impact of different gases to that of carbon dioxide – i.e. their “carbon dioxide equivalence”. Metrics simplify the complicated physics, chemistry, and biology in the full models used by the IPCC for their assessment reports and for the global 1.5°C report. The Emissions Trading Scheme and the national Greenhouse Gas Inventory are two examples of where metrics are used in New Zealand.
Different metrics are useful for different things. Each has its own strengths and weaknesses depending on the purpose it’s being used for.
Given the challenges in using metrics to compare the impacts of different gases, it’s important to keep in mind the purpose of using a metric. If the purpose is to assess the warming impact of emissions, the best approach is to consider the warming impact of each individual gas rather than use an equivalence metric.
Science can help answer some of that question, but only gets us so far. Science is also evolving in this space, as the Intergovernmental Panel on Climate Change (IPCC) report released on 4 April 2022 shows. It has found that deep greenhouse gas emissions reductions by 2030 and 2040, particularly reductions of methane emissions, lower peak warming, reduce the likelihood of overshooting warming limits and lead to less reliance on net negative CO2 emissions that reverse warming in the latter half of the century.
The IPCC have summarised the science and are clear that to limit global warming to 1.5°C, global long-lived gas emissions need to reach net zero by 2050, while global biogenic methane emissions will need to reduce by 24-47% from 2010 levels by 2050.
To reach global targets, the world could collectively invest more in reducing long-lived gas emissions, which would mean that biogenic methane emissions would not have to reduce as much. Conversely, we could globally invest more effort in reducing biogenic methane emissions, so long-lived gas emissions can reduce at a more measured pace.
There is no question that we need to take action on biogenic methane. Research from the New Zealand Agricultural Gas Greenhouse Research Centre shows that biogenic methane emissions have been the largest contribution to global warming in Aotearoa since 1840, aside from deforestation. Even if we achieve our 2050 targets, methane will still have been our largest contributor to global warming.
The NZGARC research states "New Zealand’s biogenic methane emissions currently make a bigger estimated contribution to global warming than cumulative emissions of fossil carbon dioxide and nitrous oxide combined. If gross emissions of those three gases continued at current rates, biogenic methane would remain New Zealand’s largest single contributor to global warming for the next six decades despite its relatively short lifetime in the atmosphere compared to carbon dioxide and nitrous oxide."
The science only gets us so far because there are also value judgements that need to be considered, such as how quickly should Aotearoa reduce emissions relative to other countries.
There are value judgements in many areas – not just reducing biogenic methane. In transport, for example, some of those value judgements are around questions such as “how should Aotearoa spread its investment across encouraging more people to buy or drive EVs, developing public transport, and increasing active transport like cycling and walking?”
These value judgements are important for Aotearoa, and they involve assessing a range of things – for example our historic responsibility because as a country we’ve had relatively high greenhouse gas emissions (including methane) compared to most other countries, particularly the developing world. Developed countries are encouraged to ‘take the lead’ on climate action.
Decisions on the emissions targets for Aotearoa sit with the Government of the day, as do the value judgements that are needed, as these need to reflect the democratically expressed views of the country.
Our role is to provide independent and evidence-based advice that can be considered by Government, and that can help inform the public.
The Zero Carbon Act sets out clear processes and responsibilities for both the Commission and Ministers. Our advice to date has largely been around domestic emissions budgets and emissions reductions plans to meet our legislated targets.
In 2024 we are required to review Aotearoa’s legislated targets for both long-lived gases and biogenic methane, in the light of new evidence, to provide advice on whether the targets should be changed.
The Commission will cast its independent eye across the evidence base to help inform the Government and wider Aotearoa. Science can’t tell us what our target should be, but it can help inform those decisions. A better (and wider) understanding of biogenic methane sources and potential reductions and how biogenic methane is compared with CO2 will help Government make those decisions.