Causes: Methane CH4
Dairy cows are our single largest source – image Monika Kubala | Drilliing for gas – image J. Penrose
The global monetized benefits for all market and non-market impacts are approximately US$4,300 per tonne of methane reduced. When accounting for these benefits nearly 85% of the targeted measures have benefits that outweigh the net costs. The benefits of the annually avoided premature deaths alone from a 1.5°C-consistent-methane mitigation strategy is approximately US$450 billion per year. – UN Global Methane Assessment 2021
Growing grass does not offset agricultural emissions:
Grassland carbon stocks would need to increase by approximately 25% −2,000%, indicating that solely relying on carbon sequestration in grasslands to offset warming effect of emissions from current ruminant systems is not feasible. – Wang et al, 22 November, 2023
Video 1: The Methane Budget from Global Carbon Tracker shows how methane is accelerating and reached record levels between 2017 and 2024.
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Methane is produced by single-celled ‘methanogenic’ microorganisms that feed on plants in anaerobic (oxygen-free) conditions. They are vital microorganisms because the help decay dead plants and animals. This helps to recycle the nutrients back into the food chain. In the same way that we and other animals breath out carbon dioxide as a waste product, these organisms release methane is a waste product. Methanogenic microorganisms are some of the oldest forms of life on Earth (Archaea) and they’re found everywhere, including in some trees.
Most of the methane they produce is absorbed back into the ground. Much was locked away for many millions of years along with coal (which is why it’s so often found in coal mines) and during the most recent Glacial epoch as frozen clathrates. However, clathrates are now defrosting (see the tab below) the microbes have sprung back into life, and methane is escaping into the atmosphere at a rapidly accelerating rate.
Methane that escapes into the atmosphere was assumed to take 12 years to break down into carbon dioxide (CO2) and enters the carbon cycle. However this assumption was based how quickly hydroxyl radicals (OH) in the troposphere broke down the methane. There appears to be fewer hydroxyl radicals than previously estimated, meaning that the warming effect of methane is lasting longer than anticipated.
The New Zealand Greenhouse Gas Inventory report summarised in the graph below uses a range of parameters to assess how much methane is produced by different activities. Some of this is based on estimates, others on actual measurements. Agriculture produces 53%. (Image credit: New Zealand Greenhouse Gas Inventory 1990-2023).
Fig. 2: Breakdown of New Zealand’s emissions (in Mt CO2-e) by sector in 2023
Research published February 2020 revealed a way to distinguish emission from biogenic sources—agriculture, livestock etc—and fossil fuel sources.
Click on the following tabs to find out more about the sources of methane.
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Ruminant animals, primarily sheep and cows, are the largest producer of anthropogenic methane gas in Aotearoa (see the graph in the tab above).
Methanogenic microorganisms live in the gut of these animals to breakdown the food using a process called enteric fermentation (Fig. 3).
- A sheep can produce ~30 litres of methane/day
- A dairy cow can produce up to ~200 litres/day.
Here in New Zealand, ithe table below from DairyNZ shows that in 2023-24 there were 4,701,596 dairy cows. This has and continues to result in huge quantities of methane escaping into the atmosphere directly from the animals, through effluent ponds, and also by adding fertiliser to land.
Fig. 3: The process of enteric fermentation. There are close to 1 million dairy cows in Canterbury alone (Table 3)
While enteric fermentation is natural in grazing animals, around the world, particularly in places like Brazil, humans have burned down millions of hectares of forest and wetlands that once recycled methane efficiently, with millions of domesticated ruminants that graze on grass or are fed grains from grasses.
Table: Dairy cows in New Zealand
Methane clathrate (also called methane hydrate, fire ice, hydromethane, methane ice, natural gas hydrate, or gas hydrate), is methane frozen in a crystal structure of water, forming a solid that looks like ice. Once thought to exist only in the frozen outer parts of the Solar System, it turns out to abundant in permafrost and beneath the ocean floor.
