Causes : methane

Causes: methane

(Image: Monika Kubala)

Methane (CH4)


Fig. 1: Instructions for this interactive graph (Credit: The Institute.)

  • Mouse over anywhere on the graph to see the changes in global atmospheric methane over the last thousand years.
  • To see details for time periods of your choice, hold your mouse button down on one section then drag the mouse across a few years, and release it.
  • To see how this compares to the past 771,000 years, click on the ‘time’ icon on the top left.
  • Compare this to rising global temperatures by clicking the planet/thermometer icon at the top left corner.
  • To return the graph to its original position, double-click the time icon to the left of the thermometer/planet icon

Where does methane come from?

Methane is produced by single-celled ‘methanogenic’ microorganisms that feed on plants in anaerobic (oxygen-free) conditions. They are vital microorganisms as their ‘feeding’ is in fact ‘decaying’—they help recycle nutrients. Methane is a natural bi-product of this decaying process.

Methanogenic microorganisms are some of the oldest forms of life on Earth (Archaea) and they’re found everywhere. 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 as frozen clathrates, which are now escaping into the atmosphere at a rapidly accelerating rate.

Some methane naturally escapes into the atmosphere where it takes about 12 years to break down into carbon dioxide (CO2) and enters the carbon cycle.

Fig. 2: Sheep and cows together emit 36.5% of methane emissions. Click on this graph to see a larger image (Image credit: New Zealand Greenhouse Gas Inventory 1990-2018 [April 2020].

Sources of methane (outlined on this page)

  • Agriculture
    • ruminant animals
    • adding fertiliser to land
    • effluent ponds
    • growing rice
  • Fugitive emissions
  • Burning fossil fuels
  • Forest and bush fires
  • Damming rivers
  • Land fill
  • Wastewater treatment plants
  • Methane clathrates
  • Melting permafrost

Human (anthropogenic) activities are dramatically increasing methane in the atmosphere (Fig. 1 above). The most recent New Zealand Greenhouse Gas Inventory report (summarised in Fig. 2) 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.

Research published February 2020 revealed a way to distinguish emission from biogenic sourcesrice fields, livestock etcand fossil fuel sourcesnatural seeps, industries extraction processes and burning oil, coal, and coalmethane. As this is new research, it could be a few years before it’s incorporated into the New Zealand gas inventory.

Agriculture: ruminant animals

Ruminant animals, primarily sheep and cows, are the largest producer of anthropogenic methane gas in NZ (Fig. 2).

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 and a dairy cow up to ~200 litres/day. 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 (see Forest Fires below) that once recycled methane efficiently, with millions of domesticated ruminants that graze on grass or are fed grains from grasses.

Here in New Zealand, the number of dairy cows in Canterbury increased from 490 in 1994 to 1,253,993 in 2015. This has and continues to result in huge quantities of methane escaping into the atmosphere directly from the animals and through effluent ponds and adding fertiliser to land.

Fig. 3: Ruminant animals, primarily dairy cows, are the largest producer of anthropogenic methane gas in New Zealand. Around 95% of the methane produced through enteric fermentation by ruminant animals is through belching.
Fig. 3: Ruminant animals, primarily dairy cows, are the largest producer of anthropogenic methane gas in New Zealand. Around 95% of the methane produced through enteric fermentation by ruminant animals is through belching.

Agriculture: effluent ponds

Cow dung and urine from milking sheds, concrete ‘stand-off’ pads, and permanent indoor housing is washed into these ponds. Here, bacteria and methanogenic microbes break down the effluent, producing methane and nitrous oxide (a greenhouse gas 298 times more warming potential than carbon dioxide). As dairy farming is intensified to industrial scales, more effluent will be collected this way.

Results from our investigation indicate that the national GHG Inventory is currently underestimating dairy effluent pond CH4 emissions by a factor of 1.7 to 4. Ministry for Primary Industries (MPI) noted that: ‘Currently there is widespread interest in removing animals from pastures and placing them on stand-off pads or more permanent housing.’ If this shift in animal management occurs, it would have an enormous impact on the way that dairy effluent is managed. The time that cows spend on sealed surfaces would increase, so that more manure would need to be treated by effluent ponds, resulting in higher CH4 emissions. If a much higher proportion of the total daily dairy cow waste production is collected, and manure management practices are not changed, manure CH4 emissions have the potential to equal the current main agricultural GHG sources of enteric methane and pasture nitrous oxide emissions.”  – MPI report, 2012

Agriculture: adding fertiliser to land

To fertilise the grass to feed dairy cows, nitrogen and phosphorus are added in huge quantities to the soil. Some of this fertilizer runs off into streams and rivers. These nutrients fertilise algae just as they fertilise grass, so algae flourishes (Fig. 4). Especially in summer when river levels are often lower and temperatures are higher (and increasing with climate change), the algae blooms overwhelm the water’s oxygen resources, killing other aquatic plants and animals. When the algae dies, it’s decomposed by bacteria and methanogens, which release both CO2 and methane into the atmosphere. This is major issue for NZ’s waterways, particularly Canterbury’s braided rivers (Fig. 5).

Landfill (waste)

Bacteria and methanogens decompose the organic waste (food and things like grass clippings that are thrown out with rubbish). This generates CO2 and methane that is released directly into the atmosphere. In New Zealand, landfills produce up to 4% of methane emissions.

Fig. 4: Algae bloom at Coes Ford, Selwyn river in mid-Canterbury. While there is no way to determine exactly what causes blooms each year, they are known to happen when nitrogen and phosphorous (fertiliser) from the land enters waterways, in higher temperatures, and when the volume of water is reduced. As the climate warms, higher temperatures are happening earlier each year and lasting longer, compounding the problem. (Photo: Fish and Game)
Fig. 5: Nitrate-nitrogen content in New Zealand waterways. Note the extremely high rates in Canterbury rivers (dark red) vs natural and largely uncultivated landscapes in alpine areas and Fjordland (blue). (Image: Ministry for the Environment).

Wastewater treatment plants

When you flush the toilet or wash water down the drain, the wastewater goes to where its treated.  In rural areas that’s a septic tank in your backyard. In urban areas and cities, it goes to giant Council- run treatment plants. Different industries – mining, dairy, wine, wool, leather, meat, each have their own wastewater treatment plants as well. In all cases, at some stage in the treatment process, bacteria and methanogens decompose the organic waste. This produces carbon dioxide, nitrous oxide (a greenhouse gas 298 times with more warming potential than CO2), and methane in a similar way that happens in effluent ponds. Depending on how efficient the treatment is and how the gasses are dealt with, some of these gasses escape into the atmosphere.

Burning fossil fuels

Burning coal and oil releases methane. Recently, New Zealand scientists, amongst others, have determined that burning fossil fuels is adding 35-40% more methane into the atmosphere than previously thought (ie, it is is not yet included in current inventory calculations). In addition to this, burning methane releases CO2 in the atmosphere.

Industry and manufacturing

A small amount of methane is produced during these processes, including cement and methanex manuafacturing, both of which take place in New Zealand. These are accounted for in different ways in the New Zealand Greenhouse Gas Inventory 1990-2017.


When a river is dammed, the blocked water that builds up behind it creates an unnatural, stagnant lake that kills the entire ecosystem that once existed there. Everythingplants, forests, the insects, microorganisms and fungi in the soils crucial for sequestering carbonis drowned. In the lake, bacteria and methanogens decompose the dead plants, generating CO2 and methane. This bubbles to the surface and enters the atmosphere.

Fugitive emissions

Methane is less dense than air, so its lighter, which means it can easily escape into the atmosphere during mining, transport, manufacturing, and storage—including escaping from the gas bottles used for BBQs. As it’s highly flammable, any gas that builds up too much pressure must be vented, ie leaked into the atmosphere. Some but not all mines burn some of this excess gas in a process called ‘flaring’.

In 2017, fugitive emissions from the Oil and natural gas category contributed 1,808.2 kt CO2-e (93.2 per cent) to emissions from the Fugitive emissions category. This is an increase of 799.3 kt CO2-e (79.2 per cent) from 1,008.9 kt CO2-e in 1990.”New Zealand Greenhouse Gas Inventory 1990-2017 p107

Forest and bush fires

Globally, millions of hectare of forest is burned evry year. Sometimes this is a result of climate-related rising temperatures and drought, such as in the Australian 2019-2020 bushfires, where 18.6 million hectares (46 million acres) was burned. In the Brazilian Amazon (Fig. 6), in August 2019 alone, close to 2.5 million hectares of land was burned by farmers to produce soya and beef. This releases into the atmosphere methane and also the CO2 that was once stored in trees and plants.

While forest fires are not yet a huge concern for New Zealand in terms of methane emissions, the increasing risks need to be considered in light of the billion trees project.

Fig. 6: Deliberate burning of the Amazon to grow more beef and soya (Image: Wired magazine)

Agriculture: growing rice

While very little rice is grown in New Zealand, globally ~3.5 billion people depend on rice for more than 20% of their daily calories and ~1 billion depend on it for their income (Fig. 7). The conditions in which rice is grown are ideal for bacteria and methanogens to produce methane and nitrous oxide (a greenhouse gas 298 times more potent than CO2). Perversely, techniques intended to reduce emissions while also cutting water use, may be increasing emissions, meaning methane from rice cultivation may be up to twice as bad as previously estimated.

Agriculture, particularly vast areas being converted to rice growing, is one of the reasons cited for why greenhouse gasses gradually began increasing in the atmosphere thousands of years ago, long before fossil fuels started to be burned to produce energy.

Fig. 7: Rice is is such demand globallly that in many Asian and South East Asian countries like the Philippines where this photo was taken, entire hills and mountains have been re-sculptured over centuries, possibly millennia in some places, to grow rice on terraces. (Image: Wikipedia CC license.)
Fig. 7: Rice is is such demand globallly that in many Asian and South East Asian countries like the Philippines where this photo was taken, entire hills and mountains have been re-sculptured over centuries, possibly millennia in some places, to grow rice on terraces. (Image: Wikipedia CC license.)

Melting permafrost: ‘burning lakes’

  • Permafrost can be as thin as <1m and as thick as >1,000m. It covers approximately 22.79 million km² (about 24% of the exposed land surface) of the Northern Hemisphere.
  • Melting permafrost is the result of a feedback effect of climate change, that is anthropogenic forcing is a triggering natural forcing.
  • In 2019, NOAA estimated that melting permafrost was contributing 600 million metric tonnes of net carbon (methane and carbon dioxide) per year into Earth’s atmosphere.

Permafrost is a combination of soil, sediment, and the remains of dead plants and animals that stay at or below 0°C for at least two years. Unlike ice, it doesn’t ‘melt’ once temperatures rise above 0°C. Permafrost falls apart, and the organic material decomposes, just as frozen meat or vegetables left outside a freezer will decompose if not eaten. When this decomposition happens an environment where there’s oxygen, such as outside your fridge on the sink, carbon dioxide is released. If the environment is anaerobic (lacks oxygen), such as underwater in lakes, wetlands, and the ocean, methane is released (Video 1).

Video 1: Ass. Prof. Katey Walker explains why flammable methane bubbles form in millions of lakes in the Arctic, is trapped by ice until summer, when it’s released into the atmosphere.

“By 2100, near-surface permafrost area will decrease by 2-66% for RCP2.6 and 30–99% for RCP8.5. This could release 10s to 100s of gigatonnes of carbon as CO2 and methane to the atmosphere.” IPCC, 2019

Methane clathrates: ‘burning ice’

Methane clathrate (also called methane hydrate, hydromethane, methane ice, fire ice, natural gas hydrate, or gas hydrate), is methane trapped and frozen within 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.

The United States Geological Service (USGS) estimates the amount of carbon in methane clathrates is twice the amount of carbon that exists in all the fossil fuels on Earth.

Video 2: Russian researcher Prof. Natalia Shakova explains the processes behind methane erupting from the Siberian tundra an shallow coastal waters.

While the USGS regard it as potential source of fuel, the sheer volume of what’s being released naturally as permafrost melts, has alarming consequences. As one cubic metre of methane hydrate produces between 163-180 metres of gas, the explosive potential is also high. Video 2 explores, amongst other impacts, how methane ‘burps’ from melting permafrost and methane clathrates are forming large craters in Siberia. The peer-reviewed open access paper by Shakova et al is here. Video 3 explains what happened in 2020 after Siberia experienced temperatures up to 45C.

Video 3: Record temperatures in Siberia in June 2020 have resulted in large scale meting of sub-sea methane clathrates along coastal areas.


Climate forcing:

The term comes from ‘radiative forcing’ or RF, which is the difference between the amount of solar energy reaching Earth’s atmosphere and the amount that escapes. If more solar energy escapes than arrives, the planet cools. Conversely, if less energy escapes than gets in, the planet warms. This is due to the Law of Conservation of Energy, a basic law of thermodynamics, which states that: ‘Energy can neither be created nor destroyed; rather, it can only be transformed or transferred from one form to another.’

Different climate forcings each determine how much solar energy arrives and escapes.

  • Natural Forcings are those that happen through natural changes.
  • Anthropogenic Forcings are those due to human activities.

Click here to learn about the main forcings and how they work (links to another page on this website).

Emissions Trading Scheme:

The global warming potential or radiative forcing (RF) of methane is calculated as ’25’ under the NZ Emissions Trading Scheme. This measurement is based on the average effect is has in the atmosphere over 100 years.

  • Total methane emissions from a single dairy cow/year: enteric fermentation + manure management + soil = 2060kg CO2-e
  • The average North Canterbury dairy herd is 770 cows

These emissions are currently EXEMPT from the NZTS.

Natural gas you use for BBQs and cooking is mostly methane:

It can include higher alkanes (gasses with more carbon in their molecular structure than methane), and sometimes small amounts of other gasses: carbon dioxide, nitrogen, hydrogen sulfide, and/or helium. Methane has no colour or smell, but it’s highly flammable, which is why it’s used for cooking. In many countries including New Zealand an ‘odourant’ or ‘stenching’ (ie distinctive bad smell) is added  so that it can easily be detected in case it leaks.

The bad smell coming from rotting vegetation or effluent ponds is NOT methane; it’s other gasses like hydrogen sulfide and ammonia.

Methane from soya and beef:

Millions of hectares of tropical rainforest is still being cleared specifically to sell meat to overseas buyers including McDonald’s and Burger King, which buy vast quantities of beef from Brazil. Along with Kentucky Fried Chicken, McDonald’s and Burger King also serve chicken fed a diet of soya from Brazil.

The Agricultural Revolution:

The transition of many human cultures from hunting and gathering to agriculture began ~12,000 to 15,000 years ago, around the time the last glacial maximum ended. By ~11,500 years ago the global climate began stabilising enough for agriculture to spread. By 9,000 years ago agriculture was common in many places and ~6,000 years rice growing was a common practice in many areas. At the same time, the Earth’s climate was slowly moving into a natural cooling phase. Greenhouse gas emissions from agriculture have been credited with offsetting this very slight cooling, thereby maintaining a relatively stable temperature until the Industrial Revolution, which was powered by burning huge quantities of fossil fuels, releasing huge quantities of greenhouse gasses into the atmosphere.

References and further reading