Methane (CH4)

Methane (CH4) is the simplest hydrocarbon: a molecule made entirely of carbon and hydrogen, with one carbon atom bonded to four hydrogen atoms. It is the main ingredient in natural gas, the fuel that heats most homes in the UK and powers many cookers and boilers. Methane is also the second most important greenhouse gas after CO2, and it leaks into the atmosphere from cows, landfills, rice paddies, melting permafrost and gas pipelines. Tiny molecule, huge effect.

  • FormulaCH41 carbon + 4 hydrogen
  • StateGas at room temperatureBoils at -161 C
  • Energy from burning55 MJ/kgCleanest fossil fuel
  • In atmosphereAround 1.9 ppmMuch smaller than CO2
  • Warming power28x stronger than CO2Over 100 years
  • Biggest sourceWetlands + livestockPlus oil and gas leaks

The methane molecule

Methane has a beautifully simple shape. A carbon atom sits in the middle, with four hydrogen atoms at the corners of a tetrahedron (a four-cornered pyramid). All four C-H bonds are the same length and arranged symmetrically. Each is at 109.5 degrees to its neighbours.

The symmetrical shape means methane is non-polar, so it does not dissolve well in water. It is a colourless, odourless gas. The "rotten egg" smell of a gas leak in your home does not come from the methane itself: gas suppliers add a tiny amount of a smelly sulfur compound (mercaptan) on purpose, so leaks are obvious.

Where does methane come from?

  • Underground deposits: most natural gas comes from fossil deposits formed millions of years ago from buried algae and other organic matter. The biggest reserves are in Russia, Iran, Qatar, the USA, Turkmenistan and Saudi Arabia.
  • Wetlands: bacteria living in mud at the bottom of swamps, marshes and rice paddies break down plant matter without oxygen, producing methane. Wetlands are the biggest natural source.
  • Livestock: cattle, sheep and goats have special stomachs that ferment grass with the help of methane-producing bacteria. Each cow burps out about 100 to 150 kilograms of methane per year.
  • Landfills: buried rubbish rots without much oxygen and releases methane. Modern landfills capture this and burn it for fuel.
  • Permafrost: methane is locked in frozen soils in the Arctic and Siberia. As permafrost thaws, that methane is being released into the atmosphere.
  • Gas leaks: drilling, pipelines, fracking and abandoned wells all leak methane.

Methane as fuel

Methane is the cleanest of the fossil fuels. Burning it gives mostly carbon dioxide and water:

CH4 + 2 O2 -> CO2 + 2 H2O + heat

For each unit of energy released, methane produces less CO2 than coal or oil, and far less soot, sulfur and ash. That is why natural gas has been used to replace coal in many power stations.

Even so, burning natural gas still releases significant CO2. The transition to wind, solar and nuclear is what really cuts emissions.

Fact Methane is a far more powerful greenhouse gas than CO2 molecule for molecule. Over 100 years, a kilogram of methane traps about 28 times as much heat as a kilogram of CO2. The good news is that methane breaks down in the atmosphere after about 12 years, while CO2 stays for centuries. So cutting methane leaks is one of the fastest ways to slow short-term warming.

Methane in your home

In the UK, most homes have a gas supply that brings natural gas (around 90 per cent methane) directly to the cooker, boiler and gas fire. The gas flows through underground pipes from huge offshore platforms and storage tanks. Burning the gas with the right amount of air gives a clean, hot blue flame ideal for cooking and heating water.

Modern condensing boilers extract extra heat by cooling the exhaust gases enough to condense the water vapour back into liquid, which is one of the reasons your boiler has a small white plastic pipe that drips clean water outside.

Methane and climate change

Since 1750, the amount of methane in the air has more than doubled, from around 720 parts per billion (ppb) to over 1,900 ppb today. Almost all of this extra methane comes from human activities: livestock, gas production, landfills and rice farming.

Because methane is such a strong greenhouse gas, this rise has contributed about a quarter of all the warming we have experienced since 1750. Reducing methane is a high priority. In 2021, over 100 countries (including the UK) joined the Global Methane Pledge, promising to cut methane emissions by 30 per cent by 2030.

Did you know? Cow burps and farts release so much methane worldwide that scientists are testing seaweed feeds to reduce them. Adding a small amount of red seaweed (Asparagopsis taxiformis) to cow feed has been shown to cut methane burps by up to 80 per cent in some trials. If the technique scales up safely, it could cut greenhouse emissions from cattle significantly.

Methane elsewhere in the universe

Methane is one of the most common molecules in the universe. It is found in the atmospheres of Jupiter, Saturn, Uranus and Neptune, where it gives the outer planets their blue tint by absorbing red sunlight. Methane lakes and rivers actually flow on the surface of Titan, Saturns largest moon, where the temperature is cold enough (minus 180 degrees Celsius) for methane to be a liquid.

Mars contains tiny amounts of methane in its atmosphere, and the levels seem to fluctuate. Scientists are unsure whether it comes from underground geological processes or, more excitingly, from possible microbial life beneath the Martian surface.

Try this Watch a gas hob carefully (with adult supervision). When properly adjusted, the flame should be a clean blue. If the flame is yellow or orange, the air-to-gas ratio is wrong: there is too little oxygen for complete combustion, and the burner is producing carbon monoxide and soot. Adjusting the air vent until the flame turns blue makes the cooker safer and more efficient. You are looking at chemistry in action: complete vs incomplete combustion of methane.
Deeper dive: methane hydrates, the burning ice at the bottom of the sea

One of the strangest forms of methane on Earth is methane hydrate: a solid that looks like ordinary ice but burns when you set it alight. Methane hydrates form where methane gas meets cold water under high pressure, usually under the seabed in deep ocean shelves or in permafrost regions.

In a methane hydrate, water molecules form a cage-like crystal structure with a methane molecule trapped inside each cage. A single cubic metre of methane hydrate can contain enough methane to fill 160 cubic metres of gas at the surface, all locked into a stable solid as long as it stays cold and under pressure.

The total amount of methane stored in hydrates worldwide is huge: some estimates suggest more energy than all conventional fossil fuels combined. Several countries (Japan, China, India and the USA) have explored mining methane hydrates as a future energy source. Pilot drilling has shown it is possible but very expensive and risky.

The bigger concern is what happens if these hydrates melt naturally. As the oceans and the Arctic warm, some areas of methane hydrate are starting to break down, releasing methane into the water. So far the released methane is mostly being eaten by bacteria before it reaches the atmosphere, but scientists are watching carefully. If large-scale hydrate breakdown began, the released methane could greatly accelerate global warming, which would warm the oceans even more, which would release more methane. This kind of runaway feedback is called the clathrate gun hypothesis and is one of the lower-probability but higher-impact risks of climate change.

For more, see carbon dioxide and combustion.