Methane is just one of a number greenhouse gases. Methane is CH4 (one carbon atom attached to 4 hydrogen atoms) while carbon dioxide is CO2, one carbon attached to 2 oxygen atoms.
These gases make it hard for radiant heat to leave the planet. They act like insulating blankets around the planet.
This makes them a forcing. A forcing is anything which changes the energy budget of the planet. This budget is like an ordinary cash flow budget, which measures cash in and cash out. Cash will accumulate if more is coming in than going out. The planet’s ice sheets are a very large forcing because incoming radiation (sunlight) just reflects off and back out into space. This acts to cool the planet. If the ice shrinks, then less sunlight is reflected, so more energy comes in and heats up the planet. This is also an example of a feedback. Because as the ice shrinks, more energy arrives, which heats more ice, so more energy arrives, and so on until something interrupts the loop. Some feedbacks are positive (cause warming) and others are negative (cause cooling).
But there are other things which, like ice, are not gases, but which nevertheless also impact on the energy budget. For example, small solid and liquid particles in the atmosphere are called aerosols — the same term that is sometimes used for spray cans. They can reflect sunlight coming down to the planet so that a smaller amount arrives at the surface, which stops it heating up — this is a cooling effect. Some aerosols also increase cloud cover which also has a cooling effect. Others aerosols, like the soot generated when you burn trees has a positive (heating) forcing because it prevents heat leaving the planet. When you burn coal in a power station, not only is carbon dioxide emitted, but aerosols called sulphates are generated which have a cooling impact.
In summary, some forcings heat and some forcings cool. Greenhouse gases are forcings, but plenty of things (like aerosols) are forcings without being gases.
Some humans (about 50% of us) produce methane when we fart. We all fart, but we don’t all produce methane and it isn’t the methane which smells. Natural gas, which many people use for heating or burn to generate electricity is mainly methane and the Gas company deliberately adds a smell to it, so that it’s easy to detect gas leaks.
Livestock also produce methane, but most of it is burped rather than farted. It is generated by microbes in their guts. Ruminants, like cattle and sheep, have amazing multichambered stomachs called rumens that generate vast amounts of methane during the digestion of their food.
Australian livestock generate about 3 million tonnes of methane annually. The figure in our annual Greenhouse Inventory is given as 61 million tonnes of CO2-eq (carbon dioxide equivalents). This underestimates the warming impact of the methane by a factor of about 3.
Hold a blowtorch a few inches from your leg for just 10 seconds. It will cut to the bone instantly. Will your agony diminish if I tell you that the temperature, averaged over 20 minutes, is just 48 degrees? I don’t think so. Under international rules (the Kyoto Protocol), different greenhouse gases are compared and added together by averaging their impacts over 100 years — regardless of the time it takes any particular gas to break down and become ineffective as a greenhouse gas. Averaged over 100 years a tonne of methane has 21 times the warming of a tonne of carbon dioxide — so in our greenhouse gas inventories, one tonne of methane appears as 21 tonnes of carbon dioxide-equivalents (carbon dioxide equivalents).
This is like equating a 10 second blow torch exposure to a 20 minute exposure with a 48 degree candle because the average temperature of both over 20 minutes is the same.
The principle would be fine if all gases took the same time to break down and if we had a hundred years to deal with global warming, but they don’t and we don’t. A tonne of methane breaks down quickly but has an actual warming over 20 years that is not 21, but 72 times greater than a tonne of carbon dioxide. The next 20 years may well be crucial for deciding the climate of the our children’s children, so why the Kyoto negotiators chose a 100 year averaging period is a mystery. But this Kyoto factor of 21 for methane must be music to Meat and Livestock Australia’s ears because it massively underestimates the global warming caused by its industry.
Globally, human induced (usually called anthropogenic) methane is about 350 million tonnes annually. About a quarter (96 million tonnes) of this is due to livestock which is the single largest sector. Other large producers of methane are the coal mining industry and the natural gas industry. These figures come from an international database which receives its data from UN member countries, including of course, Australia. You will need to register to access the data.
Assertion: Surely there have always been plenty of large ruminants on the planet emitting methane? and what about termites?
Truth: The global annual estimate of methane emitted by wildlife (p.542 of link) is about 15 mega tonnes and estimates of termite methane ranges from 20-30 megatonnes. Livestock has now largely displaced wildlife on the planet with the global population of cattle being 1400 million. Compare this with the estimated bison population of North America prior to white arrival of about 60 million.
Assertion: But isn’t methane from cattle (or us) just part of the carbon cycle?
Truth: Of course, but that isn’t the issue. Think about it like this. Suppose you have water in a swimming pool and you dive in. Okay. No problem. Now freeze the water and try and dive in. Ouch. No water has been added but the state of the molecules has changed and the difference is critical. Similarly take CO2 (carbon dioxide) and transform it to CH4 (methane) and you haven’t added any carbon, but changed its form and the new form traps far more heat than the old form.