Excuse me! The problem with methane gas
This topic is sponsored by the Australian Government Department of Climate Change
When you ask people about greenhouse gases, chances are they’ll focus on carbon dioxide. But there’s another, more potent gas contributing to global warming. Meet methane, the forgotten greenhouse gas.
It’s easy to see why many people don’t know much about methane. When it comes to greenhouse gases, the headlines tend to focus on carbon dioxide. Yet despite methane’s relatively low profile, it plays a crucial role in global warming. Methane (CH4) is responsible for about a fifth of the enhanced greenhouse effect. But there are many unknowns about methane, and scientists don’t always agree on the factors affecting its levels in the atmosphere.
Methane’s shorter life, greater warming effect
There is much more carbon dioxide (CO2) in the Earth’s atmosphere than methane. But methane’s global warming potential (GWP) – or warming potency compared to carbon dioxide – is around 23. That means it’s 23 times more effective at trapping heat in the atmosphere than CO2 over a 100-year period. So adding one tonne of methane to the atmosphere would have the same effect as adding 23 tonnes of CO2. And human activities are increasingly adding to atmospheric methane levels (Box 1: Methane levels).
Luckily, methane lingers in the atmosphere for only 11 to 12 years, compared to up to 200 years for CO2. With a greater potency and shorter lifetime, the impact of methane can be reduced more rapidly. This may become very important if, in the next few years, there is an increased demand for the reduction of greenhouse gases in the atmosphere.
Another advantage of methane is that it can be used as a fuel – a useful way of preventing it from entering the atmosphere.
By working to reduce the levels of a range of greenhouse gases, including methane and CO2, researchers and governments would be able to minimise the combined effects of these gases. But, as will be seen below for methane, each gas has its own set of issues when it comes to reducing emissions.
Sources of methane
Methane is lighter than air, colourless and this gas that animals burp out – despite what you might think – is odourless. It is a truly universal gas. It can occur naturally in wetlands, it’s made by animals, and it can be released as a result of human activities such as agriculture and fossil fuel production. It can also be found in many homes – that’s because the natural gas that many of us cook and heat our homes with, is about 85 per cent methane.
Methane is also known as marsh gas. That’s because it is produced when plants and other organic matter decompose in the absence of oxygen (anaerobically), such as when they are under water. This anaerobic decomposition by microorganisms (called methanogens) takes place in wetlands, swamps and marshes and is estimated to produce some 30 per cent of atmospheric methane levels.
One form of methane that’s causing concern is the vast amount locked away under the oceans and within the Arctic permafrost. The ‘clathrate gun hypothesis’, which sounds like a doomsday scenario from a science fiction novel, suggests that if the frozen stores in ocean floor sediments are released – possibly by global warming – they could trigger a global catastrophe (Box 2: The oceans’ massive methane reserves).
Methane is also produced in the gut of termites (5 per cent of global emissions) and by microorganisms in the ocean (2 per cent).
Methane from human-related activities
Methane is produced at landfills or rubbish tips, as organic rubbish decomposes underground in the absence of oxygen. Similarly, methane can be created in oxygen-free systems involving manure or sewage treatment. But rather than releasing it into the atmosphere, this methane can be put to good use. In Melbourne, for instance, the wastewater treatment plant at Werribee collects methane from its ponds. Some ponds collect up to 20,000 cubic metres of methane a day, which is then used to generate electricity to help power the treatment plant. In this way, the methane produces usable energy. This helps to lower the demand for energy from other sources such as the combustion of coal, which produces carbon dioxide.
Far away from cities, methane can also be produced as a result of the burning of biomass such as agricultural wastes, grasslands and forests (for land clearing). Rice paddies, which are covered by water for several months of the year, act like wetlands producing 10 per cent of methane emissions.
Methane can be produced by a process called enteric fermentation in the digestive system of ruminant animals such as cattle, goats and sheep. These animals burp or, less so, fart the methane into the atmosphere. Livestock are a large source of methane – producing around 14 per cent of global emissions.
Fossil fuel industries also contribute to global methane emissions. The gas is released during extraction of fossil fuels, with an estimated 45 per cent of these emissions coming from coal mines in China. Methane can also leak into the atmosphere from natural gas pipelines.
Global methane levels were relatively stable for a long time, because the total methane produced was being offset by natural methane removal methods, known as methane ‘sinks’. But atmospheric levels are now increasing because methane emissions have overtaken removals by around 22 megatonnes (Mt) per year.
The most important sink for methane is the troposphere (the lowest level of Earth’s atmosphere). Here, the hydroxyl radical (OH) acts as a ‘cleansing’ agent through its reactions with methane and other gases, effectively removing them from the atmosphere. This tropospheric oxidation removes over 500 Mt of methane per year.
Smaller amounts of methane (40 Mt) are also removed from the next atmospheric layer up – the stratosphere – again, due to reactions with OH.
Some microorganisms (methanotrophs) found in soils use methane as a source of carbon, removing around 30 Mt of methane each year.
Unlocking methane’s mysteries
Methane might have a small public profile, but it can have a big effect on climate change. Finding ways of reducing or reusing the methane produced by human activities could be a relatively quick and easy way of reducing greenhouse gas emissions (Box 3: Moderating methane).
But there are gaps in our knowledge, such as why emissions of methane have historically varied so much. Hopefully, further research into methane will help us to stall global warming. It may even avoid possible catastrophe by putting a safety catch on the trigger of the clathrate gun.
Related Academy Links
Capturing the greenhouse gang
Carbon currency – the credits and debits of carbon emissions trading
Enhanced greenhouse effect – a hot international issue
Getting into hot water – global warming and rising sea levels
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Australian Academy of Science.
Posted November 2009, edited August 2012.