Excuse me! The problem with methane gas

This topic is sponsored by the Australian Government Department of Climate Change

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).

The greenhouse gas methane comes from both natural sources and human activities.

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.

Methane sinks

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.

Box 1 | Methane levels

Since the pre-industrial era (from around 1750) there’s been a 250 per cent increase in the amount of methane in the atmosphere. Methane levels in the pre-industrial era were some 715 parts per billion (ppb), compared to 1,774 ppb in 2005. Ice core samples show that current levels are the highest in the last 650,000 years. The current increase in methane levels is due to human activities, mainly agriculture and the use of fossil fuels.

Scientists breathed a gaseous sigh of relief when global methane levels steadied out during the 1990s. But their relief was short-lived, with a recent study revealing levels started to increase again in 2007.

The concern is that methane levels may rise dramatically if global warming increases. Higher temperatures would cause melting of permafrost in the northern hemisphere. As organic matter from long-dead plants and animals is released from the thawing permafrost and becomes covered by water, more methane would be produced by microbes.

The warming of oceans may also release vast quantities of methane into the atmosphere as methane hydrates in ocean floor sediments break up (Box 2: The oceans’ massive methane reserves).

(Image: IPCC 2007)

Related sites

• Climate change 2007: Synthesis report (Intergovernmental Panel on Climate Change)

• Global methane levels move upward again (News in Science, 16 December 2008)

Box 2 | The oceans’ massive methane reserves

While atmospheric methane levels are a concern, some researchers believe the real threat is methane trapped in our oceans.

As well as the methane that exists below permafrost on the land in polar regions, there are vast reserves stored in ocean floor sediments as methane hydrates. Here, methane molecules are each surrounded by a water crystal ‘cage’, so they are basically encased in ice. Although at the high pressures experienced in the deep ocean, these can exist at temperatures well above zero degrees Celsius. These hydrates, or clathrates, are the largest reserves of methane in the world, and by some estimates consist of more than half of all fossil fuels.

The vast stores of methane hydrates under the oceans are methane molecules trapped within water crystal ‘cages’. (Image: USGS)

The concern is that rising sea temperatures will melt the hydrates, and lead to massive releases of methane. This could give way to a cycle in which the released gas results in more warming, causing more hydrates to melt, and the release of even more methane. This is what’s known as the ‘clathrate gun hypothesis’ which has been put forward to explain the mass extinction of marine and land-based life in the Permian era some 250 million years ago.

To some people, such a scenario is the stuff of science fiction, and the clathrate gun hypothesis has already featured in several novels.

But the clathrate gun might not go off – at least not just yet. Recent groundbreaking research involving Australian scientists has helped allay such concerns. The research involved scientists from CSIRO and Australian Nuclear Science and Technology Organisation (ANSTO), along with researchers from Denmark, New Zealand, and the United States. They used accelerator mass spectrometry and carbon dating to analyse the methane trapped in bubbles in ice cores from Greenland. The ice cores dated back to the end of the Younger Dryas period – known as the Big Freeze – about 12,000 years ago when there was a 50 per cent increase in global methane levels.

The good news is that the research found that the big increases in methane were produced from wetlands, and not hydrates under the ocean.

Related sites

• The trigger for the clathrate gun (Ockham’s Razor, 24 May 2009)
• Methane climate shock 'less likely' (News in Science, 24 April 2009)
• Methane on ice: A climate shock in store? (News in Science, 21 February 2003)

Box 3 | Moderating methane

Because around 60 per cent of methane is produced by human-related activities, changing the way we do some of these activities could help to reduce the amount of methane in the atmosphere.

Couth cows and kangaroo burgers

We could all become vegetarians or reduce our meat and dairy intakes. That way, there’d be fewer cattle and sheep needed. But it might be hard to convince Australians to give up their family roast or barbecued steaks. Australia has over 90 million sheep and 29 million cattle.

However, Australian scientists are tackling livestock emissions. In early 2009, the Australian Government committed nearly $27 million to research ways of reducing methane and other greenhouse gases arising from agriculture. The research involves 18 projects, including selectively breeding livestock to produce less methane; reducing methane emissions by eliminating certain livestock gut microbes; and reducing methane emissions by changing the diet of livestock.

Another suggested way of achieving lower methane emissions would be to eat meat from farmed kangaroos. Because kangaroos are not ruminants, they don’t belch out nearly as much methane. So perhaps kangaroo burgers and sausages might help save the planet – but only if you could convince lots of people to change their diet!

Drying out

Draining naturally occurring wetlands is a simple way to decrease methane levels, but this can increase carbon dioxide emissions as well as having unwanted environmental effects when animal and plant habitats are changed.

Finding different ways to grow rice – such as dry rice farming – or developing more productive rice varieties could help reduce methane levels.

One way to prevent methane being made at landfill sites is to cover the site to stop rain penetrating into the ground. Australian scientists are now looking at reducing methane levels at rubbish tips by growing plants and trees on their surface. The idea is for the plants to take up the water that would otherwise seep down and promote the anaerobic decomposition of rubbish, which produces methane. This method – known as phytocapping – has been tested successfully by researchers at the Central Queensland University.

Making use of methane

Because methane can be used as a fuel, there are several possibilities for recycling the gas and using it as a source of energy. Even though this might produce some carbon dioxide, the overall contribution to climate change would be less than if the methane wasn’t used. For years, methane produced by decomposing matter at some rubbish tips or landfill sites has been tapped as a source of the gas.

Yet another way to reduce methane levels is to lower emissions from fossil fuel industries. For instance, methane is collected from some Australian coal mines for energy production. And the CSIRO and the Australian Government Department of Climate Change have been involved in a pilot project to use methane from a Chinese coal mine to help power a gas turbine – in effect, turning a waste gas into a fuel.

Importantly, governments are now recognising the role of methane as a greenhouse gas. Methane has been included in Australia’s draft emissions trading scheme, the Carbon Pollution Reduction Scheme and the European Union is also looking at including methane (and other greenhouse gases) in its Emissions Trading Scheme which up to now has only included carbon dioxide.

Related sites

• How kangaroo burgers could save the planet (New Scientist, 25 December 2008)
• Cooperation on capturing China’s mine methane emissions (Ecos, April-May 2006)
• Putting a green cap on garbage dumps (Science Daily, 3 December 2008)
• Methane (Greenhouse gas online, United Kingdom)
• Australia's policy response (Nova: Science in the news, Australian Academy of Science)

Activities

1. Warming Earth with the greenhouse effect: An experiment

2. Changing methane levels: A worksheet

Other Activities

• Energy4me (Society of Petroleum Engineers, USA)
  • It’s a gas – natural gas – students perform a simple experiment to produce methane from decaying plant and animal matter.
• Ocean explorer (National Oceanic and Atmospheric Administration, USA)
  • Burp under the ice – students produce a report on the potential role of Arctic methane deposits in climate change.
  • The big burp: Where’s the proof? – students present information in groups on four topics related to methane hydrates, global warming and events during Earth’s history.
  • It’s a gas! – students perform an experiment that simulates gas hydrate formation and answer a series of questions on the simulation. They then conduct a webquest on gas hydrates and the importance of understanding them.
  • Animals of the fire ice – students produce a group report on animals associated with methane hydrates. They then lead a discussion about the relationships between the animals and methane hydrates.
• The Keystone Centre (USA)
  
  • Methane mitigation – students create methane through a chemical reaction then observe the combustion of methane to demonstrate the potential of waste methane as an energy source.

Activity 1. Warming Earth with the greenhouse effect: An experiment

Do greenhouse gases really warm the Earth? Find out by measuring the greenhouse effect in a ‘mini-Earth’.

Materials

• the inside (sliding) part of 2 matchboxes
• cling film (plastic food wrap)
• 2 thermometers
• plasticine
• paper towel

Procedure

1. Use a pen to poke a hole through one end of a matchbox (this is your mini-Earth) just large enough to fit a thermometer through.
2. Slide the bulb of a thermometer into the box, making sure it doesn’t touch the surface of the box (you may need to seal the hole with plasticine).
3. Repeat with another matchbox and thermometer.
4. Cover the top of one box with a single layer of cling film.
5. Place both boxes in a warm place outside, on a windowsill or under a heat lamp.
6. Record the temperature in both boxes every 5 minutes for 25 minutes.

8. Graph your results and answer the following questions.
   
   
   1. What was the effect of covering the mini-Earth with cling film?
   2. What does the cling film represent in Earth's atmosphere?
   3. Design a controlled experiment to test the warming effect of one of the greenhouse gases (Note: you do not have to conduct your experiment).

Activity 2. Changing methane levels: A worksheet

1. Use the data in the table below to produce a line graph showing how the levels of methane in the atmosphere have changed since 1850.

2. Now plot the world population and temperature data on the same graph using the secondary Y-axis.

Global changes in atmospheric methane levels, world population and air temperature from 1850-2007

3. Describe how the concentration of methane in the atmosphere has changed over time.
   
   
   

4. Is there any relationship between the levels of methane in the atmosphere and the world population? Why do you think this has happened?
   
   
   

5. What developments occurred during the 19th century that could be responsible for increasing greenhouse gas emissions?
   
   
   

6. When was the most rapid rise in methane levels?
   
   
   

7. Does this correspond to the most rapid rise in temperature? Explain your answer
   
   
   

8. What other factors could be contributing to this rapid rise in the global temperature?
   
   
   

9. If we don’t do anything to limit human-induced greenhouse gas emissions, predict methane levels in 2025.
   
   
   

10. Find out what the term ‘positive feedback’ means in science and explain how this term could be used when describing future levels of methane.

Further reading

Australian Academy of Science
August 2010
The Science of Climate change: Questions and Answers.
A document summarising the current understanding of climate change science for non-specialist readers.

Cosmos
30 October 2008
Another potent greenhouse gas on the rise
Reports on an increase in global atmospheric methane levels in 2007.

Scientific American
5 February 2010
Defusing the methane greenhouse time bomb (by Christopher Mims)
Suggests that bacteria and the fresh-water cap on the Arctic Ocean may prevent large-scale release of methane into the atmosphere.

Useful sites

Climate change science question and answer (Australian Government Department of Climate Change)

Presents a series of questions and answers regarding climate change. Question 16 provides a brief summary of the contribution of methane to the enhanced greenhouse effect.
http://www.climatechange.gov.au/en/publications/science/climate-change-science-faq.aspx

One of a series of pages on the greenhouse gases and climate change. Provides a brief summary of the role of methane in the enhanced greenhouse effect.
http://www.bbc.co.uk/climate/evidence/methane.shtml

Links to sections of a report on methane as a greenhouse gas and strategies to reduce its impact. Chapter 2 covers the climate science of methane in detail.
http://www.eci.ox.ac.uk/research/energy/methaneuk.php

Covers the sources and sinks of methane in clear language. The human impact and potential for control of methane levels are considered in each case.
http://www.ghgonline.org/aboutmethane.htm

Contains a range of information on the science of methane, sources and emissions of methane.
http://www.epa.gov/methane/index.html

• Global methane levels move upward again (News in Science, 16 December 2008)
  Reports on an increase in methane levels after seven years of no growth.
  http://www.abc.net.au/science/articles/2008/12/16/2447483.htm

• Methane - The forgotten gas (Catalyst, 25 September 2008)
  Covers the role of methane as a greenhouse gas and Australian measurements of historical methane levels from rock and ice samples.
  http://www.abc.net.au/catalyst/stories/2373958.htm

• Animal farming and greenhouse gas emissions (Science Show, 9 August 2008)
  Reports on the contribution of animal farming to greenhouse emissions and how they could be reduced.
  http://www.abc.net.au/rn/scienceshow/stories/2008/2329287.htm

• Melting permafrost spews out more methane (News in Science, 7 September 2006)
  Reports on a study showing increasing emissions of methane from the thawing of Siberian lakes.
  http://www.abc.net.au/science/news/stories/2006/1735049.htm

• Methane eruption blamed for mass extinction (News in Science, 4 September 2003)
  Proposes that an explosive release of methane could have caused a mass extinction of species at the end of the Permian era.
  http://www.abc.net.au/science/news/ancient/AncientRepublish_938770.htm

Provides a reliable and concise summary of climate change science relevant to Australia. Includes information on the predicted effects of climate change on different regions of Australia.
http://www.climatechangeinaustralia.gov.au/

Glossary

accelerator mass spectrometry (AMS). A highly sensitive technique that is used to measure the relative amount of different ions (and often isotopes) in a sample. Accelerated ions are separated by their mass to charge ratio. AMS can be used as a form of carbon dating. By measuring the different isotopes of carbon in a sample, the age of the sample can be determined.

carbon dating. A method of measuring the age of an object that contains carbon. Since living things stop taking up carbon when they die, and carbon-14 decays with a half life of 5,700 years, the amount of carbon-14 remaining in the sample can be used to determine its age.

clathrate. A compound that is formed by the trapping of molecules inside a substance rather than by forming chemical bonds between the two substances. Methane clathrates (also called methane hydrates) consist of methane molecules trapped inside a ‘cage’ of ice crystals. Methane hydrates form under conditions of low temperature and high pressure in deep oceanic sediments, where methane is produced by anaerobic microorganisms.

enteric fermentation. Fermentation that occurs in the digestive system of ruminants such as sheep and cattle. As microbes in the ruminant gut break down food, methane is released. This methane is then released into the atmosphere by the animal, mostly through exhaling or burping.

global warming potential. The global warming strength of one kilogram of a greenhouse gas relative to one kilogram of carbon dioxide over a chosen time period. Global warming potential takes into account the variation in the time that different gases remain in the atmosphere and the ability to absorb radiation.

megatonne (Mt). A megatonne is one million (1,000,000 or 10⁶) tonnes.

methane hydrate. (also called methane clathrate). A substance in which methane is trapped inside a ‘cage’ of ice crystals. Methane hydrates form under conditions of low temperature and high pressure in deep oceanic sediments, where methane is produced by anaerobic microorganisms.

methanogens. Anaerobic microorganisms that use simple organic compounds for energy, producing methane as a by-product. They occur in a range of anaerobic environments including the digestive systems of animals (particularly ruminants and termites) as well as in wetlands, marine sediments and hot springs.

methanotrophs. Microorganisms that use methane for carbon and energy (and so remove methane from the environment). Methanotrophs occur in both aerobic and anaerobic conditions. They are mostly found in soils but also occur in a range of environments associated with methane such as landfill, marshes, rice paddies and the ocean.

permafrost. Permanently frozen ground, as occurs in the polar regions.

ruminant. An animal which chews regurgitated food (cud) and usually has a stomach divided into four compartments, one of which is the rumen. Partially digested food is returned from the stomach to the mouth and chewed, allowing further digestion. Ruminants include cows, sheep, goats and deer. Their digestive system allows them to digest fibrous plant material which would be indigestible to other animals.