We often treat “greenhouse gases” and “carbon dioxide” as being almost synonymous. But this isn’t necessarily true, as previously mentioned, water vapour plays a significant part in the greenhouse gas system. But another big player is methane, it has the second largest radative force of any anthropogenic greenhouse gas.(National Oceanic and Atmsopheric Admistartion of the US Depratment of Commerce, Date Unknown)
Methane, alike carbon dioxide, has many sources, some anthropogenic and some natural. Man-made methane contributes around 70% of emissions. (NASA, Date Unknown) Whilst cattle are one of the largest anthropogenic contributors of methane, a common misconception is that it is caused by their manure and flatulence, they in fact burp 280 litres of methane in to the atmosphere each day – enough to fill 140 2 litre lemonade bottles. (Riverdeep.net, 2002) This is due to the microbes in the stomach of the cattle, which break down the cellulose in the cattle that break down the plant matter that they ingest, which produce methane. (NASA) The quantity released however brings great levels of uncertainty, for it is very difficult to trace the amount of methane a cow releases, any attempts to measure the amounts release would undoubtedly stress the animal and change the levels which are released. (Riverdeep.net) Cattle are the largest contributor of all domestic animals at 75%, however the remaining methane is from horses, camels, pigs, sheep, goats and buffalo. Overall domestic animals contribute 80 Tg of methane per annum and around half the emissions come from China, India, Brazil and the former USSR. (NASA)
The link between fossil fuels and carbon dioxide emissions is relatively well known, however its link with methane is not so. Natural gas is even almost completely methane and amounts are lost in the extraction or during distribution, where it is often even vented. Many oil fields also contain reserves of methane, if it is not economical to capture the gas for commercial usage it is often simply vented, or even burnt, which would contribute to methane and carbon dioxide levels respectively. This is an interesting feedback system as once more we see contributing factors to climate interacting. From 1983 to 1994, 3 to 4 trillion cubic feet of gas were disposed of in this manner. (Delucchi, 2003) As reserves of natural gas and oil decline, it is expected that conservation will become more economical as prices rise. Furthermore, I suspect this decrease in emissions to be compounded by the natural decrease in extraction anyway. However, the data is collected by the oil and gas companies and is a major source of uncertainty.
Rice cultivation is a further anthropogenic source, it is suspected to account for around 10% of emissions, higher yielding crops and greater production have combined to increase production by over 40%. 90% of emissions from rice paddies are from Asia.(NASA) Similarly to methane from cattle, emissions are from the decomposition of organic matter by bacteria. Factors however are very variable, yield can be affected by demand, weather and the economy. It is similarly very hard to predict as climate and the design of the landfill will effect methane output. Any bacterial decomposition of organic materials will burn methane, a process that is practised in compositing bins in many peoples gardens when not done correctly with sufficient ventilation. (Telegraph, 2007) The burning of biomass also releases methane, but similarly is unpredictable due to varying factors such as temperature, moisture and type of vegetation.
There are also many natural feedback systems, which increase dependant on the climate. The permafrost melt is often one of the most cited, methane is stored in the ice in the Arctic and Siberia, again from the decomposition of organic matter millions of years ago, when it was once wetlands. (Lemonick, Time Magazine, 2010) It is estimated that twice the amount of atmospheric methane is locked up in the permafrost (Permanently frozen subsoil – [dictionary.com]). An example of the sheer quantity of methane can often be demonstrated by piercing a hole in iced over lakes in areas of permafrost, vegetation has decomposed at the bottom of the lake producing large pockets of methane beneath the ice, holding a match near it produces a spectacular bunsen burner like flame. See figure 1 for a demonstration. A more long term fear is the methane frozen in ice under high pressure at the bottom of the ocean, known as a gas hydrate, if this were to warm and melt, this would have potentially catastrophic effects. The Arctic sea is the first indicator of this happening. It is normally covered in ice, as this melts we are seeing increasing amounts of methane being emitted. (Miller, 2009) The potential of these hydrates is still relatively unknown, but is considered a long term factor.
Figure 1 – Methane In Lakes (Youtube)
The largest methane source is the wetlands at 22% of emissions, which is around 100Tg annually through the same process of decomposition. (NASA) It is suggested that the warming of the planet would lead to increased production and hence expand this already large source. The effect of increased soil moisture is also highly uncertain at this stage (Lashof, 1997)
Termites are also a surprisingly large source at 4%, operating in the same manner as cattle as for production, as per common knowledge, they do ingest a lot of organic material. (NASA) But their levels are relatively steady.
It is important to note the radiative forcing of greenhouse gases, their prevalence alone is not enough to explain their influence on temperature. The radiative forcing of a gas is the the change between incoming irridance from outgoing irridance at the boundary between the troposphere and the stratoshere, else known as the tropopause. The troposphere and the stratosphere are atmospheric layers, the troposhere is the lowest part of the atmsophere, which extends to around 10km up, where the tropopause is, above which is the stratosphere which extends up to 50km altitude. (IPCC Glossary, Date Unknown)
The radiative force of methane is is 0.48 W/m2 compared to 1.66 W/m2 for Carbon Dioxide. This is not the whole picture however, its effects on climate are not solely dependant on its radiative forcing, the lifetime of a gas and its concentration within the atmosphere also play a role. (IPCC, 2005) The warming potential of Methane is 25 times more that of carbon dioxide when measured ton-for-ton as it is much more effective at absorbing infra-red (heat) radiation. (David Reay, 2010)
|Gas||Radiative Forcing||Atmospheric Lifetime||Global Waming Potential (GWP)|
|Carbon Dioxide||1.66 W/m²||50-200||1|
Figure 1: Comparative GWPs of Methane and Carbon Dioxide, Calculations taken over 100 years (Ewings, 2007), (IPCC Fourth Assessment)
The “Global Warming Potential” is a comparative index of all greenhouse gases. The baseline comparison is to Carbon Dioxide which has a GWP of 1. Over a 100 year period the GWP of methane is 25, meaning it is 25 times more significant than Carbon Dioxide. However, if we recalculate its global warming potential over 200 years for instance, it will be significantly less, whilst Carbon Dioxide is more likely to stay just as prominent, as it has a longer atmospheric lifetime. (IPCC Fourth Assessment)
But according to Drew Shindell of NASA, that isn’t the full story. The above statistics were taken by the IPCC, Shindell states that they are simply measurements of the gases in the Earth’s atmosphere and that due to chemical changes in the atmosphere it doesn’t give a realistic picture of the effects of each gas. Methane is in fact affected by other pollutants in the atmosphere and so changes its chemistry so this alters the greenhouse effect. Furthermore, the mixing of gases makes it hard for them to be singled out, so readings are unreliable. In a nutshell, this means that the IPCC report effectively looks at only a portion of what happens to the methane that is released, some of it forms other substances and some even induces smog (see aerosols and climate) and this feedback system is hence completely ignored. The IPCC state that methane contributes a 1/6 to the warming that we are experiencing, but new calculations suggest that it is realistically double that. (Ramanujan, 2007)