Anthropogenic sources

Anthropogenic methane sources account for over half of the total global emissions.

Uncertainties are smaller than for natural emissions and there is better agreement between top-down and bottom-up inventory techniques (Table 1.1).

1.4 Methane sources

1.4.2.1 Rice agriculture

The cultivation of rice is a signicant contributor to anthropogenic emissions of methane. Accurate emission values can be dicult to obtain due to a lack of experimental data in many of the countries producing the rice; however the IPCC AR5 report estimates 31 - 83 Tg yr^-1, which is roughly 11 % of global methane emissions (Bustamante et al., 2014). Around 90 % of rice paddies are grown in Asian monsoon countries where they provide a staple food resource for two thirds of the inhabitants there. Future emissions of methane are predicted to increase to help meet the demand for food of an increasing global population. Rice agriculture also produces methane via methanogenesis. The quantity produced is dependent on the type of paddy eld and the cultivation techniques used.

These techniques are classied according to their water regimes and summarised in Table 1.2 (Wassmann et al., 2000).

Table 1.2: Dierent agricultural cultivation systems. Methane emissions are thought to decrease in magnitude down the table (Reay et al., 2010).

Water Regime Flooded

Continuously Intermittently Irrigated

Single Aeration Multiple Aerations Deepwater

Rain-Fed Regular

Drought Prone Upland

Other factors that can aect methane production from rice agriculture include the amount of fertilizer or manure used, aeration periods and the recycling of organic waste products. In general, low methane emitting paddy elds normally produce low crop yields (Wassmann et al., 2000).

1.4.2.2 Biomass burning

Biomass burning is the deliberate burning of dry vegetation to improve agricultural productivity (Crutzen and Andreae, 1990), or the burning of

Chapter 1 Introduction to methane and the atmosphere

agricultural waste and wood fuel; both produce similar emissions. Methane is emitted as a result of incomplete combustion, which occurs when there is an insucient amount of oxygen present, or burning occurs at too low a temperature to produce purely carbon dioxide and water (Reay et al., 2010). Many compounds released during biomass burning have impacts on the climate. Carbon monoxide and methane will aect the oxidation of the atmosphere due to their reactivity with the OH radical. Nitrous oxide and many non-methane hydrocarbons will increase the concentration of ozone. Finally, particulate matter emitted can act as cloud condensation nuclei causing possible changes to the hydrological cycle and the atmosphere's radiation budget (Crutzen and Andreae, 1990).

It is thought that up to 90 % of biomass burning occurs in tropical regions and current estimated global methane emissions are around 35 [32-39] Tg yr^-1(roughly 5 % of total methane emissions, Kirschke et al. 2013). Future climates are expected to rise in temperature, which will have varying eects on the emissions from biomass burning. In some areas this will lead to an increase in the amount and size of res (this applies to wildres as well as biomass burning) thus causing an increase in methane emissions into the atmosphere. In other areas, soils will become more arid, meaning less vegetation will be produced and thus fewer res will occur; reducing biomass burning emissions (Reay et al., 2010).

1.4.2.3 Ruminants

The farming of ruminants (cattle, sheep, goats and deer) for both meat and dairy products is the largest anthropogenic source of methane emitted into the atmosphere (Reay et al., 2010). Ruminants cannot digest cellulose themselves, so they have developed a symbiotic relationship with methanogens. These microbes break down the cellulose to produce carbohydrates that both the microbial community and the ruminants use as energy. Methane is produced as a by-product of this process. The vast majority of methane is emitted orally by the ruminant (92 - 98 %, Grainger et al., 2007). Total annual emissions of methane by ruminants are estimated at 76 - 92 Tg yr^-1 (Stocker et al., 2013b).

These estimates are based on Tier 1 and 2 calculations (for more information on this see Section 1.6).

Table 1.3 shows several suggested methods of methane reduction in ruminants.

These have been classied into short term (available now), medium term (available in ten years) and long term (not commercially available for at least another ten years). Many of these suggestions have been disputed as they are

1.4 Methane sources

not economically viable, especially in developing nations. It has also been argued that with global populations expected to continue to rise the demand for meat will also increase. It is therefore considered that animal numbers will increase to meet this demand, along with methane emissions.

Table 1.3: Suggested methods for a reduction in methane emissions from ruminants.

Denitions are short term (available now), medium term (available in ten years) and long term (not commercially available for at least another ten years).

Short Term Medium Term Long Term

Reduce animal

numbers Rumen Modiers Breed animals with low CH₄ yield

Increase productivity per

animal

Select plants that produce lower CH₄ yield by the animals

Targeted manipulation of rumen ecosystem

Manipulate diet Rumen Modiers

1.4.2.4 Fossil fuels

The fossil fuel industry (natural gas, oil and coal) is estimated to emit 96 [85-105] Tg yr^-1 of methane into the atmosphere (Table 1.1, Kirschke et al., 2013). Natural gas is composed of over 90 % methane (Wuebbles and Hayhoe, 2002). The majority of this is burned for energy production, but losses into the atmosphere can occur during extraction, handling, transport and combustion processes. These emissions are expected to rise 54 % between 2005 and 2020 (US EPA, 2006), mainly due to the economic growth of countries such as Brazil and China. Oil only contains trace quantities of methane, but methane gas deposits can be found in situ when drilling for oil. Therefore 97 % of all methane emitted from the oil industry comes from the oil elds. This too is expected to rise by

~100 % between 2005 and 2020 (US EPA, 2006). Mitigation techniques have been developed by capturing the methane emitted in the drilling process and injecting it back underground. This reduces methane emissions and aids further oil extraction. Ventilated methane can also be ared to convert it into carbon dioxide, thus reducing its radiative forcing.

The coal mining industry emits 30 - 46 Tg yr^-1 of methane, which can be found in the seams of coal deposits and within the pores of the coal itself. The deeper

Chapter 1 Introduction to methane and the atmosphere

the coal deposit the higher the carbon content and the greater the amount of methane produced and stored. A surface mine is thought to emit 0.3 - 2 m³ of methane per tonne of coal, whereas an underground mine can emit up to 10 - 25 m³ of methane per tonne of coal (IPCC, 1996).

1.4.2.5 Landlls, waste and manure

Landll and waste contribute between 35 - 69 Tg yr^-1 of methane to the atmosphere (Kirschke et al., 2013). Methane is produced as a by-product of microbial processes occurring in organic matter within the waste. Recently a decrease in emissions has been observed within developed countries due to landll gas recovery systems being installed in many landlls and waste treatment systems (US EPA, 2008, DEFRA, 2012). 98 % of landll emissions are made up of methane and carbon dioxide at a 60:40 ratio. The remaining trace gases include ammonia and nitrogen (Hegde et al., 2003). Landll emissions of methane are positively correlated with temperature although this is dependent on the depth of the landll. Deeper landlls become insulated and a stabilising temperature between 25 - 45 °C can be reached. Bacterial activity decreases below 10 °C (ATSDR, 2001).

Landlls continue to produce methane for a number of years after dumping.

This period varies for individual landlls. Einola et al. (2007) recorded emissions peaking between 2-3 years post dumping, with emissions after 5 years being 0.63

% of maximum recordings. Other landlls have been recorded to have longer methane emission periods. ATSDR (2001) states the majority of methane gas is emitted within 20 years of waste being dumped although smaller emissions may be recorded for 50 years or more. Future methane emissions from landlls are predicted to rise due to developing nations producing more waste (Reay et al., 2010).

Manure waste usually either remains on the farm where it is used as fertilizer or transported into storage. Waste water is either dumped into coastal waters or taken to be anaerobically treated. Anaerobic treatment is becoming more desirable, especially in developing nations, to actually enhance the production of methane for energy use. As long as the methane produced is not lost the overall emissions of methane are reduced and a new source of renewable energy is produced (Reay et al., 2010). Economic factors and the possibility of new policies will be key determinants for the future emissions of methane from landlls and other waste sources.