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What are the main sources of methane emissions?
There are both natural and human sources of methane emissions. The main natural sources include wetlands, termites and the oceans. Natural sources create 36% of methane emissions. Important human sources come from landfills, livestock farming, as well as the production, transportation and use of fossil fuels. Human-related sources create the majority of methane emissions, accounting for 64% of the total.
Methane levels have more than doubled over the last 150 years because of human activities like fossil fuel use and intensive farming. Before the Industrial Revolution, the atmospheric concentration of methane was maintained in a safe range by natural sinks.
But for a long time now human activities have been creating methane emissions much more rapidly than the Earth can remove them, increasing global methane levels. During the last 800,000 years, methane concentrations naturally varied between 400-700 ppb but since the Industrial Revolution, methane levels have become much higher and are now 2.5 times larger.
Since the Industrial Revolution, human sources of methane emissions have been growing. Human activities such as fossil fuel production and intensive livestock farming are the primary cause of the increased methane concentrations in the atmosphere. Together these two sources are responsible for 60% of all human methane emissions. Other sources include landfills and waste (16%), biomass burning (11%), rice agriculture (9%) as well as biofuels (4%).
Figure 1:Source: Bousquet, P. et al. (2006). Contribution of anthropogenic and natural sources to atmospheric methane variability.
Fossil fuel production, distribution and use
The largest human source of methane emissions is from the production, distribution and combustion of fossil fuels. This is responsible for 33% of human methane emissions.
Methane emissions are produced wherever there are fossil fuels. It is released whenever fossil fuels are extracted from the earth whether it is natural gas (which is mostly methane), coal or petroleum. There are additional methane emission that are created during any type of handling, transportation (pipeline, truck delivery, etc.) or refinement of fossil fuels. Finally some methane is also produced during fossil fuel combustion.
By simply buying and using any fossil fuel whether it is coal, natural gas or petroleum you contribute to the most important source of methane emissions worldwide. Fossil fuel production, distribution and use creates 110 million tonnes of methane annually.
A large portion of methane emissions are caused by the extraction, processing and transportation of natural gas. Methane is the main component of natural gas, so leakage throughout the different levels of this industry releases methane directly into the atmosphere.
Coal is another important source of methane emissions. During the geological process of coal formation, pockets of methane get trapped around and within the rock. Coal mining related activities (extraction, crushing, distribution, etc.) release some of this trapped methane. Methane is emitted from active underground and surface mines as well as abandoned ones.
As with coal, the geological formation of oil can also create large methane deposits that get released during drilling and extraction. The production, refinement, transportation and storage of oil is also a source of methane emissions.
Incomplete combustion of fossil fuels also produces methane emissions. No combustion process is one hundred percent efficient, so when fossil fuels are used to create electricity, heat or power vehicles these all contribute as sources.
An important source of methane emissions is from enteric fermentation in farm animals. This is responsible for 27% of human methane emissions. Animals like cows, sheep and goats are examples of ruminant animals and during their normal digestion process they create large amounts of methane. Enteric fermentation occurs because of microorganisms in the stomach of these animals, which creates methane as a by-product that is either exhaled by the animal or released via flatus.
Because humans raise these animals for food, their emissions are considered human-related. The meat that we eat everyday contributes significantly to total methane emissions because of this. Livestock farming creates 90 million tonnes of methane annually.
Livestock related emissions has grown because of two main reasons. First the large growth of livestock populations worldwide over the last fifty years. Global livestock production has increased substantially since the 1960s with beef production more than doubling during this time.
Second, changes in natural feeding habits have largely affected farm animal emissions. In order to reduce the amount of feed needed to support livestock, humans add more nitrogen to cattle feed to make them feel more full. But increases in nitrogen help not only the cattle, but also the microbes in their stomachs which ends up creating higher methane emissions rates.
Landfills and waste
Another important human source of methane emissions is from landfills and waste. Methane is generated by the decomposition of biodegradable solid waste in landfills as well as animal and human waste streams. This accounts for 16% of human methane emissions. Landfills and waste produces 55 million tonnes of methane annually.
Landfills and open garbage dumps are full of organic matter from our garbage (things like food scraps, newspapers, cut grass and leaves). Every time new garbage comes in it is pilled over the old garbage that was already there. The organic matter in our garbage gets trapped in anaerobic conditions where there is no oxygen. This provides excellent conditions for methane producing microbes to break down the waste, which produces large amounts of methane emissions. Even after a landfill is closed, the bacteria will continue to decompose the buried waste and emit methane for years.
Wastewater from domestic, municipal and industrial sources can also produce methane emissions. Wastewater can be either released, stored or sent for treatment to remove contaminants. As with landfills, if at any of these steps the decay of organic material in wastewater happens in an anaerobic environment then this will create methane.
Livestock farming at even a modest scale will have to deal with the manure that gets produced by farm animals everyday. Livestock manure management is usually done by using large waste treatment systems and holding tanks. In many of these systems methane is produced because they promote anaerobic conditions.
Biomass burning causes a substantial amount of methane emissions. Biomass is material from living or dead organic matter. Incomplete burning of biomass creates methane emissions and huge amounts can be produced during large scale burning. This is responsible for 11% of human methane emissions.
Large open fires are mainly used by humans to destroy crop waste and clear land for agricultural or other uses. While natural wildfires can contribute to this, the great majority of biomass burning is caused by human beings. Biomass burning creates 38 million tonnes of methane annually.
Another substantial human source of methane emissions is from rice agriculture. Paddy fields for rice production are essentially man-made wetlands. They are characterized by high moisture content, oxygen depletion and ample organic material. This creates a great environment for methane producing microbes that decompose the organic matter.
Though some of the methane produced is absorbed by methane-consuming microorganisms, the vast majority is released into the atmosphere. Due to the swamp-like environment of rice fields, this crop is responsible for 9% of human methane emissions. Rice agriculture creates 31 million tonnes of methane annually.
Annually 12 million tonnes of methane are produced by biofuels making it a significant source. Biomass that is used to produce energy for domestic, industrial or transportation purposes are called biofuels. Incomplete biofuel combustion leads to the production of methane. This is responsible for 4% of human methane emissions.
An estimated 80% of biofuels are used for domestic cooking, heating, and lighting mostly in open cooking fires burning wood, agricultural waste, or animal dung. This is the single largest contributor to global biofuel emissions. Almost half of the world's population, about 2.7 billion people, use solid biofuels to cook and heat their homes on a daily basis. Most are poor, and live in developing countries.
18% of biofuels are used by low-technology, largely unregulated, micro-enterprises such as brick or tile making kilns, restaurants, etc. The balance of the biofuels are consumed for transportation uses.
Apart from being created by human activities, methane is also released into the atmosphere by natural processes. Wetlands, termites and the oceans are all natural sources of methane emissions.
The amount of methane produced by natural sources is completely offset by natural methane sinks and has been for thousands of years. Before the influence of humans, methane levels were quite steady because of this natural balance. Today, human-related sources create the majority of total methane emissions which has upset the natural balance that existed before the Industrial Revolution.
Wetlands are an important source of methane, accounting for 78% of all naturally produced emissions. Other natural methane sources include termites (12%) and the oceans (10%).
Figure 2:Source: Bousquet, P. et al. (2006). Contribution of anthropogenic and natural sources to atmospheric methane variability.
Wetlands are the largest natural source of methane. This produces 78% of natural methane emissions. The water-logged conditions of wetlands are perfect for microbes who require environments with no oxygen and abundant organic matter.
While part of wetland related emissions is absorbed by methane-consuming microbes, a large percentage escapes into the atmosphere. Wetlands create 147 million tonnes of methane each year.
Termites are a significant natural source of methane. During the normal digestion process of a termite, methane is produced. Termites eat cellulose but rely on micro-organisms in their gut to digest it which produces methane during the process. This is responsible for 12% of natural methane emissions.
Each termite produces very small amounts of methane on a daily basis. However, when this is multiplied by the world population of termites, their emissions add up to 23 million tonnes of methane annually.
Another significant natural source of methane comes from the oceans. Methane producing microbes living in the ocean create these emissions. This creates 10% of natural methane emissions. Globally, oceans create 19 million tonnes of methane annually.
The majority of oceanic methane emissions gets produced in deeper sediment layers of productive coastal areas. This accounts for 75% of the oceans methane emissions. The methane created by these microbes mixes with the surrounding water and is emitted to the atmosphere from the ocean surface.