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%).
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.
Livestock farming
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 have grown mainly because of 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.
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
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.
Rice agriculture
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.
Biofuels
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.
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