Methane CH4:
Methane (CH4) is the second-most abundant greenhouse gas in the earth’s atmosphere. Colorless, and odorless, CH4 is about 28 times more potent than carbon dioxide over a 100-year timeframe. CH4 is found in the earth’s crust, oceans, and atmosphere. As a hydrocarbon, CH4 is composed of one carbon atom bonded to 4 hydrogen atoms. This Molecular structure allows it to capture and release Infrared energy to a higher degree than CO2. The most popular industrial uses for CH4 are for heating and producing electricity.
CH4 gas comes from a variety of natural sources on earth, however; CH4 has been parabolically growing for the past 200 years. This rapid growth can be attributed to human-related development since the industrial revolution of the 1800s. Approximately 60% of all methane emissions are directly produced through anthropogenic means.
Read on for more details on industrial and natural sources of methane. Concerning methane’s nutrient cycle, CH4 sinks will be discussed as well.
Anthropogenic Sources of Methane
- Fossil Fuel Combustion
- Landfills
- Agriculture
- Water Treatment
- Livestock
Fossil Fuel Combustion
Once natural gas is extracted from earth it must be purified into a useable form that can be bought, sold, and shipped relatively safely. Fossil fuel facilities that extract natural gas are currently not great at gas containment, and leeks that plume CH4 into the air are common.
As an energy source, methane tends to generate less carbon per unit of heat than other hydrocarbons, though there are still emissions associated with burning methane. Burning methane for electricity and heat requires a combustion reaction between one methane molecule and 2 oxygen molecules. The byproducts of this reaction are CO2 and H2O water vapor.
According to the 2013 IPCC Report, fossil fuels contributed approximately 85-100 Tg of methane to the atmosphere.
Landfills
Landfills are the places where waste is discarded indiscriminately. Everything from food waste, to lawn clippings, to demolition materials might eventually find its way into a landfill. Landfills are often so tightly packed into the earth that it deprives the subterranean environment of oxygen, creating an anaerobic environment. Methane is produced when organic material breaks down in an anaerobic environment or without oxygen.
landfills released an estimated 114.5 million metric tons of carbon dioxide equivalent (MMTCO2e) of methane into the atmosphere in 2019; this represents 17.4% of the total U.S. anthropogenic methane emissions across all sectors.
Agriculture
From agriculture, CH4 emissions come from a variety of places. The obvious source is through the machines used to tend to the land and harvest crops. Those are considered indirect scope 2 emissions that are produced off sight. CH4 emissions that directly come from agriculture are often produced by waterlogged crops like rice patties.
U.S. agriculture emitted an estimated 698 million metric tons of carbon dioxide equivalent in 2018: 12.3% as carbon dioxide, 36.2% as methane, and 51.4% as nitrous oxide
Water Treatment
Similar to landfills, water treatment produces methane through anaerobic decomposition. Wastewater treatment plants use anaerobic digestion to break down sludge and other organic solids from wastewater. The byproduct of such digestion produces methane which might leak from the facility. More modern water treatment plants may reuse CH4 from the digestion phase to supplement the facility’s daily electrical needs.
Globally, methane from wastewater contributed an estimated 512 MMTCO2E of methane emissions in 2010, accounting for approximately 7% of total global methane emissions.
Livestock
Livestock animals, specifically cows, are responsible for about 14.5% of all emissions related to human activities. Cows naturally produce methane as a byproduct of their digestive process. Cows tend to partially digest their food, through a gauntlet of 4 stomaches. Each stomach is filled with bacteria that digest whatever the cow eats, producing methane along the way. Studies vary on how much methane each cow may produce, but they consistently show that cows produce more methane through belching than through their flatulence. Still, the sheer amount of methane they produce per day is comparable to the CO2 output of a car.
. Livestock contributions are placed between 85-95 Tg (1 Tg = 1 trillion metric tons) each year.
Natural sources of methane
- Wetlands
- Termites
- Oceans
- Methane Hydrates
Wetlands
Wetlands, including bogs, and marshes are the most abundant natural sources of CH4. In general, wetlands are flooded areas where methane-producing bacteria thrive. The waters that cover the wetlands tend to be low in dissolved oxygen, creating the perfect environment for anaerobic decomposition to occur beneath the surface. Wetlands make up about 80% of all naturally produced methane.
Termites
Termite, much like cows, carry methane-producing bacteria in their gut that aid in their digestion process. The amount of methane produced by termites varies considerably between species. Across the board, termites emit about 9% of all naturally sourced methane.
Oceans
The oceans generate about 8% of all naturally sourced methane. This output is spread across a variety of sources ranging from anaerobic digestion from zooplankton to volcanic vents where methane bubbles up from the seafloor.
Methane Hydrates
Methane hydrates are deposits of methane gas molecules that are trapped in ice and permafrost deposits around the world. A rough estimate of 5% of naturally sourced methane comes from methane hydrates, though considering how these frozen methane deposited have accumulated over hundreds of thousands of years, it is more likely that this is a server underestimation. Warming temperatures also threaten the stability of these frozen deposits and subterranean methane will likely become airborne greenhouse gas as global temperatures rise.
Methane Sinks
Like most naturally occurring chemicals, CH4 is part of a cycle with its own fluxes and sinks. While more research is needed on exactly where gaseous CH4 tends to pool, the most common Methane sinks are as follows:
- Methanotrophes
- Hydroxyl Radicals
Methanotrophes
Methaneotrophes refer to microorganisms that live in dryer layers of soil and above swampy soils that are saturated with water. There are also extremophile methanotrophs that thrive near deep oceanic vents where hot methane bubbles erupt from the earth. The name, “methanotroph”, refers to their food source, methane, which they capture and consume to sustain their metabolic and replication processes. In short, Methanotrophes consume methane and generate biomass.
Hydroxyl Radicals
Hydroxyl Radicals are negatively charged OH molecules and are the primary compound that naturally removes CH4 in the stratosphere. This is done through an oxidation reaction between OH and CH4 molecules. After a series of these reactions, CH4 is broken down into CO2. This scrubbing of CH4 from the atmosphere is why some might refer to hydroxyl radicals as the atmosphere’s “clean up crew”.