Methane is a naturally occurring chemical compound comprising one carbon atom bonded to four hydrogen atoms (CH4). Methane is also a greenhouse gas, which means that it absorbs radiation and warms the atmosphere. Research resoundingly shows that human activities have been releasing more methane into the atmosphere since pre-industrial times, with some estimates suggesting that human-caused methane emissions are responsible for 25% of today’s global warming.
Methane is 84 times more powerful in warming the atmosphere over a decadal timescale than an equivalent amount of carbon dioxide. As such, methane is often referred to as a short-lived climate pollutant and has become a key focus of international commitments and initiatives to limit global warming near-term. The main sectors that contribute the most to human-caused methane emissions are agriculture, energy, and waste. Reducing methane emissions from these sectors will be key to meet Canada’s Global Methane Pledge and net-zero commitment.
Cities are important sources of methane emissions, with some estimates suggesting that 21% of total human-caused methane emissions globally are from cities. Energy and waste sectors are likely responsible for the bulk of the methane emissions from cities. However, there is growing evidence from research that suggests current methods under-estimate methane emissions from cities. This represents a knowledge gap between what is measured and what is reported. To address this gap, the CURMET project will measure methane emissions from a broad range of sources and develop a new inventory-based method to report and monitor methane emissions from cities.
There are two main categories of methane emissions sources: biogenic (produced by living organisms) and thermogenic (produced by heat). Biogenic methane is produced by microorganisms called methanogens. Methanogenesis – the production of methane from methanogens – occurs in anaerobic conditions (i.e., in the absence of oxygen). Several major biogenic sources exist in cities: wetlands, landfills, and wastewater. Thermogenic methane is typically associate with geologic deposits of fossil organic material subjected to intense heating. The process, called catagenesis, cracks complex organic molecules to form hydrocarbons.
Cities have natural and human-caused sources of biogenic methane. Wetlands are the primary natural source in cities where they exist. Human-caused biogenic sources included spatially distributed sources like sanitary sewers and wastewater pumping (lift) stations, and large facility-scale sources like wastewater treatment plants and landfills (both active and closed). Landfills are understood to be major component of city-scale methane emissions, and in some cases those emissions can be seen from space.
Typically, thermogenic sources of methane in cities are associated with the natural gas distribution system, including residential and industrial end uses from combustion. It is important to consider that the primary use of natural gas in cities is as a source of energy for electricity and heating, so methane emissions from this infrastructure represents a direct loss of energy and an associated economic loss for gas suppliers and consumers.
As the primary component of natural gas, methane can escape from the distribution system through leaks, vents, and incomplete combustion. Leaks can be detected by the odor of an additive called mercaptan, which smells like rotten cabbage. Leaks can occur anywhere across the distribution system between the city gate and end-use consumption. In rare cases, natural gas leaks have been responsible for major accidents. Vented methane emissions occur from components that are designed to vent, like pneumatic components at meter stations. Incomplete combustion occurs anywhere natural gas is burned. A proportion of the methane used in equipment and appliances escapes in the flue (exhaust) gas.
First, you can make a difference in reducing methane emissions from cities. While the total volume of methane reduced may not appear very large on an individual basis, the impact adds up quickly as participation in methane-reducing actions grows. Here are some of the ways you can contribute to methane reductions.
Food waste: Anything you can do to reduce food waste can help reduce methane emissions. It is estimated that more than half of the methane emissions from landfills is from food waste. Reducing food waste has a corollary effect on household finances.
Composting: Diverting food and other organic waste to a compost facility (or to a household composter) instead of the landfill reduces methane emissions. The reason is that the waste that goes to the landfill is buried and compacted, which cuts off the supply of oxygen. Methanogens thrive in environments without oxygen. At compost facilities, the waste is aerated, which means it is exposed to oxygen, so less methane is produced.
Natural gas: Combustion-based natural gas furnaces and appliances can vary in efficiency. Small gas leaks from furnaces and other appliances contribute an estimated 9100 tonnes of methane emissions annually in the United States. Regular servicing could reduce some of those emissions by improving combustion efficiency.
Check your meter: Natural gas meters are pressurized assemblies that regulate gas pressure and track gas use. There are typically two main components: a regulator and the meter. The regulator lowers the gas pressure before reaching the meter. The meter, as the name suggests, measures the gas use. Because the assembly is pressurized, it has potential to leak. If you smell rotten cabbage near your meter, chances are that there is a leak. That smell is from an additive to the gas (typically methyl mercaptan) that helps detect leaks.