Today we review research into methane emissions from cities which along with other greenhouse gases contributes to climate warming. Cities themselves account for 70% of GHG emissions globally. Unlike CO2 however, methane emissions are more easily managed at the city level whether they come from transportation and the increased shift to natural gas as a fuel for city vehicles or, secondarily, from landfills where methane is emitted from decomposing organic materials or, thirdly, from leaks in the systems delivering natural gas to users. One of the major problems is the lack of accurate inventories of methane emissions which in some cities results in an underestimate of 50%. Some efforts being made in the transportation sector to reduce CO2 emissions include shifts to the use of propane or natural gas but these may have unintended consequences in terms of their contribution as a radiatively active gas to the greenhouse effect. Landfill emissions may be reduced by simply reducing the amount of waste generated though pricing of garbage or encouraging home composting.
Key Quotes:
“Methane differs from CO2 in that mitigation is technologically and economically feasible… Unlike CO2, a large fraction of methane is lost as fugitive emissions from engineered systems, such as leaks from natural gas pipelines.”
“some strategies to reduce CO2 emissions, such as substituting natural gas for other fossil fuels such as coal and diesel, may have the unintended consequence of increasing radiative forcing by increasing fugitive methane emissions.. methane emission rates are currently underestimated in greenhouse gas inventories … and thus it is unclear if switching to methane-based fuels provides a net benefit for climate mitigation.”
“the most important sectors for urban methane emissions are energy, waste, agriculture, and transportation, respectively …Energy and transportation primarily emit fossil methane derived from natural gas, whereas waste treatment and agriculture produce biogenic methane from the process of anaerobic decomposition”
“Natural gas vehicle use has grown rapidly over the past decade, and will continue to grow globally, particularly in developing countries in South Asia and Latin America …In the United States, use of natural gas as a transportation fuel is growing most rapidly for heavy duty and mass transit vehicles”
“Methane production can be prevented by reducing the amount of waste that ends up in landfills— cities have implemented this approach with pay-as-you-throw pricing and diversion of organic waste to alternative treatment such as composting”
“Methane from wastewater is the fastest growing emission source outside of fossil fuels, expected to increase by 19% over the next two decades as population grows, particularly in developing economies”
“recent studies have used CO and CO2 inventories to quantify methane emissions in Los Angeles, revealing emissions up to 50% larger than inventory estimates”
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