This chapter discusses anthropogenic greenhouse gases and climate forcing. It is important to note that carbon dioxide and other emitted gases mix with the atmosphere and have an effect on global atmospheric concentrations in a uniform way. Methane is emitted mainly from agriculture and landfills, or as escaped gas from natural gas pipelines’ nitrous oxide comes from fertilizer use and other industrial processes; HFCs and other industrial pollutants are used in such limited amounts that they’re still limited parts of the overall anthropogenic process. All of the six anthropogenic greenhouse gases are scored relative to carbon dioxide’s warming potential, which accounts for most of the human-induced greenhouse gas emissions. It is also important to recognize that the lifetime expectancy of one molecule of gas in the atmosphere before being reabsorbed is different for each gas, and is very long, up to thousands of years, for carbon dioxide, while it is relatively short for methane. Feedbacks, such as the ozone layer depletion and the albedo effect, and radiative forcing, also warm the planet, although not as important as carbon dioxide. Aerosols are particulate matter in the atmosphere that are disastrous for human health, but have a net cooling effect, and therefore some scientists have proposed ideas of geoengineering using aerosols to cool the climate. The total amount of emissions is rising because as the world economy is growing, more fossil fuels are used and CO2 equivalents are put into the atmosphere year after year. Methane emissions come from “fugitive gas” that is released as it is drilled for in pipelines, ruminant animals such as cows, and anaerobic respiration in rice paddies. N20 is a side effect of combustion processes and industrial activities as well as chemical changes to nitrogen based fertilizers. These greenhouse gases can be allocated to direct actions by various sectors and indirect actions from electricity generation. Agriculture, or AFOLU, meaning agriculture, forestry, and other land use, emits a lot of greenhouse gases, not only carbon dioxide. The transport and industrial sectors play major emissions roles as well. Professor Sachs shows a diagram to track where emissions come from. Two-thirds of the total radiative forcing of anthropogenic greenhouse gases is carbon dioxide from the energy sector. This video is part of the module The Basics of Climate Change Science.

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