The second most prevalent gas causing anthropogenic climate change is methane. The natural processes of methane production in wetlands are not considered anthropogenic (human-caused). Anthropogenic methane accounts for 10% of the anthropogenic GHGs in the US and 16% globally. Anthropogenic methane emissions within the US predominantly come from livestock digestive processes (basically, cow burps and to some extent, farts… no really). Methane also comes from the extraction, processing, transportation, and use of natural gas, coal, and oil. Landfills emit methane. Globally, rice production is an additional source of anthropogenic methane.

While there is less methane being emitted than carbon dioxide, pound for pound, methane is a more powerful heat-trapping gas – 28-36 times more powerful than carbon dioxide.

Similar to global carbon dioxide emissions, the atmospheric concentrations of methane are on the rise. In the figure below, it is evident that methane concentrations in the atmosphere rose by about 12% since 1984. The rate of increase roughly plateaued from the mid-1990s until around 2006 when rates began to rise more rapidly.

Like carbon dioxide, methane is not visible to the human eye. NASA has developed tools to make the sources of global methane more visible. Watch the 2-minute video below. Note the diversity of sources of methane in different locations around the world. Why is it important to better understand the different types of sources of methane?

Focused on more localized sources, see how the New York Times used a powerful infrared camera to detect otherwise invisible methane leaks at different types of fossil fuel production sites. What difference do you think it would make in our collective policy responses and individual levels of concern if we could see the active emissions of both methane and carbon dioxide every day?

It’s not just the extraction, processing, transportation, and burning of fossil fuels but even more so certain domesticated animals that contribute to the rise in atmospheric methane contributions. Researchers adapted the figure below from the Global Livestock Environmental Assessment Model (GLEAM) developed by Food and Agriculture Organization (FAO) and shows (in carbon dioxide equivalents) the GHG incidences that enteric fermentation (digestion) and manure storage have across the main livestock species raised worldwide (source). The first three columns of measures indicate the gas and source from that particular type of livestock. So, 91% of beef cattle’s contribution to GHGs comes from their digestive system, but only 11% of pigs GHG emissions comes from digestion. Most of pigs’ contribution comes from their manure. The percentages in green indicate the share of all livestock emissions that are generated from that particular source. So 45% of livestock emissions come from beef cattle, etc.

In even more depth than the figure above, researchers (cited in this source) have calculated how the full spectrum of processes involved in the production of livestock contributes to methane emissions. The production and processing of livestock feed contributes 45% of the sector’s emissions, digestive processes (enteric fermentation) produces about 39% of the sector’s emissions, manure storage contributes 10%, and 6% of the sector’s methane emissions come from processing and transportation.

Research has found that the typical meat-based American diet contributes 1485 kg (CO2-equivalent) more annual GHGs than a similar plant-based diet (vegan). The authors compare that increase in emissions to that of driving an SUV instead of an average sedan. A review of the research suggests that, on average, scientists find that a vegan diet reduces one’s GHG emissions by 45% compared to the average meat- and dairy-consuming diet. While some suggest a vegan diet as part of the effort to reduce GHGs, other researchers are working to reduce the emissions livestock produce. More on the correlation between diet and GHG emissions can be found here and here.

What is CO2-equivalent? This is simply a way for researchers and policymakers to report the effects of different GHGs in a common metric or unit of measure. When CO2-equivalent is reported for methane, it means the amount of methane they are talking about is equal to the warming power of X amount of CO2, typically over a 100-year period. They are still talking about methane, but because it is a more powerful heat-trapping gas they chose to report in terms of the equivalent amount of CO2 that would produce the warming that the amount of methane will. Instead of saying 1 kg of methane, they say 25kg of CO2-equivalent methane because methane traps heat at 25-time the rate of CO2. Sometime you may see CO2-equivalent written as CO2e. GHGs not only trap more or less heat than one another but they persist in the atmosphere for different lengths of time (more on this later).

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