The other greenhouse gas – Reuters

How methane from food waste contributes to global warming and why small efforts to stop it can make a big difference.
There are countless ways we’re advised to live more sustainably – drive an electric car, fly less often, don’t eat meat, shop local. And yet, there’s another environmental action that’s relatively unintrusive and low-cost: reducing food waste.
When it comes time to let go of that moldy peach or questionable leftover noodles, we typically turn to the trashcan and bid our foul food farewell. But climate scientists ask us to think twice before condemning our food to landfills where it breaks down and releases a significant portion of human-made greenhouse gases.
Viable food is lost at each point in the production chain as it travels from seed to supermarket. However, most wasted food journeys all the way to restaurant kitchens and homes, only to end up uneaten in the garbage. The EPA, USDA and the United Nations have each set goals to address household food waste, aiming to cut it in half by 2030.
Graphic shows the breakdown of food wasted along the production chain and how much went to landfill in 2021. Homes are the biggest food waste contributors in the United States. Almost half of household food waste is thrown in the garbage. The majority of food in landfills comes from homes.
Over time, food is buried under other waste and decomposes in anaerobic conditions (without access to oxygen), creating a favorable environment for bacteria that produce methane, a greenhouse gas 28 times stronger than carbon dioxide over a 100-year period.
“The whole characteristics of food – that they're rich in energy, rich in proteins, rich in nutrients and that they're relatively easy to extract that energy from – means that in the landfill environment, they will generate more methane than a lot of other waste components,” said Max Krause, an engineer with the EPA’s Office of Research and Development.
Graphic shows the proportion of methane potentially generated by different types of organic materials in a landfill. Food generates 39%, paper 24%, textiles 22%, other organic materials 6%, yard waste 6% and wood can potentially generate 2% of overall methane in a landfill.
The graphic then shows food getting buried in a landfill. The graphic reads, “As food is buried in landfill piles, it loses access to oxygen. Unlike other garbage, food degrades quickly and is high in moisture and nutrients. These nourishing elements and the lack of oxygen encourage methane-producing bacteria to thrive.”
Note: Data does not add up to 100% due to rounding. Potential methane generation calculated using average methane yields with a confidence interval of 95% (Source).
“Solid food waste in landfills generates incredible amounts of methane every year, just based on the fact that food is sitting in piles and it decomposes in anaerobic conditions,” said Kevin Karl, a researcher at NASA’s Center for Climate Systems Research who is also an Environmental Statistics Consultant at the Food and Agriculture Organization of the United Nations.
When we talk about global warming, we think about carbon dioxide. It’s one of the most abundant greenhouse gases in our atmosphere and is commonly the center of conversation for slowing climate change. But methane is worth some attention.
Graphic showing the contribution of different greenhouse gases to warming influence on the earth from 1750 to 2021. Carbon dioxide contributes the most warming effect, 66%. Methane comes in second, contributing 16%.
Note: Data shows the contributions of the most important greenhouse gases to the increase in “radiative forcing” or the change in heating of the earth’s surface since the pre-industrial era, 1750.
Source: World Meteorological Association Greenhouse Gas Bulletin
Reducing methane is key in achieving temperature limits set in the 2015 Paris Agreement, according to the United Nations Environment Programme and other environmental groups. Opportunities to cut methane are relatively inexpensive compared to its infamous companion, carbon dioxide. And there are two factors that might make methane most attractive to target: It is potent and it is short-lived.
Methane’s Global Warming Potential is 100 times that of carbon dioxide for its first 20 years in the atmosphere, but methane leaves the atmosphere in a fraction of the time. The carbon dioxide we emit today will continue warming our planet for up to 100,000 years, but methane only lasts for a timescale of 12 years before breaking down into less potent gases.
“One of the fastest ways to improve the quality of life for people that already exist on the planet is to reduce methane that is currently being emitted,” Karl said.
To understand, let's unpack how greenhouse gases warm our planet.
After being warmed by the sun, the earth emits heat back into the atmosphere as “infrared radiation”.
Most of the heat is absorbed by greenhouse gases in the atmosphere. Then the heat is re-emitted in all directions, out to space and back downward warming the planet.
The heat emitted from earth is made up of a range of wavelengths, and each type of greenhouse gas is picky about which wavelengths it absorbs.
Carbon dioxide is one of the most abundant greenhouse gases and absorbs the earth’s heat at a wide range of wavelengths. However, it avoids a narrow stretch along the infrared spectrum called the “atmospheric window region”.
In normal conditions, the “window” allows heat at these wavelengths to easily escape to space.
While carbon dioxide doesn’t absorb heat in this window, the problem is methane loves these wavelengths.
The earth isn’t used to having greenhouse gases absorb heat in the window. This is usually where heat can escape to space most easily, so adding even small amounts of methane can make a big difference in warming our planet.
On the other hand, adding similar amounts of CO2 (while not favorable) has a less obvious impact because there is already so much naturally-occuring CO2 in the atmosphere absorbing heat.
“That is why molecule by molecule, adding methane is disproportionally more potent than CO2 in terms of warming,” said Dr. Neil Donahue, an atmospheric chemist at Carnegie Mellon University.
Human-made methane narrows the crucial atmospheric window region, trapping heat that could otherwise escape and warming the planet.
There is not one solution that will reduce enough methane to be in line with the Paris Climate Agreement. Instead, researchers say, it will take action in many sectors to meet the goal.
Methane produced by food decomposing in landfills makes up 1.6% of all human-made Greenhouse Gas emissions. While that may not sound like much, it’s a large percentage for such a specific pollutant. When scientists look at hyper-specific categories, Karl says, anything over 1% is significant.
“Any action that can prevent food waste from sitting in untreated piles will directly lead to climate impacts being reduced.”
Reuters looked at how consumers can divert food away from landfills to help reach the goal of cutting household food waste in half by 2030:
Read how »
World Meteorological Organization Greenhouse Gas Bulletin, United States Environmental Protection Agency, ReFED, United Nations Environment Programme and Climate and Clean Air Coalition, Intergovernmental Panel on Climate Change, Neil M. Donahue at Carnegie Mellon University. Department of Chemistry, Atmospheric Chemistry and Physics: From Air Pollution to Climate Change
Clare Trainor
Julia Wolfe, Claudia Parsons
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