Marie Neubrander, an intern with Sustainable House and a mathematics and computer science student at the University of Alabama, describes how composting can avoid harmful greenhouse gas emissions.
Carbon atoms are a necessary part of life - you and I need it to breathe, eat, move, and just about everything else. However, over time, human activities have caused drastic changes to the carbon cycle, Earth’s process of balancing where carbon is stored.
The carbon cycle. From the National Oceanic and Atmospheric Association (NOAA) [1].
Human actions such as burning fossil fuels and driving release carbon dioxide, or CO2, into the atmosphere at higher than natural rates. This leads to the greenhouse effect: the high quantity of atmospheric carbon dioxide creates a cover trapping in the sun’s heat, causing rising temperatures and climate/weather havoc.
Often, when we think of pollution, these carbon dioxide emissions are at the forefront of our minds. This makes sense given that carbon dioxide makes up roughly 76% of global manmade greenhouse gas emissions, as shown in the chart below. However, as is also shown in the chart, carbon dioxide is by no means the only greenhouse gas (GHG) that contributes to global warming.
Percentage of global manmade greenhouse gas emissions by gas; data from the Center for Climate and Energy Solutions [2].
To better understand the impacts of these other GHGs, we will introduce the concept of global warming potential. Global warming potential, or GWP, is defined as “the quantity of energy the emissions of 1 ton of a gas will absorb over a given period of time, relative to the emissions of 1 ton of carbon dioxide (CO2)” [3]. Throughout the rest of this post, we use GWP to refer to GWP100, which takes this time period to be 100 years. The higher the GWP of a gas, the more it may warm the atmosphere as compared to carbon dioxide. The table below shows the GWP values of the three most common greenhouse gasses.
Table 1: GWP of three most common greenhouse gas emissions [4].
Hand-in-hand with GWP is the idea of carbon dioxide equivalent emissions, or CO2e. For a given quantity of some greenhouse gas, CO2e gives the equivalent quantity of carbon dioxide emissions [5]. CO2e is computed by multiplying the quantity of gas emitted by its GWP value. This measure gives a means for comparing the severities of different quantities of emissions of different gasses.
With this background, we can now delve into the impacts of food waste on greenhouse gasses.
When food waste decomposes aerobically, whether that be in a landfill, in compost, or somewhere else entirely, it releases carbon dioxide. Perhaps counterintuitively, this carbon dioxide is not counted as a greenhouse gas emission that contributes to global warming due to a general agreement that such CO2 emissions are of biogenic origin (definition: produced by living organisms) [6]. However, in anaerobic environments, such as those that occur in landfills, carbon tends to be released as methane. Referring back to the GWP table highlights that this is roughly 28 times as potent of an emission as carbon dioxide [6].
Landfills are more than just eyesores - they are a dangerous source of methane emissions. Image from [7].
Well-run compost systems work to avoid these anaerobic conditions. When food waste that would have otherwise been thrown out and sent to a landfill is composted, these potent methane emissions are largely avoided. This is not to say that composting cannot yield some methane production - it can. However, this production tends to be toward the bottom of the compost pile; a majority of this methane will be oxidized in more aerobic portions before leaving the compost [6, 8].
To understand more precisely the benefits of composting, it is useful to examine the quantity of methane generated by food waste in landfills. However, despite our knowledge that such emissions occur, deriving precise quantities is a difficult task and estimates vary between sources/studies.
According to a report done by the US Composting Council (USCC), after 120 days in a landfill, one metric ton of food waste may generate roughly 0.25 metric tons of methane [8]; using Table 1 this is roughly equivalent to 7 metric tons of CO2-equivalent emissions. This is a very high estimate compared to other published figures.
The ExtraFood project to end hunger in Marin County claims that every 100 pounds of food waste in landfills leads to 8.3 pounds of methane emissions [10]. In other words, this means that one metric ton of food waste generates 0.083 metric tons of methane emissions; again using Table 1, this is is roughly equivalent to 2.3 metric tons of CO2-equivalent emissions.
An article by the Government of Western Australia’s Department of Primary Industries and Regional Development states that “each [metric ton] of organic waste disposed of as landfill and broken down by anaerobic fermentation releases about one [metric ton] of carbon dioxide equivalents (CO2-e) of greenhouse gases, mostly in the form of methane” [11].
Of course, to quantify the benefits of composting, we can take a bigger-picture view than exclusively examining methane emissions.
The Sustainable Composting Research at Princeton Lab (S.C.R.A.P. LAB, based at Princeton University) is a multi-faceted group that composts uneaten food from across the university. As a component of their work, they post updates indicating the total amount of food they have diverted from landfills and the corresponding amount of greenhouse gas emissions avoided (CO2eq). In their computations, 1 metric ton of food composted corresponds to roughly 1.5 metric tons of CO2 equivalent emissions prevented; they derive their computations from the EPA Waste Reduction Model [12].
With this variety of figures in mind, we have updated our food waste calculator to perform computations similarly to those of the S.R.A.P. Lab; we assume that one kilogram of food composted corresponds to 1.5 kg of CO2eq emissions prevented.
Now, we give an example through data collected from the Chippendale footpath gardens (see more about the gardens at this Facebook page ).
Table 2 shows the quantities of each type of food waste composted each day over the course of a week.
Table 2: Food waste composted in the Chippendale footpath gardens over one week.
In total, 1040kg of food waste was composted in the Chippendale footpath gardens.
Based on the Princeton S.C.R.A.P. Lab figures, we find that this avoided roughly 1560kg of CO2 equivalent greenhouse gas emissions from being produced in a landfill; this is highlighted in the plot below.
Food waste composted and CO2-equivalent emissions prevented during one week at the Chippendale footpath gardens.
Of course, as has been mentioned previously, these numbers are only estimates; moreover, the values in the cited studies are often admittedly difficult to compute. Depending on a range of factors, such as food waste contents and compost aeration level, the true numbers presented in the above example for the Chippendale gardens may be greater or smaller than those shown.
Regardless of precise numbers, we are confident in urging you to compost whatever food waste you are unable to avoid producing.
Warnings from the Intergovernmental Panel on Climate Change are strong - our climate is changing at a more alarming rate than seen before [9]. It is essential that we come together to keep our beloved Earth as we know it; composting is just one small step at doing so.
Sources
[1] https://www.noaa.gov/education/resource-collections/climate/carbon-cycle
[2] https://www.c2es.org/content/international-emissions/
[3] https://www.epa.gov/ghgemissions/understanding-global-warming-potentials
[4] https://ww2.arb.ca.gov/sites/default/files/2018-06/Global-Warming-Potential-Values%20%28Feb%2016%202016%29_1.pdf
[5] https://ecometrica.com/assets/GHGs-CO2-CO2e-and-Carbon-What-Do-These-Mean-v2.1.pdf
[6] Nair, J. and Lou, X.F. The impact of landfilling and composting on greenhouse gas emissions – A review.
[7] https://www.moonshotcompost.com/difference-between-compost-vs-landfill/
[8] https://www.sanjoseca.gov/home/showpublisheddocument?id=198
[9] https://www.theguardian.com/environment/2022/feb/28/ipcc-issues-bleakest-warning-yet-impacts-climate-breakdown
[10] https://extrafood.org/the-need/food-waste/
[11]https://www.agric.wa.gov.au/climate-change/composting-avoid-methane-production
[12]https://scraplab.princeton.edu/2019/04/the-weekly-composter-4-2-more-partnerships-testing-data/