Nearly half of the solid waste produced globally is organic or biodegradable. Much of it ends up in landfills; there, it decomposes in the absence of oxygen and produces the greenhouse gas methane, which is up to 34 times more powerful than carbon dioxide over a century. While many landfills have some form of methane management, it is far more effective to divert organic waste to composting.
Composting ranges in scale from backyard bins to industrial operations. The basic process is the same: ensuring sufficient moisture, air, and heat for soil microbes (bacteria, protozoa, and fungi) to feast on organic material. Rather than generating methane, the composting process converts organic material into stable soil carbon, while retaining water and nutrients of the original waste matter. The result is carbon sequestration as well as production of a valuable fertilizer.
Human beings have long used compost to feed gardens and fields. Today, it is especially useful for managing growing urban waste streams. In 2009, San Francisco passed an ordinance that makes composting the city’s food waste mandatory. Copenhagen, Denmark, has not sent organic waste to landfill in more than twenty-five years, reaping compost’s win-win-win of cost savings, fertilizer production, and reduced emissions.
Albert Howard…champion of compost: Howard, Sir Albert. The Soil and Health: A Study of Organic Agriculture. New York: Devin-Adair Company, 1947.
nitrogen fertilizers…Fritz Haber and Carl Bosch: Erisman, Jan Willem, Mark A. Sutton, James Galloway, Zbigniew Klimont, and Wilfried Winiwarter. “How a Century of Ammonia Synthesis Changed the World.” Nature Geoscience 1, no. 10 (2008): 636-639.
Antoni van Leeuwenhoek; “wee beasties”: Montgomery, David R., and Anne Biklé. The Hidden Half of Nature: The Microbial Roots of Life and Health. New York: W.W. Norton & Co., 2016.
solid waste…organic or biodegradable: Hoornweg, Daniel, and Perinaz Bhada-Tata. What a Waste: A Global Review of Solid Waste Management. Washington, D.C.: The World Bank, 2012.
methane [vs.] carbon dioxide: Myhre, Gunnar, Drew Shindell, François-Marie Bréon, William Collins, Jan Fuglestvedt, Jianping Huang, Dorothy Koch et al. “Anthropogenic and natural radiative forcing.” In Climate Change 2013: The Physical Science Basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge, UK, and New York: Cambridge University Press, 2013.
San Francisco…ordinance: Howard, Brian Clark. “How Cities Compost Mountains of Food Waste.” National Geographic. June 18, 2013.
Seattle…composting requirement: Ferdman, Roberto A. “Seattle is Now Publicly Shaming People for Putting Food in Their Trash Bins.” Washington Post. January 27, 2015.
Copenhagen…[no] organic waste to landfill: Levitan, Dave. “Recycling’s ‘Final Frontier’: The Composting of Food Waste.” Yale Environment 360. August 8, 2013.
“flesh is the soil”: da Vinci, Leonardo, and Jean Paul Richter. The Literary Works of Leonardo da Vinci. London: Phaidon, 1970.
Project Drawdown defines composting as: the conversion of biodegradable waste to a useful soil amendment, while avoiding emissions from landfills. This solution replaces the disposal of biodegradable waste in landfills.
Organic wastes account for 46% of global solid waste and contribute, on average, 469 tons of carbon dioxide-equivalent greenhouse gases per million metric tons of organic solid waste due to anaerobic breakdown resulting in methane emissions. Composting is a flexible, scalable approach that reduces those emissions by more than 50%. Drawdown has defined the composting solution based on urban organic waste that is largely being managed today via landfills and, in some regions, open dumping. The mitigation impact reported in Drawdown for this solution does not consider any potential carbon biosequestration benefits from the use of compost as a soil amendment, nor any potential savings from reducing demand for nitrogen fertilizers.
To arrive at the mitigation and financial results for composting, a forecast for the total global urban organic waste production from 2014-2050 was first calculated. Next, the current composting rate of existing municipal solid waste was determined, the future plausible adoption of composting was forecast to 2050, and the emissions mitigated were calculated in comparison to a Reference Scenario that keeps adoption of composting fixed at the current percentage of global organic waste.
Total Addressable Market 
The total addressable market for organic waste is defined based on the global urban organic waste production from 2014-2050. The global market was calculated using a composite of forecasts including a linear interpolation of World Bank data from 2010-2025, an extrapolation to extend those projections to 2050, and a per capita extrapolation  using IPCC data. It is expected that by 2050, global organic waste will be around 1979 million metric tons. Current adoption  of composting in the year 2014 was estimated to be 13% of urban organic waste.
Adoption Scenarios 
Due to the lack of reliable future projections of the growth of composting, adoption estimates were made based on present urban composting rates in the US and Europe – approximately 38% and 57%, respectively – with negligible rates in developing and least-developed countries.
Impacts of increased adoption of composting from 2020-2050 were generated based on three growth scenarios, which were assessed in comparison to the Reference Scenario mentioned above.
- Plausible Scenario: For this scenario, it is assumed that composting will increase from present rates to 38% in middle- and low-income countries, and to 57% in high-income (OECD) countries by 2050. Globally, compost production increases from 106 milion metric tons per year in 2015 to 758 million metric tons per year in 2050.
- Drawdown Scenario: Here, it is assumed that composting scale and costs are optimized, the market value of compost increases, and source-separated collection is subsidized.  In this scenario, the amount of compost produced in 2050 is 1.045 gigatons.
- Optimum Scenario: This scenario considers the technical potential of the solution assuming a “zero waste” world by 2060, which targets a nearly universal adoption of organic waste composting of 1.167 gigatons.
Emissions data for each scenario was calculated using variables selected from sources which measured the comparative emissions of composting and/or landfilling. The variables were normalized to total direct emissions per million metric tons of organic waste, as a variety of sources reported complete emissions and some segmented emissions data by collection, transportation, processing, and other indirect measures. 
Financial results were reached by comparing the costs of creating and operating compost facilities to creating and operating sanitary landfills for an equivalent volume of organic waste. Collection and transportation costs were assumed to be the same for each approach. The cost of establishing new compost facilities over the period in question is calculated to be US$109 billion, which is US$64 billion less than the cost of establishing new landfills.  However, operating compost facilities costs more than operating landfills, even considering the revenue generated from the sale of finished compost. The data used for modeling both composting and landfilling was generated based on a mix of values, including worldwide averages reported by World Bank’s ‘What a Waste’ and UNEP.
For integration, Drawdown first considered the reduction of organic waste from reduced food waste and the increase in organic waste from an assumed compostable percentage of the increased adoption of bioplastic, to refactor the market for composting for each year.  Modeled compost adoption was then compared to the new organic waste market and adjusted if needed. 
Drawdown found a potential of 2.28 gigatons of carbon dioxide-equivalent greenhouse gas reductions in the Plausible Scenario over 2020-2050, corresponding to a 53% adoption of composting, with a net implementation savings of US$63.72 billion but a net operational cost of US$141.19 billion. The first cost of landfilling is more than double the cost of composting facilities; however, the operational cost of composting is 22% more than the cost of landfilling due to the maintenance incurred for aeration, moisture, and temperature control of compost piles. For the Drawdown Scenario, the emissions avoided amount to 3.2 gigatons with a 77.71% adoption, and the Optimum Scenario resulted in 3.61 gigatons of emissions reduced at an 80.66% adoption by 2050.
The impact of composting is bounded by the amount of organic waste created. There are some indicators that as regions develop, the per capita amount of waste increases then plateaus. All scenarios modeled are highly dependent on a significant increase in adoption in the Asia (sans Japan) region. Whereas less than 3% waste to landfills has already been achieved in several European countries, Drawdown has chosen more conservative growth scenarios for both China and Asia (sans Japan) to reflect the uncertainty of growth of composting, considering current investments in waste-to-energy facilities in the region.  The financial case for adoption of composting is distorted by local regulations and compost demand.  Where regulations, space, and logistics create cost barriers to landfill expansion, there is already a self-evident business case for composting. In conclusion, this analysis suggests that composting can grow to offset a significant portion of landfilling, while reducing climate emissions. This is limited by the education required for composters and consumers, as well as the realization of a financial advantage that compost may have over landfilling. 
 For more on the Total Addressable Market for the Materials Sector, click the Sector Summary: Materials link below.
 Using UN 2015 median urban population forecast.
 Current adoption is defined as the amount of functional demand supplied by the solution in the base year of study. This study uses 2014 as the base year due to the availability of global adoption data for all Project Drawdown solutions evaluated.
 For more on Project Drawdown’s three growth scenarios, click the Scenarios link below. For information on Materials Sector-specific scenarios, click the Sector Summary: Materials link.
 Perhaps through increased regulation and cost burden on landfill practices.
 Sources include the EPA WARM model, the Composting Council of Canada, and meta-analyses such as those by Zaman, et al.
 All monetary values presented in US2014$.
 For more on Project Drawdown’s Materials Sector integration model, click the Sector Summary: Materials link below.
 Reduced food waste already incorporates considerations of plant-rich diet adoption.
 E.g., adjusting adoption down if composting adoption exceeds in total or a reasonable percentage of organic waste.
 If China is slow to adopt composting, then the overall global mitigation impact of composting will likely need to be discounted further.
 Whereas the model shows a potentially uninspiring business case for adopting composting due to the increased operating cost of a compost facility over a managing a landfill, it would take only an increase in market price of finished compost (driven by demand of more climate-friendly agricultural practices) and/or a decrease in the operating costs through innovation and process design to make a compelling financial argument in favor of composting over landfill.
 Particularly in areas with informal and unregulated dumps.