US EPA's Warm Tool

EPA’s WAste Reduction Model (WARM) is a tool for assessing the GHG emissions of a baseline and an alternative waste management method for handling any of 26 materials and 8 mixed materials categories. WARM is the most common tool used to predict and estimate GHG emissions reductions from materials management strategies. WARM was created “to help solid waste planners and organizations track and voluntarily report greenhouse gas emissions reductions from several different waste management practices.” (From the WARM website). While WARM was not originally designed to be used as an inventory tool, it is possible to use WARM to estimate the life cycle GHG emissions and emissions reductions associated with a variety of material uses and end-of-life practices in a systems based approach – both projected (future) and actual (current). (WARM is available both as a Web-based calculatorand as a Microsoft Excel spreadsheet.)

WARM calculates and totals GHG emissions of baseline and alternative waste management practices—source reduction, recycling, combustion, composting, and landfilling. The model calculates emissions in metric tons of carbon equivalent (MTCE), metric tons of carbon dioxide equivalent (MTCO2E), and energy units (million BTU) across 34 material types commonly found in municipal solid waste (MSW). The emission factors represent the GHG emissions associated with managing 1 short ton of MSW in a specified manner. GHG savings should be calculated by comparing the emissions associated with the alternative scenario with the emissions associated with the baseline scenario, as opposed to simply multiplying the quantity by an emission factor. Without the comparison, part of the emissions savings or cost will be excluded.

The model takes a life cycle view and incorporates in the emissions factors for each material the emissions from raw materials acquisition, processing, manufacturing, transportation, and end-of-life management. The use phase of materials is not considered in the model’s calculations, however. For most materials, recycling is modeled as a closed-loop. For example, a plastic PET bottle is recycled into a plastic PET bottle. For those materials where there is not a dominant use of a recycled material or a lack of data, an open-looped process may be modeled. Open-loops are common for many of the paper-based material categories. Details for what is and isn’t included can be found in the EPA WARM FAQ.

WARM can be useful to communities who are working on planning, setting targets for, or measuring results of materials-related projects with anticipated GHG reductions.

WARM is one of the best tools available for state and local governments to estimate the GHG emissions associated with prevention, recycling, and composting. WARM is widely used by national, state, and local governments. Because it is commonly used, it lends some universality and comparability to the analyses that are done with it. It is a “common denominator” for solid waste GHG emissions in the US. Other available tools sometimes have drawbacks that WARM does not; they may be proprietary and accessed only through contract, may carry costs for use, and may not be as widely used.

The emissions factors that underlie the WARM tool can also be useful for estimating life cycle emissions associated with certain materials.

Although EPA's WARM tool remains one of the best options available for state and local governments to estimate the emissions reduction potential of prevention, recycling, and composting (relative to incineration and landfilling), WARM is not without limitations. Here are some that anyone using the model should be aware of:

  • WARM currently has no capacity to calculate reuse separate from source reduction. The source reduction management option assumes materials not manufactured. Using the source reduction calculations as a proxy for reuse activities only works if one assumes that the reuse actually substitutes for the mining and manufacture of virgin materials that would have otherwise been necessary. This is a shaky assumption, since some reuse activities don’t actually displace production of new materials.
  • WARM focuses on materials, not products, which leaves out some significant pieces of the solid waste stream. It doesn’t, for example, include such categories as sheetrock, textiles (which can have multiple materials in products) or household items – furniture, toys, sporting goods, electronics other than PCs. Material list is found on the WARM homepage.
  • In addition, WARM users face the challenge of reconciling their own materials category definitions with those the model employs – WARM’s assumed composition of “mixed recyclables” or “mixed plastics” for example may vary from your community’s mixture. WARM’s categories for mixed paper and corrugated cardboard remain ambiguous since there are a many materials with different emissions impacts that would fall into these categories in varying ratios.
  • Some materials management efforts are better evaluated using other methods and tools. WARM is not easily adapted to comprehensive comparisons of materials management strategies such as EPP or reuse programs. The lack of “upstream” (or production-related) emissions for food limits WARM’s utility for evaluating food waste prevention projects.
  • Methane Global Warming Potential (GWP): GWP is a concept designed to compare the ability of a greenhouse gas to trap heat in the atmosphere relative to another gas. The definition of a GWP for a particular greenhouse gas is the ratio of heat trapped by one unit mass of the greenhouse gas to that of one unit mass of CO2 over a specified time period. WARM uses 21 as the GWP for methane, which is the 100 year GWP listed in the IPCC’s second assessment from 1996. According to the EPA, November 2009, this will not be changed anytime soon as the GWP is set by the United Nations Framework Convention on Climate Change (UNFCCC) which EPA must use for national GHG inventories (and which is based on the IPCC second assessment). It is important to note that the more recent IPCC Assessment 4 (2007) uses a 100 year GWP for methane of 25. Moreover, many state and local inventory and waste professionals believe that using the 20 year horizon GWP of 72 for methane highlights the potential for important short-term emissions reduction benefits, since methane decays quickly (it has a 12 year lifetime) and thus has its maximum warming impact well before 100 years is reached. 
  • As of August 2010, a new version of WARM includes a more comprehensive analysis of composting yard and food waste than it has in the past. First, the calculation of landfill emissions from organics is based on a first-order decay rate to better measure when emissions are generated. Previous versions of the model only calculated the lifetime methane yield. In addition, landfill gas capture systems is modeled with a time element, assuming systems are phased in at landfills. With these two new elements, the model is able to estimate the amount of methane being generated at a particular time and the amount of methane being captured at that time. This new calculation methodology most affects food waste and grass. The emission factors for branches, which degrade at a very slow rate, changed very little. The new emission factor takes into account the higher soil carbon sequestration capacity for compost-improved soil as well as the GHG emissions involved in composting machinery and transportation. However, the updated model still does not include an emission factor for other compostable materials, like non-recyclable paper. WARM also does not include GHG emissions or emissions reductions associated with other co-benefits associated with the use of compost, such as water conservation and changes in fertilizer use. WARM also does not differentiate between the potential for varying emissions from compost sites themselves as a function of technology (e.g., anaerobic vs. aerobic composting, or centralized vs. home composting).
  • WARM counts long-term carbon sequestration in landfills, while ICLEI’s GHG Emissions Analysis Protocol, and the California Air Resources Board Local Government Operations Protocol do not.
  • WARM only allows comparison against a single disposal option. If your community sends waste to more than one disposal facility (for example, a landfill with gas recovery, a landfill without gas recovery, and an incinerator), then multiple runs of the model are required.
  • WARM treats international production – both of virgin and recycled materials – as if production in other countries have the same emissions factors (emissions per ton) as domestic production. Given the international flow of products and recycled feedstocks, and the potential for significant regional differences in emissions based on regional fuel mixes and technology patterns, this is a potential limitation. This is particularly acute in the forest carbon sequestration element of WARM (for paper recycling and source reduction), which is based entirely on modeling of forest management practices in the domestic US. Forest management practices, and the associated carbon benefits/impacts of reducing use of wood, likely vary widely between the US and some other areas of the world, including areas that would supply virgin fiber to foreign mills were it not for their use of wastepaper exported from the US.
  • Currently, WARM is not intended as an inventory or accounting tool. It is not sufficiently precise and it is not easily connected to other inventory protocols. As mentioned in the Inventories section, conventional "purist" inventories are based on single locations and designated timeframes. Emission savings in WARM will likely fall outside both of these boundaries.
  • WARM does not currently break emissions and emissions reductions into the years in which they actually occur. Rather, WARM rolls all future emissions and emissions reductions into a single number. While appropriate for comparing program options against each other, this limits WARM’s usefulness in inventories, since most other emissions are reported in the years in which they actually occur. Organic materials (e.g. cardboard, paper, lumber) have avoided emissions associated with source reduction and recycling that are time-sensitive.
    • Forest carbon sequestration: When paper is recycled, fewer trees are cut down. This carbon sequestration reduces the net emissions associated with paper source reduction and recycling. The reductions occur over decades, since every year following the actual recycling or source reduction event, over their lifetime, these trees absorb carbon as they continue to grow.
    • Avoided landfill emissions: When paper is recycled, less of it goes into the landfill. Landfill methane emissions are reduced, and these avoided emissions reduce the net emissions associated with paper source reduction and recycling. These reductions occur over decades, since decay in the landfill occurs over decades. The same is true for diversion of other putrescible wastes, such as food waste composting.
    • Carbon storage: WARM provides a credit for carbon stored in soils treated with finished compost as well as the non-putrescible fraction of biogenic wastes (such as lumber) placed into landfills.

Planning: Communities can use WARM to analyze possible waste management scenarios and develop optimum scenarios for reducing emissions from waste management. By design, WARM is well suited to identifying material-specific emissions by waste management scenario. Should you target corrugated cardboard for increased recycling vs. incinerating it? Or would you be better off to target personal computers for increased recycling? Which materials will give you the bigger emissions reductions if source reduced? If a community has good data on your solid waste and how it is managed currently, WARM can be used to provide good relative comparisons of emissions reductions potentials of source reducing or recycling various materials versus landfilling or incinerating them. As such, it can be useful in setting programmatic direction. The State of Minnesota Pollution Control Agency, for example, was charged with working with stakeholders to develop a plan for meeting a given emissions reduction target for solid waste management. The stakeholder group used WARM to run several scenarios for possible changes in the system and determine which would reach the target.

Setting targets: Some communities use WARM to help set their targets as either tons of materials or GHG. Fort Collins (Colorado), for example, used WARM to determine the anticipated GHG savings from their existing recycling programs (such as curbside recycling) and to set the targets for GHG reduction in 2012 and 2020 from new programs such as enhanced PAYT (pay-as-you-throw) and starting a drop-off site for construction and demolition debris. In these cases, it was first necessary for Fort Collins to estimate the amounts of materials that would reduced or recycled through these programs and then run those estimates through the WARM model. (See Fort Collins example here.)

In sum, WARM is one of the best tools available for state and local governments to estimate the GHG emissions associated with prevention, recycling, and composting. WARM is not without limitations. More about WARM and its limitations is summarized here.