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Frequently Asked Questions

Who should use WARM?

WARM was originally developed for small to moderate-scale waste managers enabling them to understand how their “business-as-usual” waste management practices compare to alternative practices such as recycling, source reduction, or composting in terms of greenhouse gas emissions and energy use. Its user base has expanded to include various community officials, EPA WasteWise partners, and municipalities interested in learning more about the climate and waste connection. However, the results garnered from using WARM are estimates and the model approach is not appropriate for use in inventories because of the diffuse nature of the emissions and emission reductions within a single emission factor calculated in WARM.

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What information do I need to use WARM?

The most basic information needed to run WARM includes data on the amount of waste handled by material type (ex. aluminum cans, corrugated cardboard, etc) the associated waste management practice (recycled, landfilled, combusted, or composted) used currently and the alternative practice.

Optional extra information which would enhance the accuracy of the GHG Emissions and Energy Use include:

The tool provides default data on the “national average” amount of landfill gas recovery, landfill gas collection efficiency, and transportation distances; however, the results of WARM runs will be more accurate if users enter site-specific information, to the extent that it is readily available.

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How credible are the results generated by WARM?

WARM has been in development for over 10 years and relies on information from leading scientists and technical experts. The methodology and data has been peer reviewed at several stages; some of the peer review processes are detailed in Background Document C: Review Process for the Report. A lengthier review process including public comments and responses is detailed in Background Document D: Comment-Response Document.

The field of life-cycle analysis has expanded dramatically since WARM was originally developed and interest in life cycle studies and supply chain impacts is at an all-time high. For that reason, EPA is in the process of updating many of the emission factors and assumptions embedded in WARM. As new updates and improvements become available, EPA will post new versions of the model and explanations of revisions. To learn more about the data sources and methodology employed in WARM, please consult the latest edition of EPA's research report: documentation for the emission factors. We have also provided an overview of the model's history.

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Can WARM be used as a GHG accounting or inventory tool?

Although emissions estimates provided by WARM are intended to support voluntary GHG measurement and reporting initiatives, WARM is intended as planning tool and not an accounting tool. Its proper use entails comparing the current waste management practice with an alternative waste management practice and obtaining the impacts associated with changes in waste management practices. For further information, EPA has prepared a short summary on the differences between life-cycle analysis (such as WARM uses) and GHG inventories.

Example Calculation for WARM GHG Emissions Benefits

For instance, the GHG savings of recycling 1 short ton (2,000 lbs) of aluminum as opposed to landfilling would be calculated as follows:

Alternative approach - "Business as Usual" approach =
GHG Emissions Benefits

Emissions associated with Recycling - Emissions associated with Landfilling =
Difference in GHG Emissions

[1 short ton x (- 13.61 MTCO2E/short ton)] - [1 short ton x (0.04 MTCO2E/short ton)] =
- 13.65 MTCO2E total GHG savings

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Why are the values for "barrels of oil" different when you compare the GHG results to the energy results?

Common Energy Conversion Factors


Million Btu per Barrel of Oil: 5.8
Gallons Oil per Barrel of Oil: 42
Million Btu per Gallon of Gas: 0.125

GHG Emissions per Barrel of Oil combusted in MTCE: 0.116 MTCE

Cars ("average" passenger car over one year):

Fuel Consumption (gallons of gas): 502
CO2 Emissions (tons): 4.6

It is sometimes difficult to understand the magnitude of GHG emissions savings in terms of metric tons of carbon equivalent (MTCE). Therefore, WARM includes the capacity to convert traditional units for reporting GHGs emissions into more units that are more familiar to WARM users. These units include the combustion emissions equivalent in barrels of oil and the average combustion emissions per passenger car in one year. In other words, WARM uses a conversion factor which compares the GHG Emissions Benefits to the amount of GHG emissions that are released when a barrel of oil is burned or the amount of GHG emissions that are emitted from a typical passenger vehicle in one year.

Caution must be used when converting GHG Emissions Benefits to common equivalents such as barrels of oil for two reasons. First, the GHG savings reflect energy and non-energy savings. Second, GHG Emissions Benefits are accrued through a range of fossil fuel feedstock.

For more information, please consult Waste Management and Energy Impacts.

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Why does recycling have lower emissions than waste prevention for some material types?

In WARM, the recycling emission factors reflect the difference between making a product with virgin inputs and making a product with recycled raw material inputs. This means that the virgin inputs that would have been necessary to create the specific material are no longer required because this material is being recycled. In contrast, source reduction is assumed to displace the current mix of recycled and virgin raw material inputs used to manufacture a given material. This means that when a material is reduced (source reduction), the inputs needed to create this material come from both recycled content and virgin content.

For more information, please see "Why Recycling Some Materials Reduces GHG Emissions More than Source Reduction."

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How could composting a material require more energy than landfilling the same material?

Based on the available data, EPA has found that the energy used during the composting process may exceed the energy needed to landfill the same materials. This is based on a comparison of the data in WARM that represents energy requirements for transportation and equipment operations in landfills and centralized composting facilities.

All of the emission factors in WARM include a transportation component. The "transportation" value for any given disposal/material pair (e.g., composting yard trimmings) reflects the energy needed to transport the material to the landfill or centralized composting facility as well as the equipment used to operate the landfill or composting facility. Energy requirements are defined as the amount of diesel fuel required to operate the machinery associated with each process which is then converted into energy units per ton of material (BTU/ton). The transportation/equipment operation values vary by waste management type, but not by material type.

The energy assumptions underlying the transportation component of the emission factors for composting and landfilling materials in WARM are presented below.

Transportation Energy (BTU/ton)
Strategy Collection Vehicles Equipment Total

Because composting requires more energy in terms of transportation and equipment operation, the overall energy impact of composting is positive as compared to landfilling.

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What components make up the emissions factors associated with each process?

Each of the five waste management practices contains assumptions about the emissions associated with each material in its post-use life cycle. Generally, these assumptions encompass the different stages of each material's life cycle including the emissions associated with raw material acquisition and manufacturing, forest carbon sequestration, waste management emissions, transportation, soil carbon sequestration, combustion emissions, and composting emissions. A complete breakdown per waste management practice is outlined below.

GHG Sources and Sinks

MSW Management Strategy

Process and Transportation GHGs from Raw Materials Acquisition and Manufacturing Forest Carbon Sequestration or Soil Carbon Storage Waste Management GHGs

Source Reduction

Decrease in GHG emissions, relative to the baseline of manufacturing

Increase in forest carbon sequestration



Decrease in GHG emissions due to lower energy requirements (compared to manufacture from virgin inputs) and avoided process nonenergy GHGs

Increase in forest carbon sequestration

Process and transportation emissions are counted in the manufacturing stage


No emissions/sinks a

Increase in soil carbon storage

Compost machinery emissions and transportation emissions


Baseline process and transportation emissions due to manufacture from the current mix of virgin and recycled inputs


Nonbiogenic CO2, N2O emissions, avoided utility emissions, and transportation emissions


Baseline process and transportation emissions due to manufacture from the current mix of virgin and recycled inputs


CH4 emissions, long-term carbon storage, avoided utility emissions, and transportation emissions

a No manufacturing transportation GHG emissions are considered for composting of food discards and yard trimmings because these materials are not considered to be manufactured.

NA = Not Applicable

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Can I use WARM retroactively and discover "avoided emissions" from a waste management practice switch already undertaken?

Yes, as long as the information concerning the previous "business as usual" waste management practice is accurate. Note that WARM is intended to inform waste managers about the GHG Emissions benefits associated with implementing changes in their waste management practices and to support voluntary GHG measurement and reporting initiatives. As such, estimates of "avoided emissions" are for informational purposes.

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What does "source reduction" mean?

Source reduction has many environmental benefits. It prevents emissions of many greenhouse gases, reduces pollutants, saves energy, conserves resources, and reduces the need for new landfills and combustors. Practices including two-sided copying of paper and the reduction of transport packaging by industry have yielded substantial benefits through source reduction.

In WARM, source reduction is considered a specific waste management practice that can be compared to a baseline waste management practice (recycling, landfilling, combustion, or composting). As such, there is a GHG emission factor (in MTCE per ton of material); this factor represents the estimated GHG implications of not producing that material. The general components of this emission factor are outlined in the table below. Some of the source reduction components do not apply to certain material types (i.e., plastics and inorganic materials do not increase forest carbon sequestration. Therefore source reduction of this type would not have an impact on forest carbon sequestration).

The default method in WARM for source reduction provides an emission factor associated with not manufacturing a certain material from the current mix of inputs, which means a material-specific ratio of recycled and virgin inputs. Users may choose to modify the default assumption for source reduction by calculating source reduction benefits as compared to production of a material from 100% virgin inputs (rather than the current mix of virgin and recycled).


Management Strategy

GHG Sources and Sinks

Process and Transportation GHGs from Raw Materials Acquisition and Manufacturing

Forest Carbon Sequestration or Soil Carbon Storage

Waste Management GHGs

Source Reduction

Decrease in GHG emissions, relative to the baseline of manufacturing

Increase in forest carbon sequestration (for certain material types)


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What is Landfill Carbon Storage?

When biogenic materials are placed in landfills, the carbon in these materials is removed from the atmosphere. Landfill carbon stocks increase over time because much of the organic matter placed in landfills does not decompose, especially if the landfill is located in an arid area. However, not all carbon in landfills is counted in determining the extent to which landfills are carbon stocks. For example, the analysis does not count plastic in landfills toward carbon storage. Plastic in a landfill represents simply a transfer from one carbon stock (the oil field containing the petroleum or natural gas from which the plastic was made) to another carbon stock (the landfill); thus, no change has occurred in the overall amount of carbon stored. On the other hand, the portion of organic matter (such as yard trimmings) that does not decompose in a landfill represents an addition to a carbon stock, because it would have largely decomposed into CO2 if left to decay in the environment. For more information, please see Landfill Carbon Storage in EPA's Waste Reduction Model.

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Why can't mixed materials (e.g. mixed organics, mixed plastics, mixed MSW) be “source reduced” in the alternative management scenario?

The mixed materials in WARM are composed of substrates of different material types on a percentage basis. This is to facilitate the comparison of waste streams that are unseparated. Source reduction of mixed material types is harder to conceptualize (how do you source reduce mixed MSW) and even harder to quantify. The mixed material emission factors reflect the mix of materials present in the US municipal solid waste stream, but information on the relative fractions of each material in the mixed categories would not necessarily be consistent with the proportions of the specific materials that are source reduced.

Users interested in calculating benefits of source reduction will need some information on the quantities of individual material types that are being source reduced, then material-specific source reduction factors can be applied to those inputs and used to calculate the benefits.

For mixed paper, the composition of each of the three mixed paper types (broad, office, and residential) is listed in Exhibit 3 of the documentation chapter on paper products. The mixed paper material types cannot be "source reduced" because of the variation in the many paper types.

The mixed metals source is composed of steel cans and aluminum cans and the mixed plastics are composed of HDPE, LDPE, and PET. Both mixed materials are estimated by taking a weighted average of the current recovery rates for the respective material components. Again, these mixed material types cannot be "source reduced" in WARM because of the variation of component material types.

The mixed MSW and recyclables are household materials that are typically discarded or recycled, respectively. Due to their diverse component mixture, these material types cannot be "source reduced" in WARM.

How can I model product reuse in WARM?

Users can model reuse in WARM by a modified use of the source reduction emission factors. EPA has prepared a short guide on how to do this.

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