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Versions of the Waste Reduction Model (WARM)

The availability of updated information has required updates to the energy and emission factors used in the Waste Reduction Model (WARM). Since some companies and waste managers have used older versions of the tool, the purpose of this page is to explain the changes incorporated into each version.

Below is a brief chronology of changes made to WARM. Starting with the most recent edition, brief summaries of changes and updates since the previous version are provided. The objective is to provide users with a transparent picture of the evolution of the tool and to provide context for comparisons of results obtained from different versions of WARM.

On this page:


Current WARM Tool - Version 14

This version of WARM(1 pg, 3 MB) , released in March 2016, contains a new material management pathway and revisions to existing numbers and methodology. The majority of the updates in WARM version 14 concern the addition of the anaerobic digestion materials management pathway.

EPA added energy and emission for anaerobic digestion of organic materials, including food waste, yard trimmings, and mixed organics. These emission factors include:

  • the energy and emissions associated with transporting materials,
  • operating the digester,
  • avoided utility emissions from biogas combustion,
  • avoided fertilizer application and
  • fugitive emissions and soil carbon storage resulting from application of digestate to agricultural soils.

The anaerobic digestion pathway in WARM includes options for the user to model both a wet and dry digester system. WARM also allows the user to model application of digestate to agricultural soils with and without curing. Additionally, EPA revised the method for calculating energy and emissions from transportation of materials to landfills, combustors, composting facilities, and anaerobic digesters. Each of these pathways use a consistent source for transportation impacts per mile and ton of waste transportation.

In addition, EPA made regular updates to various factors in the model using current data sources. The latest 2014 statistics on the carbon content of fuels, landfill methane generation distribution (by type of landfill), and landfill gas recovery and flaring rates have been incorporated from EPA's Inventory of U.S. Greenhouse Gas (GHG) Emissions and Sinks: 1990-2014. The non-biogenic carbon content of mixed municipal solid waste (MSW) was updated using annual waste disposal data from EPA's Municipal Solid Waste in the United States, Facts and Figures. Various aspects of the U.S. average electricity mix were updated based on the U.S. Department of Energy (DOE) Energy Information Administration's (EIA's) 2015 Monthly Energy Review while state electricity grid emission factors were updated based on the 2015 update to the eGRID database. EPA updated material properties for biodegradable materials used in modeling anaerobic digestion and landfilling based on recent papers and research. These properties include carbon content, carbon storage factors, and methane yield. These updates resulted in changes to the majority of emission factors in WARM.

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WARM Version 13

Waste Reduction Model (WARM) Version 13(736 K) , released in June 2014 and updated in March 2015, contains a variety of new material categories and revisions to existing numbers and methodology. The majority of the updates in WARM version 13 concern the model’s handling of organics, including new source reduction emission factors for food waste, an updated methodology for estimating landfill gas emissions, the inclusion of fugitive gas emissions from composting, and updates to the global warming potential (GWP) values in the model.

Additions in March 2015 Update

EPA added source reduction emission factors for two meat food waste types: beef and poultry. These emission factors include the energy and emissions associated with the upstream production of food products from farm to retail. EPA also updated the weighted average mixed food waste categories to include three options: "Food Waste", a weighted average of all food waste types in WARM; "Food Waste (meat only)", a weighted average of beef and poultry; and "Food Waste (non-meat)"; a weighted average of grains, fruits and vegetables, and dairy products. Source reduction was added as an alternative material management pathway for mixed paper (all types), mixed metals, and mixed plastics. EPA corrected errors that had applied incorrect landfilling emission factors when users selected certain landfill management options or scenarios in the tool. EPA also corrected an error that caused the incorrect regional electricity grid to be applied when some states were selected.

Additions in June 2014 Update

EPA added source reduction emission factors for several non-meat food waste types: grains, bread, fruits and vegetables, and dairy products. The revisions to the landfilling waste management pathway, prepared using a landfill gas Monte Carlo analysis model developed by James Levis and Morton Barlaz, more accurately estimate the fraction of total produced landfill gas that is used beneficially, flared, and vented to the atmosphere at landfills that manage landfill gas. This analysis improves upon the landfill gas collection efficiency modeling in WARM and updates the methane oxidation rates. In addition, the Excel version of WARM now allows users the option of selecting and reviewing results based on California regulatory gas collection scenario as one of four landfill gas collection scenarios. For information on the major changes to the landfill emissions methodology for WARM version 13, please see the Landfilling Chapter and the Landfill Gas Monte Carlo Model Documentation and Results. The composting pathway has been updated to include emissions of CH4 and N2O occurring during the composting process based on recent literature estimates.

GWP values in WARM have been updated to include those from the Intergovernmental Panel for Climate Change (IPCC) Fourth Assessment Report. In addition, EPA made its regular updates to various factors in the model using current data sources. The latest 2012 statistics on the carbon content of fuels, landfill methane generation distribution (by type of landfill), and landfill gas recovery and flaring rates have been incorporated from EPA's Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2012. Retail transport emissions for various materials were updated based on the Bureau of Transportation Statistics (BTS) 2012 Commodity Flow Survey. The non-biogenic carbon content of mixed MSW was updated using annual waste disposal data from EPA's Municipal Solid Waste in the United States, Facts and Figures. Various aspects of the U.S. average electricity mix were updated based on EIA's 2014 Monthly Energy Review while state electricity grid emission factors were updated based on the 2014 update to the eGRID database. These updates resulted in changes to the majority of emission factors in WARM.

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WARM Version 12

Waste Reduction Model (WARM) Version 12(289 K) , released in February 2012, contains several updates and improvements from the previous WARM Version 11. In this latest version of WARM, the interface displays results in metric tons of carbon dioxide equivalent (MTCO2E) as the default unit for GHG emissions, but results are still available in units of metric tons of carbon equivalent (MTCE). Additionally, this version of WARM no longer maintains macros. The removal of macros does not affect the results or functionality of the tool. All of the energy and emissions (both MTCO2E and MTCE) results are displayed automatically.

New emission factors were added for four plastics, linear low-density polyethylene (LLDPE), polypropylene (PP), polystyrene (PS) and polyvinyl Chloride (PVC). Additionally, new emission factors were developed for polylactide (PLA) and aluminum ingot. Several other emission factors were revised:

  • the emission factor for aluminum cans has been updated to include additional can manufacturing steps in the updated life cycle data as well as factor in industry-specific electricity grid mix assumptions,
  • the combustion and open-loop recycling pathways for residential broadloom carpeting now incorporate new data from Dr. Matthew Realff and
  • the emission factors for three plastics, high-density polyethylene (HDPE), low-density polyethylene and polyethylene terephthalate (PET) were updated using new life-cycle data.

The mixed recycling and mixed plastics recycling factors have both changed due to:

  1. revisions to the underlying numbers in the virgin and recycled HDPE and PET emission factors,
  2. the removal of the LDPE recycling pathway and
  3. updates to the waste generation and recovery numbers based on EPA's "Municipal Solid Waste (MSW) in the United States: Facts and Figures" report.

For information on the major changes to the updated Aluminum Cans and new Aluminum Ingot material GHG emission and energy Factors for WARM version 12, please see the Aluminum FAQ document

In addition, EPA made its annual updates to various factors in the model using current data sources. The latest 2010 statistics on the carbon content of fuels, landfill methane generation distribution (by type of landfill), and landfill gas recovery and flaring rates have been incorporated from EPA's Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2009. The non-biogenic carbon content of mixed MSW was updated using annual waste disposal data from EPA's Municipal Solid Waste in the United States, Facts and Figures and BioCycle's State of Garbage in America report. Various aspects of the U.S. average electricity mix were updated based on EIA's 2010 Annual Energy Review while state electricity grid emission factors are updated based on the eGRID database.

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WARM Version 11

This version of WARM, released in August 2010, contains several updates and improvements from the previous WARM Version 10. In this latest version of WARM, EPA modified the interface to display results in metric tons of carbon dioxide equivalent (MTCO2E) as the default unit for GHG emissions, but results are still available in units of metric tons of carbon equivalent (MTCE).

New emission factors were added for six construction and demolition (C&D) materials: asphalt concrete, asphalt shingles, drywall, fiberglass insulation, vinyl flooring, and wood flooring. Emission factors for tires were also updated: the tire recycling pathway now encompasses ground and shredded rubber applications and no longer includes retreading as a recycling application.

Additionally, the Excel version of WARM now incorporates region-specific electricity grid factors to more accurately model emissions associated with avoided generation of electricity due to landfill gas recovery in the landfilling pathway and waste-to-energy in the combustion pathway. The Excel version of WARM also includes an updated method for estimating the landfill gas collection efficiency, allowing the user to select between three landfill gas collection efficiency scenarios based on specific landfill recovery characteristics:

  • typical operation,
  • worst-case collection and
  • aggressive gas collection.

Component-specific decay rates were added for all organic materials to more accurately model the rate at which each material decays within a landfill under given landfill moisture conditions. The component-specific decay rates modeled in WARM are also available as a user-defined input in the Excel version of WARM and are based on selecting one of four landfill climate/moisture characteristics: dry, average, wet, or bioreactor. The updated research from Dr. Morton Barlaz on component-specific decay rates and landfill gas collection efficiency, upon which these new factors are based, is described further in the memorandum to EPA entitled “WARM component-specific decay rate methods,” available among the WARM background documents.

In addition, EPA made its annual updates to various factors in the model using current data sources. Statistics on the carbon content of fuels, landfill methane generation distribution (by type of landfill), and landfill gas recovery and flaring rates have been incorporated from EPA’s Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2008. The non-biogenic carbon content of mixed MSW was updated using annual waste disposal data from EPA’s “Municipal Solid Waste in the United States, 2008 Facts and Figures” and BioCycle’s “State of Garbage in America” 2008 report. These updates resulted in changes to the majority of emission factors utilized in WARM.

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WARM Version 10

This version was released in November 2009 and contains updates and improvements from the previous version 9. Both the web-based and Excel versions of WARM were updated to a new “side-by-side” interface to facilitate user input entry. The latest statistics from 2008 on national average electricity generation fuel mix, transmission and distribution losses, coal weighting for electricity generation, and generation per fuel type were added. Also added were updates on the carbon content of fuels, landfill methane generation distribution (by type of landfill), landfill gas recovery and flaring rates, and waste generation and recovery rates. The non-biogenic carbon content of mixed MSW was updated using annual waste disposal data.

The landfill carbon storage factors for mixed MSW were revised to reflect new data from Dr. Morton Barlaz and the GHG equivalencies were updated to match EPA's GHG Equivalency Calculator. New GHG equivalencies were added to show the change in emissions calculated by the user in terms of gallons of gasoline, cylinders of propane, railway cars of coal, as a percentage of the annual CO2 emissions from the U.S. transportation sector, and as a percentage of the annual CO2 emissions from the U.S. electricity sector.  The recycling emission factors for the mixed paper material types were modified to include updated recycled boxboard data. These updates resulted in changes to the majority of emission factors utilized in WARM.

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WARM Version 9

This version was released in August 2008 and contains updates and improvements from the previous version 8. The latest statistics from 2007 on national average electricity generation fuel mix were added along with updates on the carbon content of fuels, landfill methane generation distribution (by type of landfill), and waste generation and recovery rates. The landfill carbon storage factors were revised to reflect new data from Dr. Mort Barlaz and the GHG equivalencies were updated to match EPA's GHG Equivalency Calculator. Also, the 1605(b) functionality in the Excel version of WARM was removed since 1605(b) no longer supports reporting of savings from waste reduction. Finally, the waste-to-energy combustion pathway energy values (MMBTU) incorporates a revised methodology that considers the ratio of mass burn combustion facilities (17.8 percent) and the national average electric utility electricity combustion efficiency (32 percent). These updates resulted in changes to the majority of emission factors utilized in WARM.

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WARM Version 8

This version was released in August 2006 and contains several updates and improvements from the previous version. The latest statistics from 2006 on national average electricity generation fuel mix were added along with research on landfill methane generation and forest carbon sequestration, and tires were added as a new material type. These updates resulted in relatively minor changes to the majority of emission factors utilized in WARM.

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WARM Version 7

This version was released in August 2005 and contains several updates and improvements. Copper wire was added as a new material type providing additional resolution to the metals category. The latest statistics on national average electricity generation fuel consumption were added along with recent research on landfill methane generation and carbon sequestration associated with organic matter; the retail transportation component of the life-cycle was also added to the methodology. The computational methodology for landfill carbon storage and methane generation was adjusted slightly to reflect the carbon content of methane. Generation and recovery percentages were also updated based on the MSW in the United States: 2003 Facts and Figures report. These updates resulted in relatively minor changes to the majority of emission factors utilized in WARM.

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WARM Version 6.1

This version was released in December 2004 and included a couple of minor revisions to the prior WARM version 6. The emission factor for transportation of waste to landfill by combination truck was revised. In addition, revisions were made to the landfill-gas-to-energy offsets for users interested in reporting to the U.S. Department of Energy’s 1605(b) program. The landfill-gas-to-energy offsets for 1605(b) users were revised to be phased in over 30 years, rather than the total offset being applied in year one.

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WARM Version 6

This version was released in March 2004 and incorporated several updates and improvements. Five new material types were added: personal computers, carpet, clay bricks, aggregate, and fly ash. Emission factors for all other materials were updated based on:

  • new information related to the current mix of recycled and virgin inputs;
  • new data on municipal solid waste generation and recovery from the 2001 Facts and Figures; and
  • the energy and greenhouse gas equivalencies were updated based on more recent statistics from DOE's EIA.

These equivalencies are intended to put results in units that may be more “tangible” than British thermal units or metric tons of carbon (e.g., number of barrels of oil, number of households’ annual energy consumption, and number of gallons of gasoline).

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WARM Version 5

This version was released in December 2003 and included revised estimates for perfluorocarbons (PFC) emissions and carbon anode carbon dioxide emissions associated with aluminum production based on data reported in the Inventory of U.S. Greenhouse Gases and Sinks: 1990-2000.

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WARM Version 4

This version was released in May 2002 and included the latest data on municipal solid waste characterization based on the 2000 Facts and Figures Report, and electricity generation data for the year 2000 from the DOE’s EIA. Additionally, this version uses life-cycle energy data sources from EPA's Office of Research and Development (ORD) (Research Triangle Institute (RTI), rather than the Office of Solid Waste (OSW) (Franklin) as used in previous versions. In general, ORD's data set of energy, fuel mix, and loss rates is likely to be more up-to-date than some of the information from OSW. The ORD data was used for those materials with a complete set of ORD energy intensity and fuel mix data. This information was not available for wood products or steel because ORD did not develop emission factors for wood products and the ORD steel data was not sufficiently disaggregated to replace the existing OSW data.

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WARM Version 3

This version was released in November 2001 and reflected several improvements made to the previous version of WARM (version 2). For participants in the U.S. DOE’s 1605(b) program, results and summaries could now be viewed by gas, phased over time, and phased over time by gas. The ability to view results by gas shows emissions for carbon dioxide, methane, nitrous oxide, tetrafluoromethane, hexafluoroethane specifically, rather than only total emission reductions in terms of metric tons of carbon or carbon dioxide equivalent.

The phased results and summaries show the emissions of the waste management practices for year one, for years 2-15, for years 16-30, and for all years. Results and summaries phased by gas shows results for the five gases for year one, years 2-15, years 16-30, and for all years. New materials were also added to this version and the transportation to landfill calculations were updated using data on carbon dioxide emissions per mile of freight transport.

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WARM Versions 1 and 2

The first version of WARM was released in 1998, followed by the release of version 2 in December of 1999. At this stage, WARM was undergoing rapid change and growth in terms of its capabilities. These first versions included 17 material types (metals, plastics, organics, and mixed paper and recyclables) as well as the basic options still available in WARM such as the ability to specify current mix or virgin materials, the type of landfill gas control system, and the transportation distance to the waste management options.

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