The WARM model quantifies avoided lifecycle GHG emissions due to source reduction, reuse, and recycling activities. The emission factors have components that represent changes in GHG emissions that may not necessarily occur in the year during which the source reduction, reuse, or recycling takes place. No precise methodology currently exists for addressing the timing of emissions.
The methodology for addressing the timing of emissions is unfortunately not straightforward, and will require future work. Alternate approaches might take into account:
- Waste decay rates. Emissions may be highest in the first year or two after the waste is deposited in the landfill. For example, given a typical waste decomposition half-life of 18 years, the methane generated in the first year is 1/27th of the total methane generated over the waste's lifetime. Decay rates, however, are complicated by differences by material and climate.
- Yearly waste history. Considering source reduction, reuse, and recycling activities over multiple years gives credit for avoided GHG emissions in a given year, regardless of when the waste reduction activity took place. With enough historical information, Waste-In-Place models can estimate emissions that would have occurred in a given year. In the absence of yearly waste history, there is a mathematical model that assumes annual equal installments of waste.
- Forest carbon sequestration. It is not clear to what extent the WARM emission factors for deforestation are sensitive to the lifetime of trees, their rate of carbon sequestration, and the timing of forestry practices.
For an example of an early effort to account for the timing of emissions and emissions reductions associated with waste disposal and waste reduction, click here.