Timing+of+Emissions

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.**//

ICLEI's CAPPA tool is based on WARM's lifecycle emission factors. In order to make waste reductions comparable to the wide variety of other types of emissions reducing activities from the energy and transportation sector, CAPPA presents landfill methane reductions in terms of an approximate annual reduction based on the ultimate generation figure for a waste type over the time period it would likely be emitting. This simplified approach is intended to be applied to a portion of materials that would be diverted year over year from an ongoing program rather than a single year of diverted material, which would have a variable rate of resultant emissions as it degrades over time. By estimating the portion of landfill emissions that would have occurred during the year in which the waste reduction takes place, //**CAPPA begins to address the timing of**// //**emissions,**// though as a matter of the presentation for policy relevance rather than from an emissions modeling point of view.

The methodology for addressing the timing of emissions is unfortunately not straightforward, and will require future work. Alternate approaches might take into account:
 * 1) 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.
 * 2) 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.
 * 3) 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.