Methods of Modeling Construction Emissions Have Large Impacts on Short-Term Warming Assessments

Switching from greenhouse gas-emitting energy sources to lower-carbon energy systems is necessary to reduce global warming emissions, but constructing the infrastructure required for new technologies generates greenhouse gas emissions. When addressing short-term warming impacts, it is important to reliably characterize the payback or breakeven times required for the accumulated technology switching benefits to match the climate impacts of construction emissions. Case studies have demonstrated that analysis methods used to account for the time required for construction and the temporal behavior of radiative forcing of short-lived climate forcers, such as methane, significantly impact payback or breakeven times. This work explores whether the findings from these analyses can be generalized. Analyses of approximately 100 scenarios indicate that methods that do not take the timing of infrastructure construction and the temporal variation of the global warming potency of methane relative to carbon dioxide into account are likely to underestimate infrastructure construction breakeven times by a factor of 2 or more over a wide range of scenarios. There are, however, scenarios where the underestimates could be larger or smaller by more than a factor of 2. Relatively small changes, particularly in operational emissions, can have a substantial impact on infrastructure construction breakeven times as well as long-term behavior. Scenarios illustrating these extremes in behavior are presented. A spreadsheet tool for calculating infrastructure construction emission breakeven times is provided. Reliable estimates of breakeven times require that (1) infrastructure construction emission inventories account for different species of greenhouse gases separately and (2) temporal variations in the greenhouse gas potencies of different species relative to that of carbon dioxide and the timing of construction emissions are taken into account.