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.