Note that not all IAMs reported all ten of the possible
scenarios (Table 1). The failure of the IAMs to report
scenarios is attributable to a wide range of factors, ranging
from technically not being able to find a solution, to having
found a solution but with the carbon price being “too high,”
to simply not attempting to run the scenario. As such,
individual modeling teams were required to report the
feasibilities of the scenarios that they were asked to run, by
indicating “NR” (not run) for the not-attempted scenarios,
“INF” (infeasible) for the scenarios that failed to solve or
generated an excessively high carbon price, and “F” (feasible)
for all other scenarios. 57 feasible scenarios out of 90 possible
scenarios were reported and used for the analysis. Note
that the number of IAMs that find a scenario feasible varies
widely among technology availability assumptions, so that
conclusions drawn from direct comparison across alternative
technology availability cases in this study may be subject
to sample selection bias. We will refer to Table 1 to discuss
feasibilities in various scenarios.
3. CO2 emission pathways toward 450 ppm CO2e
Achieving 450 ppm CO2e stabilization in 2100 requires
major changes in the global energy system. The character of
the transformation pathway reflects near-term climate policy
assumptions as well as the long-term goal [17]. To the extent
that the HST pathway undertakes less near-term emission
mitigation than the OPT pathway, it reduces the remaining
allowable emission budget.
3.1. Transition pathways with optimal near-term policies (OPT)
In the FullTech cases, bioenergy (BE) with CO2 capture
and storage (CCS) technology is available for all IAMs. The BE
and CCS (BECCS) technology combination facilitates energy
production with net CO2 removal from the atmosphere.
This in turn means that a fixed budget can be met through
“overshoot” pathways, in which the long-term target is
temporarily exceeded but eventually reduced to target levels
through BECCS.
In the FullTech-OPT scenarios, nearly all IAMs report negative
net global emissions by 2070 (Fig. 1). Negative emissionsmostly
come from BECCS and also terrestrial sequestration through
land-use change such as afforestation and soil carbon enhancement.
By 2050, emissions have been reduced to a small fraction
of reference levels.