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46 Managing Global Warming

2.4.2 Zero emissions energy only

The ZEE-only scenario represents a policy of maintaining TFC growth at historical
levels and relying on reducing emissions by transitioning to ZEE to stay within the
carbon budget. The amount of ZEE required varies according to assumptions about
the level of residual FF emissions. If these emissions are brought to zero within
30 years, the reduction must begin in 2022 (Scenario 2) or 2028 to meet the 1.8°C
and 2°C targets, respectively. The 1.5°C target requires emissions to peak in 2017
and reduce to zero within 16 years.
If FF emissions are assumed to stabilize at 2Gt(C)/yr, about 20% of 2015 levels,
and peak in 2025, meeting the 2°C target would require them to reduce to this level by
2044 (Scenario 3). 1.5°C is unachievable and 1.8°C requires emissions to be reduced
to 2Gt(C)/yr within 1year. With residual emissions of 4Gt(C)/yr, the 2°C target
requires that emissions peak in 2019 and reduce to 4Gt(C)/yr within 1year.
In all cases, the continuing land use emissions mean that at 2100 the cumulative
emissions remain on a rising trend breaching the carbon budget thereafter. Only
the introduction of GGR can ensure that cumulative emissions plateau below the
relevant carbon budget.
It is also noteworthy that in all the variations of the ZEE-only scenario, the ZEE
Index exceeds 12. The decline in emissions is so rapid that the bulk of TFC must come
from ZEE.

2.4.3 Zero emissions energy/Reducing energy consumption
ZEE/REC introduces the policy of reducing energy consumption (REC). The impact
of REC depends upon when it begins and how deep it is. In scenarios with an early,
rapid, and deep transition away from FF, REC has almost no impact on cumulative
emissions, but it does significantly reduce the amount of ZEE required later in the
century. For example, for the 1.8°C scenario with emissions peaking in 2020 and
reducing to 1Gt(C)/yr within 25 years, a reduction in energy consumption starting
in 2018 and reaching 50% within 30 years has no significant impact on cumulative
emissions but reduces the ZEE Index from 14 to 6 (Scenario 4). This arises because
the calculator assumes that if energy consumption falls, established energy suppliers
with low marginal costs will respond to the lower demand by reducing prices, in line
with conventional supply and demand market economics. Those investing in new
energy sources do not have this price flexibility because their marginal costs will
be less elastic. For this reason, short-term reductions in energy consumption are
assumed to reduce the consumption of ZEE in preference to FF, and therefore have
little impact on emissions.
There are two scenarios in which these assumptions would be invalid. First, if the
marginal cost of ZEE falls consistently below that of FF; and second, if governments
use policy levers to force the early retirement of FF energy production. The impact of
these options on emissions can be examined in the calculator by varying the FF reduc-
tion rate parameter. In the last example, if emissions were reduced over 12 rather than
25 years, the 1.8°C target could still be met either with residual emissions reducing to