Page 79 - Managing Global Warming

Greenhouse gas removal and zero emissions energy production 51

global scale over many decades. Although reducing emissions to zero is climatically
the optimum outcome, this may not be politically or socially feasible given the extent
to which FF are embedded in modern life, and our commitment to the never-ending
increase in consumption implicit in the prevailing imperative for continuing economic
growth. I suggest a range between 0 and 10Gt(C)/yr for the plausible value of the
eventual stable level of emissions. This wide range reflects the uncertainty, and pro-
vides an opportunity to examine the policy implications of values at the extremes of
the range. It does not preclude emissions peaking at higher levels before stabilizing
within this range, although it ignores the potential negative impacts of hysteresis in
overshoot scenarios.
The final variable in this group is the time it takes for emissions to stabilize at the
chosen level. The drivers here are sociotechnological and political. Many historical
precedents suggest that major structural changes to deeply embedded systems take
more than a single generation. For example, Fig. 2.4A and B show the transition from
wood through FF to modern renewables. These figures are based on US data but a
similar, albeit shorter, story is apparent from global data [14]. As much wood and coal
are burned today, as were ever burned. New fuels have struggled to keep pace with
economic growth and never supplanted previous energy sources. Fig. 2.4A shows that
coal took 60 years to reduce wood’s market share from 100% to 10%, oil took 70 years
to reduce coal’s share from 70% to 15%, and the combined share of coal, oil, and nat-
ural gas has reduced only marginally in the last 50 years, a period in which energy
consumption has more than doubled. There is clearly a great deal of inertia in the
global energy economy. This is unsurprising given the high levels of investment in
relatively long-lived energy assets and the costs of retrofitting legacy assets to cope
with new fuels. It seems prudent to assume that it is not only plausible but also prob-
able that new fuels will follow these repeated historical trends (see Section 2.7).
In setting a range of plausible values for the stabilization period, a reasonable sup-
position is that the lower the eventual level of emissions, the longer it will take to
reach. Fig. 2.5 illustrates a plausibility range according to the degree to which emis-
sions are to be reduced below their peak. This plausibility range is difficult to specify.
Given the slow progress of the international community in reaching the Paris Agree-
ment, it seems implausible that, at one extreme global emissions could be eliminated
in less than 40 years, or at the other, that it could not be done within 120 years.

2.5.2 Reduction in energy consumption (REC)
Population and consumption per capita are the primary drivers of TFC, but it is also
influenced by energy efficiency. As already noted (see Fig. 2.2 supra), the long-term
trend in energy consumption reflects an annual reduction of about 1% p.a. in the
energy required to produce each unit of GDP. This trend is reflected in the extrapo-
lation of future energy demand and therefore the factor under consideration here is the
potential for an incremental increase in efficiency. It is unclear to what extent policy
interventions can further improve the underlying trend toward greater energy effi-
ciency and simultaneously prevent it leading to greater consumption through the
rebound effect [7].