The CO2 concentration limitation below 500 ppmv in case of constant future emissions raises the question: Which economic growth rate will be possible from the intended transition to renewables?
To pursue a calculational example, the constant emissions scenario is considered as target. Furthermore, the total CO2 emissions are considered as originating from fossil material usage, land use change, and cement manufacture. Land use change and cement manufacture are estimated to contribute with 17 % and 6 % to the total emissions, respectively. Additional contributions from volcanoes are negligible and therefore disregarded. As mentioned, natural intake and uptake of atmospheric CO2 are presumed to be in unchanged balance since pre-industrial times.
Details on the world energy consumption are taken from [6, 7], the respective CO2 allocations from [2]. For year 2020, the total globally delivered energy is estimated at 119,000 TWh per year, with a share of 23 % in electric energy and 77 % in direct primary energy (Fig. 2). The latter comprises the fossil material usage for other applications than electricity generation. Of the electric energy, 65 % is generated from fossil fuel and 35 % by the further technologies, i.e. particularly hydropower, nuclear energy, wind, biomass, photovoltaics, and geothermics. The present growth rate of annual energy demand is of the order of 1,6 % per year.
As renewables scenario, it is assumed that all electricity generation from fossil fuel is being replaced by renewable technologies at constant pace within 25 years. Taking CO2 investments for provisioning of the renewable equipment into account, the CO2 emissions typically are reduced by 43 % within the first 25 years, and by 95 % afterwards. These figures are derived for average total CO2 emissions of 30 g CO2 per renewables-generated electric kWh. This emissions value has exemplary character, representing the actual low end for photovoltaics and 1.5 times the anticipated conditions for wind power. The renewables electricity generation is taken continuous, disregarding the need of a buffering system. Within this parameter set, the implicit optimistic and conservative biases are expected to be balanced.
At present, 27 % of the total annual global CO2 emissions originate from fossil fuel electricity generation. The above-mentioned emissions reduction of 95 % in this sector corresponds to a reduction of 26 % with respect to the total emissions. On a time horizon of 100 years, this reduction corresponds to an annualized reduction rate of 0.3 %/year. In result, the transition from fossil fuel to renewables in electricity generation opens an overall growth allowance of the order of 0.3 %/year.
Due to the efficiencies of fossil fuel usage in electricity generation, about 2.8 times the generated energy needs to be provided by the raw material. In applications others than electricity generation, this efficiency factor is highly specific to the particular process, rendering its explicit consideration inappropriate for an order-of-magnitude view; hence, the delivered energy is taken as the energy contained in the raw material. On this base, the residential and commercial sectors are estimated to contribute 7 % to the total CO2 emissions. If – to explore the extreme – all these emissions were eliminated and this regarded on a 100-years horizon, it would translate to an annualized reduction rate below 0.1 %/year relative to the total present emissions.
Presently, 66 % of the total energy consumption originate from the non-electricity applications in the sectors industry and transportation, corresponding to 43 % of the total CO2 emissions. Replacement of fossil techniques by low-emission ones appears less straightforward than in other application areas. As indication, we know of the difficulties to size the real CO2 footprint of electric vehicles. Also, new technologies in heavy transportation are hardly in sight yet, and the ship engines are just at the verge of a first transformation stage. In the industry sector, e.g. existing process heat techniques may need to be migrated to new methods which is to be considered challenging.
Assuming 30 % of the CO2 emissions in these two sectors (industry and transportation) can be reduced within 30 years – perceived achievable – this corresponds to a reduction of 0.4 %/year relative to the total emissions, bearing an overall growth allowance of the same size.
Since 23 % of emissions originate from cement manufacture and land use change (see above), reductions in these areas can add valuable additional growth allowances. A detailed elaboration is disregarded in lack of currently imaginable reduction mechanisms.
The thus described transition to renewables leads to carbon emissions as summarized in Fig. 3, relative to the present emissions. The residual emissions from electricity production as well as in the sectors residential and commercial nearly vanish; the sectors industry and transportation retain 30 % of current levels; emissions from cement manufacture and land use change are carried forward from the present as reasoned; 46 % are considered as feasible reduction relative to the present.
Conclusion: A total economic growth window of 0.8 %/year may be opened by the migration from fossil material usage to new techniques. This growth allowance represents the upper boundary of ‘Green Growth’, first due to the optimistic translation from carbon emissions into atmospheric CO2 concentration. Second, the upper limit of 500 ppmv for the future CO2 concentration has exemplary character; the temperature projected for this concentration may need to be judged too high as a target to aim for. Third, presumption is that carbon emissions remain constant from the presence which may be assessed unrealistic. Fourth, effects not regarded, e.g. from induced ice/snow albedo change, are to be accounted for with further temperature contributions (from risk point of view, of the order 1 to few °C, perhaps partly substantiating on the long-term horizon).
Of the total growth window, about 40 % is anticipated from the renewables migration in electricity generation. To 50 %, the growth allowance is attributed to migration efforts in the sectors of industry and transportation. This reveals that transitions in these sectors are an important key if mankind is to continue their abiding striving for economic growth. Regarding the existing societal mechanisms, globally homogeneous taxation of CO2 emissions appears as a viable tool to stimulate potential transformations.