Harald Heinrichs · Pim Martens Gerd Michelsen · Arnim Wiek Editors

5 Scenarios and Pathways Towards Future Sustainable 
Energy Systems
Over the last years many outlooks or scenarios have been developed by various 
stakeholders ranging from 
Greenpeace (2012
) and the WWF (Deng et al. 
2012
) to 
the International Energy Agency (IEA) and others which try to quantitatively 
describe the future development towards a global energy system that is compatible 
with the target to prevent dangerous climate change. For the energy system this 
means that global greenhouse gas emissions from the energy system (and other 
sources) have to be cut by at least 50 % by the mid of the century compared to emis-
sions in 1990.
Figure 
19.5
shows a result from a scenario analysis commissioned by the WWF 
(
Deng et al. 2012
) which can be used as an example. It reveals that in a baseline 
scenario – if no intensified climate and environmental goals are pursued globally – 
energy use and related GHG emissions will constantly increase over the next
decades (dotted line in Fig. 
19.5
) putting global climate at high warming risks and 
causing a range of other problems.
Contrasted to this is a 100 % renewable energy scenario with two major strate-
gies towards a more sustainable system – (i) higher end-use efficiency, energy sav-
ings and electrification (“sufficiency” and “efficiency”) and (ii) the substitution of 
fossil fuels by renewable sources (decarbonisation or “consistency”):
The Rebound Effect in Energy Policy
Despite widespread acknowledgement of energy efficiency as a core strategy 
for sustainable energy systems, it is often claimed that rebound effects will 
absorb a large part of the energy savings or even lead to higher energy con-
sumption in the long run.
There are however different effects that are mixed under the term rebound. 
Direct rebound effects which are directly and short-term linked to energy effi-
ciency measures (e.g. heating a well-insulated building to a slightly higher 
temperature or the more frequent use of a fuel-efficient car) are estimated in 
several studies for OECD countries to be between 0 and 30 % and expected to
decline. This means that more than 70 % of the energy savings achieved 
through energy efficiency remain.
The effect of other, often more long-term and structural, so-called indirect 
and macroeconomic rebound effects as well as their attribution to energy effi-
ciency policy is, however, more unclear. While orthodox economics believe 
them to be small, ecological economists have argued that they are likely to be 
large or even “backfire” (Sorrell 
2010
). We, however, are convinced that 
given the narrow absolute global limits of natural resources, using them as 
efficiently as possible is a necessary strategy to supply everybody with ade-
quate amounts of energy services.
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• About 50 % of the energy demand expected by 2050 under baseline conditions 
can be avoided by exploiting the large existing potentials for energy efficiency in 
households, commercial and industrial sectors as well as in transportation and in 
the energy sector itself.
• The remaining energy demand is almost fully decarbonised by the expansion of 
renewable energies. The main share of the remaining future energy demand will 
be delivered by renewable electricity from wind energy, PV cells and other solar 
energy, hydropower and other sources, while transport and heating fuels will 
come from biomass, geothermal energy and other renewable sources.
Together, this makes it possible to almost completely phase out fossil fuels and 
nuclear energy globally.
As can be seen in Fig. 
19.5
, the use of fossil and nuclear energy will not decline 
before 2020. The reason for this is that all technical strategies towards a sustainable 
energy system need some time to gain momentum.
As energy systems consist in part of very long-lived infrastructures and long- 
lived goods and equipment, it takes time to make the complete shift to improved 
technologies such as plug-in hybrid-electric cars (few decades), LED lighting (some 
years) and efficient buildings (many decades). Furthermore, saving almost 50 % of 
the energy and supplying the rest from renewable sources will require the develop-
ment of new and improved technologies. The costs and, or, reliability and perfor-
mance of many technologies still have to be improved, and production lines have to 
be established.
Thus, in order to realise such ambitious strategies as depicted in many of the 
scenarios, most of the low-carbon technologies will be needed soon, and to avoid 
delays in the realisation of the scenarios, the remaining time for successful R&D is 
already short. Therefore, it is necessary also to revise the priorities of (public) R&D 
funds which were in the past mainly focussing on nuclear energy, fossil fuels and 
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