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1. Introduction
1.1 EU goals
A so-called 20-20-20 target was set by the EU to achieve 20 per cent energy savings, 20 per cent increase in production of renewables and 20 per cent carbon emission reductions by 2020. This ambition was followed by the EU Energy Building Performance Directive that by 2020 all newly constructed buildings need to be “nearly” zero energy. These aims have now been surpassed by the Paris Climate Treaty Agreements, which aim for a carbon neutral built environment by 2050.
The greatest challenge lies in the approach of the existing built environment. Started in 2014, the EU FP7 funded project called City-zen addresses this urban energy transition assignment. It tests new technologies and develops methods and tools, spatial-technical as well as social-economical and legal-political, to help cities getting the transition started. As part of City-zen, so-called Roadmaps are developed for the two partner cities, Amsterdam and Grenoble. These Roadmaps should pave the way towards a desired future state.
1.2 Definitions of sustainable ambitions
Before sustainability-related targets and goals can be set, a clarification of possible ambitions is needed, because in debates there is a mix-up and confusion about terms such as zero carbon, zero energy, fossil free and circular. Therefore, we first state our practical definitions of these terms.
(Net) zero carbon, carbon neutral, climate neutral
In a living system as we know on earth, total absence of carbon emissions is impossible, so when people talk of “zero carbon”, they mean “net zero carbon” or “carbon neutral”, related to a certain period, mostly one year. Since the ambition of carbon reduction is related to climate change mitigation, the goal is actually to become “climate neutral”, encompassing also other greenhouse gases (GHG). Simplification to “carbon neutral” therefore requires a conversion of other GHG emissions to “carbon equivalents”. A clean definition of (net) zero carbon, carbon neutral or climate neutral is that, over a year’s time, the net GHG emission of the system considered is zero. For this purpose, carbon emissions from fossil fuels may be sequestered, for instance by storage in the underground, or by functional use in horticulture or industry, or compensated by planting green. Indirectly, in order to become climate neutral, compensation is possible through carbon trading (of CO2