  Information and communication technologies (ICT) are increasingly seen as key
enablers for climate change mitigation measures. They can make existing
products and activities more efficient or substitute them altogether.
Consequently, different initiatives have started to estimate the environmental
effects of ICT services. Such assessments, however, lack scientific rigor and
often rely on crude assumptions and methods, leading to inaccurate or even
misleading results. The few methodological attempts that exist do not address
several crucial aspects, and are thus insufficient to foster good as-sessment
practice. Starting from such a high level standard from the European
Telecommunication Standardisation Institute (ETSI) and the International
Telecommunication Union (ITU), this article identifies the shortcomings of
existing methodologies and proposes solutions. It addresses several aspects for
the assessment of single ICT services: the goal and scope definition (analyzing
differences between ICT substitution and optimization, the time perspective of
the assessment, the challenge of a hypothetical baseline for the situation
without the ICT solution, and the differences between modelling and case
studies) as well as the often ignored influence of rebound effects and the
difficult extrapolation from case studies to larger populations.
