  The mitigation of climate change requires a fundamental transition of the
energy system. Affordability, reliability and the reduction of greenhouse gas
emissions constitute central but often conflicting targets for this energy
transition. Against this context, we reveal limitations and counter-intuitive
results in the model-based optimization of energy systems, which are often
applied for policy advice. When system costs are minimized in the presence of a
CO2 cap, efficiency gains free a part of the CO2 cap, allowing cheap
technologies to replace expensive low-emission technologies. Even more striking
results are observed in a setup where emissions are minimized in the presence
of a budget constraint. Increasing CO2 prices can oust clean, but expensive
technologies out of the system, and eventually lead to higher emissions. These
effects robustly occur in models of different scope and complexity. Hence,
extreme care is necessary in the application of energy system optimization
models to avoid misleading policy advice.
