Serious concerns about the existence of a price bubble within the European Union Emissions Trading System
(EU ETS) emerged during its third trading period. Existing bubble tests based on costs for switching from
cheap, polluting to costly, clean energy sources are restricted to situations of market certainty. This limitation
is unrealistic, considering the ongoing CO2 reduction measures. Additionally, the fundamental value is not
uniquely identified, leading to inconclusive empirical findings. We apply a robust approach to infer bubbles
in the EU ETS. Our findings do not support the presence of a bubble in the third or fourth trading period.