  We explore the class of trilevel equilibrium problems with a focus on
energy-environmental applications and present a novel single-level
reformulation for such problems, based on strong duality. To the best of our
knowledge, only one alternative single-level reformulation for trilevel
problems exists. This reformulation uses a representation of the bottom-level
solution set, whereas we propose a reformulation based on strong duality. Our
novel reformulation is compared to this existing formulation, discussing both
model sizes and computational performance. In particular, we apply this
trilevel framework to a power market model, exploring the possibilities of an
international policymaker in reducing emissions of the system. Using the
proposed methods, we are able to obtain globally optimal solutions for a
five-node case study representing the Nordic countries and assess the impact of
a carbon tax on the electricity production portfolio.
