.. _policy_overview:

Policies
========
A number of different energy- and climate-related policies are, depending on the scenario setup and the research question addressed, explicitly represented in |MESSAGEix|. This includes the following list of policies:

* GHG emission pricing
* GHG emission caps and trading emission allowances
* Renewable energy portfolio standards (e.g., share of renewable energy in electricity generation)
* Renewable energy and other technology capacity targets
* Energy import tariffs
* Fuel subsidies and micro-financing for achieving universal access to modern energy services in developing countries (via linkage to the `MESSAGE-Access model <https://iiasa.ac.at/web/home/research/researchPrograms/Energy/MESSAGE-Access.en.html>`_)
* Air pollution legislation packages (fixed legislation, current and planned legislation, stringent legislation, maximum feasible reduction via linkage to the `GAINS model <https://iiasa.ac.at/web/home/research/researchPrograms/air/GAINS.html>`_)

In general, these policies are implemented via constraints or cost coefficients (negative and positive) in the optimization problem (see Section :ref:`policy` for more details). In the case of air pollution policies, the different legislation packages are implemented via a set of emission coefficients and associated costs derived from the `GAINS model <https://iiasa.ac.at/web/home/research/researchPrograms/air/GAINS.html>`_. The cost coefficients are, however, not part of the optimization procedure, but instead allow an ex-post quantification of air pollution policy costs for a specific energy scenario.