MESSAGEix

MESSAGEix is a framework for building models of energy systems and for integrated assessment, the kind of model that ties energy, the economy, the environment and climate together.^1,2,8 It is made by the Energy, Climate, and Environment Program of IIASA, which has developed MESSAGE models for decades.^1,2 MESSAGE itself is a linear optimisation model for strategic energy planning: it finds the lowest-cost way to meet given demands for energy within technical and policy limits.^2,3,5 It can be linked to MACRO, a model of the whole economy, so that energy prices feed back into how much energy people demand.^2,3 The framework and its data platform, ixmp, are described by Huppmann and ten co-authors in Environmental Modelling & Software, 2019.^5,8

A model built with it describes a "reference energy system": fuel supply chains, the plants that convert primary energy into fuels and electricity, transmission and distribution, and final demand for energy services.³ It can track the age of each plant, early retirement, and how variable renewable energy fits into the system.³ It can solve with perfect foresight over the whole period or myopically, one period at a time.^3,7 The tutorials use small single-country models; IIASA's global model is far more detailed.^2,3

IIASA's global model, MESSAGEix-GLOBIOM, links the energy model to the land-use model GLOBIOM, the air-pollution model GAINS, MACRO and the simple climate model MAGICC, and it was used to develop the Shared Socioeconomic Pathway scenarios.⁵ It divides the world into 11 regions, sometimes 14.⁶ It usually runs from 2010 to 2110 in periods of five or ten years.⁷ Its code and documentation are a separate package, message-ix-models.^1,10

message_ix is a Python package, fully usable from R, under the Apache-2.0 licence, installed from conda-forge or with pip.^1,2 The model equations are written in GAMS, a commercial modelling language that needs a licence to solve.^3,4 A free demonstration licence, available on request from GAMS, is enough for the tutorials and tests.⁴ The documentation includes the full mathematical formulation, generated from the code, and tutorials for self-study and classrooms.² Published work must cite the framework.² The latest release is version 3.11.1, from June 2025.⁹

Someone building a model of a country's or region's energy system can start from the tutorials and add its own plants, fuels and demands, then compare scenarios of how that system could change under different technical limits and policies.^2,3 For other global integrated assessment models, see GCAM and REMIND.