Ce document constitue le rapport détaillé du projet de recherche GECS (EVK2-CT-1999-00010), faisant partie du 5ème Programme cadre de la Direction générale de la recherche de l'Union européenne (Thème : environnement et développement durable). Il comprend sept chapitres : 1- General introduction ; 2- The international climate negotiation and the long term GHG constraints (GECS work package 1.) ; 3- GHG emission trends, abatement technologies and marginal abatement cost curves (GECS work package 2a.) ; 4- GHG emissions from agriculture: world projections with the IMAGE model and the modelling of GHG constraints with the AGRIPOL model (GECS work package 2b.) ; 5- The development of the POLES world energy model of the GEM-E3 economy model for multi-gas assessment studies (GECS work package 3.) ; 6- Multi-gas assessment studies with the POLES world energy model and the GEM-E3 economy model (GECS work package 4.) ; 7- General conclusions