EU Heads of State and Government agreed in March 2007 that the EU will reduce its greenhouse gas emissions to 30% below 1990 levels by 2020 within an international post-2012 climate agreement, provided that other developed (Annex I) countries commit to comparable reductions. Within this context, this paper first explores the pros and cons of many possible conceptual approaches to assess the comparability of the mitigation efforts by Annex I countries. We selected six approaches for further analysis, which represent efforts well and are technically feasible. The implications of each of these six approaches were analysed in terms of the reductions and abatement costs that must be made by different Annex I countries to meet an aggregate reduction of 20% and 30%, respectively, below 1990 levels by 2020. The analysis indicates that significant reductions are necessary for all developed countries. This study shows that reductions by the EU of at least 30%, combined with comparable reduction efforts by other developed countries to meet the aggregate Annex I reduction target of 30% by 2020 and support of developed countries for developing countries to keep their emissions 15-30% below the baseline, are sufficient to achieve the EU climate goal of 2 degrees C. (C) 2009 Elsevier Ltd. All rights reserved.