In current policies, targets for GHG (greenhouse gas) and more specifically CO2 emissions are set on the basis of annual emissions. However, warming effects associated with global average temperature rise depend on accumulation of GHG in the atmosphere. Due to the quantity and longevity of CO2 in the atmosphere there is increasing awareness that taking into account cumulative CO2 emissions in defining targets for restraining the growth of CO2 emissions would be particularly effective. The notion of effectiveness is linked to measuring the degree of achievement of the objectives. Considering CO2 emissions targets set over a few decades, the path of emissions reduction contains relevant information that cannot be captured by a classical measure like the distance to the target. The main contribution of this paper is the definition of an original measure of carbon dioxide reduction effectiveness, which allows comparison of specified CO2 reduction paths expressed in deterministic or probabilistic ways. Appropriate metrics are used to illustrate the proposed measure which in particular captures the importance of early action. The chi(0) measure is applied to simple what-if scenarios for the EU27 electricity and heat sector to 2050, evaluating the impact of each scenario with respect to a reference case. (C) 2011 Elsevier Ltd. All rights reserved.