The article analyses CO2 emissions from fossil fuel combustion and provides tools for estimating the target for national carbon intensity of energy by 2050. A case study of the energy system of Poland is presented. It is revealed that if carbon emissions are to be reduced by 80% from 1990 to 2050, the energy mix of Poland will require significant structural changes. This will be however challenging, because Poland has a huge coal sector and limited potential for harvesting renewable energy sources, excluding only biomass. The carbon intensity of primary energy would have to be reduced in Poland from 20 g C MJ(-1) in 2009 to about 4.1 g C MJ(-1) in 2050 (including LULUCF). Further, the study suggests the national energy mix suitable for achieving this energy decarbonisation target. It is found that a significant share of coal/peat can be retained only when CCS is applied to all fossil fuel-fired power plants and to large-scale industries. Besides, the share of biomass, other renewables and optionally nuclear energy must be significantly increased which will be both costly and technologically challenging. Further, deployment of carbon negative bioenergy as well as CO2 recycling are suggested as promising energy decarbonisation options. (c) 2012 Elsevier Ltd. All rights reserved.