Climate change is one of the most important issues our world faces today and it is responsible for a number of natural disasters that threaten human life and existence. Carbon dioxide, produced from almost every energy consuming activity, is the dominant greenhouse gas responsible for global warming. Water desalination is an energy intensive activity, and when it is powered by conventional energy sources, significant amounts of CO2 are released. For every cubic metre of fresh water produced, there is a 2 kg of CO2 reduction if renewable energy sources (RES) are used instead of electricity from the local grid. On the other hand, the cost of fresh water produced by desalination is much less if conventional sources of energy are used. Making appropriate policy choices require information on both costs and benefits. So here we estimate the critical CO2 cost, above which desalination units should use renewable energy instead of conventional energy sources. It was found that the critical CO2 emissions cost can be close to the CO2 capture cost and in many cases less than the penalties imposed by the European Commission. Several case studies of water desalination in the Aegean islands verify the conclusions. (C) 2010 Elsevier Ltd. All rights reserved.