Elevated energy efficiencies seem politically beyond dispute as measures against anthropogenic climate change. Energy efficiencies, however elevated they become, remain within the applied energy system, though. The paper draws attention to the exergo-thermodynamical system change of splitting up energy into exergy and anergy where exergy is the maximum of available technical work drawn from energy. Consequently, it is the elevated exergy efficiency, which deserves the attention of energy engineers helping to reduce greenhouse gas emissions. In this context, hydrogen energy is irreplaceable, hydrogen exergizes energy! It makes more technical work available (=exergy), it avoids exergy destruction and the built-up of huge amounts of useless anergy. (C) 2007 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.