The paper introduces basic concepts and terms of energy that deal with adaptation to and mitigation of climate change. Descriptions are also provided of the more commonly used energy units and measurements. The global energy system is described, and energy consumption patterns and CO2 emissions data are presented. The concept of energy efficiency improvement potential is introduced. A historical perspective of energy consumption and CO2 emissions for the world is made, with a comparison of current energy consumption and estimates of fossil and nuclear energy reserves and resources, and the potential of renewable energy sources.