The major part of worldwide energy production concerns fossil energy. Therefore, concomitant carbon dioxide production is a problem. What are suitable alternatives? Here we present considerations for selection of energy sources needed for future worldwide energy consumption. Plants are collectors and converters of sunlight and are indispensable in the carbon cycle. This cycle converts sunlight to chemical energy, and next frees this energy by biochemical oxidation in humans and other organisms, or by chemical oxidation. Currently, the carbon cycle is strongly disturbed by abundant use of fossil fuels. Sunlight- and
wind energy-collecting and -converting systems seem to be the best energy-generating systems in near-by future, but in far future also nuclear fusion will become important.