Local effects of energy-conversion systems have already been observed, and the influence of cities can already be quantified. The global climate system is, however, a complex nonlinear system, and the large number of feedback loops between its different components mean that it is sensitive to large-scale changes in any one part.

The global climate system consists of five subsystems: the atmosphere, oceans, cryosphere (ice and snow), biosphere and land. These components interact with each other through a wide variety of processes, such that the total climate system is highly complex and nonlinear. Despite the inability to understand all aspects of the climate system and to predict climatic changes, it is possible to make preliminary estimates of the impacts of energy conversion and consumption on the system. These estimates can be made on the basis of observed responses of the system to perturbations and on the basis of results from climate models. Although many uncertainties about the impacts remain, as is illustrated in this article, it is nevertheless clear that human activities could become an important climate regulating force in the near future.