A natural or artificial system which absorbs carbon dioxide (CO₂) from the environment at a faster rate than it releases CO₂ into the environment. Forests, oceans, and soil are the most important natural carbon sinks. Artificial carbon sinks usually refer to existing or proposed facilities for storage of captured CO₂ generated in production processes. Carbon sinks are seen as important tools to combat global warming. The efficiency of natural carbon sinks can be artificially enhanced; for example, fertilization of certain regions in oceans by mineral nutrients can increase growth of biomass and thus increase the rate of carbon dioxide absorption. Creation and improvement of carbon sinks, or carbon sequestration, can be viewed as a technological complement to other environmental policies such as a carbon tax. They have not been widely used, primarily because of the high economic cost or concern about potentially dangerous unintended effects on the environment.