A hypothetical ideal state in which stored carbon persists in perpetuity. Absolute permanence is not attainable, but real-world residence times of carbon in soils, forests, geology, and products can be compared against the ideal of permanence. A distinction can be drawn between physical permanence (as defined above) and contractual permanence, which is the use of legal and financial contracts to simulate permanence by holding someone responsible for remediation in the event of a reversal. Permanence is sometimes used to mean “residence time”, or the actual duration in years that a GHG is expected to remain out of the atmosphere.