Why a 100-year time horizon should be used for global warming mitigation calculations

Global warming mitigation calculations require consistent procedures for handling time in order to compare 'permanent' gains from energy-sector mitigation options with 'impermanent' gains from many forest-sector options. A critical part of carbon accounting methodologies such as those based on 'ton-years' (the product of the number of tons of carbon times the number of years that each ton is held out of the atmosphere) is definition of a time horizon, or the time period over which carbon impacts and benefits are considered. Here a case is made for using a time horizon of 100 years. This choice avoids distortions created by much longer time horizons that would lead to decisions inconsistent with societal behavior in other spheres; it also avoids a rapid increase in the implied value of time if horizons shorter than 100 years are used. Selection of a time horizon affects decisions on financial mechanisms and carbon credit. Simple adaptations can allow a time horizon to be specified and used to calculate mitigation benefits and at the same time reserve a given percentage of weight in decision making for generations beyond the end of the time horizon. The choice of a time horizon will heavily influence whether mitigation options such as avoided deforestation are considered viable.