Accounting for time in mitigating global warming through land-use change and forestry

Abstract

Many proposed activities for mitigating global warming in the land-use change and forestry (LUCF) sector differ from measures to avoid fossil fuel emissions because carbon (C) may be held out of the atmosphere only temporarily. In addition, the timing of the effects is usually different. Many LUCF activities alter C fluxes to and from the atmosphere several decades into the future, whereas fossil fuel emissions avoidance has immediate effects. Non-CO2 greenhouse gases (GHGs), which are an important part of emissions from deforestation in low-latitude regions, also pose complications for comparisons between fossil fuel and LUCF, since the mechanism generally used to compare these gases (global warming potentials) assumes simultaneous emissions. A common numeraire is needed to express global warming mitigation benefits of different kinds of projects, such as fossil fuel emissions reduction, C sequestration in forest plantations, avoided deforestation by creating protected areas and through policy changes to slow rates of land-use changes such as clearing. Megagram (Mg)-year (also known as 'ton-year') accounting provides a mechanism for expressing the benefits of activities such as these on a consistent basis. One can calculate the atmospheric load of each GHG that will be present in each year, expressed as C in the form of CO2 and its instantaneous impact equivalent contributed by other gases. The atmospheric load of CO2-equivalent C present over a time horizon is a possible indicator of the climatic impact of the emission that placed this load in the atmosphere. Conversely, this index also provides a measure of the benefit of not producing the emission. One accounting method compares sequestered CO2 in trees with the CO2 that would be in the atmosphere had the sequestration project not been undertaken, while another method (used in this paper) compares the atmospheric load of C (or equivalent in non-CO2 GHGs) in both project and no-project scenarios. Time preference, expressed by means of a discount rate on C, can be applied to Mg-year equivalence calculations to allow societal decisions regarding the value of time to be integrated into the system for calculating global warming impacts and benefits. Giving a high value to time, either by raising the discount rate or by shortening the time horizon, increases the value attributed to temporary sequestration (such as many forest plantation projects). A high value for time also favors mitigation measures that have rapid effects (such as slowing deforestation rates) as compared to measures that only affect emissions years in the future (such as creating protected areas in countries with large areas of remaining forest). Decisions on temporal issues will guide mitigation efforts towards options that may or may not be desirable on the basis of social and environmental effects in spheres other than global warming. How sustainable development criteria are incorporated into the approval and crediting systems for activities under the Kyoto Protocol will determine the overall environmental and social impacts of pending decisions on temporal issues.