Under Kyoto, biological activities that sequester carbon can be used to create CO2 offset credits that could obviate the need for lifestyle-changing reductions in fossil fuel use. Credits are earned by storing carbon in terrestrial ecosystems and wood products, although CO2 emissions are also mitigated by delaying deforestation, which accounts for one-quarter of anthropogenic CO2 emissions. However, nonpermanent carbon offsets from biological activities are difficult to compare with each other and with emissions reduction because they differ in how long they prevent CO2 from entering the atmosphere. This is the duration problem; it results in uncertainty and makes it difficult to determine the legitimacy of biological activities in mitigating climate change. While there is not doubt that biological sink activities help mitigate climate change and should not be neglected, in this paper we demonstrate that these activities cannot be included in carbon trading schemes.