Under Kyoto, forestry 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 forest ecosystems and wood products, although CO2 emissions are also mitigated by delaying deforestation, which accounts for one-quarter of anthropogenic CO2 emissions. Non-permanent carbon offsets from forest activities are difficult to compare with each other and with mitigation strategies because they differ in how long they prevent CO2 from entering the atmosphere. In this paper, we investigate issues of carbon sequestration in detail, but in particular we expand in comprehensive fashion on earlier work comparing carbon mitigation activities according to how long they can lower atmospheric CO2 levels. The duration problem is modeled theoretically. Meta-regression analysis with 1047 observations from 68 studies is then used to determine whether the duration problem leads to inconclusive results between carbon-uptake costs and carbon sequestration. In addition, from the regression analysis, it is possible to estimate potential costs of carbon uptake via forestry activities for various scenarios. It turns out that forestry activities are competitive with emissions reduction in tropical regions and, perhaps, in boreal regions, but certainly not in Europe.