The Kyoto Protocol (KP) requires signatories to reduce CO2-equivalent emissions by an average of 5.2% from 1990 levels by the commitment period 2008–2012. This constitutes only a small proportion of global greenhouse gas emissions. Importantly, countries can attain a significant portion of their targets by sequestering carbon in terrestrial ecosystems in lieu of emission reductions. Since carbon sink activities lead to ephemeral carbon storage, forest management and other activities that enhance carbon sinks enable countries to buy time as they develop emission reduction technologies. Although many countries are interested in sink activities because of their presumed low cost, the analysis in this paper suggests otherwise. While potentially a significant proportion of required CO2 emission reductions can be addressed using carbon sinks, it turns out that, once the opportunity cost of land and the ephemeral nature of sinks are taken into account, costs of carbon uptake could be substantial. Carbon uptake via forest activities varies substantially depending on location (tropical, Great Plains, etc.), activity (forest conservation, tree planting, management, etc.), and the assumptions and methods upon which the cost estimates are based. Once one eliminates forestry projects that should be pursued because of their biodiversity and other non-market benefits, or because of their commercial profitability, there remain few projects that can be justified purely on the grounds that they provide carbon uptake benefits.