Magnitude, distribution and causes of terrestrial carbon sinks and some implications for policy

Recent analyses continue to modify our understanding of terrestrial carbon sinks. The sinks are large and variable enough to account for much of the variability in the growth rate of atmospheric CO2. They are distributed throughout both northern mid-latitudes and the tropics. Identification of the factors influencing an observed sink is extremely difficult; methods for attribution are reviewed. Although various ecological mechanisms (e.g. CO2 fertilization, nitrogen deposition, climatic variability) have been shown experimentally to have short-term effects on physiological processes controlling the amount of carbon in terrestrial ecosystems, it is unclear which of these mechanisms has been most important in the past 10-100 years and which will be most important in the future. The decades-long supposition that CO2 fertilization has been a major driver of terrestrial carbon uptake is being challenged. A major portion of the sink in the northern mid-latitudes (although probably not in the tropics) is a result of recovery from past changes in land use and management. To the extent that these direct human actions explain most of the current (and future) sink, attribution and thus accounting become more tractable, but the continued functioning of the sink is limited and largely dependent on deliberate actions (e.g. afforestation, sustainable forest management and preservation). (author)