Can Trees Buy Time? An Assessment of the Role of Vegetation Sinks as Part of the Global Carbon Cycle

Atmospheric CO₂ concentrations can be reduced by storing carbon in vegetation. However, this lowers the concentration gradient between the atmosphere and other potential carbon reservoirs, such as the oceans, and thereby reduces the subsequent inherent rate of removal of CO₂ from the atmosphere. Hence, storage of carbon in temporary reservoirs can reduce atmospheric CO₂ concentrations in the short term, but if the carbon is released again, it will increase concentrations in the long term. It must, therefore, be considered when, or, indeed whether, to store carbon in vegetation sinks.To determine an optimal strategy, the exact nature of climate-change impacts needs to be considered first. Impacts can be mediated by:1. the direct and instantaneous effect of CO₂ and its associated temperature;2. the rate of change in CO₂ and its associated temperature;3. the cumulative effect of CO₂ and its associated temperature.Carbon stored in permanently maintained vegetation sinks can lower atmospheric CO₂ concentrations, but this can be done most effectively if sequestration occurs close to the time when atmospheric concentrations are to be lowered. Similarly, maximal rates of change can be most effectively reduced by carbon sequestration close to the time of anticipated maximal rates of change. For reducing impacts via cumulative forcing, however, early sink activity would be more effective than delayed activity.Temporary carbon stores would only be beneficial for climate change impacts related to the cumulative impact of CO₂, but it could even worsen impacts mediated via the instantaneous effect of temperature or those related to the rate of change. Hence, the planting of trees is only beneficial in reducing climate-change impacts if the most serious impacts are those related to the cumulative effect of increased temperature. If other impacts are more serious, then the planting of trees would bring greater benefits if it is delayed until closer to the time when the most severe impacts are to be expected. However, if serious land degradation would result from deforestation, or from a failure to plant trees in the near future, then trees should still be planted in order to maximise the amount of carbon stored on land.