Higher atmospheric CO2 increases plant growth - you might remember this from your school days. It's very important as it means that the higher CO2 as a result of human activities causes the Earth's ecosystems to photosynthesise and grow faster, drawing down CO2 and slowing the rate that it accumulates in the atmosphere. In fact, currently just under 50% of the CO2 we release is taken up by enhanced ocean and terrestrial carbon sinks (a combination of higher photosynthesis and greater dissolution of CO2 in seawater).

However, a major control on the effectiveness of this negative feedback is what happens to this extra carbon locked up in plants. It isn't just enough for plants to grow faster, if the extra carbon they are taking up is returned to the atmosphere at an equivalent faster rate (the cycle of carbon through the biosphere would have effectively been speeded up, without a change in the size of the stored component). For example, if trees grow faster but die younger, then average tree size may be the same, with no increase in biomass (carbon storage).

This concept is known as the residence time - the average amount of time that a carbon atom fixed through photosynthesis remains as living or dead biomass, before returning to the atmosphere. And it is something that we don't know very much about...

This study looks at what happens to the carbon in soils what atmospheric CO2 is higher. The authors show that although the input of carbon to soils increases (due increased inputs of dead biomass (leaves, twigs, branches, roots etc., all as result of higher growth rates and more living biomass), higher CO2 also increases the flux of CO2 from the soils back to the atmosphere. They show that this reduction in soil carbon residence time is due to higher rates of decomposition in the soil.

In the long run, this suggests that the amount of carbon that can be stored in the soil in finite, and if similar feedbacks also apply to living biomass (i.e. faster growing forests have lower living carbon residence time), then it suggests that the potential for ecosystems to continue to "mop up" after us is limited...