"Since the agricultural revolution human activities have significantly degraded soils globally so that the amount of carbon stored in soils has been depleted. Carbon sequestration practices just need to partially restore soils to their "natural" carbon content to sequester huge amounts of carbon.

Land management should always be viewed as a short-term bridge to the deployment of low carbon energy sources (which will take some time). In other words not a permanent solution."

And, with regard to CO2 and temperature: "How atmospheric CO2 will respond to increases in temperature is indeed uncertain. Future climate simulations take this uncertainty into account and this is, in part, responsible for the range of temperature changes quoted in assessments like that of IPCC."

Thanks for your great questions (and apologies for the delay in answering them).

Studies show that improved land management can substantially increase the amount of carbon in soils. This is true even in ecosystems like grasslands, because not all of the plant biomass is decomposed. A small fraction of each year's growth makes its way into soil storage pools that have turnover times of thousands of years.

The risk of losing biologically stored carbon (for example, through fire) is certainly something that has to be addressed in any legitimate offset project. This is typically done through "insurance" in which the offset provider holds some credits back or has a pool of unsold offsets.

You also asked whether warming could counteract biological sequestration. It's true that respiration can increase with temperature. The exact balance between land management and temperature's effects on soil carbon will vary from system to system, and this is an important area of ongoing research. However, at least one study has shown that some soils can acclimate to warming, meaning that soil carbon losses may not always increase with warming.

- Martha & Lisa

Of course, all trees eventually die (100-1000 years?), so most of the CO2 they sequester will eventually be released back to the atmosphere except for that stored in the ground by the roots. But I guess we should just keep replanting. I visit the cypress swamps in eastern Arkansas frequently and can just 'see' the carbon being reclaimed.

Ultimately, the choice of tree species will be affected by many factors, including local climate, soils, and co-benefits landowners want to get from an offset project, but here's a summary of the general differences between using evergreen and deciduous trees for forestry offsets.

Many evergreen trees store more carbon, and do so more quickly, than deciduous species. (There are exceptions; check out Figure 6.2 in the book excerpts Martha mentioned.)

However, even if they store less carbon or do so less quickly, deciduous species have a lot of co-benefits. For example, deciduous forests generally have a rich "understory" community below the main forest canopy, and allow multiple uses, such as grazing animals in the spring. Deciduous trees tend to use less water, too.

Hope that helps. Happy holidays!