It is clear that much land needs to be kept for agricultural use but it is also possible that the area required for food production will stabilize in the future.

Tropical forests hold the largest terrestrial carbon store and are active carbon sinks. Reducing emissions from deforestation and degradation is a vital component of tackling dangerous climate change. In addition, tackling illegal and ill-managed logging will be an important part of reducing emissions from forestry.

Carbon cycle, carbon fluxes and stocks.

Terrestrial ecosystems store about 2100 Gt C in living organisms, litter and soil organic matter, which is almost three times that currently present in the atmosphere.

Temperate forests are active carbon sinks and deforestation in the temperate zone has largely stopped. Where demand for land and/or water allows, reforestation would enable carbon sequestration and could provide other benefits including higher biodiversity and recreation opportunities.

The boreal forest biome holds the second largest stock of carbon; most of this is stored in the soil and litter. The draining of boreal forest peatlands, inappropriate forestry practices and poor fire management may all cause significant losses of the carbon stored in this ecosystem.

Peatland soils store a large amount of carbon but there is a grave risk that much of this will be lost as peatland ecosystems worldwide are being converted for agriculture, plantations and bioenergy. Conservation and restoration of tropical peatlands should be considered a global priority.

Without the contribution of oceans and coastal ecosystems to global biological carbon sequestration today’s CO2 concentration in the atmosphere would be much larger than it is. But the uptake capacity of oceans and coasts is both finite and vulnerable. Minimisation of pressures, restoration and sustainable use are management options that can help these ecosystems maintain their important carbon management function.

Tundra ecosystems are dense in carbon. They have little potential to gain more carbon but a huge amount could be lost if the permafrost were to thaw. Prevention of climate change is currently the only failsafe method of minimising this loss.

Dividing the world into seven biomes, we estimate that tropical and subtropical forests store the largest amount of carbon, almost 550 Gt.

Temperate forests are active carbon sinks and deforestation in the temperate zone has largely stopped. Where demand for land and/or water allows, reforestation would enable carbon sequestration and could provide other benefits including higher biodiversity and recreation opportunities.

Timber forestry can be adapted to increase the amount of carbon held in plantations.

The large surface area of drylands gives dryland carbon sequestration a global significance, despite their relatively low carbon density. The fact that many dryland soils have been degraded means that they are currently far from saturated with carbon and their potential to sequester carbon can be high.

Savannas cover large areas of Africa and South America and can store significant amounts of carbon, especially in their soils. Activities such as cropping, heavy grazing and increased frequency or intensity of fires can reduce carbon stored in these systems.

Human needs and ecosystem conservation

Living systems play a vital role in the carbon cycle. Photosynthesising organisms – mostly plants on land and various kinds of algae and bacteria in the sea – use either atmospheric carbon dioxide or that dissolved in sea water as the basis for the complex organic carbon compounds that are essential for life.

There are competing demands for land use. Any policy that aims to promote ecosystem carbon management must resolve conflicts between different land uses and take care not to disadvantage the poor.

There is great potential to restore carbon in tropical agricultural soils through management practices that, in the right circumstances, can also increase productivity. Agroforestry can offer particularly large carbon gains, although it can increase water demand. Agricultural carbon sequestration policies will need to be tailored to particular circumstances to allow farmers to benefit.

Agricultural systems in the temperate zone tend to occupy fertile soils that would have formerly supported temperate grassland or forest. Land clearance for croplands and pasture has greatly reduced above ground carbon stocks from their original state and soil carbon stocks are also often depleted as tillage disrupts the soil, opening it to decomposer organisms and generating aerobic conditions that stimulate respiration and release of carb...

Tropical forests hold the largest terrestrial carbon store and are active carbon sinks. Reducing emissions from deforestation and degradation is a vital component of tackling dangerous climate change. In addition, tackling illegal and ill-managed logging will be an important part of reducing emissions from forestry.