Land and the carbon cycle

Posted: 31 March 2009

Land cover affects the amount of solar energy absorbed and reflected by the Earth as well as the amount of greenhouse gases in the atmosphere through the global carbon cycle. Forests perform significant roles in greenhouse gas absorption, and tropical forests are sequestering one-fifth of the emissions from fossil fuel combustion. Prevent forest degradation and promoting reforestation and afforestation are strong options for mitigating climate change.

Carbon cycle diagram
Carbon cycle diagram
The role of land use change and forestry in the global carbon cycle. Source: International Energy Studies, Berkeley Lab. Click to enlarge
Seven to 10 per cent of the land area on Earth is covered by tropical forests.

  • The terrestrial ecosystem depends on a number of cycles - carbon, nutrient, hydrological, etc, all of which can be affected by human actions.

  • Carbon moves between the atmosphere and terrestrial ecosystems through these primary channels: photosynthesis, respiration, decomposition, and combustion.

  • Human activity changes the flows of carbon and carbon stock through land use, land-use change and forestry (LULUCF).
  • Global carbon stocks
    Global carbon stocks
    Global carbon stocks in vegetation and soil up to 1 metre in depth. Source: IPCC, Land Use, Land-Use Change, and Forestry, 2000.

    • Between 1850 and 1998, fossil fuel combustion and cement production have emitted 270 ± 30 gigatonnes of carbon as carbon dioxide. 136 ± 55 gigatonnes of that carbon was from land-use change, mostly in forests.

    • About 43 per cent of these emissions remain in the atmosphere. The remaining emissions are taken up, about equally, by the ocean and terrestrial ecosystems.

    • Nearly the same amount of carbon emitted from the terrestrial system - from natural or human causes - have been sequestered in the terrestrial ecosystem, making net emissions relatively small or negative.
  • This pattern of net negative emissions may change in the future -
    • Ecosystem capacities could become overtaxed and nutrient-limited.
    • The rate of photosynthesis in some plants might increase and taper off or plateau, while plant respiration will continue to increase and emit carbon dioxide as temperatures increase.
    • Ecosystem degradation is a serious concern.

    • Deforestation and other unknowns could alter this carbon sink.
  • Forests relate to climate change in 4 primary ways, according to the Food and Agriculture Organization of the United Nations (FAO) -
    • Forests release emissions into the atmosphere when they are cleared, overused, or degraded.

    • LULUCF also influence methane and nitrous oxide emissions, mostly through wetland restoration, burning of biomass, and fertilizing the forests.
    • Forests react to changes in the climate. With global warming, these changes could be in the form of altering the composition of forests or forest die-back.

    • When managed sustainably, forests provide an alternative energy fuel to fossil fuels like coal. It should be noted that burning of forest products, or biomass, also emit greenhouse gases but to a lesser extent.

    • Forests serve as a carbon sink, absorbing greenhouse gases and removing them from the atmosphere.
  • Old-growth forests in the tropics act as a carbon sink, sequestering carbon at a rate of 1.3 petagrams of carbon annually. This is a phenomenon that spans all tropic regions, and scientists have found that the amount of carbon these forests trap annually is increasing.
  • Tropical forests absorb nearly one-fifth of carbon dioxide emissions from fossil fuel combustion - that's 4.8 billion tons, removing the greenhouse gas from the atmosphere.

  • African forests have sequestered carbon at a rate of 1.2 billion tonnes of carbon dioxide annually. Each hectare of tropical forests in Africa is absorbing carbon at an additional rate of 0.6 tonnes annually. It is suspected that the additional carbon sequestration is a result of increased carbon dioxide in the atmosphere, but this trend is not expected to continue indefinitely, as these trees can only grow so much.

  • Of global greenhouse gas emissions from human activity, nearly half remains in the atmosphere. Of the remaining emissions, half is taken in by the oceans, and the rest is sequestered in land carbon sinks. Tropical forests are responsible for half of the land carbon sink sequestration, or one-eighth of total emissions.
  • Related Links:

    Desertification and degraded lands (Food and Agriculture section)

    Land Use, Land Use Change and Forestry (LULUCF) under the UN Framework Convention on Climate Change

    IPCC Special Report on Land Use, Land Use Change and Forestry

    United States Lawrence Berkeley National Laboratory: Climate Change and Forestry

    Thomas Lovejoy, Climate Change's Pressures on Biodiversity (PDF), in State of the World 2009: Into a Warming World, (Worldwatch Institute)