Plants on land and in the ocean take up carbon during photosynthesis and release it through respiration. Experimental research has shown that some land ecosystems respond to higher atmospheric concentrations of CO2 by taking up and storing more carbon in plant tissues, soils, and sediments. Based on a combination of ecosystem models and observations, it has been estimated that for the period 2000 to 2008, land ecosystems removed roughly one-third of the CO2 emitted by human activities. However, roughly half of this carbon sink was offset by changes in land use that resulted in net CO2 emissions back to the atmosphere (mainly through tropical deforestation).
If the balance between CO2 absorption and emissions by ecosystems were to change in response to either future climate changes or changes in management, this could lead to a significant positive or negative feedback on atmospheric CO2 levels. For example, the warming of ocean surface waters across much of the world may represent a positive feedback on climate change, because warming of surface waters commonly reduces the uptake of CO2 by phytoplankton, which could lead to less ocean uptake of CO2, faster CO2 accumulation in the atmosphere, and accelerated greenhouse warming. However, a number of factors influence the storage of carbon in ocean- and land-based ecosystems. For instance, the availability of nutrients and water can limit uptake by land plants, and increases in temperature or large wildfires can increase GHG emissions from land-based ecosystems to the atmosphere. Other important factors modulating the carbon sink provided by terrestrial ecosystems include species redistributions and changes in growing season lengths, drought, insects, pathogens, and land use. As a result of this complex interplay of factors, projections of the future land-based carbon sink are uncertain.
Changes in terrestrial ecosystems could also potentially lead to abrupt climate changes. For example, increasing temperatures are leading to warming and thawing of permafrost (frozen soils) across the northern latitudes. These frozen soils store vast amounts of carbon. As permafrost continues to thaw, this carbon may be released to the atmosphere in large quantities in the form of the GHGs CO2 and CH4, which would significantly amplify global warming (and since this warming would then lead to further permafrost thawing, this represents a potential positive feedback). Other such carbon-climate feedbacks are possible, and this area of research is garnering increasing attention and concern.
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