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CO2 Enrichment: Effects on Ecosystems

Year of publication: 2002

Volume 2, The Earth system: biological and ecological dimensions of global environmental change

Page(s): 9
Number of pages: 9
Author(s): Prof. Christian Körner

We live in a carbon world, which means that almost half of all dried organismic tissue consists of the element carbon, C. If organismic tissue or debris is oxidized (decomposed), carbon is emitted as carbon dioxide, CO2 . Although CO2 is called a trace gas, because only ca. 360–370 ppm of the atmosphere is currently represented by CO2, it is the major substrate which green plants absorb by photosynthesis. Photosynthesis, in essence, reduces CO2 to sugar, utilizing solar energy (release of oxygen, O2 ). Non-green living parts of plants, green parts at night, all animals and nearly all microbes obtain their energy requirements from re-oxidizing carbon compounds (respiration), and thus consume oxygen and emit CO2 . Photosynthetic binding of CO2 and respiratory release of CO2 are the two major components of the biological carbon cycle of the globe, which turns over approximately 100 Gt (i.e., billions of tons) of carbon year -1 .
Whenever the magnitude of the two processes is unequal, carbon either accumulates in the atmosphere or in the biosphere (including the pedosphere, i.e., the soil humus, and organic ocean sediments). Net biospheric carbon accumulation during the last 0.5 billion years has created enormous fossil carbon reservoirs below the ground, a large part of which mankind is releasing within ca. 300 years, starting around the year 1800, with dramatic increases of emission in recent decades, further enhanced by deforestation (the release of current biosphere carbon). This is why the atmospheric CO2 concentration is increasing so rapidly and thereby is changing the biosphere’s diet at an unprecedented speed. Atmospheric CO2 is plant food, but at the same time, it absorbs long wave (infra-red) radiation and, together with other gases, contributes to an enhancement of the global greenhouse effect. The latter phenomenon may create a warmer climate, for which the greenhouse gas CO2 is best known, but this synthesis aims at explaining the direct biological consequences of CO2 enrichment, irrespective of any indirect influences on organisms via climatic change. Elevated CO2 contributes to the biodiversity crises in a slow, invisible manner. It is one of the prime needs of global change science to visualize such possible effects.

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