Two large cycles determine global dynamics of carbon mass transport in the biosphere. The first of these is provided for by the assimilation of CO2 and decomposition of H2O through photosynthesis of organic matter followed by its degradation to yield CO2. The second
Turnover of planktonic photosynthesis 50 organisms
CO2 uptake by ocean 30
CO2 release by ocean 30
Co deposited in precipitation 0.08
Cc deposited in precipitation 0.16
Biological cycle (photosynthesis- 85
degradation of organic matter) HCO- ion mass exchange between land and troposphere
Supply to troposphere 0.136
Rainfall washout from troposphere 0.139 Stream loss of
Transport of oceanic airborne HCO- 0.003 ions to land cycle involves the uptake-release of carbon dioxide by natural waters via chemical reactions of CO2 and H2O leading to buildup of a carbonate-hydrocarbonate system. The cycles are intimately related to the activity of living matter. The living matter of the biosphere, the global water cycle, and carbonate-hydrocarbonate system regulate the cyclic mass exchange of carbon between atmosphere, land, and ocean. These global carbon fluxes are shown in Table 4.
A specific feature of these two major biogeochemical cycles of carbon is their openness, which is related to the permanent removal of some carbon from the turnover as dead organic matter and carbonates. The carbon burial in the sea deposits is of great importance for biosphere development.
There is a suggestion that the alteration of glacial and interglacial periods in the Pleistocene was mainly due to fluctuations of CO2 in the atmosphere. It may be hypothesized that the spread of land ice and the drastic reduction of forest areas with their typically high biomass were favorable for an elevated content of carbon dioxide in the atmosphere and the subsequent climatic warming up. In its turn, the resulting contraction of glacial areas and reforestation was attended by an increased CO2 uptake from the atmosphere and by its binding to the biomass and soil organic matter. The resulting effect was a gradual cooling and the onset of a new glaciation followed by reduction of forest areas and a repetition of the whole cycle.
The role of carbon dioxide in the Earth's historical radiation budget merits modern interest in raising atmospheric CO2. There are however other changes of importance. The atmospheric methane concentration is increasing, probably as a result of increasing cattle population, rice production, losses during natural gas exploration and transportation, and biomass burning. Increasing methane concentrations are important because of the role they play in stratospheric and tropospheric chemistry. Methane as a greenhouse gas (GHG) species is also important to the radiation budget of our planet.
Analyses of ice cores from Vostok, Antarctica, have provided new data on natural variations of CO2 and CH4 levels over the last 220 000 years. The records show a marked correlation between Antarctic temperature, as deduced from isotopic composition of the ice and the CO2/CH4 profiles (Figure 7).
Clear correlations between CO2 and global mean temperature are evident in much of the glacial-interglacial paleo-record. This relationship of CO2 concentration and temperature may carry forward into the future, possibly causing significant positive climatic feedback on CO2 fluxes.
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