Conclusions

• During the period 1860-1994 altogether (241 ± 20) · 10¹⁵ g·C has been emitted into the atmosphere by fossil fuel combustion and the present (1990) rate is (6.0 ± 0.5) · 10¹⁵ g·C·yr^-l
• During the same time (150 ± 50) · 10¹⁵ g·C has been released due to deforestation and changing land use, the present rate being (1.6 ± 0.8) · 10¹⁵ g·C·yr^-l
• Atmospheric CO₂ concentrations have increased from 275 ± 10 ppmv in the middle of the 19^th century to 370 ± 1 ppmv in 2001.
• We understand the basic features of the global carbon cycle quite well. It has been possible to construct quantitative models which can be used as a general guide for the projection of future $CO$₂ concentrations in the atmosphere as a result of given emission scenarios.
• The uncertainties of projections of likely future $CO$₂ changes on the basis of a given emission scenario are considerably less than those of the emission scenarios themselves.
• On the basis of a range of emissions scenarios, we conclude that it is unlikely that atmospheric $CO$₂ concentrations will double (i.e. exceed about 550 ppmv) before the middle of the 21^st century and such concentrations may not be reached until after 2100.
• The major uncertainties of these projections are due to inadequate knowledge about the importance of the following processes:
  • rate of water exchange between surface, intermediate and deep layers of the sea.
  • the sensitivity of marine primary production to changes of nutrient availability in surface waters.
  • burying of organic matter in the sediments in coastal regions (and lakes).
  • change of alkalinity, and thus the buffer factor of sea water due to increasing amounts of dissolved inorganic carbon (DIC) (in a long term perspective also due to carbonate dissolution on the continental shelves).
  • fertilization and increase of biomass and organic matter in soils in terrestrial ecosystems due to increasing atmospheric $CO$₂ concentrations and possibly deposition of nutrients emitted from anthropogenic sources.
  • enhanced rate of decay of organic matter in soils, particularly in the process of forest exploitation.
  • charcoal formation during biomass burning.
• A slow down of the increase of fossil fuel use during the next decades would greatly widen the range of options in the future use of fossil fuels without reaching atmospheric $CO$₂ concentrations twice pre-industrial levels.

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