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 last century to 360 ± 1 ppmv in 1994.
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 next century and such concentrations may not be reached until after the end of the next century.
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.