• The clouds, which are remarkable for the variety of processes they bring into play, in effect, act at the same time on solar and terrestrial radiation, thus determining the amount of energy available for the continental land masses or the oceans.

• The models of the carbon cycle used in SAR WGI, and relied on in this Technical Paper, include CO2 fertilization of plant growth as the sole interaction between environmental conditions and terrestrial carbon.

• Hence, the terrestrial uptake may be due more to change in uses and management of land than to the direct effects of elevated CO2 and climate.

• However, during these two decades, terrestrial ecosystems may have served as a small net sink for carbon dioxide.

• They may also affect the total sequestration of carbon in ocean and terrestrial ecosystems, which can affect the global carbon cycle and the concentration of greenhouse gases in the atmosphere.

• The climate system consists of the following major components: (a) the atmosphere; (b) the oceans; (c) the terrestrial and marine biospheres; (d) the cryosphere (sea ice, seasonal snow cover, mountain glaciers and continental scale ice sheets); and (e) the land surface.