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Commentary | Global CO2 Distribution

This page describes the characteristics of CO2 mole fractions for the surface and upper-air with particular focus on global and hemispheric scales based on the numerical simulation data for Global CO2 Distribution.

   The panel on the left of Figure 1 shows the global average of annual mean CO2 mole fractions on the surface (red) and in the upper-air at a height of around 6 km (blue). Both show a continuous increase. This is primarily attributed to human activity such as fossil-fuel combustion, which caused CO2 emissions exceeding the amount absorbed by plants and oceans. The excess is widely spread, both laterally and vertically, via atmospheric transport. This is why CO2 mole fractions are increasing in the upper-air, while remaining slightly lower than those on the surface, as shown in the figure.

   The panel on the right of Figure 1 shows corresponding monthly mean mole fractions. Surface and upper-air values are both smaller in (boreal) summer and larger from winter to spring. This seasonal cycle is closely related to plant and microbial activity. Active plant photosynthesis results in massive CO2 consumption in summer, while plant respiration and organic-matter decomposition in soil become dominant in winter. Due to vertical transport of CO2, surface variations are reflected in upper-air values with a smaller amplitude of variation. In addition, the time taken for this transport makes the yearly maximum of mole fractions appear later in the upper-air than on the surface.

   Variations in monthly mean mole fractions exhibit considerable differences between the Northern Hemisphere and Southern Hemisphere. Figure 2 shows values separately for the former (left) the latter (right). Periodic cycles are prominent in the Northern Hemisphere, while no clear seasonal variability is observed in the Southern Hemisphere. This is mainly because the former has a larger land area, parts of which are characterized by high concentrations of human or plant activity. As a result, the global average mole fractions are dominated by the characteristics of the Northern Hemisphere.

   In the Southern Hemisphere, monthly mean mole fractions are usually slightly higher in the upper-air than on the surface. This indicates an influence from the atmosphere of the Northern Hemisphere on that of the Southern Hemisphere through transport in the upper-air

   Figure 3 shows annual mean CO2 mole fractions on the surface (red) and in the upper-air (blue) for the Northern Hemisphere (dots) and the Southern Hemisphere (triangles). The gap between lines of the same color indicates the interhemispheric difference in mole fractions, which is smaller in the upper-air than on the surface. This is mainly because the atmosphere at the surface level in the northern hemisphere is more readily affected by human activity and tends to be characterized by high annual mean mole fractions.