up the lighter C12, and do so at a higher rate than they take up C13, with the result that carbon compounds formed from the products of photosynthesis, including plants, animals and fossil fuels have a lower C13 to C12 ratio than does the atmosphere.  Introducing a large quantity of CO2 derived from photosynthesis would cause the  C13/C12 ratio to decline.  In contrast, CO2 introduced from volcanoes or from outgassing from the ocean would not significantly affect the C13/C12 ratio.  In fact the global C13/C12 ratio has declined, which is very strong evidence the source of the CO2 increase has was C12 enriched, ie, derived from photosynthesis.  Therefore it is very strong evidence that it comes from the biosphere or fossil fuels, rather than from volcanoes or oceanic outgassing. 6) Declining oxygen concentration Because the change in solubility of O2 in water with change in temperature significantly differs from that of CO2, the change in O2 concentration is not affected by other possible CO2 sinks. That means the decline in CO2 concentration means any large unknown natural sources of CO2 must not come from a source of combustion but must come from a low C14 source generated by photosynthesis. These facts together almost completely preclude the existence of such putative natural sources. Because of the importance of the O2 decline, it is worthwhile looking at the chart below from the IPCC TAR which shows it: The observed decline in O2 is straightforward. The diagonal arrow from the start point marked