Atmospheric evidence for a global secular increase in carbon isotopic discrimination of land photosynthesis.

A decrease in the C/C ratio of atmospheric CO has been documented by direct observations since 1978 and from ice core measurements since the industrial revolution. This decrease, known as the C-Suess effect, is driven primarily by the input of fossil fuel-derived CO but is also sensitive to land and ocean carbon cycling and uptake. Using updated records, we show that no plausible combination of sources and sinks of CO from fossil fuel, land, and oceans can explain the observed C-Suess effect unless an increase has occurred in the C/C isotopic discrimination of land photosynthesis.

Interannual variability in the oxygen isotopes of atmospheric CO2 driven by El Niño.

The stable isotope ratios of atmospheric CO(2) ((18)O/(16)O and (13)C/(12)C) have been monitored since 1977 to improve our understanding of the global carbon cycle, because biosphere-atmosphere exchange fluxes affect the different atomic masses in a measurable way. Interpreting the (18)O/(16)O variability has proved difficult, however, because oxygen isotopes in CO(2) are influenced by both the carbon cycle and the water cycle. Previous attention focused on the decreasing (18)O/(16)O ratio in the 1990s, observed by the global Cooperative Air Sampling Network of the US National Oceanic and Atmospheric Administration Earth System Research Laboratory.