- Northern Hemisphere d C13/C12 observations show a seasonal cyclicity, rising in the summer and falling in the winter. The cycles are consistent with light C12 being absorbed by plants during the growing season, and with light C12 being released to the atmosphere by plant oxidation and more fossil-fuel use during the winter.
- The cycles show both positive and negative deflections in the northern hemisphere, relative to the southern hemisphere baseline. The absorption of C12 by plants in the growing season is the strongest and sharpest part of the cycle.
- The long term trend is toward more negative d C13/C12, consistent with the accumulation of atmospheric CO2 from fossil fuel use.
6) The Keeling Curve Summary: Seasonal CO2 cycles and Global CO2 Distribution
Atmospheric CO2 Carbon Isotope Data: