Spectral indications of unexpected contributors to atmospheric CO2 variability?

The frequency spectrum of Northern Hemisphere atmospheric CO2 concentration shows strong periodicities at 6.2, 1.0 and 0.5 years. The 6.2 year cyclici ty has been attributed to non-linear interactions between the annual cycle and the atmospheric pole tide. The yearly signal is linked to terrestrial p hotosynthesis; phase analysis shows that annual CO2 minima occur at a lag o f approximately 85 days from insolation maxima. Variations in the nominal y early period occur during times of anomalous anthropogenic CO2 production. Periodicity at 6 months represents a deviation of the yearly signal from an annual sinusoid. If the yearly signal is largely sinusoidal, a plausible s ource of the 6 month periodicity could be cyclic phytoplankton productivity driven by natural insolation modes. Together, the three largest spectral p eaks account for over 99% of the variation in detrended CO2 data. Copyright (C) 1999 Royal Meteorological Society.