A re‐evaluation of the coherence between global‐average atmospheric CO 2 and temperatures at interannual time scales

Frequency‐dependent coherence between atmospheric CO 2 and historical temperatures reveals climate feedbacks within Earth's carbon cycle. Coherence between interannual fluctuations in global‐average temperature and atmospheric CO 2 has changed over time. Since 1979, at Mauna Loa and other observation sites, interannual coherence exhibits a 90° phase lag that suggests a direct correlation between temperatures and the time‐derivative of CO 2 . The coherence transition can be explained if the response time of CO 2 to a global temperature fluctuation has lengthened from 6 months to at least 15 months. A longer response time may reflect saturation of the oceanic carbon sink, but a transient shift in ocean circulation may play a role. Coherent annual‐cycle fluctuations in CO 2 and temperature are evident in the 1958–1988 time series, but not since 1979. Coherence of interannual CO 2 variations with gridpoint temperature anomalies are strongest in the tropical oceans.