Frequency-dependent coherence between atmospheric CO₂ and historical temperatures reveals climate feedbacks within Earth's carbon cycle. Coherence between interannual fluctuations in global-average temperature and atmospheric CO₂ 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 CO₂. The coherence transition can be explained if the response time of CO₂ 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 CO₂ and temperature are evident in the 1958–1988 time series, but not since 1979. Coherence of interannual CO₂ variations with gridpoint temperature anomalies are strongest in the tropical oceans.