We analyzed the evolution of the atmospheric CO2
mean annual cycle simulated during 1950–2300 under three scenarios designed to separate the effects of climate change, CO2
fertilization, and land use change. CO2
fertilization in boreal and temperate ecosystems drove mean annual cycle amplification over the NH midlatitudes during 1950–2300. Boreal and Arctic climate change drove high-latitude amplification before 2200, after which CO2
fertilization contributed nearly equally to amplification.