Drivers of the Increasing Mean Annual CO2 Cycle in the CESM

The observed increase in the amplitude of the CO2 annual cycle indicates that climate change and CO2 fertilization are altering patterns of terrestrial carbon exchange. We investigated the drivers of trends in the mean annual cycle of atmospheric CO2 in the Community Earth System Model (CESM) in simulations run until 2300. In these simulations, the amplitude of the mean annual cycle (averaged over the Northern Hemisphere) increases by 10% over the historical period and by 50% by 2100. Growing season productivity gains are responsible for most of the amplification, resulting in a strengthening global land carbon sink. We separated the effects of CO2 radiative forcing (climate change) and fertilization by enhanced CO2 mole fractions by analyzing two separate simulations, one fully-coupled and one in which CO2 exerted no additional radiative forcing. Climate change and fertilization both increase the Northern Hemisphere CO2 annual cycle amplitude until the year 2300, with the largest amplitude gains occurring in the mid and high latitudes. CO2 fertilization drives the amplitude increase over the northern midlatitudes through 2300 and over the Arctic after 2100. Climate change is responsible for most of the amplitude increase over the Arctic before 2100. In contrast, climate change reduces the CO2 annual cycle amplitude over the tropics. While the mid latitude fertilization effect dominates the global CO2 amplitude response, the shift of the terrestrial biosphere to carbon source at the end of the 23rd century indicates that the tropical climate change drives the net terrestrial carbon flux.