Description: Continued increase in atmospheric CO 2 seasonal amplitude in the 21st century projected by the CMIP5 Earth system models Earth System Dynamics, 5, 423-439, 2014 Author(s): F. Zhao and N. Zeng In the Northern Hemisphere, atmospheric CO 2 concentration declines in spring and summer, and rises in fall and winter. Ground-based and aircraft-based observation records indicate that the amplitude of this seasonal cycle has increased in the past. Will this trend continue in the future? In this paper, we analyzed simulations for historical (1850–2005) and future (RCP8.5, 2006–2100) periods produced by 10 Earth system models participating in the fifth phase of the Coupled Model Intercomparison Project (CMIP5). Our results present a model consensus that the increase of CO 2 seasonal amplitude continues throughout the 21st century. Multi-model ensemble relative amplitude of detrended global mean CO 2 seasonal cycle increases by 62 ± 19% in 2081–2090, compared to 1961–1970. This amplitude increase corresponds to a 68 ± 25% increase in net biosphere production (NBP). The results show that the increase of NBP amplitude mainly comes from enhanced ecosystem uptake during Northern Hemisphere growing season under future CO 2 and temperature conditions. Separate analyses on net primary production (NPP) and respiration reveal that enhanced ecosystem carbon uptake contributes about 75% of the amplitude increase. Stimulated by higher CO 2 concentration and high-latitude warming, enhanced NPP likely outcompetes increased respiration at higher temperature, resulting in a higher net uptake during the northern growing season. The zonal distribution and spatial pattern of NBP change suggest that regions north of 45° N dominate the amplitude increase. Models that simulate a stronger carbon uptake also tend to show a larger increase of NBP seasonal amplitude, and the cross-model correlation is significant ( R =0.73, p 〈 0.05).