Insights on carbon cycle dynamics using spatially resolved Northern Hemisphere CO2 variations from seasonal to multi-decadal timescales

The amplitude and phasing of the atmospheric CO2 annual cycle are important diagnostics that characterize land-atmosphere carbon exchange, which reflects a balance between terrestrial uptake via photosynthesis and release via respiration and natural or human-induced disturbances. Observations of atmospheric CO2 concentrations date to the 1950s and monitoring networks have expanded worldwide during recent decades, which allow improved spatiotemporal analysis of variations in CO2 seasonal cycles. In this study, we investigate the long-term trend and interannual variation in CO2 seasonal amplitude and phasing at Northern Hemisphere (NH) flask sampling stations with records longer than 15 years. We show that there is site-to-site variability in both the trend and temperature sensitivity of the seasonal cycle amplitude. At any given site, decadal scale variations are evident in both quantities when we modulate the start and end date for our analysis. We explore the underlying drivers for the interannual variations of CO2 seasonal amplitude superimposed on the long-term increasing trends in the NH, and possible links between changes in CO2 amplitude and phases to gain new insights into the dynamics of carbon cycling in the NH. Finally, we demonstrate the application of these observational diagnostics for benchmarking Earth System Models and terrestrial ecosystem simulations.