Evolution of natural and anthropogenic fluxes of atmospheric CO₂ from 1957 to 2003

An analysis is carried out of the longest available records of atmospheric CO₂ and its ¹³C/¹²C ratio from the Scripps Institution of Oceanography network of fixed stations, augmented by data in the 1950s and 1960s from ships and ice floes. Using regression analysis, we separate the interhemispheric gradients of CO₂ and ¹³C/¹²C into: (1) a stationary (possibly natural) component that is constant with time, and (2) a time-evolving component that increases in proportion to fossil fuel emissions. Inverse calculations using an atmospheric transport model are used to interpret the components of the gradients in terms of land and ocean sinks. The stationary gradients in CO₂ and ¹³C/¹²C are both satisfactorily explained by ocean processes, including an ocean carbon loop that transports 0.5 PgC yr⁻¹ southwards in the ocean balanced by an atmospheric return flow. A stationary northern land sink appears to be ruled out unless its effect on the gradient has been offset by a strong rectifier effect, which seems doubtful. A growing northern land sink is not ruled out, but has an uncertain magnitude (0.3-1.7 PgC yr⁻¹ centred on year 2003) dependent on the rate at which CO₂ from fossil fuel burning is dispersed vertically and between hemispheres.