Using atmospheric CO₂ data to assess a simplified carbon-climate simulation for the 20th century

The CSIRO biosphere model has been coupled to an atmosphere model and a simulation has been performed for the 20th century. Both biosphere and atmosphere are forced with global CO2 concentration and the atmosphere is also forced with prescribed sea surface temperatures. The simulation follows the C4MIP Phase 1 protocol. We assess the model simulation using atmospheric CO2 data. Mauna Loa growth rate is well simulated from 1980 but overestimated before that time. The interannual variations in growth rate are reasonably reproduced. Seasonal cycles are underestimated in northern mid-latitudes and are out of phase in the southern hemisphere. The north–south gradient of annual mean CO2 is substantially overestimated due to a northern hemisphere net biosphere source and a southern tropical sink. Diurnal cycles at three northern hemisphere locations are larger than observed in many months, most likely due to larger respiration than observed.