Impact of atmospheric small-scale fluctuations on climate sensitivity

Climate change scenarios are based on numerical models with finite spatial and temporal resolutions. The impact of unresolved processes is parameterized without taking the variability induced by subscale processes into account. This drawback could lead to an over-/underestimation of the climate sensitivity. The aim of this study is to investigate the impact of small-scale atmospheric fluctuations on the modeled climate sensitivity to increased CO2 concentration. Using a complex coupled atmosphere-ocean general circulation model (ECHAM5/MPI-OM) climate response experiments with enhanced small-scale fluctuations are performed. Our results show that the strength of the global warming due to a CO2 doubling depends on the representation of small-scale fluctuations. Reducing the horizontal diffusion by a factor of 3 leads to an increase of the equilibrium climate sensitivity by 13%. If white noise is added to the small scales, the climate sensitivity tends to weaken. The largest changes in responses occur in the upper troposphere.