The feedback causing the high climate sensitivity of a version of the HadSM3 climate model: undiscovered or just unrealistic?

One member of the Met Office Hadley Centre single-parameter perturbed physics ensemble- the so-called "low entrainment parameter" member- has a much higher climate sensitivity than other members of the ensemble. We show that this is because the concentration of stratospheric water vapour in this ensemble member, which differs markedly from observations, also increases significantly when climate warms. The direct relationship between the bias in the control state (elevated stratospheric humidity) and the cause of the high climate sensitivity (a further increase in stratospheric humidity) places further doubt on the realism of these particular integrations. This, together with other evidence, lowers the likelihood that the climate system's physical sensitivity is significantly higher than the likely upper range quoted in the Intergovernmental Panel on Climate Change's Fourth Assessment Report.