Abstract
Abstract. It has been shown previously that 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 individual parameter perturbations. Here we show that the concentration of stratospheric water vapour in this member is over three times higher than observations, and, more importantly for climate sensitivity, increases significantly when climate warms. The large surface temperature response of this ensemble member is more consistent with stratospheric humidity change, rather than upper tropospheric clouds as has been previously suggested. 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) lends further doubt as to the realism of this particular integration. 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.
Highlights
Much discussion has centred on the likelihood of the sensitivity of the physical climate system being significantly larger than the 2–4.5 K range quoted in the Intergovernmental Panel on Climate Change (IPCC)’s Fourth Assessment
We present results from four integrations of the HadSM3 model: a standard-parameter control run and an LEP run with pre-industrial CO2 (STD1 and LEP1 respectively) as well as
We find that the high sensitivity of this climate is due to a large increase in stratospheric water vapour in the 2×CO2
Summary
Much discussion has centred on the likelihood of the sensitivity of the physical climate system being significantly larger than the 2–4.5 K range quoted in the Intergovernmental Panel on Climate Change (IPCC)’s Fourth AssessmentReport (AR4) (IPCC, 2007). When entrainment rates in the model’s convection scheme are set to low values, the climate sensitivity is approximately 7 K on doubling CO2 from pre-industrial values, which is much higher than the IPCC range of 2–4.5 K quoted above, and much higher than any other member of the single-parameter Murphy et al (2004) ensemble. In the ensemble considered here where just one model parameter is perturbed at a time (labelled S-PPE-S in Collins et al, 2010) the performance of the low entrainment is competitive with other members of the ensemble. It could certainly not be described as an outlier. The spread of global mean biases and the magnitude of RMS errors are both smaller in this
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
