final_program_abstracts[1]

11 IMSC Session Program
The spread and skewness of climate sensitivity: revisiting
some commonplace assumptions
Thursday - Poster Session 2
Alexis Hannart
Both models and observations yield probability distributions for climate sensitivity –
the global mean temperature expected for a doubling of CO2 – that exhibit two main
characteristics: they are widespread (likely range of 2°C-4.5°C) and positively skewed
(small but finite probabilities of very large values). Both of these characteristics have
recently received attention in order to determine their causes and implications. Several
authors have proposed a simple framework to explain these characteristics based on
the uncertainty on feedback and on the inverse relationship between feedback and
sensitivity.
Introducing more formal statistical definitions and concepts within this framework,
explicit relationships between the spread and the skewness of the PDFs of sensitivity
and feedbacks are derived. Based on these relationships, we discuss the implications
on sensitivity PDFs spread and skewness of further reducing the spread on feedback.
We also discuss the validity of the assumption of a non-skewed PDF for feedback,
which is commonplace. We show that this assumption is not supported by AR4
multimodel ensemble data and that it is questionable based on this evidence.Note : A
limited part of this study has been published (Hannart, A., J.-L. Dufresne, and P.
Naveau (2009), Why climate sensitivity may not be so unpredictable,Geophys. Res.
Lett.) but the scope of this proposed talk is a bit broader and does not merely restrict
to a comment on a specific article.
Abstracts 278