Can we believe in high climate sensitivity?

TL;DR

This paper critically examines observational estimates of climate sensitivity, arguing that many are based on implausible assumptions and that a moderate sensitivity around 2.5°C is more credible than extremely high estimates.

Contribution

It demonstrates that many existing high climate sensitivity estimates are unreliable and provides a reasoned argument supporting a moderate sensitivity value.

Findings

Many high sensitivity estimates rely on implausible assumptions

A moderate climate sensitivity (~2.5°C) is statistically justifiable

High sensitivity values (>4.5°C) are very unlikely

Abstract

The climate response to anthropogenic forcing has long been one of the dominant uncertainties in predicting future climate change (Houghton et al, 2001). Many observationally-based estimates of climate sensitivity (S) have been presented in recent years, with most of them assigning significant probability to extremely high sensitivity, such as P(S>6C)>5%. However, closer examination reveals that these estimates are based on a number of implausible implicit assumptions. We explain why these estimates cannot be considered credible and therefore have no place in the decision-making process. In fact, when basic probability theory is applied and reasonable assumptions are made, much greater confidence in a moderate value for S (~2.5C) is easily justified, with S very unlikely to be as high as 4.5C.