8.6 Climate Sensitivity and Feedbacks

8.6.1 Introduction

Climate sensitivity is a metric used to characterise the response of the global climate system to a given forcing. It is broadly defined as the equilibrium global mean surface temperature change following a doubling of atmospheric CO₂ concentration (see Box 10.2). Spread in model climate sensitivity is a major factor contributing to the range in projections of future climate changes (see Chapter 10) along with uncertainties in future emission scenarios and rates of oceanic heat uptake. Consequently, differences in climate sensitivity between models have received close scrutiny in all four IPCC reports. Climate sensitivity is largely determined by internal feedback processes that amplify or dampen the influence of radiative forcing on climate. To assess the reliability of model estimates of climate sensitivity, the ability of climate models to reproduce different climate changes induced by specific forcings may be evaluated. These include the Last Glacial Maximum and the evolution of climate over the last millennium and the 20th century (see Section 9.6). The compilation and comparison of climate sensitivity estimates derived from models and from observations are presented in Box 10.2. An alternative approach, which is followed here, is to assess the reliability of key climate feedback processes known to play a critical role in the models’ estimate of climate sensitivity.

This section explains why the estimates of climate sensitivity and of climate feedbacks differ among current models (Section 8.6.2), summarises understanding of the role of key radiative feedback processes associated with water vapour and lapse rate, clouds, snow and sea ice in climate sensitivity, and assesses the treatment of these processes in the global climate models used to make projections of future climate change (Section 8.6.3). Finally we discuss how we can assess our relative confidence in the different climate sensitivity estimates derived from climate models (Section 8.6.4). Note that climate feedbacks associated with chemical or biochemical processes are not discussed in this section (they are addressed in Chapters 7 and 10), nor are local-scale feedbacks (e.g., between soil moisture and precipitation; see Section 8.2.3.2).