Question vi): Transient sensitivity

What is the transient sensitivity of the climate system to an evolving greenhouse gas signal (transient climate response)?

The transient climate response (TCR), is a measure of the strength of the transient warming in response to greenhouse gas increases (see Meehl et al., 2007). It is relevant for predicting near-term climate change for given levels of greenhouse gas increase and is also an important climate system property that describes the short term response of the system to changes in radiative balance (Held et al., 2010). Its magnitude is determined by atmospheric feedbacks and by how much of the energy imbalance with increasing forcing is taken up by ocean warming.

The period since the Little Ice Age is ideally suited to sharpen estimates of TCR as it is one of long-term warming and change in radiative balance. Results estimating the greenhouse gas signal from the full instrumental record narrow uncertainties compared to those based on shorter records (Gillett, 2012) and analysis based on reconstructed Hemispherically averaged temperatures show that the response to greenhouse gas changes, together with aerosols, can be detected by 1950 (Hegerl et al., 2007; 2011). In contrast to estimates available to date, TITAN makes use of spatial and temporal long-term evolution over the last two centuries, accounting for observational, model and forcing uncertainty.

Only if estimated uncertainties are realistic, can they be used to determine, for example, the probability of remaining under 2oC warming given a particular greenhouse gas concentration path.