Question vi): Transient sensitivity
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.
The response to greenhouse gases is detectable in observations throughout the instrumental record and supports model based estimates of the transient climate sensitivity. Due to large uncertainties in aerosol response pattern, and due to the so far neglected effect of model uncertainty, we couldn’t further constrain the transient sensitivity but showed that standard approaches to do so underestimate uncertainty if not including model pattern uncertainty (Schurer et al., 2018). Overall, human influences have been the key drivers of longterm variability over the Anthropocene, but natural variability played an important role in regional and shorter term trends, summarized in a review paper (Hegerl et al., ERL 2019).