Transient climate response (TCR) is defined as the change in global temperature after a doubling of CO2 during a simulation where atmospheric CO2 increases at 1%/year. But, is TCR a constant?
The previous post described a set of simulations with the FAMOUS climate model in which we ran 230 different realisations of increasing CO2 at 1%/year for 140 years. [The difference between the types of ensemble (MICRO & MACRO) are not important for this post.]
The figure below shows TCR estimated in the usual way, over the first 70 years of the simulation (black bars). Note that there is a range of TCR from the different realisations of future climate. This uncertainty is around 0.1K, demonstrating that estimating TCR from a single member (as is normally done) is reasonable – this is a much smaller uncertainty than the range between models for example.
It is also possible to estimate TCR over all later 70-year periods in which CO2 also doubles. It is clear that, in this model, TCR increases as the climate warms – i.e. TCR estimated from years 71-140 is larger than from years 1-70. Although this might be due to committed warming adding to the later estimates, it is also possible that TCR may not be constant.