Understanding Variability in Transient Climate Response

Transient climate response (TCR) is the change in global mean surface temperature in response to a 1% increase in CO₂ for 70 years. We analyze a 100-member ensemble of runs of a fully coupled global climate model (MPI-ESM1.1) with identical forcing and show that internal variability can lead to large differences in TCR between the ensemble members. We further show that most of the spread between the ensemble members is due to heat transport in the oceans — ensemble members that transport more heat to the deep ocean have lower TCRs. Another important factor is the climate feedback parameter λ, which determines how much of the forcing is radiated back to space — ensemble members that radiate more energy back to space (more negative λ) also have lower TCRs. Together, these two factors explain almost all of the spread among the ensemble members. Because of this, one must be careful not to assume that TCR estimated from observations of the 20^th century represents the Earth system’s actual TCR.