Estimating climate sensitivity and future temperature in the presence of natural climate variability

We use an initial condition ensemble of an Earth System Model as multiple realizations of the climate system to evaluate estimates of climate sensitivity and future temperature change derived with a climate model of reduced complexity under perfect conditions. In our setup, the mean and most likely estimate of equilibrium climate sensitivity vary by about 0.4-0.8 degrees C (1 sigma) due to internal variability. Estimates of the transient climate response vary much less; however, the effect of the spread and bias in the transient response on future temperature projections increases with lead time. Future temperature projections are shown to be more robust for central ranges (i.e., likely range) than for single percentiles. The estimates presented here strongly depend on a delicate balance between a particular realization of the climate system, the emerging constraints on the estimates as well as on the signal, and the decreasing uncertainties in ocean heat uptake observations.