Transient climate response estimated from radiative forcing and observed temperature change

Observations and simulations (using the HadCM3 AOGCM) of time‐dependent twentieth‐century climate change indicate a linear relationship F = ρΔT between radiative forcing F and global mean surface air temperature change ΔT. The same is a good description of ΔT from CMIP3 AOGCMs integrated with CO2 increasing at 1% per year compounded. The constant “climate resistance” ρ is related to the transient climate response (TCR, ΔT at the time of doubled CO2 under the 1% CO2 scenario). Disregarding any trend caused by natural forcing (volcanic and solar), which is small compared with the trend in anthropogenic forcing, we estimate that the real‐world TCR is 1.3–2.3 K (5–95% uncertainty range) from the data of 1970–2006, allowing for the effect of unforced variability on longer timescales. The climate response to episodic volcanic forcing cannot be described by the same relationship and merits further investigation; this constitutes a systematic uncertainty of the method. The method is quite insensitive to the anthropogenic aerosol forcing, which probably did not vary much during 1970–2006 and therefore did not affect the trend in ΔT. Our range is very similar to the range of recent AOGCM results for the TCR. Consequently projections for warming during the twenty‐first century under the SRES A1B emissions scenario made using the simple empirical relationship F = ρΔT agree with the range of AOGCM results for that scenario. Our TCR range is also similar to those from observationally constrained model‐based methods.