London, 31 August: A recent paper published in Nature has received international media attention because of its claim that human-induced CO2 emissions caused global temperatures to start increasing around the 1830s, much earlier than generally accepted.
In a critical analysis of the paper by Abram et al. (2016) and published today at the influential Climate Audit blog, Nicholas Lewis, an independent climate researcher, demonstrates that the evidence that supports the claimed anthropogenic origin of the early warming onset is inappropriate and does not substantiate that claim.
Lewis said: “The authors’ claim that the start of anthropogenic warming can be dated to the 1830s flies in the face of the best estimates of the evolution of radiative forcing; those given in the IPCC 5th Assessment Report (AR5) show that anthropogenic forcing was only 0.01 W/m2 in 1840, and was still under 0.1 W/m2 in 1870. It is not credible that a negligible 0.01 W/m2 increase in forcing would have had any measurable effect on ocean or land temperatures globally; it is doubtful that an increase of 0.1 W/m2 would do so. Most of the evidence given for the anthropogenic origin claim, which is entirely model-simulation based, ignores the industrial era increase in aerosol forcing, the dominant negative (cooling) anthropogenic forcing; the remaining evidence appears to be invalidated by a simulation discontinuity in 1850.”
He added: “Recovery from the heavy volcanism earlier in the century and an upswing in Atlantic multidecadal variability, superimposed on a slow trend of recovery in surface temperature from the LIA as the ocean interior warmed after the end of the particularly cold four hundred year period from AD 1400–1800, appears adequate to account for warming from the late 1830s to the final quarter of the 19th century.”
Lewis also pointed out that, ironically, should the study’s finding of anthropogenic warming starting as early as circa the 1830s be correct, it would imply that anthropogenic aerosol forcing is weaker than estimated in IPCC AR5, and therefore that observational estimates of climate sensitivity (both transient and equilibrium) based on AR5 forcing values need to be revised downwards.