Mid-19th Century Warming Likely To Be Natural,
Not Human-Induced, Says Independent Climate Scientist
London, 1 September: 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.
This seems to me to be a very unlikely claim, as so little extra CO2 had been emitted that even a climate alarmist would find it difficult to make a case for it. Nick Lewis has shown that this is correct.
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.
Nicholas Lewis: Was early onset industrial-era warming anthropogenic, as Abram et al. claim?