Ocean heat content is from Box 3.1, Figure 1 of AR5.
The temperature history shows an obvious millennium scale temperature oscillation, indicating that natural climate change accounts for a significant portion of the temperature recovery since the LIA. Richard Lindzen writes, “Lewis does not take account of natural variability, and, I suspect, his estimates are high.” Fredrik Ljungqvist prepared a temperature reconstruction of the Extra-Tropical Northern Hemisphere (ETNH) during the last two millennia with decadal resolution [Ljungqvist 2010] here emissions to 1900 were insignificant.
The approximate temperature trends during each of the periods identified in figure 1 were estimated.
Climatologist Nicholas Lewis used an energy balance method to estimate the Equilibrium Climate Sensitivity (ECS) best estimate at 1.45 °C from a doubling of CO after allowing the oceans to reach temperature equilibrium, which takes about 3000 years.
A more policy-relevant parameter is the Transient Climate Response (TCR) which is the global temperature change at the time of the CO at the current growth rate of 0.55%/year would take 126 years.
The long time between these periods has the effect of averaging out the effect of short-term ocean oscillations such as the Atlantic Multi-decadal Oscillation (AMO) and the Pacific Decadal Oscillation (PDO), but it does not account for millennium scale ocean oscillations or indirect solar influences.
The 5th assessment report (AR5) of the Intergovernmental Panel on Climate Change (IPCC) gave a best estimate of aerosol forcing of -0.9 W/m [WG1 AR5 page 571].
The analysis gives the TCR best estimate at 1.21 °C with a likely range [17 – 83%] of 1.05 to 1.45 °C.
Energy balance estimates of ECS and TCR use these equations: , where F.
The two periods used for the analysis were 1859-18-2011.
They were chosen to give the longest early and late periods free of significant volcanic activity, which provide the largest change in forcing and hence the narrowest uncertainty ranges.
The transient climate response (TCR) to greenhouse gas emissions, the warming when carbon dioxide (CO) doubles in about 125 years, was estimated by climatologist Nicholas Lewis at 1.2 °C using an energy balance approach and the new aerosol estimates but assuming that there was no natural long-term warming nor any urban heat contamination of the temperature record.