So lets see if we have an agreement on the following.
The Medieval and probably the Roman warms were clear warming events. Recent sea level approach suggests they might have been overall warms globally.
The recent warm is warmer (x1.2, x1.5, x2?) than the two previous warms. The amount is important but I think we both agree the older warms are hard to calculate precisely at the moment.
The recent warm started before significant man-made CO2 in the mid 1940s.
All previous warming events were not due to man-made CO2. Natural CO2 role is fuzzy but is likely an effect of raised temperatures and not a cause.
Solar/volcanic natural variation are the likely drivers of the Med/Roman warms.
The recent warm has some natural component of warming.
Here's where we may diverge at least in magnitude.
Volcanic is only accurate to a judgement on small, medium or lots in the geological record.
Total Solar Irradiance can be measured by proxies and has a 11 year cycle.
This short period cycle does not fit with the longer temperature/sea level variations we see in the past 2000 years.
Herrera 2013 that I site makes an attempt to extract a longer period cycle and then model forward. https://www.researchgate.net/publication/264671225_Reconstruction_TSI_NA
His research shows a solar grand maxima from 1900 to 1990 and then declining to a predicted minima in ~2040
The power index he derives for the current warm is about the same as the Medieval warm.
The following is speculation and unproven but I believe is worth taking a closer look at.
If TSI is the main driver of warming in the past - how much did the maxima peaking in 1990 naturally warm the planet? One would guess to the about the same level of the Medieval warm.
By TSI alone, the planet should be cooling so there is likely another factor warming it further - I believe this factor is likely man-made CO2.
The questions I am currently exploring - can the residual warming be estimated, how does this affect CO2 forcing estimates, and are the current predictive models in the ballpark?
A first pass check on the current IPCC estimate of human activities radiative forcing and Solar irradiance shows the ratio of CO2 warming is estimated at
Therefore shouldn't the current warm which has a high TSI and presumed CO2 warming be at least x5 the warming of the Medieval warm?
There are other factors and lots of detailed work to take in account but the forcing strengths of CO2 and solar seem out of balance.
Changing these will of course change the forward prediction of models.
