By Douglas Moreman. Modified on
See also Epistemological Concerns

The Error of Incompleteness in Climate Models
Suppose the the Medieval Warming Period, MWP, is real.
Earth warmed into that Period and remained warm for more than 300 years.
"The Little Ice Age": Starting around 1400, Earth cooled and remained cool until about 1850.
Since 1850, possibly 100 years earlier, Earth has been intermittently warming.
One concludes that some Medieval Warming Mechanism MWM, involving the Earth and or the Sun, warmed the Earth during the MWP and then some Latest Colling Mechanism LCM (possibly MWM in reverse) cooled the Earth in the Little Ice Age."

Since atmospheric CO2 was not a greenhouse factor until about 1950, the warming from 1750 or 1850 to 1950 might have been due to MWM?

It seems that neither of MWM nor LCM is modeled in any climate model that has been used to advise governments that the Globe is facing a great danger.
The increase of CO2 in our atmosphere is the main mechanism of these models. But, CO2 is not relevant to Medieval Warming or the Little Ice Age
nor relevant to the warming from about 1800 to 1950.
No model explains these recent climate events?
Each relevant climate-model hides its "incompleteness" by being an elephant model.

In a bad climate model, one or more parameters is adjusted so that the model's prediction of past climate is consistent with observations of past climate over some interval of time. The model is then used to predict the future. But, the model is built upon the assumption that the model's set of a few earthly mechanisms is the reason for the observed data used in the tuning of the model - and that no MWM or LCM (as above) is relevant. But, knowing neither MWM nor LCM, we cannot know that MWM and LCM are both irrelevant to events that created that data. We cannot know that the tuned parameters are correct. We cannot know that the predictions the model makes of the future are correct.
It does not take degrees in climate "science" and mathematical reasoning to understand this?

It might be, for readers, worth noting the lack of relevance of whether the Medieval Warming Period was hotter, as some say it was, or colder, as some say it was, relative to today.