Global Circulation Models are claimed to be a physical model of Earth's climate.
I prefer to call them Global Climate Models.
They currently claim as CO2 increases, Earth's surface temperature warms in a predictable way.
They make that claim because that's what the climate modelers believed BEFORE they developed the models -- the models represent their beliefs.
In the satellite era, some aspects of climate have been measured, such as cloudiness.
GCM models can be tested by making them reproduce the known past climate of Earth instead of the future climate.
GCM back testing "predicting" cloudiness, compared with actual cloudiness, has reflected errors of at lest +/-10%.
Climate change involves the absorption and re-emission of about 200 watts per square meter.
The average energy impact of clouds on Earth climate, by partially blocking sunlight, is about -27.6 watts per square meter
That means a ±10% error would produce a ±2.76 W/m2 uncertainty in climate projections.
That uncertainty equals about ±100% of the claimed warming effect of man made CO2
In less than the first five years of a climate prediction, total uncertainty about cloudiness would exceed the entire prediction of CO2-caused global warming (typically +3.0°C after 100 years).
From uncertainty about clouds alone, all the UN's IPCC climate projections have uncertainties larger than their projected greenhouse temperature increase.
Include uncertainty about cloudiness, and Earth may become a winter wonderland in 100 years ... or a tropical paradise!
No one knows.
Small uncertainties about GCM inputs ... rapidly expand over time (as you move forward in years).
The annual mean global temperature would be tough to predict one year in advance.
Yet the UN's IPCC makes predictions 100 years ahead.
GCM's have dozens of parameters, and up to a million variables.
All of them have some sort of error, or uncertainty.
GCM predictions are so unreliable they offer no objective reason to claim any of the warming since 1940 was due to man made CO2 ... or that CO2 will warm the climate in the future.