CO2 can only produce a minor effect in glaciers because the measured change in CO2 (roughly one third of a doubling) is much too small to account for the large observed temperature changes.
The study of the climate before 1850 is mainly from climate proxies such as ice cores.
Proxies are not thermometers.
We can't determine an accurate average global temperature from a few proxies.
Dozens of proxies provide rough estimates.
The only high confidence conclusion from proxies is that Earth's climate has always been changing, and there is no evidence CO2 levels caused temperature changes.
The Milankovitch Theory describes the effects of Earth’s orbital variations on the glacial cycle.
It has been popular since the early 1970’s.
An ice age is a period with extensive ice sheets over vast land regions, as we have now over Antarctica.
The last four ice ages have taken place roughly 150 million years apart.
We don’t know why the Earth is in an ice age, but we do know about 10% of the time there is a milder condition known as an interglacial.
Milankovitch Theory
In the 1920s Serbian genius Milutin Milankovitch calculated the Earth's insolation at different latitudes due to orbital variations ... without computers.
His theory was not accepted until 1970, when geological evidence was found on multiple glacial-interglacial cycles.
Proper dating of glaciations during the past 3 million years showed they usually take place at intervals of 41,000 years.
Three types of orbital changes affect Earth’s insolation (solar radiation that reaches Earth's surface) over the long term:
Eccentricity:
Movements of the other planets, specially Jupiter and Saturn, cause the Earth’s orbit to slightly change its eccentricity.
The eccentricity changes have a major beat of 413,000 years, and two minor beats of 95,000 and 125,000 years.
The changes in eccentricity alter the amount of solar energy Earth receives as the distance from the Sun changes.
Earth’s orbit is close enough to being circular that the change in insolation is small, currently about 6.4% peak to trough.
Changes in eccentricity shorten and lengthen the seasons as the Earth speeds up at Perihelion (Earth closest to the sun) and slows down at Aphelion (Earth furthest from the sun).
Obliquity:
This cycle is from changes in the inclination of Earth’s axis, or axial tilt, that varies between 22.1° and 24.3°, in a cycle that takes 41,000 years.
The current tilt is 23.44° and decreasing.
The higher the obliquity, the more insolation in the poles during the summer, and the less insolation in the poles during the winter.
High obliquity promotes interglacials, while low obliquity is associated with glacial periods.
Obliquity changes add a significant amount of warming at high latitudes year after year over thousands of years, and can have an enormous cumulative effect.
The temperature proxy record shows temperatures decreasing during periods of low obliquity and increasing during periods of high obliquity.
When obliquity starts rising during a glacial period, it starts moving energy from tropical to polar areas.
Its effects on global average temperatures are not noticeable for many thousands of years.
The hypothesis that obliquity drives the glacial cycle solves most problems of Milankovitch Theory.
The 100,000 year problem is solved because there is no 100,000 year cycle, just a 41,000 year cycle that skips one or two beats.
Precession:
There are two precessional movements.
The axial precession is Earth’s slow wobble as it spins on its axis due to the gravitational pull on its equator by other solar bodies.
Precession doesn’t change the total amount of insolation that the Earth receives during the year.
The interaction of the various components of precession produce cycles at 19,000 22,000 and 24,000 years, with an average of roughly 23,000 years.
Precessional changes greatly affect the amount of insolation during a three-month period, but that change is offset over the following three months, leaving total annual radiation unchanged.
Problems with The Milankovitch Theory
The current theory explaining glaciations paced by the 100,000 year eccentricity cycle is supported by a scientific consensus.
In the past one million years, glaciations have taken place at 100,000 year intervals.
But between one and three million years ago, glaciations were taking place at 41,000 year intervals, pointing to obliquity as the main factor.