[Georgia (OP)]

Donald has been in touch to ask:

"The early atmosphere of the earth was high in CO2. How much did this affect the Earth's temperature? Was it really 400,000 ppm? Vs 380 ppm today?"

What do you think?

[chiralSPO]

The early atmosphere had a LOT of CO₂!

To first approximation, all of the carbon currently contained in all of the biomass, all of the fossil fuel deposits and all of the limestone (and other carbonate minerals) used to be in the atmosphere. It's hard to know how much of these there are directly, but because all of the oxygen in the atmosphere corresponds molecule for molecule to net carbon dioxide that has been consumed by photosynthesis, this accounts for biomass and fossil fuels together--that alone means the atmosphere must have had at least 20% (200,000 ppm) CO₂...

Primordial life developed (or arrived) when the world was in this state, and quickly took advantage of this plentiful resource, converting it into calcium carbonate or organic molecules.

[evan_au]

400,000 ppm = 40%.

Whether this is likely depends on the surface temperature:
- If the surface temperature was > 100C, all of the oceans would have been vaporised, so a large fraction of the atmosphere would have been water vapor.
- Once the temperature dropped well below 100C, the water would have condensed onto the surface, leaving Nitrogen and Carbon Dioxide. 40% CO₂ sounds feasible if most water were removed.

There are debates about where Earth's volatiles came from (and when), but one common theory is that they were delivered by icy comets. Typical volatiles found on comets include H₂O, CO, CO₂, CH₄, and NH₃.

See: https://en.wikipedia.org/wiki/Comet_nucleus#Composition

[Leitwolf]

There is one very important thing about the early (actually not so early) atmosphere, that is usually being forgotten. Keep in mind that insects were much larger the farther you go back in "history". The only reasonable way to explain this, is by assuming a much denser atmosphere. Well, higher concentrations of O2 could have done that too, but they were never that much higher.
 
So a 100 or a 200 million years ago (and so on) we not just had higher concentrations of CO2, but also a multiplicator as there was much more atmosphere all over. Next to higher temperatures, and accordingly more vapour in the atmosphere, that meant a greenhouse gas bonanza.

That brings up the question, how the planet was not fried back then. Unless of course we question the GHE itself.

[Ophiolite]

There is one very important thing about the early (actually not so early) atmosphere, that is usually being forgotten. Keep in mind that insects were much larger the farther you go back in "history". The only reasonable way to explain this, is by assuming a much denser atmosphere. Well, higher concentrations of O2 could have done that too, but they were never that much higher.
 
So a 100 or a 200 million years ago (and so on) we not just had higher concentrations of CO2, but also a multiplicator as there was much more atmosphere all over. Next to higher temperatures, and accordingly more vapour in the atmosphere, that meant a greenhouse gas bonanza.

That brings up the question, how the planet was not fried back then. Unless of course we question the GHE itself.

The OP is asking about the primordial atmosphere, not the one that existed recently in the Carboniferous. Thus your post appears to be off-topic.

Meanwhile, back on the proto-planet, the original primordial atmosphere was removed either by the solar outflow in its T-Tauri phase, or as a consequence of the moon-froming impact. As noted by evan_au the subsequent atmosphere was probably delivered by comets (or asteroids), though a proportion certainly arose from mantle degassing. Last time I looked, debate continued as to whether or not the atmosphere was reducing and, if it was, to what extent. I'll look for a recent review paper and post a link.