Check out this answer from Consensus:

Based on the available data, there is no evidence to suggest that CO2 levels in Earth's atmosphere have ever exceeded 800 ppm during the past 23 million years. The highest recorded levels during this period were significantly lower than 800 ppm, with present-day levels already surpassing historical maxima. Further research and more comprehensive CO2 proxies are needed to explore CO2 levels beyond this time frame and to understand the full extent of natural CO2 variability in Earth's history.

The concentration of carbon dioxide (CO2) in Earth's atmosphere has been a subject of extensive research due to its significant impact on climate change. Understanding historical CO2 levels can provide insights into the natural variability of Earth's climate system and the potential future impacts of anthropogenic emissions. This article explores whether CO2 levels in the Earth's atmosphere have ever exceeded 800 parts per million (ppm) by examining data from various geological periods.

Historical CO2 Levels

Current atmospheric CO2 concentration is known to be higher than it has been during the past approximately 800,000 years, based on direct measurements of CO2 within ice cores1. However, when we look further back in geological time, the picture becomes more complex due to the lack of direct CO2 proxies that can be applied across very long spans of time.

CO2 Levels in the Past 23 Million Years

A study examining the past 23 million years of Earth's history using the δ13C value of terrestrial C3 plant remains provides a comprehensive CO2 record. This method is applicable to the entire 400 million-year history of C3 photosynthesis on land. The study found that CO2 levels likely ranged between approximately 230 ppm and 350 ppm, with a 68% confidence interval suggesting a range of 170-540 ppm. The highest levels were observed during the early and middle Miocene, and CO2 levels were likely below present-day levels during the middle Pliocene1. These findings indicate that present-day CO2 levels (412 ppm) exceed the highest levels experienced at least since the Miocene epoch.

CO2 Levels During the Last 8000 Years

Another study focused on the last 8000 years, a period known as the Holocene. This research suggests that the 20 ppm rise in atmospheric CO2 content over this period was partly due to a 500 Gt C increase in terrestrial biomass early in the present interglacial period. This increase contrasts with a proposed 200 Gt C decrease in terrestrial biomass during the latter part of the Holocene2. However, this study does not provide evidence of CO2 levels reaching or exceeding 800 ppm during this time frame.

CO2 in the Thermosphere

Observations of CO2 concentrations in Earth's thermosphere, the uppermost layer of the atmosphere, reveal that CO2 is the primary radiative cooling agent and significantly affects the energy balance and temperature of this high-altitude layer. Anthropogenic CO2 increases are expected to propagate upward throughout the entire atmosphere, leading to a cooler, more contracted thermosphere. However, observed trends in thermospheric mass density derived from satellite orbits are generally stronger than model predictions, indicating an incomplete understanding of these changes3. While this study highlights the increasing CO2 levels in the upper atmosphere, it does not provide historical data on CO2 levels exceeding 800 ppm.

Has the CO2 in the earth's atmosphere ever been over 800ppm?

Robert L Kelly has answered Near Certain An expert from University of Wyoming in Anthropology, Palaeoecology

Yes, about 60-55 million years ago during the Paleocene-Eocene thermal maximum. To the best of my knowledge we don't have actual measures of the ppm, but it might have been higher than 800, given the evidence for large forests in arctic regions, the almost complete lack of ice anywhere in the world, and an average annual temperature at least 8 degrees C higher than today.