Scientists who study Earth's climate history in the past half-billion years believe CO2 levels were between two to four times higher than CO2 levels today.
CO2 levels have been declining in the past 175 million years … but the three biggest CO2 peaks in that period were during periods when Earth was cool.
There's no evidence high CO2 levels ever warmed the planet.
The middle Eocene climate 43 million years ago was up to +5 degrees C. warmer than today, yet the CO2 level was about the same as today (400 ppm, "infered from boron isotope composition in planktonic foraminifera").
60 million years ago CO2 was 3,600 ppm
47 million years ago CO2 was 500 ppm
43 million years ago CO2 was 1,400 ppm
The UNs IPCC completely ignores real-world climate proxy observations of CO2 when making wild guess computer game projections of the future climate.
Their projections assume CO2 is the 'climate controller' even though no real world historical observations ever showed CO2 to have that effect.
Trends of CO2 levels and air temperature from Antarctic ice core data show air temperature always increased before CO2 levels increased, during the past 250,000 years.
The CO2 level rose 500 to 1,500 years after the planet began to warm, because global warming causes CO2 outgassing from the world’s oceans.
Earth is currently 3 degree C. cooler than it was during the peak warmth of the prior four interglacials, when the CO2 level was about 300 ppm.
That 3 degree C. of cooling happened while two greenhouse gasses, CO2 and methane, were increasing by one-third and 2.5x, respectively.
Rapid sea level rise, as land-based glacier ice melted after the glaciation peak 19,000 years ago, started 3,000 years before CO2 levels began rising.
The Greenland Ice Sheet Project showed, for the past 5,000 years:
(1) In the first 4,800 of those 5,000 years, the temperature varied widely, but CO2 was stable from 275 and 285 ppm.
(2) In the past 200 years, the temperature varied much less, but CO2 rose from about 275 ppm to 400 ppm.
CO2 content has never been a major controller of Earth’s temperature.
CO2 content may not even be a minor controller of Earth's climate
If CO2 has any effect at all, even the IPCC agrees the next +100 ppm CO2 increase will have significantly less effect than the last +100 ppm CO2 increase.
The history of climate change has one conclusion: Global warming from natural causes gradually warms the oceans, and warming oceans gradually outgas CO2.
Warming causes CO2 levels to increase with a time lag.
The reverse theory, that CO2 causes warming, is a "green" fantasy with no supporting evidence in 4.5 billion years of climate history.