What is the current CO₂ level in the atmosphere, and has it ever been higher in Earth’s history?

Today, global CO₂ is around 420–430 ppm (parts per million). As of April 2025, Mauna Loa measurements are about 430 ppm [gml.noaa.gov]. This is the highest CO₂ concentration in at least a few million years [factcheck.org]. To put that in perspective, the last time CO₂ was persistently above 400 ppm was about 3–4 million years ago, during the mid-Pliocene era [factcheck.org]. Back then, Earth was warmer and sea levels were much higher. Yes, CO₂ has been higher in the distant past – if you go back tens of millions of years (e.g. the age of dinosaurs or earlier), there were periods with CO₂ well above 1000 ppm [factcheck.org]. For example, many hundreds of millions of years ago in the Cambrian period, CO₂ might have been several thousand ppm. However, those prehistoric climates were radically different, and life (including humans) was adapted to those conditions. The key point: in human history (and even human prehistory), CO₂ has never been as high as it is now. Our current CO₂ level is unprecedented for at least 3 million+ years of Earth’s timeline [factcheck.orgfactcheck.org]. Moreover, the rate at which CO₂ is rising today is extremely rapid – faster than any known natural increase in Earth’s past [factcheck.org]. This rapid change is what makes it hard for ecosystems (and human society) to adapt easily.