What is the approximate highest concentration of CO2 measured in the Earth's atmosphere in modern times?

The highest concentration of CO2 measured in the Earth's atmosphere in modern times is approximately 400 ppm (parts per million). This figure represents a significant milestone in the study of climate change and atmospheric science.

CO2 levels have been monitored extensively, especially since the late 20th century, through various means, including direct measurements at monitoring stations like the Mauna Loa Observatory. The concentration of around 400 ppm is particularly important as it reflects the impact of human activities, such as fossil fuel combustion and deforestation, which have driven CO2 levels higher than they have been for hundreds of thousands of years.

This figure is critical for understanding the greenhouse effect and its implications on global warming. As CO2 levels rise, they contribute to the trapping of heat in the atmosphere, leading to climate change, making this measurement both a significant indicator and a concern for environmental science.