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

The highest concentration of CO2 in the atmosphere in modern times has indeed reached around 400 ppm (parts per million). This level has been recorded consistently since the early 2010s, with measurements taken from various observatories, notably the Mauna Loa Observatory in Hawaii. The increase in CO2 levels is primarily attributed to human activities such as the burning of fossil fuels, deforestation, and industrial processes.

In the context of climate science, 400 ppm is significant as it crosses a critical threshold that many scientists believe affects climate-related processes and impacts. This level has been associated with heightened global temperatures and is a focal point in discussions about climate change and efforts to reduce greenhouse gas emissions.

The other concentrations listed—350 ppm, 450 ppm, and 500 ppm—represent either levels that have not been recorded in the recent past or are predictive estimates for future scenarios based on current trends. 350 ppm is often cited as a goal for stabilizing climate change, while 450 ppm is considered an upper threshold for avoiding severe climate impacts, and 500 ppm could indicate severe and potentially irreversible effects on the climate system.