In the last 400,000 years the concentration has fluctuated from about 180ppm to about 300 many times.[1] My historical reference in the previous post was the last 500,000,000 years. In that context we are low.
Since our species' collective life comprises the last 400,000 years, and we are 420ppm now, do you think it's also fair to say that we are historic highs?