[thackney]

The pre-industrial concentration was 280, the rise to 400 is due to human activities.

And all the previous rises and falls were also due to human activity, going back millions of years? Or just this one?

Burning fossil fuel to release carbon means that carbon was previously in the atmosphere before the ancient plants, algae, etc captured it.

[palmer]

No, all the previous rises and falls were natural due to warming and cooling that was mostly solar in origin. For example see:

where a temperature rise from -80 to -60 (F) causes a CO2 rise from 200 to 300. In the past 200 or so years we have seen a temperature rise of maybe 2F. That would cause a CO2 rise of maybe 20ppm. A better estimate from solubility equations is 10ppm. That much of the rise we have seen is natural and the rest is manmade.