Atmospheric CO2 measurements have been taken every hour of every day at the top of Mauna Loa in Hawaii since the 1950s. The accumulated data show quite convincingly that CO2 levels have risen rapidly over the past 60 years and are now higher (compared to historical averages estimated by other, indirect methods) than the levels at any time over the past three million years.
It is widely believed that the rising CO2 levels of modern times are a direct result of human activities, including especially the burning of fossil fuels (coal, oil, and gas). 400 ppm is just a number, of course; it does not necessarily signify an important level in and of itself. It’s just another signpost on the road of continuously rising CO2 concentrations. Despite all the talk over the past decade or so about the need to stop it to prevent global warming, little action has been taken so far.
So CO2 levels will probably continue to rise for awhile, until we actually get serious about doing something about it.