Atmospheric carbon dioxide concentration hits 4oo ppm. Ditto Kevin Bullis:
There are two things to keep in mind about carbon dioxide. First, it stays in the atmosphere for a long time—hundreds to thousands of years. If we stopped burning fossil fuels today, the concentration of carbon dioxide will start falling, but it will take over a thousand years to get back to pre-industrial levels.
The second is that it takes a long time for the earth to respond to higher levels and reach a new state of equilibrium. So we won’t know for many decades—or even over a hundred years– what impact 400 parts per million will have.
That means that, while it will be a long time before we know what 400 parts per million will do to the planet, whatever it does we’ll be stuck with for a long time.
Marinate on that.