The National Oceanic and Atmospheric Administration and Scripps Institution of Oceanography say that the carbon dioxide concentration in the atmosphere hit the symbolic milestone of 400 parts per million yesterday, up from about 280, the level it was at for thousands of years before the Industrial Revolution.

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 (see “Climate Change: The Moral Choices”).

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.

Here’s the takeaway from Scripps:  “There’s no stopping CO2 from reaching 400 ppm,” said Ralph Keeling [a geochemist at Scripps ]. “That’s now a done deal. But what happens from here on still matters to climate, and it’s still under our control. It mainly comes down to how much we continue to rely on fossil fuels for energy.”