404 Why it's a bad number
This is the Mauna Loa data, the Keeling Curve, for the year to May 2015. It's a bit noisy from week to week but concentrate on the smoothed average shown by the blue/white boundary. CO2 concentration peaked at about 404ppm this year, a rise of over 2ppm on this time last year, and will now drop back a little as the northern temperate forests come into leaf, start photosynthesising and sequester CO2.
The extraordinary thing is that despite everybody knowing that global warming could end civilisation we continue to burn fossil carbon so next May the concentration could reach over 406 ppm.
We've known since 1824, thanks to Joseph Fourier, that the atmosphere kept us warm, and since 1864, thanks to John Tyndall, that adding gasses such as carbon dioxide and methane to the atmosphere would warm the planet further. In 1896 Svante Arrhenius calculated just how much warmer and by 1938 Guy Callendar had measured both the rise in CO2 concentration and the rise in temperature, added them together and gave us the correct answer.
Before humans started burning a lot of fossil carbon, CO2 concentration was around 280ppm. By the time Charles Keeling started the Mauna Loa observations in 1958 it had already risen to around 315ppm. James Hansen made the number 350 famous, the level we really should not be above if human civilisation is to have a sustainable future. Yet here we are with a new record set and a discussion amongst climate scientists as to whether the rate on increase is increasing. Doh!
Moral: We have to stop burning fossil carbon. Now. And then we need to get the concentration back down below 350ppm by sequestering carbon.