Settled science: 350 ppm or maybe 450ppm or maybe some far larger number would be too much CO2 in our atmosphere

Greenhouse gas levels highest in 3m years

The 450 ppm level is considered to be the point at which the world has a 50 per cent chance of avoiding dangerous climate change. Any higher and the odds of avoiding searing temperature rises of 4 or 5 degrees by the end of the century become prohibitively risky.

The Science: 350 ppm - Solutions to climate crisis by James Hansen from his book, Storms of My Grandchildren

over the past several years through his research he has come to the conclusion that we must reduce the CO2 in the atmosphere to 350 ppm in order to avoid disaster for coming generations.

Tom Nelson: 350: The most brain-dead campaign of your life

Take a careful look at the black line in this graph: