Why is it 350?

To see the origins of ‘350’ ppm see the following paper:

Target Atmospheric CO2: Where Should Humanity Aim?- December 2008, James Hansen et al, Goddard Institute for Space Studies

and a quote:

Atmospheric carbon dioxide is already 385 parts per million (ppm) [as of Dec 2008] and it is increasing by about 2 ppm each year as a result of the burning of fossil fuels (coal, oil, and gas), with a smaller contribution from burning of forests. The authors use evidence of how the Earth responded to past changes of CO₂ and on-going climate changes to show that atmospheric CO₂ has already entered the dangerous zone.

The authors suggest that global policies should have an initial target for atmospheric CO₂ of 350 ppm. They note that the optimum CO₂ level for maintaining a planet similar to that on which civilization developed is likely to be less than 350 ppm, but a 350 ppm target already reveals that dramatic policy changes are needed urgently. By the time such fundamental changes are achieved, knowledge will exist to help fine-tune the target CO₂.

As of August 2019 we were at 415ppm.