Answer:
Step-by-step explanation:
In any event, CO2 concentrations have already touched 400ppm in 2015 and continue to rise about 2ppm every year. You have probably read material from the 350.org website and movement, which indeed claim we need to stay at 350ppm in order not to endanger the climate system.
More in general, the IPCC and countries who are working towards reaching a meaningful climate change agreement this year in Paris think that 450ppm is an acceptable limit, one that would still carry with it some damage (global warming within 2 C) but 'manageable'.
In terms of tangible effects of higher CO2 concentrations, the recent report by NASA