You are both wrong. CO2 is not harmful to humans in the same way that CO is. The threat of a CO2 rich environment is that there won’t be enough O2 to breathe. The CO2 doesn’t kill you, the lack of oxygen does. O2 is normally 21% of the atmosphere but most people can survive in O2 depleted air down to 12-15%.

Now as far as the effect on the climate, one example. NASA scientists have warned for years that warming oceans would bring on changes to the ocean’s conveyor belt that takes heat from the Southern Atlantic and delivers it to Northern Europe. The prediction was that global warming could lead to a deep freeze in Europe. And now twice in the last 4 years Europe has experienced epic record breaking cold spells across the continent that killed hundreds of people.