Good news, everyone! ~450 ppm is far from the 10,000+ PPM that is toxic to humans or animals. (That's 4.5%, for the math challenged.) At this rate of increase you'll be "drowning" in a thousand or more years, I guess.

It's also 1/4 of the approximate 1,600 PPM that is optimal for a large number of plants to grow. In other words, the current levels are actually very low, compared to what the plants evolved to use.

Increased plant growth from more CO2 results in more biomass, thus more CO2 uptake, so it's a virtuous cycle and you'll be OK after all.