Why do we need less CO2? If you plan to feed the world a vegetarian diet, most crops are at peak productivity around 1000-1300 ppm of CO2. They also use 10-15% less water for that output.

We are currently at 450ppm CO2. Plant death occurs at 150ppm CO2.

Methane's half life in the atmosphere is about 9 years. It breaks down into CO2 and H2O.