Co2 makes up around 0.04% of the Earth's atmosphere, of which we as man contribute around 3%, which is around 0.00012% of the Earth's atmosphere, or 12PPM. Yes, it's likely to be a greenhouse gas. No, I don't think this is the cause of the world warming. The great big ball in the sky is somewhat more convincing. It's also theorised that co2 rises are a reflection of global temperature rises, not the temperature rising due to co2. We simply don't know exactly, but I'd find the former more convincing than the latter.