I have read that CO2 has a diminishing return on heat retention after about 50 ppm. In other words, once you hit the 50ppm level, you're getting the bulk of heat retention that CO2 provides already. Ancient CO2 levels have been higher than 3000 ppm in the past and did not always correlate with temperature.

Somewhat alarmingly, at least in my opinion, CO2 levels dropped to about 180 ppm during the recent Little Ice Age. Plants require 150 ppm to order to grow, so that's cutting it pretty close. CO2 levels have been dropping for a long, long time. And the Earth is usually a lot warmer than it is today.

Plants will also grow dramatically larger if you give them more CO2, and they require less water. It is clear that some, but not all CO2 recently added to our atmosphere came from human activity. Not a lot of people want to admit it, but adding that CO2 might be a good thing.