Happer's key points: 400 ppm of CO2 is nothing special, 1,000 ppm of CO2 would be beneficial for the productivity of agriculture – which has already gone up a little bit, thanks to the slightly increased CO2 levels. The Earth has seen concentrations as high as 4,000 ppm or higher. It wasn't during the era of humans – but the era of humans is a tiny fraction of the Earth's history and when CO2 was around 4,000 ppm, our primate ancestors – whose physiology and climatic preferences don't really differ from ours – were already alive.
Worried about 400 ppm CO2? Distinguished Princeton physicist Dr. William Happer says 1000 ppm would be beneficial to the planet
CO2 Nears 400 ppm – Relax! It’s Not Global Warming ‘End Times’ — But Only A ‘Big Yawn’ — Climate Depot Special Report