1) Man’s direct GHG emissions are dominated by the man-made release of CO2; primarily from the production of energy, land-use changes, etc. Excess CO2 dumped into the atmosphere does indeed drive the anthropogenic contribution to climate change. CO2 is a GHG. It acts to raise the temperature of the atmosphere all by itself (this is basic physics). It also acts to increase the amount of water vapor that resides in the atmosphere, which in turn increases the atmospheric temperatures even more.

2) Current levels of CO2 are not bordering on levels that would be toxic, or even detrimental to, plant life. During the last ice age, CO2 levels averaged about 180 ppm – far lower than current levels (about 53% of current levels). During those thousands of years, plant life survived very well.

3) The question is not whether human physiology can tolerate various amounts of CO2. The question revolves around how the climate, and the environments that are important to humans (most if not all of them), will respond as humans continue to dump ever more fossil CO2 pollution into the system.

4) Also of note is the fact that trace gases can have large effects. For example, stratospheric ozone is a trace gas. But a reduction in that trace gas causes large changes in the UV flux reaching the Earth’s surface. That in turn can cause unhealthy effects for environments the world over – which is why an international treaty introducing cap and trade controls on ozone-depleting emissions was put into effect so successfully. We currently need similar international efforts to reduce our CO2 emissions, in order to avoid the worst of the anthropogenic components of climate change.