CO2 may have been present in the atmosphere in greater concentrations than right now, but the problem is the time frame for how quickly we’ve gotten there. In the past, there’s been gradual rises and declines. We’ve condensed the rise into the space of about 100 years. That can induce a tipping point from which the global environment cannot recover from.

In the link I posted above, Phil Plait takes the deniers to task for ignoring the combination of time and concentration, or the argument that having more CO2 will benefit plants - ignoring that temperatures and climate will shift where and what plants can survive - turning previously fertile areas (think the breadbaskets of the world) into areas that are facing persistent droughts and water shortages.

That’s why the ongoing persistent droughts across the US are so worrisome. Farmers are already seeing this effect, and consumers are seeing it too - in the form of higher beef, chicken, and produce prices.