Vera Prescott: I had trees. They suck up all the oxygen.
Brantley Foster: Actually, trees produce oxygen.
Vera Prescott: Who are you, Mr Wizard?
--The Secret of My Success
Research findings suggest that higher atmospheric CO2 levels have facilitated greening of the planet. It is well known that higher CO2 levels increase plant productivity and growth. Part of the productivity gain relates to reduced plant water requirements which improves efficiency.
Meta-analysis of over 1700 measurements collected from 21 sites found a positive correlation between ambient air CO2 and soil moisture content. Moreover, the effect was greater in drylands than in wetlands, Considering the inherent water limitations in drylands, additional soil water availability brought about by higher CO2 levels should be a driver in increased vegetative greenness.
These findings are supportive of comments made on these pages about the robustness of nature to a wide range of circumstances.