In plants, stomata control water loss and CO 2 uptake. The aperture and density of stomatal pores, and hence the exchange of gases between the plant and the atmosphere, are controlled by internal factors such as the plant hormone abscisic acid (ABA) and external signals including light and CO 2 . In this study, we examine the importance of ABA catabolism in the stomatal responses to CO 2 and light. By using the ABA 8′-hydroxylase-deficient Arabidopsis thaliana double mutant cyp707a1 cyp707a3 , which is unable to break down and instead accumulates high levels of ABA, we reveal the importance of the control of ABA concentration in mediating stomatal responses to CO 2 and light. Intriguingly, our experiments suggest that endogenously produced ABA is unable to close stomata in the absence of CO 2 . Furthermore, we show that when plants are grown in short day conditions ABA breakdown is required for the modulation of both elevated [CO 2 ]-induced stomatal closure and elevated [CO 2 ]-induced reductions in leaf stomatal density. ABA catabolism is also required for the stomatal density response to light intensity, and for the full range of light-induced stomatal opening, suggesting that ABA catabolism is critical for the integration of stomatal responses to a range of environmental stimuli.