Stimuli-responsive macromolecules have inspired much interest in polymer science. Inputting an external stimulus to these polymers can modulate their chain structures and self-assembled architectures for functional outputs. This appealing feature has made this class of polymer materials promising for many emerging applications. In order to apply these polymer systems in organisms and further make them adaptive to physiological environments, it is important to explore new stimulation modes. In this Feature Article, we review the recent development of using carbon dioxide (CO₂) as a stimulus for tuning or controlling block copolymer (BCP) self-assembly. We show that a series of CO₂-responsive functionalities can easily be incorporated into BCP structures, and that rationally designed BCPs can have their self-assembled structures undergo drastic changes in size, shape, morphology and function, controlled by the amount of CO₂ in aqueous solution. This gas stimulus has some distinct advantages over other conventional stimuli: it is truly “green” for the environment of the target polymer system without any chemical contaminations; the stimulating strength or magnitude can be precisely adjusted with the continuous gas flow; and, being a key metabolite in cells, it provides a convenient physiological signal to allow synthetic polymer systems to mimic certain properties of organelles and act as intelligent macromolecular machines and devices.