Top: schematic of the physical locations in the disk corresponding to different carbon carriers (dust, molecular gas, and atomic gas), with labels for terms used in sketches below. Notional locations of midplane water ice snowline and N₂ ice snowline are indicated by dashed blue lines that, for clarity, fade out with radius. Middle: illustration of the carbon release scenario, which returns the inner disk gas carbon abundance to the original value. Bottom: three options for the carbon locking scenario: (a) a deadzone blocking accretion through the midplane, and (b) dust traps and (c) planetesimal formation preventing the radial drift of volatile-rich dust grains. See text in Sect. 4.1.2 for more information.