The CDA foams degraded rapidly under coastal seawater conditions (Figure ). During the 36week incubation, the extent of degradation for a 98-mil thick, low-density CDA foam was visually discernable as a progressive increase in the foam's transparency (Figure ). The medium and low-density foams lost more than 35 and 65% of their mass by 36 weeks, respectively (Figure ). At the microstructural level, the opening of the pores within the medium-density CDA foam was visible (Figure ). Similarly, the cell walls and edges of the low-density CDA foams showed evidence of their degradation after 36 weeks (Figure ). Comparatively, the PS foams had no measurable mass loss (Figure ) and no microstructural changes (Supplementary Figure ). Values of 𝑘 𝑑 for the PS foams were constrained to less than 5 µm/yr based on calculated mass loss trajectories for different values of 𝑘 𝑑 and the foam's geometric dimensions (i.e., length, width, and thickness). The values of 𝑘 𝑑 for the thick CDA foams (30-115 mil) were substantially greater than those of the thin, solid CDA films (1-10 mil). The low-density CDA foams had values of 𝑘 𝑑 that were ~15-fold greater than that of solid CDA (Supplementary Table ). Accordingly, while a 1-mm thick solid CDA film would take an estimated 15-20 years to fully degrade in the coastal ocean, a low-density foam of the same dimensions would take less than a year. The value of 𝑘 𝑑 for the medium-density foam was double that of solid CDA (156  7 µm/yr; Supplementary Table ). Comparatively, a prototype foam of a similar formulation and density investigated in Mazzotta et al. had a value of 𝑘 𝑑 about seven times that of solid CDA (445  26 µm/yr; Supplementary Table ). The difference in the value of 𝑘 𝑑 for these two medium-density foams likely stemmed from differences in their pore size, shape, and distribution (Supplementary Figure ). The medium-density foam evaluated in Mazzotta et al. had smaller pores (average diameters of 46 v.s. 68 µm) and more of them (cross-sectional areal densities of 177000 v.s. 99000 pores/cm 2 ) than the medium-density foam used in this study (Supplementary Figure ). Compared to foaming, plastic formulation (the use of filler and plasticizer) had a minor effect (<25% change) on the value of 𝑘 𝑑 (Supplementary Discussion 2, Supplementary Figure ).