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Recent research has found that the subsiding Sunda Shelf (Southeast Asia) was permanently exposed prior to ca. 400 ka with initial submersion-exposure cyclicity, associated with interglacial-glacial sea-level cycles, beginning between 400 and 240 ka. We analyzed the impact submersion-exposure cycles on regional environment and climate through a 640 k.y. leaf-wax carbon isotope (δ¹³C_wax) reconstruction at Andaman Sea Site U1448, representing relative changes in C₃/C₄ plant abundances. Prior to ca. 250 ka, the Sunda region was inhabited by a stable C₃ (forest) biome, after which submersion-exposure cycles initiated with the deglacial sea-level rise at ca. 250 ka. During subsequent glacial-age sea-level drops, the newly exposed shelf was rapidly colonized by C₄ grasses, followed by slow transitions back to C₃ forests, representing a tenfold increase in the variability of C₃/C₄ vegetation in the …