The effects on photosynthesis of CO2 and desiccation in Porphyra haitanensis were investigated to establish the effects of increased atmospheric CO2 on this alga during emersion at low tides. With enhanced desiccation, net photosynthesis, dark respiration, photosynthetic efficiency, apparent carboxylating efficiency and light saturation point decreased, while the light compensation point and CO2 compensation point increased. Emersed net photosynthesis was not saturated by the present atmospheric CO2 level (about 350 ml m−3), and doubling the CO2 concentration (700 ml m−3) increased photosynthesis by between 31% and 89% at moderate levels of desiccation. The relative enhancement of emersed net photosynthesis at 700 ml m−3 CO2 was greater at higher temperatures and higher levels of desiccation. The photosynthetic production of Porphyra haitanensis may benefit from increasing atmospheric CO2 concentration during emersion.