Atmospheric CO₂ has increased by 25% in the past 200 years and its concentration is predicted to double by 2100. Much of the research effort into the impacts of elevated atmospheric CO₂ concentrations on algae has focused on marine algae and very little work has been conducted using freshwater algae or freshwater algal viruses. Furthermore, little is known about the combined impacts of future CO₂ and temperature increases on the infectivity of algal viruses, although the two impacts are closely connected. In this study, Plectonema boryanum IU597 was grown under four different conditions: (1) 25°C+400μmol/mol CO₂ (control); (2) 29°C+400μmol/mol CO₂ (elevated temperature); (3) 25°C+800μmol/mol CO₂ (elevated CO₂); (4) 29°C+800μmol/mol CO₂ (greenhouse effect). The cultures were incubated for one week, one month or three months before analysis. After incubation, the biomass of each culture was measured by the Chl a labeling method and by direct counting of cell numbers. The cell size was determined by microscopy measuring, the adsorption rate of cyanophage PP was determined using the centrifugation method, and the burst size and lytic cycle were affirmed by measuring the one-step growth curve. Results indicated that different culture conditions had no significant influence on the cell size of P. boryanum. However both the elevated CO₂ and the greenhouse effect conditions could significantly promote the cell number and total Chl a concentration of P. boryanum. The elevated CO₂ condition also decreased the Chl a content per cell by 31.35%-42.42% compared with the control condition. The latent period of cyanophage PP was about 260 minutes under control condition, while under the elevated CO₂, elevated temperature, and greenhouse effect conditions, the time was brought forward to 200, 170-200, and 140 minutes, respectively. The lytic cycle of cyanophage PP was about 380 minutes under the control condition, while in the elevated CO₂, elevated temperature, and greenhouse effect conditions, the time was brought forward to 320-380, 320, and 260-320 minutes, respectively. This indicated that elevated temperature and elevated CO₂ conditions had a synergistic effect on the latent period and lytic cycle of cyanophage PP. The first hour's adsorption rate of cyanophage PP was an average of 2.7% in the control condition, while under the elevated CO₂, elevated temperature, and greenhouse effect conditions, adsorption rates were an average of 1.7%, 4.8%, and 7.3%, respectively. This indicates that the elevated temperature and elevated CO₂ conditions had a synergistic effect on absorption rates of cyanophage PP. The burst size of cyanophage PP was an average of 32 PFU/cell in the control condition, while in the elevated CO₂, elevated temperature, and greenhouse effect conditions, the average burst size was 24, 51, and 32 PFU/cell, respectively. This indicates that the combined effect of elevated temperature and elevated CO₂ on burst size of cyanophage PP could be counteracted. These results not only suggest that the infectivity of cyanophage PP could be significantly promoted under the greenhouse effect conditions, but also imply that the combined effect of elevated CO₂ and elevated temperature on the infectivity of algal viruses was complex and caution is needed when studying the overall responses of freshwater virus-mediated processes to global change.