The study presents quantitative niche models for the marine Cyanobacteria *Prochlorococcus* and *Synechococcus*, assessing their present and future global abundances and distributions. The models, based on neural network, nonparametric, and parametric analyses, use over 35,000 discrete observations from major ocean regions. They predict cell abundance based on temperature and photosynthetically active radiation (PAR), with distinct responses for each lineage. The models estimate global biogeographic patterns and seasonal variability, with maxima in warm, oligotrophic gyres of the Indian and western Pacific Oceans and minima at higher latitudes. The annual mean global abundances are 2.9 ± 0.1 × 10^27 cells for *Prochlorococcus* and 7.0 ± 0.3 × 10^27 cells for *Synechococcus*. Using climate projections, the models predict increases in cell numbers of 29% and 14% for *Prochlorococcus* and *Synechococcus*, respectively, by the end of the 21st century. These changes are geographically uneven, with expansions in some regions and declines in others. The findings suggest that oceanic microbial communities will experience complex changes due to projected future climate conditions, which may have significant impacts on ocean ecosystems and biogeochemical cycles.The study presents quantitative niche models for the marine Cyanobacteria *Prochlorococcus* and *Synechococcus*, assessing their present and future global abundances and distributions. The models, based on neural network, nonparametric, and parametric analyses, use over 35,000 discrete observations from major ocean regions. They predict cell abundance based on temperature and photosynthetically active radiation (PAR), with distinct responses for each lineage. The models estimate global biogeographic patterns and seasonal variability, with maxima in warm, oligotrophic gyres of the Indian and western Pacific Oceans and minima at higher latitudes. The annual mean global abundances are 2.9 ± 0.1 × 10^27 cells for *Prochlorococcus* and 7.0 ± 0.3 × 10^27 cells for *Synechococcus*. Using climate projections, the models predict increases in cell numbers of 29% and 14% for *Prochlorococcus* and *Synechococcus*, respectively, by the end of the 21st century. These changes are geographically uneven, with expansions in some regions and declines in others. The findings suggest that oceanic microbial communities will experience complex changes due to projected future climate conditions, which may have significant impacts on ocean ecosystems and biogeochemical cycles.