The most important groups of modern red calcareous algae are the Mg-calcite secreting Corallinales and Sporolithales, and the aragonitic Peyssonneliales and Nemaliales. They are common on the world's shelves and are vulnerable to the global warming and the lowering of pH of sea water, caused by the ongoing increase in anthropogenic CO₂. Among them, coralline algae are ecosystem engineers and major producers of carbonate sediment, of particular importance in temperate and cold seas. Corallines respond to marine acidification and rising temperature showing decreased net calcification, decreased growth and reproduction, as well as reduced abundance and diversity, leading to death and ecological shift to dominant non-calcifying algae. Despite their key ecological and sedimentological role, and because of their vulnerability to marine warming and acidification, our knowledge of the distribution of coralline-dominated habitats and the quantification of their carbonate production is not adequate to allow proper environmental management and confident modelling of a global carbon budget. Locating the algal carbonate factories around the world, then describing them, e.g., evaluating their extent and their production, are a priority for future research.