Carl Sagan once proposed an interesting concept of terraforming Venus by floating some kind of bacteria in its atmosphere. The bacteria would be constantly drifting down to the surface and dying, but with no predators and ample resources, multiplying quickly enough to replace the losses, and small enough to be kept aloft by air currents. Gradually they would transform the carbon back into solid mass, and bring the atmospheric pressures and temperatures down to earth levels.

Later on in his career he decided against that scheme, but with our progress in biotechnology, it might be the only feasible approach.

https://en.m.wikipedia.org/wiki/Terraforming_of_Venus