Phytoplankton fix perhaps 40% of the Earth's carbon, they like CO2 and warmer temps but cannot be force-fed. They also depend upon ocean currents (helped by temp differential between poles and equator, currently in decline) and tectonic movement, also by receding ice which frees up necessary minerals. Too much phytoplankton is also notoriously bad for fish, etc. Thus, (along with Milankovitch Cycles) the rather cyclical nature of planet warming/cooling, yet at times green mass has grown out of control, turning us into a snowball. Perhaps in the future (when all is known), there will be a CO2-driven "thermostat" (bio-machines that sequester carbon when there's warming, release when there's cooling), yet this isn't the only factor. There may be a need to artificially generate ocean currents and mineral release, control insolation, methane release. Bottom line, I fear that problems may go beyond fossil fuel burning, and that we still have much to learn about the rather complex elements of climate change.