Artificial photosynthesis is absolutely required in several sectors, if you want to eliminate the use of fossil fuels. International air travel is the main exa

Artificial photosynthesis is absolutely required in several sectors, if you want to eliminate the use of fossil fuels. International air travel is the main example, so is space travel, and global shipping is a little behind. There's no way to fly passenger jets over the Atlantic and Pacific Ocean using battery power; the density is just too low. Likewise, if you want to have SpaceX continue to launch large methane-fueled or RP-1 fueled rockets without fossil fuels, capture of atmospheric CO2 and conversion to hydrocarbon fuel will be needed. It's also a way of storing energy for long periods of time, i.e. if you're in the near pole regions of a planet, and want to capture solar energy in the summer and use it in the winter, batteries aren't going to work on those timescales. Some other means, generally a stable chemical conversion, will be needed. Converting carbon dioxide to methane or gasoline, converting nitrogen to ammonia (a bit dirty), even converting iron oxide to reduced iron, are all possible methods of doing this. As far as converting atmospheric carbon dioxide to a form that won't eventually get back in the atmosphere, that's pretty difficult (you have to make limestone, CaCO3, or perhaps carbon fiber building materials, or even better, diamond).