technically we can invest in green energy now and a lot of government buildings are increasingly using renewable sources. However the problem with renewable energy is that it is very costly.
The reason it is so expensive in comparison to other sources of energy is twofold:
1. The infurstucre for distribution and production is not in place (for example, lets say i want to buy a hydrogen car but there is no hydrogen fuel station near where i live, that means it wont be wise for me to buy a hydrogen car. Or i could be a business man and I want to build a hydrogen fuel station in a town, but no one in that town has hydrogen cars, so if i do i will lose all my money)
2. Green energy is not being subsidized by the government (at least not to any significant degree). you see renewable sources and fossil fuels cost about the same amount of money, but because the government heavily subsidizes fossil fuels, it seems that they are a lot less expensive
so the solution to the problem may seem simple, move money from coal and oil to green energy, use it to pay for infrastructure and encourage the use of green energy. The reason we cant do this is because oil, coal, and gas companies have massive amounts of money and have control over a lot of politicians, so much control that they can block new laws that would make green energy cost effective.