...They were hard in 1980. So far as I know, they're all solved, and usually solved multiple ways. (Eg., ammonia synthesis and flow batteries for storing renewable energy.)
Also, we cannot keep burning coal and natural gas. We're already somewhere around 50/50 on breaking agriculture. Break agriculture and that's it. Earth reverts to conditions where the maximum human population is around 500 million and probably not that. (Pre-industrial peak was about a billion. We've damaged a lot of soil.)
This is a survival problem, not an agricultural problem.