The tree problem
From the emergence of Homo sapiens until the last 200-300 years, trees have been the major source of energy (and other materials and food). Even 20,000 years ago, the ratio of trees to humans was about 1.5 million to one, but it has now fallen drastically to about 400 to one (and is still falling). To save the planet, and humanity, we need food and energy systems that are efficient in terms of supplying both of those outputs while improving the quality and sustainability of the biosphere. This could best be done by reversing this declining ratio of trees to humans.