It is substantiated by centuries of human history. And by physics and economics too. Particularly, economics teaches us that when resource's price raises, attractiveness of substitutes and thus investment into improving these substitutes raise considerably. And physicists are working on solving problems that block our progress for finding alternatives right now (e.g. on increasing energy density for electrical storage, increasing energy efficiency of solar cells) - and there are some encouraging signs there too. E.g. this one: http://www.nature.com/nature/journal/vaop/ncurrent/full/natu... may allow producing solar energy much cheaper than before.