The same types of innovations that lifted us above our problems in the 20th Century will form the blueprint for the 21st. The rise of cheap desalination, innovations in biotechnology and GMO's, more sophisticated extraction methods for rare earths and other minerals, new deposits of oil and gas, revolutions in battery technology, perhaps even developments in renewables like Solar (not at present), and so much more. We live in a century of fantastic possibility. Our only realistic hope is that our genius measures up to the task. Planning for massive global legislation and controls for carbon, water management, or whatever the issue of the day may be is not only unrealistic but counter-productive as it retards growth which breeds the dynamism that allows that aforementioned genius to rise to the surface.
We're already in the infancy of a commercial space program with a solar system of resources at our disposal--people are too quick to err on the side of pessimism given our repeated history of success.
Plus if we really, really needed to there are ways to throw a ton of mass into orbit (and beyond) that we could opt to.