The Sustainability Challenge  framework is an extremely simplified visual representation of the most challenging predicament & problem facing humanity: 

If at all humanity somehow manages to muster the necessary global political will (to over the predicament) by agreeing to implement the Strong Sustainability Measures i.e. Controlled & Voluntary Collapse then:

“How do we radically reduce Global Throughput  (TP) (i.e. material & energy footprint of humanity) and still sustain current  population levels?“

(Population at current size assuming no uncontrolled collapse & die-offs take place)