James Lovelock developed the Gaia hypothesis -
that Earth and its biological systems behave as a huge single entity. This entity has closely controlled self-regulatory negative feedback loops that keep the conditions on the planet within boundaries that are favourable to life.
He also postulated that the earth could support ~0.5bn people at an acceptable standard (6% of current levels). Alternative estimates suggest that 5 earths would be needed to support the global population at US levels of consumption.
A working proposition may be that the planet is overpopulated by a factor of (say) 10.
Climate change is just one element of environmental degradation, caused largely by human consumption. A return to a simpler less damaging lifestyle embraced by (or forced on) all, may not mean a full return to (say) 19th century living standards as technology has evolved.
Whilst for the UK and many developed countries a simpler, greener existence may be attractive, for much of the world it would still be aspirational. The global overpopulation factor of 10 may reduce to 3-5. Can anything else can be done:
- "green" initiatives (energy, recycling, insulation, food production etc) will prolong the window for action but not avoid impending population stresses.
- the urge to reproduce is a fundamental characteristic of all animals. Global populations are projected to continue increasing to 11.2bn by 2100.
- third world and developing countries understandably aspire to developed country standards including housing, food, clean water, lighting, refrigerators, education, choice etc.
Limiting populations is the only realistic solution to long term sustainable existence, but is not even on the agenda. Instead some religions still believe contraception intrinsically evil, and pro-life who think abortion a punishable act. The "one child" policy in China has been dismantled because even they could not make it work.
The probability of the next 100 years passing without events materially reducing human populations is low. It may be conflict over increasingly scarce resources (water, food, materials), pandemic, war (lebensraum!), climate change making some parts of the world uninhabitable.
It will not be the end of humanity - many millions (possibly a billion or two) will survive. They will be those with education, skills, contingency plans etc - the weak as always will perish. An uncomfortable, but probably objective, outcome.