I often disagree with pdq on issues, so it is nice when I can just roundly agree with him....population IS the overriding 'keystone' of the Earth's problems.
We can find better ways to generate power and really USE them , create new types of grain with better yields, farm certain types of fish, learn to recycle materials, get better at controlling chemical pollution, change our habits regarding automobiles and even learn to do without certain things.....but IF population increases ---or even remains the same!!--- we will not be able to sustain a comfortable quality of life for most people. We do not NOW have the ability to feed everyone properly or provide them decent living space.
This situation is more apparent in 3rd world countries, but if you look in the right places, developed nations are straining to keep up in providing clean water, growing enough food and dealing with disasters. And if, as seems likely, this warming trend continues, (no matter WHAT you feel is causing it!!!), it will exacerbate almost all those other problems.
A microcosm of the situation can be studied by just looking at Easter Island during the last several hundred years. They made some of the same errors we are making now...both physically and culturally... and paid by having their small civilization collapse. No, they did not 'all' die, and neither will we, but their lifestyle was severely affected...as ours will be... if we don't reduce our population to a level capable of sustaining life at a comfortable level.