- don't use toilet paper - don't drink milk - don't travel more than a few miles from their birthplace

etc etc, and that will be shifting quite quickly, I believe. It took centuries for industrialisation to get e.g. Europe past such reference points of wealth, after two hundred thousand years of most of humanity living roughly without significant improvements in wealth. But it only took a few decades for e.g. China to lift hundreds of millions out of poverty and towards some form of middle-class.

You can debate some of the details for sure, my overal point is that we're not just going to add a few billion people to this planet, more importantly, we're going to add a few billion middle-class people to this planet. It's not the billions who live off of subsistence farming, who's impact on the environment is not all that much more substantial than any other animal grazing in a field, that is the big environmental issue. It's the fact that those people become more like us, drive cars, run refrigerators, eat energy-inefficient meats, have a shower in the morning and evening, live in large homes that must be cooled and warmed, eat more than necessary etc.

If you look at some figures that state we consume 10x more energy (making up a number here) than a subsistence farmer... it's more the economic shift from poor to rich we may need to worry about from an environmental, than raw additions to our global population.

(disclaimer: not implying I have anything against poor people getting richer, purely looking at it from an environmental perspective. Also, there's opportunities in a richer planet to build environmentally sustainable infrastructure, too, but on aggregate it'll bring some big issues to the table, far bigger than adding 1.6 billion poor people to our population, I think.)

The (human) carrying capacity of Earth was estimated to be around 2 billion, which was surpased sometime in the late 1920's. Contrary to other comments here, that does not mean that after you hit population 2,000,000,0001 we all die (we clearly did not). Instead, what it means is that given the technological level we had at that time, we'd consume renewable resourses faster than they can renew themselves, and we'd also produce waste faster than the environment can degrade it. Otherwise, more than 2B people would cause environmental degradation, which would itself reduce the carrying capacity in the long term.

Please note this definition is tied with humans technological level. It is not set in stone, since we have some degree of control over our impact in the environment, and we have the ability to use the same resources in a more efficient way. The big tragedy of 20th century is that this fact was not recognized but for a handful of theorists, and therefore it was not a political and economic goal to explicitly manage the carrying capacity of Earth. As of 2016, the situation is still the same.

By example, we gained a bunch of technologies that allows us to do the same stuff more efficiently. Given explicit economic incentives, we might have... maybe doubled our carrying capacity (CC=4B). Unfortunatelly, because this was not a goal itself, we engaged in a buch of economic practices that negated much of this benefits, so if we are generous these might have been reduced by half (e.g. CC=3B). Also given that population growth was not arrested back in the 1970's, but only slowed down, the carrying capacity has not improved at all (CC=2B).

Currently, environmental degradation is going in overdrive. We have lost a lot of time, and the resources we need to make an orderly transition are already commited to keep the system going. Population will go down, one way or the other. I don't believe in a single sharp die off many apocalyptic thinkers profetize, but adding and extra 1.6B mouths to feed will make the downward tendency of the curve more steeper than it needs to be.