" Governments would have to go up against the very corporations that made them rich to lay down laws of environmental priority"
Yes exactly - or a "global government" - if we wanted to see drastic and immediate change there would in fact have to be a kind of environmental war (if not an actual war even) that empowered an Authority with the necessary political and military muscle to override national and corporate interests. That's hard to envisage without some cataclysmic upheaval which would no doubt involve a large loss of life.
Perhaps there are really only two choices - die quickly over a few decades, and maybe have a small chance of rebuilding or die slowly over a couple of centuries, but die comprehensively and finally. The other alternative which involve some miracle technological breakthrough and happy human families ever after look increasingly unlikely to me.
You touch on one of the main elements in the argument - that of over consumption.
It might be helpful to trot out the Wikipedia definition of sustainability:
"Sustainability is the capacity to endure. In ecology, the word describes how biological systems remain diverse and productive over time. Long-lived and healthy wetlands and forests are examples of sustainable biological systems. For humans, sustainability is the potential for long-term maintenance of well being, which has environmental, economic, and social dimensions.
Healthy ecosystems and environments provide vital goods and services to humans and other organisms. There are two major ways of reducing negative human impact and enhancing ecosystem services. The first is environmental management; this approach is based largely on information gained from earth science, environmental science, and conservation biology. The second approach is management of human consumption of resources, which is based largely on information gained from economics.
Sustainability interfaces with economics through the social and ecological consequences of economic activity. Sustainability economics involves ecological economics where social, cultural, health-related and monetary/financial aspects are integrated. Moving towards sustainability is also a social challenge that entails international and national law, urban planning and transport, local and individual lifestyles and ethical consumerism. Ways of living more sustainably can take many forms from reorganising living conditions (e.g., ecovillages, eco-municipalities and sustainable cities), reappraising economic sectors (permaculture, green building, sustainable agriculture), or work practices (sustainable architecture), using science to develop new technologies (green technologies, renewable energy), to adjustments in individual lifestyles that conserve natural resources."
Ok, it's a bit long winded, but essentially it makes the central point that the consumption and production of resources in a system has to be "in balance" to be sustainable - i.e. to last indefinitely (or as indefinitely as anything material can last!).
The Earth's ecosystems were doing pretty well even as late as a couple of centuries ago. A balance of sort, ignoring the slow grind of evolution and natural/cosmic catastrophes) had developed and had persisted for hundreds of thousands of years.
That's not the case any more. Humans have created conditions, via industrialisation and the ability to "insulate" ourselves against the Earth's natural cycles, where we can breed and consume and breed and consume in a manner which takes us "outside" ecosystems that have developed over the millenia.
This is patently dangerous because the easiest way to maintain this "bubble" of human existence is to create our own sub-system - which not only is proving patently incapable of long term sustainability, but also actively damages or conflicts with the existing natural order. Human arrogance, being what it is, has pushed ambition way beyond understanding. So that, long before we have any real clue as to what we are tampering with when we extinguish habitats, flora and fauna, we do it anyway and damn the consequences as long as it serves to maintain our hallucinogenic sub-system bubble.
Mushrooming population is a direct result of the cocoon we have placed around ourselves, but it isn't per se the cause of our unsustainable situation. To answer your question directly, I don't think that we can say that any particular population level is sustainable or unsustainable without looking at how that population is behaving, living, travelling, breeding etc. The fact is that humans, as distinct from other lifeforms, consciously manipulate, the ecosystem. That in itself, might not be a disastrous thing to do provided, and it is a very big provided, we actually have sufficient understanding to predict accurately. We patently don't have this ability at present, and yet our shortsightedness, selfishness, uncaringness, ignorance, arrogance ... whatever! allows us to carry on as if none of our actions will adversely affect ourselves or other animal life.
What has changed in the last 50 years or so is that the majority of intelligent humans are waking up to the fact that we can't behave like this and survive. The problem is that they don't agree on a solution. Accordingly, it seems patently obvious to me that until we DO understand what we are dealing with better, we need to make some drastic changes to try and reduce the impact of our human sub-system bubble on the remainder of the natural world and the Earth itself. Population control as part of a package of measure, would be a very good start imo.
Thinking this through, I got the title of this thread wrong. It should have read:
"Can life on Earth survive us?"