After thousands of years of unfettered human expansion, we've hit the wall. Humanity has to develop a different paradigm than endless growth and the rape-pillage-and-burn approach to extracting materials, energy and resources, which represents the current corporate push to "grow". Changing this requires a different kind of accounting system and a shift in values. Key to this is reversing the equation with respect to population levels, done in an intelligent fashion. This immediately raises the specter of overarching bureaucracy, violation of individual rights and invasion of privacy. These reactions are valid if governmental control is used indiscriminately to abuse individuals on the basis of reactionary, brute-force methods. But there's a better way, through consensus.
Interestingly, this consensus can be forged out of the current blowback to the UN protocols that have engendered the "smart growth" strategies for urban development. In the US, the state of Alabama has just outlawed this approach in order to protect private property from eminent domain and undercut pollution controls and ecological preservation; it has effectively prohibited the UN Agenda 21. The Agenda 21 document says the world is one world; that its resources are finite; that environmental problems in one country affect its neighbors and the world at large; that over-population imposes a great strain on the world's finite resources; that a concerted, global response is more likely to be effective than a disjointed one; and that every nation had to find its own way to live up to the resolution. Further discussion of this political impact is here in an article from Huffpo.
If it's understood that development must be curtailed, as do the scientists who most recently stepped up to take a public position, then a very obvious way to preserve our remaining open lands, natural areas, lakes, rivers, oceans and forests is to relieve population pressure. You can't have both, there's a limit to the budget, it's very simple. So a global public policy needs to be leveraged in a way that preserves the quality of life and the natural resources in all countries at the same time as it rewards negative population growth. There's many positive ways to accomplish this, as the Sierra Club points out.
In terms of a global agreement that allows countries to preserve their natural resources and individual freedoms, it is necessary to establish a baseline population standard that gives incentives for people to preserve their way of life in a manner that regenerates their land resources, hence their wealth. Self-interest in happiness and abundance for individuals is then governed by limitations on population through public policy and taxation measures that make it clear that lower populations will benefit all of society and provide the necessities of life for everyone. One way this can be accomplished is laid out here.
An agreement around these issues, hopefully during Rio+20 this month (download a draft here), is imminently needed before Nature collapses the pie for all of us.