The first major test of my Half-Earth Hypothesis is in progress. Analysis of new data indicates that, if the hypothesis is correct, within a year humanity will begin consuming the ecological producers that maintain those ecological supporters that enable our basic survival. This will likely result in several hundred million casualties in the next decade, followed by several billion in the first half of the following decade, potentially leading to our effective extinction soon after.
This "hard shutdown" is a consequence of our historical behavior, but we may still have a chance of converting it into a "safe" shutdown by controlling both our population size and the amount of resources lost by our excess consumption. If competition for resources is the main cause of the initial casualties, we might in the best case be able to eliminate it and keep our population constant while reducing our individual consumption to a sustainable level and maintaining it there. This assumes that the lost producers can be recovered, and that their loss hasn't triggered a cascade of further environmental degradation.
If we can't control the population loss, then it may be kept from growing back as individual consumption continues to fall. In this case, protective policies might also prevent further casualties, and the drop in individual consumption may be stopped before it jeopardizes the maintenance of a civil society.
In my opinion, the best case future is about as improbable as the one most governments and businesses appear to expect, which is predicated on limitless growth. Based on that expectation, the other possibilities represent risks that merit little attention in the form of tweaks to their plans that may account for only a few-percent of additional costs in the distant future ("distant" being more than five years out).
Combining the scenarios discussed here and using my own estimates of their probabilities, I anticipate that the world will experience a serious food crisis just as the U.S. presidential race reaches its peak. During the new president's first term, the death toll will mount into the millions and people will attempt to grow much more food, exacerbating the problem. The next election will occur just as the population begins to recover, but consumption will have already begun falling. How far it falls, and whether we will suffer a much more massive loss of life, will be determined during the following decade.
Whether or not these scenarios are accurate, they provide a useful context for discussing how carbon emissions may decrease, voluntarily and involuntarily. The obvious preference should be for the best case; and we should put the mechanisms for creating it in place, regardless of motivation. We can similarly study the mechanisms involved in creating the disastrous alternatives so we can reduce their probability of becoming reality.