Sometime in the latter half of this century, human population will peak. Having swelled to a bit over nine billion people, our numbers will begin to drop as people age and women worldwide pass through the urban transition, gain control over their own life-choices and have fewer children.
After that, population will proceed to decline by the middle of the 22nd century to a number somewhere between 8.5 billion and 5.6 billion (depending it seems largely on whose assumptions about longevity growth you find most credible).
That’s pretty much the consensus position among demographers (though there is a range of belief about when the peak will happen and whether we can expect to more or less plateau at 8.5 billion or experience a long bumpy slope to a stable-state population of about 6 billion). Note that we don’t need to assume any sort of apocalypse here: this is the orderly progression of human beings passing through a post-industrial demographic threshold you can already see in cultures from Japan to Italy to Finland.
The standard response to these facts is that some new technology will “save” us, and make limits irrelevant. But I am consistently impressed, when I speak with folks who are hard at work in the fields of biotechnology, molecular engineering and software design, at how real a sense of limits actually exists among the smarter ones. There are things we don’t know how to do now and may never (in any foreseeable time span) know how to do; there are others that seem like good ideas until you start doing them and encounter the unintended consequences; there are still others that work, but work in ways that mean something different than we expected. Where in the 90s we expected emerging technologies to unleash the boundless, more contemporary thinking about these technologies seems to me to be all about seeing them not as magic but as tools: profoundly useful, if used right, but perhaps far less transformative than once we hoped. They may greatly extend the range of actions we can take within the fundamental limits we face, but they most likely won’t change the limits themselves.