What is the real question here?
There may really be two questions to address within the conceptualization of our doom. One is, Are we dooming millions—perhaps billions—to premature demise thanks to our current trajectories of population growth and energy demands, and who are they? The second is, Are we dooming ourselves to extinction?
Are millions of us doomed to premature demise?
To the first question, I’d hazard a “pretty likely." My students have often asked me what factors would contribute most to widespread destruction of human life, and I’ve always answered, “Resource availability and disease.” Not really that insightful of a response, as these are factors that, given a striking enough imbalance, negatively influence many eukaryotic species. Indeed, they have destroyed pockets of humanity since humanity existed. While some black swan of a microbe may be lying in wait to wipe out millions, our most immediate threat involves existing diseases and resource availability and inequity. Current resource inequities already result in the deaths of millions every year, often because of accessibility-related factors—especially water—that ultimately lead to disease. Thus, resource accessibility and disease remain interacting threats, and we, as animals, are not above succumbing to them in huge numbers. It’s something we already do. Who succumbs depends on a number of factors, most of them related to relative wealth and access to medicine and technology.
So, are we all doomed?
And that takes me to the second question I’ve posed here: In part thanks to that wealth and technology, are we dooming ourselves to extinction? Although disasters like Fukushima seem to portend a global doom, in the absence of a planet-wide apocalyptic event, I’d have to say, “Highly unlikely.” Consider the factors required for us to become extinct. In the end, 7 billion+ of us must die. Disease isn’t going to do that. Even a disease that devastated billions wouldn’t reach every pocket of humanity on Earth and delete H. sapiens in a swift blow. At the very least, some of the most isolated populations or the most resource-rich and technologically and medically enabled would avoid it. We are, after all, the species that has survived the Black Death and smallpox and millennia of high rates of infant and child mortality before we had such tools at hand. It would have be a mighty yet subtle microbe of a sort never before seen that could evade all the potential resistance—genetic, technological, and medical—humanity has and wipe us off Earth’s face.
Energy and resource deprivation won’t do it, either. While depletion might lead to a chain reaction of events that cause mass fatalities because of a lack of resource access, it won’t take us out to the very last human being. Humanity in the form of Homo sapiens has survived eras of disease and deprivation without effective medical therapies or a hint of advanced technology. Given our current tools, even if we bombed ourselves back to the Stone Age, in the immortal words of our masters of war, that’s an age we managed to survive with far less in the way of technical savvy or tools.
Mother, do you think they’ll drop the bomb?
Speaking of bombs, what about the fear that has lingered over my entire existence as a child of the Cold War era, Red Dawn, and The Day After? A nuclear winter would have to destroy to completion all food supplies globally. It would have to efface all available resources, worldwide, and preclude their renewal, and not everyone’s on board with even more minor versions of these outcomes. Disease effects from globally distributed radioactivity would have to kill everyone before they reproduced or make reproduction impossible, something that has not been seen in animals, at least, in the aftermath of perhaps the worst nuclear disaster, Chernobyl. The idea of the end of human reproductive capacities is a Children of Men scenario that highlights the only flaw in that gem of a film (and book): How can an inability to reproduce become universal among a globally distributed population of 7 billion organisms with unprecedented technological capacities? Even our sperm counts aren’t actually falling. In the face of a nuclear winter, pockets of humanity would likely survive and reproduce. Perhaps not abundantly. Perhaps not with comfort or cheer. But reproduction and a continued chain of being are all that’s required, and as agile and wily exploitative generalists, we’ve proved to be remarkably good at that.
For us to become wholly extinct—not only extirpated from some areas, not just experiencing truncated lifespans—would require a planet-wide catastrophe of the magnitude that took out the dinosaurs 65 million years ago or even worse, that caused the Permo-Triassic extinction, both of which cleared out species that didn’t have large brains, technology, or opposable thumbs. It would have to be an event that would reach all corners of the Earth and take every last human life, either in the immediate aftermath or over time with severe global climate change. Even with a planet-scale disaster brought courtesy of an errant asteroid, some small populations likely would remain, just another bottleneck in the history of the H. sapiens species. I note that such catastrophes historically have still left some behind. After whatever ended the age of the dinosaurs, some individuals survived to perpetuate the archosaur lineages whose descendants wander and fly the globe today. Their chain of being also continues.
The fault is in ourselves
A look into the past may also provide a look at the future. What has led to the extinction of previous, highly able Homo species such as Homo erectus and Homo neanderthalensis? The exclusion of previous Homo species may have involved interspecies competition between rivals in the Homo genus, namely us (a.k.a., "anatomically modern humans") vs. Them, possibly combined with some regional catastrophes. We outcompeted Them, and here we are. Might some other Homo species arise and return the favor? Given the usual requirements for speciation in animals, it’s unlikely. Isolated populations would have to remain isolated—that means no boats, no planes, no overland migration—for many, many generations. Knowledge of technology would have to vanish. The other apes are unlikely to hop onto to two feet any time soon and rise to a Planet of the Apes scenario, either. In other words, we’re all we’ve got, and it’s likely to stay that way.
Today, we are lonely Homos, and the only real threat we have to H. sapiens is ourselves. But how deep does that threat from Us go? History and rational evaluation suggest that it is not deep enough to erase H. sapiens from the face of the Earth. That doesn't mean that even as we survive, we won’t do severe damage to our global population numbers, all through our own devising. We are fully capable of bringing partial ruin on ourselves, with the potential for unimaginable pain and suffering and loss of human life. But will the ruin be complete and spell our ultimate doom? I don’t think so.