In January of 1995, Russia detected a nuclear missile headed its way. The alert went all the way to the president, who was deciding whether to strike back when another system contradicted the initial warning. What they thought was the first missile in a massive attack was actually a research rocket studying the Northern Lights. This incident happened  after the end of the Cold War, but was nevertheless one of the closest calls we’ve had to igniting a global nuclear war. With the invention of the atomic bomb, humanity gained the power to destroy itself for the first time in our history. Since then, our existential risk— risk of either extinction or the unrecoverable collapse  of human civilization— has steadily increased. It’s well within our power to reduce this risk, but in order to do so, we have to understand which  of our activities pose existential threats now,  and which might in the future. So far, our species has survived 2,000 centuries, each with some extinction risk from natural causes— asteroid impacts, supervolcanoes, and the like. Assessing existential risk is  an inherently uncertain business because usually when we try to figure out how likely something is, we check how often it's happened before. But the complete destruction of humanity has never happened before. While there’s no perfect method to determine our risk from natural threats, experts estimate it’s about  1 in 10,000 per century. Nuclear weapons were our first addition to that baseline. While there are many risks associated with nuclear weapons, the existential risk comes from the possibility of a global nuclear war that leads to a nuclear winter, where soot from burning cities blocks out the sun for years, causing the crops that humanity depends on to fail. We haven't had a nuclear war yet, but our track record is too short to tell if they’re inherently unlikely or we’ve simply been lucky. We also can’t say for sure whether a global nuclear war would cause a nuclear winter so severe it would pose an existential threat to humanity. The next major addition to our existential risk was climate change. Like nuclear war,  climate change could result in a lot of terrible scenarios  that we should be working hard to avoid, but that would stop short of causing extinction or unrecoverable collapse. We expect a few degrees Celsius of warming, but can’t yet completely  rule out 6 or even 10 degrees, which would cause a calamity of possibly unprecedented proportions. Even in this worst-case scenario, it’s not clear whether warming would pose a direct existential risk, but the disruption it would cause would likely make us more vulnerable to other existential risks. The greatest risks may come  from technologies that are still emerging. Take engineered pandemics. The biggest catastrophes in human history have been from pandemics. And biotechnology is enabling us to modify and create germs that could be much more deadly than naturally occurring ones. Such germs could cause pandemics  through biowarfare and research accidents. Decreased costs of genome sequencing and modification, along with increased availability of potentially dangerous information like the published genomes  of deadly viruses, also increase the number of people and groups who could potentially create such pathogens. Another concern is unaligned AI. Most AI researchers think this will be the century where we develop artificial intelligence that surpasses human abilities across the board. If we cede this advantage, we place our future in the hands of the systems we create. Even if created solely with humanity’s  best interests in mind, superintelligent AI could pose  an existential risk if it isn’t perfectly aligned  with human values— a task scientists are finding extremely difficult. Based on what we know at this point, some experts estimate the anthropogenic existential risk is more than 100 times higher  than the background rate of natural risk. But these odds depend heavily on human choices. Because most of the risk is from human action, and it’s within human control. If we treat safeguarding humanity's future as the defining issue of our time, we can reduce this risk. Whether humanity fulfils its potential— or not— is in our hands.