The world as we know it has ended. A particularly virulent strain of avian flu finally breached the species barrier and hopped successfully to human hosts. Or tensions between India and Pakistan reached the breaking point, culminating in the use of nuclear weapons. Or a rocky asteroid, only around a mile across, slammed into the Earth and fatally changed atmospheric conditions.
As recently as the last century, people made the things they used every day. Yet in the span of just a couple generations, we have become a society of consumers rather than makers. Thanks to today's modern conveniences, we have become disconnected from the basic skills and knowledge on which our lives and our world depend.
Here, then, are a few of the skills you'll need to survive in the post-apocalyptic wasteland.
Survive the immediate aftermath
Aside from dodging raiding bandits, the single most important thing you can do to stay alive in a post-apocalyptic world without antibiotics is to stop yourself picking up infections in the first place. Ensure your drinking water is not contaminated — boil it if necessary, or even disinfect using diluted bleach scavenged from any abandoned household. Soap is enormously effective at protecting against gastrointestinal and respiratory infections, and can be made by treating animal fat or plant oil with quicklime (roasted chalk or limestone) and soda (see below).
Scavenge what you need
For a certain grace period you’ll be able to dine-out on the left-overs of our fallen civilization — stockpiles of canned food in the supermarkets — before you need to redevelop agriculture to stop yourself starving to death. You’ll need viable, preserved seeds, and the Global Seed Vault on the Norwegian island of Svalbard will be well-worth a post-apocalyptic recovery expedition. This is a doomsday-proof facility dug deep into the arctic permafrost and represents an ideal agricultural SAVE file.
Reconstruct the calendar
The bane of our working lives today, the calendar is in fact critical to reliable agriculture and survival as it allows you to track your passage through the cycle of the seasons and so predict the best time for planting and harvesting.
In the northern hemisphere, summer solstice is the day the sun rises from its northern most point on the horizon (which you can in turn determine with a magnetized needle) — this falls around 21st June and so you can use this observation to peg the rest of the calendar. As your agriculture becomes increasingly efficient it’ll demand a lower and lower fraction of your population, freeing people to specialize in other skills and for your society to grow in complexity and capability.
Restart a chemical industry
Advancing civilization is not just about ensuring food surplus or exploiting windmills or steam engines to ease human labor, but also about providing vital substances. One of the most crucial classes of chemicals throughout history has been alkalis like potash (potassium carbonate) and soda (sodium carbonate), as these are needed in making glass, paper and soap. Potash can be simply extracted from the ashes of a wood fire by soaking water through them. Discard the insoluble minerals that settle on the bottom, and then recover the dissolved potash by evaporating away the water. Soda is made in the same way, from burning seaweed.
Once all the remaining gasoline and diesel is gone you’ll struggle to drill for your own oil: the easily-accessible reserves have already been pumped dry. But that doesn’t mean you’ll have to abandon automobiles and mechanization — astonishingly, the internal combustion engine can be run on flammable gases released by the thermal-breakdown of lumber. Wood gasifier cars were common during WWII, with a tall combustion chamber strapped on the back and a pipe delivering the flammable gases into the engine cylinders.
Reestablish contact with remote communities of other survivors
If there are no functioning radios left, you can create your own receiver with surprising simplicity from scavenged materials, as was demonstrated by POW ingenuity during WWII. The key component is the rectifier that strips the sound away from the carrier wave: the contact between a pencil and rusty razor blade functions for this. A crude transmitter can be built for Morse code broadcasts using a spark generator.
How to relearn all else
By far the most important thing to try and protect and preserve through the apocalypse is the technique you need to apply to relearn everything else for yourself, to rediscover how the world works and then exploit that knowledge for developing novel technology and improving your life. This tool is the scientific method. The core principle is that you can only reliably understand the world by observing it first-hand and by quizzing it with carefully constructed questions ("experiments") to test which of your explanations works best.
Lewis Dartnell is a UK Space Agency research fellow at University of Leicester and author of The Knowledge: How to Rebuild the World from Scratch (The Penguin Press). Read more at the-knowledge.org.