So the world has been destroyed. Pick your poison. We have probably 300 different scenarios lying around here somewhere.

What's Left:

  • The world is a desolate wasteland, dry and crispy and barren
  • Temperatures are significantly higher (Greenhouse gasses)
  • Wildlife is all but wiped out (something always survives). I suppose the oceans are still somewhat populated with life.
  • Humans live in a few isolated bubble enclaves. Current era technology survives but is not easy to replace.
  • Humans collected and have stored DNA for basically all life forms on the planet (think Titan AE). Lets assume we can effectively create plants/animals/life from their DNA in this reality.
  • The goal of the remnants of humanity is to restore the world to livability and ecological stability.
  • The isolated habitations are in regular contact at the government level. Person to person global communication is no longer common place, either via phone or net.

Scientifically speaking how does humanity go about restoring what it destroyed, and what kind of timeline are we looking at?

  • $\begingroup$ No nuclear winter in this scenario? Outlasting that would be one of the longer pieces of recovery $\endgroup$ – Twelfth Dec 23 '14 at 17:41
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    $\begingroup$ @Twelfth My thought is that as the climate is changed by us people the rate at which it changes will accelerate. Adaptation is all well and good but if you kill off the basic building blocks of the food chain it doesn't matter how well the rest adapts. And again, pick your poison, I am more interested in how to fix things at this point than how it got there. $\endgroup$ – James Dec 23 '14 at 19:08
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    $\begingroup$ Climate change will not make land crispy and barren. But the way we are going, Alaska might have tropical forest and alligators. Most of the species (including plants) which need colder temperatures will go extinct. And how to fix things depends what is broken, and how badly. $\endgroup$ – Peter M. Dec 23 '14 at 19:50
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    $\begingroup$ Do the remnant "bubbles" of humanity have enough contact to make a concerted effort or is it just every bubble for itself? $\endgroup$ – the_OTHER_DJMethaneMan Apr 15 '15 at 18:34
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    $\begingroup$ Nobody mentioned Svalbard ? en.wikipedia.org/wiki/Svalbard_Global_Seed_Vault $\endgroup$ – Vincent Apr 18 '15 at 19:36

11 Answers 11


I'll give a go here...

First thing would be for the surviving humanity to take full inventory of what they are working with here. The globe is a huge place and though some regions will be quite hostile, there should be some regions that are relatively 'nice' to be living. For example, arctic currents flow south from Alaska down the North Amercian coastline and should provide a cooling effect to that coast, making it most likely that a large area of semi usable land still exists in northern canada and into the Yukon/Alaska (this assumes the increased heat and additional fresh water doesn't alter oceanic currents). Getting to these area's, colonizing them, and getting humanity itself back onto it's feet will be a top priority. The Antarctic continent is likely another location that would be well suited land now...apparently under it's currently ice landmass is the largest lake in the world, providing fresh water supplies and the necessary resources to reestablish human population.

From there, it's a 2 part effort to bring temperatures back down.

  1. Albedo. In short, we want to discover methods of which to reflect incoming sunlight back into space without being absorbed here on earth. Unfortunately the best method of doing this is Ice!...which in this scenario wouldn't exist. So we'd need one of two methods here

    • Vegetation. Plantlife that is light in color (or white leaf all together) that is prolific and can be spread across large locations of the globe would theoretically work. I'm not sure what exists for this in present day, but genetically engineering a white leaf plant and covering large amounts of land could work
    • Water / Clouds. This is a feed back mechanism that already exists...Dry crisp and Barren is not really a truth here...a rise in temperature ultimately see's more water transitioning from liquid to gas (which actually cools the globe as well...this is in part my comment on just global warming coming to these effects, the properties of water insulate the globe relatively well)...a warmed globe is ultimately a wetter globe. More clouds in the air = more clouds reflecting sunlight. Once again in the theory range, but anything humans can do to cover the globe in cloud (getting into weather modification here) will help.
  2. Reduce insulation. I think your question is on the basis that CO2 is the primary greenhouse gas responsible for this warming effect. If this is the case:

    • Plant life. Plants refine the co2 from the air and turn it into a variety of other compounds. Not the quickest solution
    • Stop burning fossil fuels. Faster we quite adding to the CO2 total, the faster it can be brought under control
    • CO2 Sequestering. CO2 'nets' can be used to capture CO2 from the atmosphere. This CO2 is then brought under high pressure which liquefies it. From here, it can be injected into Earth (oddly enough, into the wells where it likely came from in the first place). Look up carbon capture and co2 sequestration if you want more info on the process.

It's hard to get into timelines without defining what extent it has gotten to...but one assumption to challenge...the earth is 'supposed' to be anything. The Earth is in a constant state of flux, shifting from one shape to the next over the course of years. There is no 'natural' state for it to be in, outside of this constant change and trying to prevent change is unnatural as the change we are inflicting upon it. Even an attempt to get it to 'most positive for humans' comes at great benefit to some regions and quite costly to others...restoring the cradle of life to it's dawn of humanity state leaves England under ice.

Edit to add:

It is possible that our efforts in cooling the globe will overshoot as well...any method of cooling the globe that worked too well could just throw us the other direction and into an ice age. Climate is quite the balancing act.


Life is nothing if not amazingly good at rebuilding after major disasters, so to regain livability and ecological stability, all we probably need to do is let the world be. It will eventually regain equilibrium, though the equilibrium it reaches post extinction will likely be quite different from what there was before.

If we want to rebuild the same sort of ecosystems, we'll have a bit more of a challenge on our plate. Reintroducing animals, and breeding animals back from very small populations, is difficult. In addition, large animals such as tigers or bears generally require a healthy, mature ecosystem to support them. We'll have to regrow the forests first, then reintroduce our large herbivores, and finally start rebuilding carnivore stocks.

In addition, we'll have to get everything that springs up on its own out of the way. All of the current invasive species are likely to be some of the prime repopulators of the barren, crispy wastes. If we want to reintroduce broadleaf forests in the southern US, for example, we'll have to clear the forests of kudzu vine that have sprung up in their place. In many places, it may be best to build a new ecosystem, based on what is already growing, rather than try to reestablish what existed before the catastrophe.

Before we can do any of this, though, we'll have to stabilize the human population. What we'd probably want to do is set up whatever industry is needed for long term support of agriculture first, and move on from there. If the crispy wastes start sprouting grass, it may be a good idea to bring back some large ungulates, such as bison, to provide an additional food resource that we don't have to actively manage, with the same logic holding true for fruit trees.

The survivors will have a leg up on modern humans in terms of getting technology back online, though, because all of the processed resources are still there. Even bombed out and depopulated, a city is full of all of the aluminum, steel, and silicon that a fledgling culture would need to get back on its feet.

  • $\begingroup$ If there's no grass in this crispy wasteland, then no bison. And if Kuzdu survives, we could just feed everyone, including us, that. Its edible, from what I understand. $\endgroup$ – Oldcat Dec 22 '14 at 22:13
  • $\begingroup$ Huh, never knew kudzu was edible. Apparently, it makes good animal feed too, so I may yet be able to have my bison :) $\endgroup$ – ckersch Dec 23 '14 at 15:08

You say that the world is a desolate wasteland, dry and crispy and barren. Repairing this could happen naturally but it would take a long time, likely thousands of years.

It would take a very long time even with intervention from humans, your going to have to go through the process of growing grasses then shrubs, bushes, small trees and big trees.

It would be quite hard to replicate the current ecosystem, especially when some plants are dependent on others and must come first. And then finally after you have sufficient plants you could start introducing animals, but again it would have to be in a specific order and you would have to let some animals populate the area before introducing a animal that is higher up in the food chain or you would run the risk of running a species extinct.

But you have also mentioned that the temperature is significantly higher, which means we probably would not be able to get it back to the way it was before. We may have to plant plants that thrive in hotter environments.

It will be a very long process, and lots of hard work, or it just may happen by itself.

  • $\begingroup$ Last time (PETM) it took 170,000 years. $\endgroup$ – user3082 Feb 3 '15 at 9:04

Before I start answering your question in earnest, I just want to elaborate on a key point of an earlier answer which is important to understanding the climate of your world:


All of the precipitation that falls originated as water vapor that was evaporated from the surface of the Earth. It is always raining somewhere on the Earth, just as evaporation is always occurring over most of the Earth's surface. At any given time, precipitation covers only about 2% to 5% of the surface of the Earth, while evaporation is occurring over the remaining 95% to 98% of the Earth. Thus, as water vapor slowly evaporates over most of the Earth, an approximately equal amount gets "concentrated" into relatively small rain systems that turn some of the vapor into precipitation.

In your world, it is very hot and most of the land is arid or dry and deadish, which means that:

  1. The air is able to hold a lot of humidity before precipitation will occur. When it does it will be violent, causing flooding and damage.

  2. Desert arid areas are caused by a lack of precipitation. Because most of the land is arid, these precipitation events must be occuring very rarely or are concentrated in just a few places (the latter would be easier for humanity to work with).

You cannot just introduce life into an arid environment and hope it will catch on. You either have to cool down the whole damn world so that precipitation can be more evenly spread, or irrigate parts of it so that it gets the water it needs to develop. (This has been done in the Imperial valley, which has turned a desert region into a succesful farming region.)


I suggest that your remaining human colonies concentrate on surviving on irrigation, while very gradually cooling the climate (Don't want another snowearth). Once precipitation is more evenly spread you can think about the order to introduce other forms of plant and animal life.

To cool down the earth, most people have focussed on removing C02, but I suggest you focus on adding oxygen to the atmosphere instead. (This may actually be a very critical concern, since a lot of the earth's oxygen producing plant life will have been destroyed). Luckily, 50% of the earth's oxygen is created by photoplankton. These suckers love sunlight and lots of nutrients. The nutrients are easier to get in coastal regions and cold waters - it is easier for nutrients to rise up from the ocean floor. Since the water is warmer now, humanity could give a helping hand by plankton farming (adding nutrients) or dredging the ocean floor.


How long will this take? You don't want to rush it. I would aim for a very slow cool, at LEAST several hundred years.

  • $\begingroup$ I imagine that human enclaves would be split into two parts - the areas where precipitation falls, which would have to be developed to withstand the damage caused by extreme events, and the areas where the precipitation was channelled, which would be the farmland, safer, and more controlled. $\endgroup$ – Kristy Apr 16 '15 at 15:14
  • $\begingroup$ While researching my answer to this question, I had a look at the plants and animals that survive in more arid regions. I was intrigued to discover that the animals are all quite poisonous, becaues there is an evolutionary advantage to kill your prey with as little effort as possible. Lightbulb moment! So that's why Australia is home to so many deadly snakes and spiders. $\endgroup$ – Kristy Apr 16 '15 at 15:19

My idea - and that's simply because I'm trying to do it in my garden :) - would be to use some simple permaculture techniques in order to create sustainable gardens.

With access to fresh water and enough nutrients to start, people could plant trees, bushes and vegetables in such a way that a micro-ecosystem would emerge. Trees would shield the garden from too much wind and sun, remains of one year's crops would become compost for the next year's plants and human work would ensure that the garden would not die off.

But, at least at the beginning, gardens will not produce enough food for large communities. Ideally, people will spread with each family, or a group of families, trying to set up their own garden. It will be a lot, lot of work, and they will live in very harsh conditions, maybe for generations, but eventually their gardens will grow enough that they will be able to climb above subsistence agriculture and direct a part of their time and energy into recreating more complex forms of society, luxuries, technologies, etc.

And then there will be another global war...

You can also take a look at this: http://en.wikipedia.org/wiki/Open_Source_Ecology


The first step in your path to rebuilding the world would have to be planting a crap load of trees. the reason being is that trees are a hell of a lot better at Carbon sequestration than other plants. While I will leave the exact species up to you, the type of tree used would preferably grow quickly and live for a long time. You can use the tree in this ad as a prime example of what you want. I don't actually know how long it would take for this strategy to pay off, but with the new forests growing I would say anywhere from 150 to 200 years (coincidentally this is roughly the amount of time it took us to screw Earth up).

The biggest problem might be replacing entire ecosystems and habitats and cleaning up land pollution. If we want a healthy world we will have to get rid of the 270,000 tons of garbage on our oceans. Furthermore, since the world has 2.6 trillion pounds of garbage that used to be generated annually, and much of that was not bio-degradable (e.g. Plastic) the remnants of humanity will have a heck of a time cleaning that up. And that is not even factoring in the amount of garbage our panicking, collapsing societies made directly before the great fall.

It will take many years to regrow some of the extinct animals. The average lifespan of a lion is 10-14 years. The average for an elephant is between 30 and 70 years. These animals and other large animals like them will be endangered for at least a hundred to two hundred years.

While I am not a mathematician I could probably safely say that to clean the atmosphere and build a basic regional ecosystem would take around 300 to 1000 years.


Note: for the purposes of this answer, I have assumed no tech remains. If you do have tech surviving, then many of these steps will be easier.

There seem to me to be a number of steps that you must go through to rebuild a planet.

1. Water

Water is the most basic human requirement and without it everyone dies very quickly. The first priority is to find a source of fresh water that people can drink. I do hope your apocalypse wasn't nuclear or this is going to be very hard.

2. Food

Food is the next requirement: while you can survive longer without food than you can without water, you're quickly going to need some to be able to keep rebuilding. Look for old stashes, if there are any: anything in a tin will have kept. As soon as possible you want to start getting some basic farming going. Fishing will also be a good option, especially if you're near the sea.

2.5: Tools

Tools are also fairly essential. To open tins and kill things to eat you're going to need some, even if it's just a sharp bit of rock. Start your search for tools quickly, preferably at the same time that someone else is out looking for food.

3. Shelter

Start building. You need somewhere to survive. If you're already in a cave you're in a fairly good way, but you might want to consider partially blocking entrances to keep warmth in. The more people in one shelter, the better, as it will be warmer inside. However, caves can't sustain you forever and you're going to need to start building other shelter soon, but that requires wood.

4. Start Growing

Trees, plants, food, start growing it. You'll need them all to survive, and many plants we think of as useless do actually have very useful functions in a survival situation: nettles can be used as string for bows and you can make tea from them (though that may be a while off yet). Trees will be essential for building better shelters in the future.

5. Start Exploring

Now that your camp is reasonably well set up and sustaining itself, get a team out every day, exploring the surrounding landscape. They should look for:

  • Food sources
  • Water sources
  • Better shelters
  • Raw materials
  • Other people

You've got a good setup for your little group, so you can now try to make contact with other groups of survivors. There might be one 2 miles over the hill; your explorers should find them. Any lost souls you find, you can take in: you should have some surplus food and water, and it strengthens your group.

6. Start Building

This should now be several years down the line, where you have food, water and materials stored up. You can now start building: homes for people, storerooms so your cave and homes aren't so cramped. As you get new materials in, invest them in building to last - you want what you build to survive as long as possible so you don't have to expend materials repairing it.

7. Start Innovating

This is even more years down the line. You should have a permanent camp set up and teams going out every day to get food, water and materials. Set up another team or few at camp to start making things that might be useful for you:

  • New tools
  • New storage methods
  • Methods of communication

Obviously we're not talking about radios and all that yet (unless you happen to have the materials and engineers - but they're still pointless unless someone else has one), but any way of getting your voices heard further and further away. Perhaps a primitive voice amplifier? Go up to the top of the nearest hill and shout. See if you get any responses.

8. Join Forces

Your group is now strong. You should be looking out for other groups and other lone survivors so you can join up with them. You want to get more and more people together, because then you can start rebuilding civilization.

I should also note that in most of these steps, you're going to need more and more advanced tools than the last. You might want to set up a team of engineers at base to make you new tools.


Okay, I'll bite; your world is crispy and (mostly) barren, but you can go outside and wander around as long as you brought a picnic. And you have the ability to rebuild life. Sounds super ideal for those struggling with terraforming Mars, for example.

Step One: Soil

You need soil to grow most things, and this is going to be a big struggle - I think people dreaming up terraforming Mars (for example) downplay its importance and difficulty. You'll find some, if you dig deep around and hydroponics, etc. will help at the beginning. Get your bugs and worms making soil from any organic matter and rocks that you can find.

Step Two: Create Ecosystem(s)

An ecosystem doesn't happen overnight, but I can give you one example where it worked quickly. In Abu Dhabi (where I work), a very desert climate, they were able to unintentionally establish (unstable) ecosystems just out of landscaping and forestry projects: the plants were fed oasis or aquifer water, and later desalination. Within 5 years, the palms and gaff trees, etc., were somewhat mature, bringing bugs, then birds, and later non-migratory birds and small predators.

Find a series of locations to do this more regionally: pine trees and associated flora & fauna in higher elevations, tropical ecosystems in rain shadows and tropics. Continue your pilot projects, but don't bring back the mosquito, please.

The things that thrive best will expand outward until they reach their biological limitations. Document everything.

Step Three: Agriculture

Presumably your small bubbles of people have the means to make their food just for them, and the conditions are ready for them to venture out. Take your soil-production facilities and start finding the best land for agriculture. Don't get too hasty on the livestock, but focus on your staple foods that grow fast and in abundance (and can make alcohol out of...): rice, wheat, and so on.

Step Four: Infrastructure

With agriculture and ecosystems, populations can start growing more stably. Time to build your bigger power plants (hopefully renewables this time), water collection & storage, wastewater treatment, recycling. You don't have wood, but you can make concrete-block housing and buildings; hire your urban planner right about during this step.


Since the answers upthread have lots of detail I will only elaborate on two points:

  1. Gathering water can be done using large "fog catchers", such as is done in the Atacama Desert in Chile. Small amounts of moisture accumulated over a long period will serve to moderate microclimates and make planting more successful in at least designated areas.

  2. Planting will have to be dine much differently. A company called Afforestt (http://afforestt.com/index.html) uses a high density planting and cultivation technique to generate dense and self sustaining forests in a short period of time (most dramatic is the claim their technique will do in a few decades what takes almost a millennium to occur in nature.)

While it is questionable if this claim would work in the environment you describe, even areas where the forest dies injects biomatter into the environment, and a simplified ecology of moss, lichen and fungus might still survive, creating a patch of soil for the next attempt.

The best way to do about this would be to create very small colonies of plants and animals, more like carefully tended gardens than forests, and allow them to gradually spread out around the edges. As time passes, more of these garden spots can be planted in a checkerboard pattern, infilling the empty squares as they become self sustaining and the micro (and eventually macro) climates moderate.

  • $\begingroup$ I like the idea of fog catchers. I also imagine that moss and lichen would grow quite well without precipitation so long as there was humidity. Some lichens have even survived in conditions similiar to Mars! $\endgroup$ – Kristy Apr 17 '15 at 10:12

I assumed climate change - global warming. Any other apocalypse would have different effect on Earth and different way to fix it.

Naturally, it will take some 10K-15K years to bind most of the CO2 from atmosphere to rocks. I will dig up link later.

After sea level increase of about 200 feet (70 meters) in about 500-700 years, most of humanity cities (which are most often close to sea - advantage for trading) would be devastated by sea water, together with big part of agricultural land. Increased temperature will shift livable zones (equatorial areas would be practically unlivable).

Increased acidity of water will topple ecological balance: many small creatures will not be able to build shell (it would dissolve - small organisms have bigger ratio of surface to volume) and food pyramid in oceans would collapse. Algae and jellyfish would benefit.

Introducing extinct species will not help if they cannot survive in more acidic water.

It took millions of years in carboniferous period to bind all that carbon from atmosphere to coal, oil and gas. Any idea that we can save planet by just planting few trees is delusional: you will need to bury them underground for millions of years to repeat the results.

Restoring delicately balanced ecosystem is hard. Part of the problem is that we do not understand all the relationships, and not all species are easy to preserve and breed to populate newly opened niches. Most are not.

Another cause of problems might be forgetting to introduce natural enemies of a species, like rabbits in Australia, or asian carp in Missisippi. Without population control, any species can dominate ecosystem. Every insect has parasitic wasp living off it. So you need to introduce them all.

Our attempt to fix just much simpler limited ecology in Biosphere 2 failed miserably.

I would not bet that in 20K years we will have our current Earth back - it is unlikely.

One of the problems would be that even if we can create animals, we cannot recreate the communities they lived in, their "culture". Say elephants, or orcas, or bonobos. Those are smart animals, they rely on more than bare instincts.

Another one of those answers of mine where people downvote it because do not like facts mentioned. Yup, if we screw it is THIS bad. Unless you allow to apply magic and/or magical thinking.

Say aliens collected human DNA, PC and wikipedia. They created babies, get them potty trained. Why babies don't get on PC and read the wikipedia? What aliens missed?

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    $\begingroup$ While I appreciate the response this doesn't really answer the question... $\endgroup$ – James Dec 23 '14 at 16:13
  • $\begingroup$ Answer is: depends how much we will screw up, and how much resources we will have to fix it, it might take 10K-20K years, but getting back to "almost pristine" Earth of today - answer might be NEVER. $\endgroup$ – Peter M. Dec 23 '14 at 16:45
  • $\begingroup$ What part of "Scientifically speaking how does humanity go about restoring what it destroyed" does your answer really address?, this seems more like a semi on-topic rant $\endgroup$ – Twelfth Dec 23 '14 at 20:33
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    $\begingroup$ I guess I am in wrong, because I cannot scientifically prove how to fix unknown apocalypse (with unknown and undefined consequences) using unknown (but future and cool) methods. Oh I know, just use bit of magical thinking! $\endgroup$ – Peter M. Dec 23 '14 at 23:59
  • $\begingroup$ Seems to answer the question to me. How do we fix the Earth? Re-sequester all the carbon that escapes. How long will it take? Tens of thousands of years. $\endgroup$ – ckersch Dec 24 '14 at 0:12

This brings to mind Sid Meyer's Civilization II game that has been running for more than ten years, and then posted as a save file, spawning a subreddit. Some people have picked up the challenge of ending it on a happy note.

http://www.reddit.com/r/theeternalwar/comments/1os2g4/4300_71_years_world_peace_part_1/ is a good example.

Really, how life rebuilds is entirely dependent on what happened. If it's nuclear warfare, think upwards (gravity'll draw radioactive particles down). If it's global astral strike, or solar conflagration, dig down (but not straight down).

So, likely resources:

  • there will be a decided lack of wood. All trees will be vaporized, burnt, withered, rotted, or dissolved. The next best thing would be bones, of which there should be enough of to start carving into rock. If building upwards, the lack of wood is even more annoying: you have to climb down every time.
  • Water: Humans can go three days without water. If needed for any reason, we can filter our own urine to drink.
  • Food: Humans can go three weeks without food. If digging down, get used to eating moles.

In any case, a key to survival would be mosses.

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    $\begingroup$ "IF FOR ANY REASON WE SUDDENLY LACK AIR, WE DIE IN THREE SECONDS." Untrue. Most people can hold their breath for 30 seconds minimum, and death by asphyxiation is estimated to take twice as long as you can hold your breath: 1 minute. And what about people who can hold their breath for 5 minutes? $\endgroup$ – ArtOfCode Apr 18 '15 at 19:30
  • $\begingroup$ WELL, that is, if humans suddenly run out of air, and then run our of air in our lungs, we wind up dead. This is further compounded and/or exasperated if we lose the atmosphere: not only do we lose air, we lose pressure. $\endgroup$ – Nefer007 Apr 19 '15 at 16:49
  • $\begingroup$ The fact remains: we can survive without air for longer than 3 seconds. $\endgroup$ – ArtOfCode Apr 19 '15 at 17:40

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