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This is one of the questions I get asked most often since I started to cover the topic of asteroid impacts. Will humans will become extinct within a decade, or within a century? And can this happen through natural disasters? For instance if you watch the movies you may think there's a chance of a giant asteroid impact which will make us extinct. But what's the real situation?

I'll look at natural disasters first, and then results of our technology that could happen in the next ten or twenty years. Technology advances so rapidly that you can't look much further than that, as you can see by predictions of the present from thirty or more years ago. It would be nice to look further ahead, but what may seem major issues for the future now may never happen and other things that we haven't even thought of might be the big issues of the day in the 2040s.

And this is about whether we can go extinct, not about things like famine or war. Even an all out nuclear war leading to a nuclear winter would not make the tropics as hard to live in as the Arctic - so some humans would surely survive. And the radioactivity could also be dealt with, enough so that some humans would survive it. Of course we must not let that happen. But it wouldn't make us extinct, and that's the topic here. Would anything else do this? What about climate change, or asteroid impacts? I've written this for anyone - so if you have a scientific background, do excuse me when I occasionally venture into the more "wacky" ideas that bother some people though any scientist would see that there is no possibility of them happening.

Well, first, we are an extremely adaptable species, able to survive anywhere from the Kalahari desert to the Arctic, with only stone age technology. We had already colonized most of the world by the end of the neolithic period.

Overview of Pre-modern human migration - there is debate and controversy about the details, but generally agreed that humans were already present world-wide by the end of the neolithic period (which ends around 2000 BC), or shortly after.

So, as long as we retain at least stone age technology, there isn't much that could make us extinct.

Homo Sapiens is listed in the IUCN Red list of threatened species - as one of the species of least concern

"Listed as Least Concern as the species is very widely distributed, adaptable, currently increasing, and there are no major threats resulting in an overall population decline."

The species is at least 100,000 years old. In that time, there have been volcanoes, earthquakes, tsunamis, floods - but no massive asteroid impacts.

So, what about those, could those make us extinct? Or could any other rare event make us extinct - something so unusual that it hasn't happened in the last 100,000 years?

GIANT METEORITE IMPACTS

If you ask someone what could make us extinct, they might well say, an asteroid impact. It tends to be top on everyone's list. But if you look into the topic closely, it's not possible.

We haven't been hit by anything this big for over three billion years. The big craters on the Moon, Mars and its moons, Mercury, a very old and huge eroded crater on Earth also - they all date back to over three billion years ago. Back then the solar system was still settling down into its current state - towards the end of the "late heavy bombardment".

The Aitken basin at the lunar South pole. It's believed to be over 3.8 billion years but the exact date is hard to pin down. Impact of an asteroid perhaps 170 km in diameter.



An asteroid that large hitting Earth would indeed be seriously bad news and probably make humans extinct. But those big craters on the Moon and other bodies in the inner solar system date from well over 3 billion years ago when our solar system was still settling down. We have no impacts that big on any of Mars, Mercury, Earth, our Moon, moons of Mars and what we have of the geological history of Venus since its volcanic resurfacing.



There are thousands of asteroids up to around 10 km or so in diameter that do close flybys of Earth. They are similar in size to the Chixculub meteorite, up to around the size of New York - too small to make a species as adaptable as homo sapiens us extinct. The larger asteroids like Vesta, Ceres etc are all in stable orbits, at least for a few hundred million years (there's a 0.6% chance of Vesta hitting Ceres in every billion year period, but it's known they can't hit for millions of years into the future).

The reason is that Jupiter protects us, so the simulations suggest. It breaks up really big comets, or they hit Jupiter or the sun, or get ejected from the solar system before they get into orbits close enough to ours to be any problem.

Details: the thing is that large comets or asteroids from the outer solar system would almost certainly not be exactly aligned with the Earths orbit, but pass well above us or below us on the way in and out (Earth is a tiny target). And our gravity is not enough to make a big difference there except with a very close flyby. But Jupiter's is, so they are bound to pass close enough to Jupiter to be affected after a few orbits.



It's only after many flybys of Jupiter that they would end up in the same plane as the other planets in the inner solar system.



It's true that Jupiter does also send many smaller asteroids and comets our way. But in the process, many also hit Jupiter, or hit the sun, or evaporate in the Sun, and the larger ones get torn apart through tidal forces when they pass close to the Sun or Jupiter.



We saw just this happen with Comet Shoemaker Levy. It got split into numerous smaller comets through tidal effects of Jupiter Then it hit Jupiter, leaving these marks in its upper atmosphere, which gradually faded away. At the time it was thought that this was a rare once in a century type impact. But then another impact was discovered in 2009. We now know that Jupiter is regularly hit by quite large comets, and the models show that it tears others apart, as does the sun.



This seems to be the explanation of the cratering record and so the reason why Earth has not been hit by hundreds of kilometers diameter asteroids and comets for billions of years. More about this, with links to the scientific literature, in my Giant Asteroid Headed Your Way? - How We Can Detect And Deflect Them - scroll down to the section on " WHAT ARE THE CHANCES OF GETTING HIT BY A REALLY LARGE TEXAS SIZED ASTEROID AS IN THE MOVIE ARMAGEDON?"

They'll say - look at the meteorite strike that ended the dinosaur era!

But we are not dinosaurs. Without any technology, turtles, crocodiles, alligators, small mammals, flying dinosaurs (the birds), dawn redwood trees, pine trees, many lifeforms survived that impact. And humans with the barest minimum of our technology are able to survive anywhere from the Arctic to the hottest of deserts, or in tropical rainforests. We would survive, some of us, a giant impact like that.



River turtle, Boremys basking on a Triceratops dinosaur skull, Credit: Brian T. Roach, Yale Peabody Museum

How Tough Turtles Survived Dino-Killing Meteor



It did become extinct eventually, but not through asteroid impacts. Probably because it was unable to retract its neck and succumbed to predators.

And it is also extremely unlikely that we are hit by anything even that big. We have found all the 10 km diameter asteroids between Jupiter and the Sun already. Found 90% of the 1 km ones also. The 10 km ones happen only every 100 million years. And given that we have found all except the ones currently way out further away than Jupiter, that makes it 99.99999% certain we won't be hit by one of those this century.

So - that's both extremely unlikely, and also would not make us extinct. There would be dramatic effects on life on Earth for sure. In the mass extinction at the end of the dinosaur era, probably at least partly caused by the asteroid impact, 80% of the species on Earth were made extinct.

However, humans can live on many things, not just on land, also in the sea and along the sea shore, in lakes, underground, and in the air. Hard to think of many animals as adaptable as humans with the most primitive technology. Fruit, insects, nuts, grains, roots, shellfish, fish, animals, including rodents and other small animals, birds, seaweed, etc. If you lived exclusively on some of those you might not live very long but you just have to live into your late teens to have children. With some elders around a bit older to teach them how to survive.

We are an unusual species in that respect, there aren't that many creatures that are as omnivorous as humans are with basic tools. And then after the worst of the disaster, the world recovers again, the remaining species grow abundantly on land vacated by the disaster, and humans would expand along with them.

It's true that humans did go through a genetic bottleneck and nearly went extinct over a hundred thousand years ago (most likely). But that's in early times when they were confined to Africa and had only early stone tools.

A species as adaptable and versatile as humans with the minimum of technology would surely be amongst the 20% that would survive a giant asteroid impact.

For details about asteroid impacts, see my Giant Asteroid Headed Your Way? - How We Can Detect And Deflect Them

SUPERNOVA, GAMMA RAY BURST ETC

A supernova also would not make us extinct. None near enough to be deadly to earth anyway. But we are also protected by our atmosphere, and there would be many people on the other side of the world at the time.

Also, our sun is a stable quiet G type sun, far too small to go supernova. It will become a red giant eventually in the future, when it runs out of its hydrogen fuel and starts to burn helium (at higher temperatures and pressures in its core) and will throw off its gaseous envelope and become a white dwarf. But meanwhile for the next several billion years it's in a quiet phase of its life cycle, for the next several billion years.



The same also applies to a gamma ray burst. And both very unlikely. The galaxy is a hundred thousand light years in diameter - so the next supernova is almost certain to be far too far away to harm us - and gamma ray bursts are very focused. and would need to be pointed directly at us - which again is very unlikely.

And for any disaster of this type, radiation from space, there would be at least a fair few people in submarines, very hard to think of any disaster that could kill everyone in submarines.

They would also survive asteroid impacts. Tsunami are a surface phenomenon and a hundred meters or so below the surface you wouldn't even know it is happening chances are.

BLACK HOLES, ETC

As for black holes, well there can't be many mini black holes in the universe or we'd see stars blinking out.

Also there are no large ones near to the sun, as we'd spot them by the accretion disk. They are much rarer than stars anyway, and not likely to pass close to our solar system as a result.

And not likely to create one ourselves, because the galaxy has natural particle accelerators the size of stars and larger - the fast particles they create which hit Earth regularly don't make mini black holes, or if they do, they are harmless - and we are nowhere close to duplicating those energy levels. We'd need to be able to build something like CERN larger than a star before it's a concern.

OUR ENTIRE UNIVERSE DYING

Sometimes readers of my articles worry about the universe itself dying. So I thought I'd just put in a word or two about this.

Our universe is young so far. The galaxies are giving birth to lots of stars, which only young galaxies can do. As they get older they merge with each other and have less "baby stars" than before. And though some galaxies have already gone through to that stage, there are many very young spiral galaxies like our one.

They go through to the next stage by collision - when two galaxies collide together then they have a big rush of star birth, then nearly all the gas is used up as stars and it is over. Still have new stars from time to time, but only a few - and that's the old age of a galaxy. if that doesn't happen, then they will produce stars less and less often and the older stars gradually die and turn into white dwarfs, or black holes, or neutron stars. But who knows, maybe new lifeforms will arise then that find neutron stars, white dwarfs etc just fine as places to live?

And for trillions of years into the future there will be plenty of Earth like planets being born. 92% of Earth type planets are still to come according to one estimate. Nobody knows quite what happens longer term than that.

With most ideas of how the universe would end they have it expanding for trillions of years, getting colder and dimmer as it goes. It might eventually collapse again, but there's no sign of that at present and it would probably need new physics for that to happen. Roger Penrose thinks it might expand for ever and the matter decay until there is nothing left but light - at which point, there is nothing to produce any sense of scale, so it can then turn into a new Big Bang and start again - this is his "conformal mapping" theory. Freeman Dyson thinks there is a chance that life could continue to survive endlessly in a continually expanding universe, by making better and better use of the small amounts of energy remaining.

There are other ideas which could lead to the universe ending sooner. The Big Rip is a genuine theory in cosmology - though just one of many ideas about how our universe could end. It predicts that the universe could tear apart - but - this is 22 billion years in the future. Our sun has only been here for 4.7 billion years, and has had complex multicellular life for only 0.5 billion years. Most cosmologists think the universe will last for trillions of years, so compared with that, this is a near future scenario. Still, not something that most people will be concerned with except cosmologists.

So we don't need to worry that the universe is going to die any time soon. As with anything, our universe is probably not going to last for ever. But over a timescale of billions of years, nothing is likely to happen. We are not likely to become extinct as result of the entire universe dying.

UNIVERSE IS A SIMULATION AND IT IS HALTED - OR IS A DREAM AND WE WAKE UP

It's hard to know where to put this, it's not really through our activities within the universe, yet does it really count as natural causes? Anyway it is more closely connected to the idea of the universe dying so I thought I'd put it here.

So first of all the reasoning behind the idea that the universe is a simulation is quite interesting. The idea is that as time goes on then a technological civilization will eventually reach the point where it can simulate universes, as a kind of virtual reality, and then its beings can live inside the universe. Which could run at a much faster time rate than our universe, and have many more beings in it.

Either it's just our senses that are stimulated as if in a real universe, like virtual reality goggles but much more immersive - or else - that our entire beings are simulated in some advanced physics engine a bit like physics engines in computer games.

Then the idea is that if that happens then the number of beings in simulated universes within other universes would be far larger than the number of beings in real universes (by a "universe" here I mean a reality that seems to be everything there is to its inhabitants - not necessarily that it really is all that there is - that then makes the idea of a "simulated universe" make sense, otherwise it would be an oxymoron - a concept that contradicts itself).

So, according to this argument, then by probability we are likely to be in a simulated universe. Then, the idea is that suddenly someone decides to halt the simulation. Which could happen at any time.

I don't think myself that humans can be equivalent to a computer program - more on this later. But still, the simulated universe wouldn't need to be programmed. Maybe it has non computable physics in it? In a way that makes the idea more plausible. Because if programmed - you can ask, why would advanced beings put themselves or their descendants into a universe that has so much suffering? If it is non computable, maybe it was not intended this way and it is just how it turned out?

Still - that would be my main argument against this idea. If they could simulate any universe, as surely they could, why simulate this one, with so much suffering - including accidental suffering? Why not simulate one where you can be impervious to harm, or where if you are hurt you are just instantly revived like characters in computer games?

If it is non computable physics that is simulated, it might not be possible to do that of course. But still - why this particular universe, surely they could dream up something much better? Is this the best they could do - I mean - in many ways this world is amazing, but - it doesn't seem optimized for humans, as you would expect it to be if we it was built just for us as a simulated reality.

That's not a knock down argument though :).

Runewell on Reddit has just commented on this article with the idea that the simulation could be like a computer game, where we are not the main focus of the simulation. Maybe even Earth is not, just one simulated world of numerous ones that the game players can visit.

There are many science fiction short stories about simulated worlds, including futures where people play with simulated universes as a game. Such as Philip K. Dick's Trouble with Bubbles. See Simulated reality in fiction.

You can hear Nick Bostrom talking about the idea that we are living in a simulation here: Radio Motherboard. He suggests that if we live in a simulation, then it would be impossible to tell - unless it was designed to let you find out. Because the simulation could be designed to paper over the cracks if you find bugs in it including erasing memories etc.

Then there is the idea, that it is just a dream and you might wake up at any time. This is a philosophical view that is impossible to disprove really. It's more compelling if you are someone who has vivid and lucid dreams. A lucid dream is one where you know that you are dreaming and so can look at the dream carefully, knowing that it is a dream.

Richard Feynman, famous Nobel laureate physicist, was able to do this when young - later he decided to stop doing this. But while he could do it, he investigated his dreams like a scientist.

"...The next time I had a dream, there was a girl lying in tall grass, and she had red hair. I tried to see if I could see each hair. You know how there's a little area of color just where the sun is reflecting--the diffraction effect, I could see that! I could see each hair as sharp as you want: perfect vision!



"Another time I had a dream in which a thumbtack was stuck in a doorframe. I see the tack, run my fingers down the doorframe, and I feel the tack. So the "seeing department" and the "feeling department" of the brain seem to be connected. Then I say to myself, Could it be that they don't have to be connected? I look at the doorframe again, and there's no thumbtack. I run my finger down the doorframe, and I feel the tack!"

(long quote from Feynman on Reddit.com)

And you can experience pain during dreams also. Including pains that you don't have while awake. It's a rare experience, but does happen. Some dreamers can wake from a dream with high levels of pain, and the pain vanishes because it was just a dream pain.

And many people have the experiencing of waking from a dream, only to find that they are still dreaming. Sometimes the experience is so vivid that even when they finally wake properly, after several false awakenings, they may be unsure whether they have woken up properly yet, or are still in a dream. See this web survey of false awakening.

With that background, what if this is a dream?

There isn't really too much more to say here, philosophers have argued it for centuries, and nobody has a knock down way to prove that you are not dreaming. You could dream all the science and experiments. And - it seems coherent - but is it? All you really have is the present moment in a dream - and the rest is your memories which are dream memories if it really is a dream.

I don't think there is any way to prove that it isn't. Except to say, that if it is a dream, you'd probably want to wake up from it. And to say that most people who worry about human extinction don't worry that we might be in a dream or a simulation.

SUPER VOLCANOES, SOLAR STORMS ETC

A super volcano would plunge us into a period of darkness and cold for many years. It's similar to a nuclear winter. But again, this would not make a species as adaptable as Homo Sapiens extinct, with at least stone age level tools.

A solar storm from some types of star would be totally devastating. But ours is a quiet G type star in the middle of its life. It can't unleash a solar flare large enough to make us extinct.

The main risk here is for our power lines and any other long distance conducting cables. The great solar storm of 1859 happened at a time when the only long distant cables were telegraph cables. It caused sparks to fly in some cases giving them electric shocks. With our modern technology it could cause multi million dollar electrical transformers to fail - and more importantly - those would be hard to replace, and if so, power might be down for weeks or months. So the main question there is whether our power grid is sufficiently robust enough to withstand such an event. It would not make humans extinct.

Tsunami obviously only affect coastal regions and low lying places.

Earthquakes are local in their effect and can't possibly make us extinct. Actually, they are much more limited in their effect than movie fiction suggests. It's just not possible for an earthquake to make California fall into the sea or even for it to swallow people up into cracks in the ground. See a geologist's critique of the movie 10.5.

As for clusters of earthquakes - well they can't set each other off, they are too far apart. This is just natural random clustering.

See List of earthquakes in 2015

If you look a the table, then there were 143 earthquakes magnitude six upwards. So it must be quite common for two to happen in a day. There are only 20 magnitude 7 upwards. Still there's a strange mathematical result - if you have a party with 23 people in a room, though the chance that you share a birthday with one of the others is only 22/360, the chance that any two of them share a birthday with each other, when you take account of all possible pairings, is 50%. See Birthday problem.



So at 20 or so a year, it must also be quite common that you get two of the magnitude 7 and upwards earthquakes in a day too.

Earthquakes often cause other aftershocks as they send tremors through the earth that can cause other faults to slip - and occasionally they can be as large as the original earthquake. So earthquakes could cause each other - but probably only over ranges of hundreds of miles, not so likely over thousands of miles. See this story from 2012: Are 4 Big Earthquakes in 2 Days Connected?

Earthquake Facts&Earthquake Fantasy

SO SAFE FROM EXTINCTION FROM NATURAL DISASTERS

So - natural disasters, it seems, can't do it at all. We are safe from extinction at least, not just for a century, but for thousands, even millions of years. Hundreds of millions of years into the future, then yes, we could go extinct through disasters of one type or another. Eventually the sun will get too hot - though - we could postpone that by using shades, or maybe even by moving the Earth itself further out. See David Brin's Let's Lift the Earth!. And for Don Korycansk's repeated Jupiter flybys gravity tug, see Planet Earth on the move

There's a tiny chance of Mercury getting deflected from its orbit. There may be other things that could threaten us on a hundreds of millions years timescale.

But - it is very unlikely that the human species lasts that long. Some genera do survive for hundreds of millions years - rare for a species to though. Even horseshoe crabs have changed in their detailed structure over hundreds of millions of years. And hominid species typically last for only a few million years before the next one - no reason really to suppose we are different.

If somehow we continue as a species for hundreds of millions of years, then I think you can assume we have high technology that stabilizes our genome and probably if humans have survived for that long, as a high technology species, there isn't that much you need to be concerned about as regards natural events making them extinct.

Meanwhile we live at a quiet time in the life of a stable G type star with a reasonably stable system of planets in a quiet backwater of our galaxy. There are many dangerous stars with planets such as stars about to go supernova, or stars that have extreme solar flares. And there are much more dangerous parts of our galaxy also. But we are lucky enough not to live in those places.

DISEASES

That leaves, diseases. But you generally get a few immune to it if it is a natural disease.

If it kills too quickly, it won't spread. If it kills slowly with an incubation period, then some will be likely to be immune to it. The thing is, it is an advantage to the disease virus or microbe also to keep humans alive, so the disease also will adapt to humans if it can. It's mainly diseases that are recently transferred to humans that are most lethal.

It is possible for species that are already rare and weakened by other stresses to go extinct through disease. But humans are widespread and occur in many different habitats, and a few groups of humans are geographically isolated, in the uncontacted tribes. So that's a different situation from some rare frog species that only exists in one particular habitat.

We have made some species of insect locally extinct by the sterile male technique of releasing large numbers of males that are sterile so overwhelming the population of fertile males . But that wouldn't work with humans :).

It would be possible surely to genetically engineer some bug to intentionally make us extinct - but since it would have a latency period, a few people would be naturally immune, and there would also be others that never are contacted. If nothing else, then the uncontacted tribes, which still exist in a few islands and forests, would emerge from their forests bewildered to an uninhabited world :), That would make a fun sci fi story though I don't think it is likely in reality.

CLIMATE CHANGE

Climate change won't do this. It's effects are much exaggerated by a few people who go over the top, opposite to the climate skeptics, climate excessers ??

See for instance, How Guy McPherson gets it wrong

You also hear that we can turn Earth into Venus through climate change. This just can't happen. We would need to release and burn many times the entire global inventory of coal, oil, methane etc. We couldn't do it even if we never took any precautions.

Our world is technically in the middle of an ice age, compared with most of the Earth's history, because we have ice at both poles. That's because we happen to have a continent at the South pole and an almost enclosed ocean at the North pole, both of which help to cool down the Earth. Normally there is no permanent ice anywhere except on mountain tops and the world is much warmer than it is now.

So the main issue is not the temperature of the world, but the speed of the change. So, the world isn't going to become uninhabitable or suffer any kind of apocalyptic event as a result of climate change.

And - Earth gets plenty of energy from sunlight. Just a small area of the desert set aside for solar power collection could supply all our energy and be an end to the problem completely. But that's too expensive with present day technology. As time goes on and the technology improves, then it may well be we can do that; get all our energy from solar. There are other options also. It's definitely doable, lots we can do about it.

THINGS WE CAN DO TO OURSELVES

That leaves things we can do to ourselves.

ARTIFICIAL INTELLIGENCE

The idea of an AI intelligence taking over the world, well I don't find that plausible at all. Here I'm voicing my personal view and opinion.

I think we are a long way away from that, indeed that we will never build computers that can understand truth in the way a human can do. Indeed if we do have strong AI, I don't think it will be through programming, but rather through genetics, biology, or some such. Or some approach that is somehow part biology, part machine. Which would involve many ethical issues. E.g. enhancing the intellectual capacity of a whale and giving it the ability to speak like a human - is that an acceptable thing to do? What kind of life would such a creature live - would it not potentially be very unhappy and miserable? Anyway we aren't close to that capability yet AFAIK.

Why Strong Artificial Intelligences Need Protection From Us - Not Us From Them

I can't see this happening within ten or twenty years for ethical reasons. In the long term, yes maybe. Could we genetically engineer our own future replacements? It's a while away yet though. And I expect if it happens at all, that it will be through genetics or something similar, artificial life, or could be half computer half living - but not through computing alone.

BLACK HOLES (AGAIN)

Some people worried that the experiments with the LHC at CERN would create a black hole that would destroy the Earth.

So first, most people don't think it will create a black hole. It is just some very unusual physics that would let that happen.

The idea is that if you hit two things together with enough energy - then by Einstein's E = mc^2, then that energy is equivalent to mass. With a fast enough collision, it could create enough mass in a small enough space to create a tiny tiny black hole, smaller than an atom.

The amount of energy when such tiny particles collide, even at close to the speed of light, is the same as the energy in two mosquitoes bumping into each other. So you can't create anything very big. And what everyone expects from modern physics is that a mini black hole that small would just form briefly and instantly fall apart, through Hawking radiation or other processes.

But in assessing safety they had to acknowledge that this is not absolutely proven, of course, is always possible that some new physics we don't know about comes into play.

To answer that, they looked at mini black holes that would be created by cosmic rays hitting neutron stars and white dwarfs - very dense stars. These stars could trap even neutral mini black holes with no charge - so if there were any black holes created in a process like that and they were stable and able to swallow up the Earth they'd be created in huge numbers by cosmic radiation hitting white dwarfs and neutron stars and those very dense stars then wouldn't be able to survive.

Yet they do, because we can see them, very ancient and very dense stars. So even if we are completely wrong about Hawking radiation, and mini black holes are stable - then the continuing existence of these very dense stars shows they can't be dangerous to the Earth.

NANOTECHNOLOGY

As for nanotechnology - we could make tiny nanomachines, already do. But we are nowhere near able to make a nanite - a nano replicating machine. Nowhere near.

We can't even build a "clanking replicator" - a factory or solar panel or some other big machine or device able to create a copy of itself. The advances in 3D printing take us partly there, but still need humans to source the materials they use, and to build the copy of the printer, as well as not being quite at the stage of printing out computer chips.

In the near future, perhaps eventually we get something like this self replicating lunar city?

But probably it will need assistance from humans to keep going.

That could happen, nanoreplicators, see no reason technically why not. But surely not in just ten years. Don't think that is long enough to get to a "clanking replicator" many steps to be filled in though there are ideas about how we could do it.

So again this is for further into the future, can't see us getting there in ten or twenty years. By the time it is possible, then we may well also have the means to control it too. I think it is just too far ahead to concern ourselves with in any detail.

After all in any such projection so far ahead we are probably going to miss future developments as obvious as mobile phones are to us today. Hardly anyone thought they were likely when I was a child in the 1960s. Almost anything one says is bound to be out of date by then. Ideas for ways that nanites could be made safe is all speculation when we have no idea what technology we would have at that stage and what nanites would be like in detail.

But - just to sketch briefly for those who are scared by it: perhaps require them to have a maximum number of generations, equivalent of a telemore, require that they have an "always on" signal so that they stop working and self destruct if they no longer receive that signal, have antibody type nanites that police for rogue nanites that don't self identify as safe, etc etc. But - any projections like that may well seem naive and silly to someone reading projections about nanites, say 50 years into the future.

POPULATION GROWTH

If the problem is population growth - how could that lead to 0 people? Boom and bust perhaps in very worst case, like lemmings, but how could it lead to 0 people?

In more detail, just to cover it briefly, actually, we are no longer in the exponential phase of population growth. We reached "Peak child" in 2005. Since then we have entered a new phase, where the population is growing only because people are living longer. The adult population is growing, especially the elderly people, while the number of children is projected to be around the same in 2050 as it is now.

The middle of the road population now levels off around 2100, like a sigmoid curve. The lower projection reaches a peak around 2050.

The most rapid growth projection has exponential growth continuing. For that to happen then the number of children being born has to increase again.

That could happen. Nigeria has a larger birth rate and has a large population. If you just project from that into the future assuming the birth rate won't change, you get massive population growth just from the Nigerians.

But for the population to explode mainly through Nigerian children, Nigeria would need to reach a population larger than China, which seems unlikely. Surely its birth rate will reduce eventually, as has already happened with many other similar countries?

Many countries now have negative growth, or have birth rate slowing down - in countries of all religions and philosophies and systems of government - correlated mainly with wealth that the wealthier countries have lower birth rates. For a runaway exponential population growth, that trend would have to reverse in some way.

We need to keep on track with that, and the main way seems to be through raising the living standards of poor people as when their conditions are better they have less need for large numbers of children, because most children survive and because they have support also from their society.

But the topic here is extinction risks.

Lemmings do boom and bust cycles and you get years with very few, then a sudden boom, then they nearly all are gone with hardly a lemming to be seen, then the same happens again. It looks as if we will avoid that and the population will level off. But if it does do a boom and bust, then in nature that doesn't lead to extinction.

Population growth, I think, can be dismissed as not an extinction risk.

EXPERIMENTS WITH LIFE

The one thing that I do think needs great care is life itself. Experiments like this one:

Synthetic bug given 'fewest genes' - BBC News