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Let's say a Moon colonist in the future has discovered an alien time machine. A small remnant of humanity still exists on colony worlds in the solar system, but due to runaway climate change, they are extinct on Earth. What event or events would the time traveler have to change in the past in order to make sure climate change as we understand it today never happens? Oh, the time traveler would go back in time on the Moon and has a shuttle that would get him to Earth in that past period.

EDIT (2-14-2019): It's clear that the basic premise of my question is flawed, since there is not one event or small number of events that, if changed, would alter the impending climate change as we understand it. Not only that, but it seems that opinion vary widely on exactly how damaging climate change will become, even if not significantly curtailed. That said, the information here could fuel quite a number of different tales, so thank you all for your input (and you don't have to stop now).

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closed as off-topic by Mołot, Rob Watts, Gryphon, James Feb 17 at 8:00

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    $\begingroup$ The premise itself seems flawed: it's much easier to survive in habitats on a completely ruined planet, than in space habitats. You still have an entire planet as a spaceship around you, much easier to maintain a habitat. $\endgroup$ – vsz Feb 11 at 5:35
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    $\begingroup$ @vsz, perhaps earth's surface was rendered uninhabitable in the nuclear water wars during the climate catastrophe chaos. $\endgroup$ – crobar Feb 11 at 11:45
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    $\begingroup$ Easy: kill every protohuman who tries to leave the African continent. $\endgroup$ – Carl Witthoft Feb 11 at 15:33
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    $\begingroup$ If the goal is to save humanity, this wouldn't be such a hot solution (no pun intended). $\endgroup$ – James Feb 11 at 15:41
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    $\begingroup$ As said by others, the premise is flawed. Climate change might kill hundreds of millions of people at the very most, but definitely not billions, at least in the next several hundred years. Oceans will not cover all land on earth, crops will not become impossible to grow, and heat won't become unsurvivable. I'd recommend expanding on how climate change destroyed the world in your story. $\endgroup$ – Goose Feb 12 at 13:44

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Go back to the cold war era and start a "Russians are trying to warm the planet" scare. You will need a lot of money to fund some big advertising campaigns. You also want to seed a few specific technologies like nuclear and solar power to try and push them along. Let ignorance and paranoia work towards the betterment of mankind for a change.

It's believable to the average citizen since every American knows Russia is cold so warming it seems like a good idea for the Russians. Make it clear Russians don't care about pollution and things like that, make pollution a sign of communism. The reds are trying to make the planet too hot for Americans use special gasses is good. It doesn't matter if it is real as long as the fear is real. Simple slogans like "don't let the reds turn up the heat" are good.

The more believable the scare the better, create fake data that will mirror real data. Then americans will invest absurd amounts of time and resources in researching ways to reduce and counteract greenhouse gasses as well as reduce clean . It will create PR problems for major polluters. The US did some breathtakingly expensive and difficult things to combat the "red threat" many of which are still around. They become synonymous with nationalist views. These methods will steadily spread to the rest of the world. as technology is exported, especially as they are made cheaper.

There are dozens of smaller way to help it along.

If you make Americans think Russians are opposed to nuclear power because it doesn't help with warming and is too clean you can encourage nuclear power which will go a long way to replacing coal.

Show people farming practices and land usage can be used to combat greenhouse gasses, and real Americans will use those methods to screw over the Russians.

Fund some ice core drilling or other science to back up your claim, a little evidence goes a long way with propaganda. It will also allow for a later transition to a different message "its not just the Russians we are doing it to ourselves unknowingly"

Make the Russian way seem like the brute force way while real Americans make things that are more efficient, large brutish Russian cars vs sleek efficient american four cylinders, ect. Make it clear Russians burn coal while americans use clean methods. Combatting fear of nuclear power will help.

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    $\begingroup$ You'd have to be very careful to avoid critical paradoxes here. The space program might never exist, since rockets expel a lot of pollution and you already have the climate to use as the proxy battleground. No space program could mean no space colonist on the moon to go back and trigger the whole process in the first place. Don't Marty McFly yourself out of existence! $\endgroup$ – bta Feb 11 at 17:10
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    $\begingroup$ @bta fundamentally, this guy is trying to modify himself out of existence. A world where the earth wasn't consumed by global warming is a world that isn't sending back a time traveler to solve that particular problem. $\endgroup$ – Ben Barden Feb 11 at 17:43
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    $\begingroup$ There could even be a kernel of truth behind the claims - when my family visited East Germany in 1991 shortly after reunification, it was striking just how many dirty smokestacks they had all over the place, even close to cultural sites like Wittenberg - as opposed to what I was used to in the US, and in West Germany where I spent a couple months. $\endgroup$ – Daniel Schepler Feb 11 at 19:16
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    $\begingroup$ It could also work like this: 1: Print en.m.wikipedia.org/wiki/1950 and take it with you to June 1950. 2: Find a phone booth with a phone book, contact a local FBI field office. Tell them to meet you on an urgent matter related to Soviets. 4: hand them your printout -- this proves to them (in a few weeks at least) that you are a time traveler. 5: Tell them that in the future the Soviets have rendered the Earth uninhabitable due to the global warming. 6: Profit. $\endgroup$ – ZeroOne Feb 11 at 23:19
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    $\begingroup$ During the cold war, everyone was afraid of the looming ice age. My father was in full paranoid survival mode, stocking up to survive multiple years of winter. $\endgroup$ – pojo-guy Feb 12 at 19:05
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The mad scientist sledgehammer option for this particular nut.

Kill a very large slice of the world population.

It worked when Europe colonised the Americas, so many natives were killed it actually changed the global climate.

America colonisation ‘cooled Earth's climate’

He travels back in time to the height of the cold war at its most unstable & dangerous time with some small thermonuclear devices & uses them to provoke a full on nuclear third world war.

The massive resulting reduction in human population successfully reduces industrial production & agricultural drivers of climate change sufficiently to reverse global warming & delay its resumption from human causes until long after we develop cleaner infrastructure & technologies.

We know from Chernobyl that nature won't have a problem with this & he's clearly not worried about his own existence as such a major change in the worlds history (fixing climate change) will write him out of existence one way or another anyway no matter how it's achieved.


Taking back a flask or two of some really virulent engineered virus could work as well.

But one big advantage of the nuclear option is it's going to (perhaps, hopefully) leave vast swathes of territory largely uninhabitable to any humans (if they want to live much past 20 without dying of cancer & want their children born without genetic defects), great tracts of undisturbed forest to help suck CO2 out of the atmosphere for centuries to come before the land is safely habitable again.

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – L.Dutch Feb 11 at 3:59
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    $\begingroup$ A large nuclear war could plunge the earth into a nuclear winter. That solves the problems of global warming, but may cause humans to go extinct much earlier. $\endgroup$ – hat Feb 11 at 5:29
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    $\begingroup$ So Genghis Khan was a time traveller then? The game here being that the further back in time you go, the fewer people you have to personally stick a knife into. $\endgroup$ – Separatrix Feb 11 at 12:21
  • $\begingroup$ @hat : check out the previous comments that were moved to chat, that was already discussed. $\endgroup$ – Pelinore Feb 11 at 15:21
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    $\begingroup$ @elPolloLoco "what if one of his ancestors gets killed in the war?" already covered "he's clearly not worried about his own existence as such a major change in the worlds history (fixing climate change) will write him out of existence one way or another anyway no matter how it's achieved" any significant change to history automatically wipes him (or the version of him that would have existed in this new timeline) out. $\endgroup$ – Pelinore Feb 11 at 15:36
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Here is an idea: What if Henry Ford had built his assembly line for an electric car rather than a gas powered car? Before the assembly line brought down the price of the Model T, electric cars were actually less expensive than gas cars. The assembly line would have made these even cheaper. Electric cars and infrastructure would need more electric power plants. Today that means burning more coal but perhaps your time traveler could also steer us toward nuclear. Perhaps the introduction of and sodium cooled reactors earlier could have alleviated environmental concerns allowing nuclear plants to be more prevalent. This could have lead to thorium reactors reducing nuclear waste concerns and maybe lead to viable fusion type reactors.

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  • $\begingroup$ Yes, but would shifting from the internal combustion engine for autos to electric have been enough? Also, the time traveler can't hang around for decades, nor does he/she have an incredible influence over the course of long term events, such as "guiding" nations to greater reliance on nuclear. We're talking about going back either to one time or several and promoting/preventing specific innovations, even if it meant saving/killing the innovator (desperate time traveler). $\endgroup$ – James Feb 10 at 15:56
  • $\begingroup$ Nuclear power in the 20es? $\endgroup$ – L.Dutch Feb 10 at 15:57
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    $\begingroup$ My daughter send me a cartoon about this: Mum and kid walking past power station belching out black smoke. "Mum, what's all that smoke?" "That's the exhaust from Dad's new electric car". $\endgroup$ – RedSonja Feb 12 at 11:29
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    $\begingroup$ But that's offset by the transmission inefficiencies, the increased workload on the electrical infrastructure (ask California how the increased population of electric vehicles is working out), the low energy density of batteries compared to hydrocarbons, and the long term disposal of battery waste. At best electric versus gas is close to a zero sum game. $\endgroup$ – pojo-guy Feb 12 at 19:01
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    $\begingroup$ All transportation only makes up about 5% of the total energy consumption in today's world. It's not enough to even make a dent, even if you ignore the fact that we'd power those cars with coal power plants anyway. The problem with cars isn't the greenhouse gases they produce, it's the pollution they bring to cities. $\endgroup$ – Luaan Feb 13 at 11:16
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Direct bootstrap of nuclear fission technology in the 1700s.

Sounds crazy right? Not so fast.

In order to reliably prevent runaway climate change, we must prevent the situation that caused it, namely cheap coal and oil power. This is quite well accomplished by getting there first with uranium, plutonium, and thorium reactors all at once. Since mining won't be so well developed yet, starting with breeder reactors to extend the fuel supply is a must.

Yes I know what all this entails; an immediate gift of 1950s physics, metallurgy, manufacturing, etc. This is an overwhelming change to society but totally worth it.

I'm just going to assume you have to go about this the long way and can't bring much pre-manufactured stuff with you. The evidence of the re-entry vehicle itself will suffice to prove future origin and that you posses actual knowledge they can't match. Everything will be in fifty pounds or so of books and blueprints. Every piece of metallurgy required starting from the blacksmith to titanium working (you should be able to start a reactor without it but it will be most convenient for mass roll-out). The 1700s are a convenient time because the manufacturing technique is right at the cusp of being able to make a lathe that makes a lathe better than itself. You will need basic electronics, light bulbs, how to build a Geiger counter, how to locate uranium (and if you can get it, the locations of good deposits), safe handling of radioactive components, electric motors, early steam power (to crank generators if nothing else), lead-acid batteries, and quite a bit more I can't think of right off the top.

The idea is to seed this stuff so that by 1859 the response to selling fuel oil is "How crude" (pun very much intended) and nobody wants it because they already have enormous power at their fingertips.

This is a ridiculously valuable gift. Choose wisely which nation gets it. Many of the nations in good shape to receive it now were not back then.

MOD: please leave comments because comment voting.

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    $\begingroup$ at least the Thirty Years' War would be much shorter if they use nuclear weapons. $\endgroup$ – Henning M. Feb 10 at 21:26
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    $\begingroup$ @Carl Witthoft: Funny how no major news outlet makes any such claim. $\endgroup$ – Joshua Feb 11 at 15:38
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    $\begingroup$ @CarlWitthoft: I am well-versed in science and expect the effect of the waste heating to be too small to notice due to the vast size of the sky over the oceans. $\endgroup$ – Joshua Feb 11 at 16:06
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    $\begingroup$ @CarlWitthoft: Direct solar heating is about 1000W/m2. All of this is re-radiated into space, plus all of the geothermal heat, in order to have a stable global temperature. You'd need to put in a mindboggling amount of heat to exceed solar heating. No, Global Warming is directly caused by the blocking of the re-radiation without blocking the direct solar radiation. And this happens because the re-radiation peaks around 300K while solar radiation peaks around 6000K, i.e. IR versus visible light. CO2 very much block IR re-radiation $\endgroup$ – MSalters Feb 12 at 10:54
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    $\begingroup$ @Joshua Even if the science were known, the infrastructure for it wouldn't be there - you need uranium mines, ways to transport and refine that uranium, manufacturers of the cement, steel, and plastics used to build nuclear power generators, manufacturers of wire to transmit the electricity, and consumer/commercial uses of that power. $\endgroup$ – David Rice Feb 12 at 16:34
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Keeping traveling back even when incentive is lost

A big problem with solving problems trough time travel is that once it is fixed the incentive to travel back in time is lost and thereby no one will travel back in time to keep the timeline fixed.

So what the time traveler has to do is leave a note. Either to himself, or if he never gets born in the new timeline, he has to leave it for someone who he trusts to keep the "fixed" timeline.

How to prevent climate change

He gives working fusion technology to people in 1950 and within 20 to 30 years the amount of CO2 produced per capita will be comparable to the 1820s. Power generation through fusion is supposed to be scaled up and down however you want once we figured out how to keep it running for more than fractions of a second.

So, it will be possible to power all of our electronics from it as well as our cars and everything else that simply needs energy and doesn’t rely on chemical reactions (like our own body).

Side notes

It is possible to provide fusion energy even earlier than 1950 but then your time traveler would need to provide more and more knowledge for people to able to built fusion reactors.

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    $\begingroup$ As far as we know today, stable fusion seems to require some unobtainium - where to obtain that in the 1950's? $\endgroup$ – Hagen von Eitzen Feb 10 at 18:42
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    $\begingroup$ @HagenvonEitzen This is technology the time traveler would of course need to bring with him as well but the groundwork would already be laid in the 1950's. $\endgroup$ – Soan Feb 10 at 18:47
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    $\begingroup$ @HagenvonEitzen YBCO is likely the "unobtainium" in question, and could definitely be made with 1950s tech. $\endgroup$ – eyeballfrog Feb 10 at 23:23
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    $\begingroup$ It might be possible to slow coal down if you stopped the nuclear disasters like chernobyl. 3 mile and Fukishima, which scared the public and has bogged nuclear power down in regulations. Which might of meant that more plants constructed are nuclear slowing down the effects of global warming. $\endgroup$ – Shadowzee Feb 10 at 23:37
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    $\begingroup$ @HagenvonEitzen - as far as we know today, time travel is fundamentally impossible, so if they solved that part, I guess fusion is a high school science project. $\endgroup$ – Tom Feb 12 at 16:03
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Prevent Chernobyl and Three Mile Island so that adoption of nuclear power isn't regressed. This might not completely solve the problem but if it cuts enough emissions to buy a couple decades so that renewables and electric cars and other technologies become economical soon enough to prevent cataclysmic warming.

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  • $\begingroup$ +1 Very nice idea. There are a few other major accidents and events that need to be resolved, but few enough one can imagine preventing all of them. $\endgroup$ – Keith Feb 11 at 22:37
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    $\begingroup$ Without these accidents they would still be rather cavalier about nuclear safety. Nuclear is not the answer, it is the ultimate curse. Humanity has exceeded the natural carrying capacity of the earth and adding cheaper energy will just grow their numbers. The cheap food is fading and some people think the fish will increase and the jungles grow back if we can breed more human mouths to feed. Cheap energy has been the cause of the population explosions. There is no counter force before starvation. $\endgroup$ – KalleMP Feb 11 at 22:57
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    $\begingroup$ @KalleMP: Fair point - you've got your own answer right there. Introduce birth control way earlier, possibly as early as the 15th century. $\endgroup$ – MSalters Feb 12 at 11:02
  • $\begingroup$ "Prevent Chernobyl and Three Mile Island so that adoption of nuclear power isn't regressed" => Multiplication of nuclear power station across the globe = multiplication of the risk one (or more) of them goes off ... long term down the line, we're extinct because of a nuclear overwhelm. Agree it's different from going extinct by global warming, but it's not going to be a much nicer habitat ! $\endgroup$ – Hoki Feb 12 at 12:54
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    $\begingroup$ It might not help to prevent these incidents. If we never learned from these disasters, then we might build nuclear power plants with fewer safety features, and then a substitute of Chernobyl might occur just a few years down the line. By preventing the problem, you might prevent the solution and re-engage the problem. $\endgroup$ – John Feb 13 at 5:00
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It would be quite hard to change the history by gifting technology, as we already had the ecological solutions, but time and time again we have rejected them in name of either profit or convenience. For the same reason bringing knowledge of the global climate change work, and it could even accelerate the process.

I think the easiest way would be to infect the humanity with an engineered retrovirus causing one small change- humans would be now much better at recognizing co2 levels, and anything above 250 ppm long term would cause serious psychosomatic issues.

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  • $\begingroup$ An interesting thought, Lumberjack. Maybe "recognizing" wouldn't be as effective as a retrovirus that would result in severe medical consequences (mass deaths, for instance) should CO2 levels rise above 250 ppm. The problem is that you can't change just one thing. In making such a biological change in all people everywhere, what else would change? There are probably too many variables to manage with this answer. $\endgroup$ – James Feb 10 at 18:35
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    $\begingroup$ One interesting unintended consequence would be that Apollo 13 would not have ended favorably. The biggest problem in the middle stages of the crisis was the rising CO2 levels in the capsule were pretty close to critical levels even for humans as we are today. They spent considerable time rigging up a makeshift CO2 scrubber to lower the levels $\endgroup$ – Justin Thyme the Second Feb 10 at 21:34
  • $\begingroup$ James, as far as I know we can already detect dangerous levels of co2, so it would be easier (though by no means easy) to put that sense into overdrive. As to serious consequences, I would rather avoid that and settle for just making the sensation unpleasant, as there might be other triggers, and we to cause human extinction. $\endgroup$ – Lumberjack Feb 10 at 22:33
  • $\begingroup$ “as we already had the ecological solutions, but time and time again we have rejected them in name of either profit or convenience.” Agreed. But if you introduce advanced battery, solar and and wind power technology before combustion engines take over it would be cheaper, more convenient and also more environmentally friendly to use them. Early combustion engines were surprisingly bad. $\endgroup$ – Michael Feb 11 at 11:44
  • $\begingroup$ @Michael As were early steam engines. Efficiency is always a trade off between costs and expected value. In the US, combustion engines were all the rage, because there were vast reserves of oil, and gasoline was one of the byproducts of the oil industry (mostly built around city lighting). In Europe, combustion engines (despite being developed there) were rather slow to displace steam engines - coal was much cheaper. But either were cheaper than horses, and that's what really matters :) You can't make decisions like "95% fuel efficiency is better than 80%". There are other costs. $\endgroup$ – Luaan Feb 13 at 11:45
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Not at all

Unless the time-traveler is not a time traveler but a dimension traveler, the mere fact that he has experienced a world with the climate change makes it impossible to prevent climate change because of causality:

  • He had experienced climate change
    • so he took a time travel and went back to fix it.
  • He fixes it.
    • He changed the past and the person doesn't ever experience climate change.
    • So he doesn't take a time travel and go back to fix it.
  • He never went back to fix it, so he experienced climate change... Back to the start.

You see, true time travel is impossible as you can't cheat causality. The only safe and sound time travel that does not cause causality errors is one that goes back to try to fix time... and fails, leaving the time traveller with a need to do the time travel in the first place. He goes back to see if he can fix the past... and utterly fails.

Dimension hopping

If you go for dimension hopping on the other hand, your past becomes independent from the past of the mirror-verse you show up in. Suddenly causality can't throw you a wrench into the gears and the traveler can save the alternate reality you ended up in. Though in this case there could be two entirely different entities of the time traveler: the one originating from the universe that experienced climate change... and (unless he destroys the lineage that would lead to the alternate him), his double that never experienced it. But at this point, you are no longer time traveling at all.

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    $\begingroup$ "But at this point, you unpacked Magic, so anything goes" You already did that at the start when you introduced time travel. ~ I generally assume you travel back down your own time line & automatically generate a new time line the instant you arrive ~ suddenly it's exactly the same world it was at that time in the past, plus a few billion atoms.. [pauses, Googles].. huh! sorry "seven billion billion billion" atoms that weren't there last time. $\endgroup$ – Pelinore Feb 10 at 17:20
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    $\begingroup$ ^ [Continued] if you travel forward from there you're going up the new time line & can never really get back to your own original time-line, just one that looks exactly like it, so if you do manage to go back & retro-fix whatever changes you made the first time there's always a copy of you there already. $\endgroup$ – Pelinore Feb 10 at 17:31
  • $\begingroup$ "Not at all" : I presume from that you share my choices when it comes to viewing how time travel would actually work & your point is that the original time-line he left from won't change one jot & the new time-line he's in is the one that benefits from his changes? $\endgroup$ – Pelinore Feb 10 at 17:43
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    $\begingroup$ But what if you go by 85 mph while chasing 9.1 Jigawatts through a flux compensator? $\endgroup$ – Hagen von Eitzen Feb 10 at 18:39
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    $\begingroup$ I feel like responding to a question about time travel with "time travel can't be done because of a paradox" isn't really answering the question. The OP isn't asking if the method of time travel to solve the problem is possible, they are asking what changes to the past with time travel can be done to get the desired effect. $\endgroup$ – MetalJimmor Feb 11 at 8:16
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A time traveler should work from within the system, thus he first needs a proper foundation. And as he cannot just do a bank transfer from the future, he should take a business model from the past that made people rich before and would be easy to replicate without a lot of resources, e.g. an internet payment company.

Given that the time traveler knows the technology from the future and now has proper funding he can now pretend that he is this genius inventor that just succeeds at everything he does, even if it is not in his area of expertise. That means he can take the money to invest into new initiatives that will help mitigate the global climate catastrophe:

  • A solar energy company
  • Electrical car company
  • Public transport company
  • As a backup plan make spaceflight cheaper and go to Mars
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  • $\begingroup$ It is true that a time traveler could go back and manipulate key stocks thus amassing wealth, but even if you had the Einstein or Steve Jobs of genius/innovators, there would be the matter of influencing, not only the US, but the world. I'm beginning to conclude that under most circumstances, a single person could not travel back in time and make a few actions that would alter the trajectory of climate change as we understand it today. $\endgroup$ – James Feb 11 at 20:08
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    $\begingroup$ If the public transport company involves digging a lot of tunnels, then it also can develop the tunneling equipment for Martian ice mines and radiation shelters. Additionally, tunnels on Earth can provide shelter during a variety of dramatic disasters other than climate change. $\endgroup$ – Robyn Feb 13 at 6:26
  • $\begingroup$ @James An omniscient time traveler could. A human, even with vast wealth of knowledge and the ability to keep trying over and over again, not really. The problem is predicting the result of your intervention - there might be one tiny thing you need to change to fix everything, but that doesn't mean it's easy to find and perfectly predict the results. And more importantly, it doesn't mean it'd make a good story, which still trumps everything else (and mind, there have been many sci-fi stories about time travelling aliens who fixed/broke the world with one small change) :) $\endgroup$ – Luaan Feb 13 at 11:53
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How did I get here?

Your question implies a little foreknowledge in the here-and-now. We currently have global warming, and we know in the future this wipes out humanity. So first, how do we get from here to there?

Current state of affairs

The current state of affairs is that almost all nations are on-board with solving Global Warming. We have China reducing their coal usage, Russia has launched its Ecology program to reduce carbon emissions, everyone is pretty much on board. Only one nation really stands out in present-day in their opposition to climate change science, and that is the United States.

Since everyone else seems to be on board, and the US is not, it seems safe to assume that the global disaster in this storyline is likely caused by the US.

So now what?

The way to fix the problem would be to get the US to follow a different path, the earlier the better. So how do we do that?

You're not going to like my solution.

Al Gore. He was an inch away from the Presidency, and one of the early voices of warning. If he had become president instead of George Bush, we would have that early start against global warming that we require to avert the disaster. The United States would be a leader in the fight against it, instead of the last denier. This would likely avert the global disaster.

My solution? Travel back in time to October 2000, and assassinate George W. Bush.

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    $\begingroup$ Not sure where you get your information, but in doing a quick Google search, I found that both Forbes and EESI (Environmental and Energy Study Institute) agree that the US is leading the world in greenhouse gas reduction, and Scientific America reports that "Group of 20 nations will fall short of the Paris Agreement’s goals." Granted, I'm writing fiction, but I do depend on facts as a basis, not politics. $\endgroup$ – James Feb 11 at 21:35
  • $\begingroup$ @James, that is true. NASA is in particular a part of the United States government that has never wavered in their support of reducing carbon emissions. But the current state of affairs is that we have a President that thinks it's a Chinese conspiracy theory, and has stated so publicly. It's not really politics I'm chasing here, I'm just looking at what people right now are saying. If someone wants to write a story where a global warming event destroys the world, that seems like a likely starting point. $\endgroup$ – BoredBsee Feb 12 at 14:27
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    $\begingroup$ Killing Bush could have led to a large increase in sympathy votes for the Republican ticket, and President Dick Cheney could have been much worse than Vice President Dick Cheney. In the 2000 election, someone actually lost a senate race to a dead man (the dead man's wife took his seat, IIRC). To actually get Gore elected, I would say mire Florida's secretary of state Katherine Harris in some scandal before the election (maybe even before her election in 1998) such that she couldn't certify the recount in Bush's favor, and the Bush v. Gore Supreme Court case would never have been decided. $\endgroup$ – dissemin8or Feb 12 at 16:11
  • $\begingroup$ I do think there's a chance that a Gore administration could have made a huge environmental impact; especially with a cooperative congress. Considering the slim margin of Bush's victory, there are probably much more subtle ways to nudge the outcome, such as donating some money to the Gore campaign, posting an anti-Bush conspiracy theory on the internet, or planting a manufactured anti-Bush scandal (e.g. Hillary's October surprise pneumonia, but give Bush pneumonia). Or alter one event in the Clinton administration that helps tilt the scales for both Gore and congressional Democrats. $\endgroup$ – John Feb 13 at 4:44
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    $\begingroup$ Related: you could kill Kennedy to prevent America from entering the Vietnam War; and in doing so, maybe you keep liberals in power for longer. But you mis-timed it and killed him AFTER we entered the Vietnam War (therefore the traveler is and always has been the true assassin). The (22nd - 23rd century?) time traveler could have a fuzzy grasp of 20th century history, and have no idea that they are stumbling into recreating events that have already passed. Then they could be stuck in the 60's with limited resources trying to improvise a new plan. $\endgroup$ – John Feb 13 at 4:57
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Climate Change Is Not Trivially Preventable In This Way

Not to be a downer, but most of the answers so far seem to assume that a tiny change to the United States' (or other similar industrial powers') behavior at some specific point in time, all will eventually turn out OK. The problem with these approaches are:

  • They incorrectly assume that climate change is a regional problem. e.g if we somehow get England not to discover that coal is awesome for all sorts of things, nobody else in any other place will ever figure that out. This is mistaken because scientific discoveries and technological innovations are being worked on by multiple people across the world and it's largely accidental who is the first one to discover it. The modern-day version of this assumes that the United States is solely responsible for climate change because it's not doing enough, when the United States is only 1 country out of 195, and many of those other countries are poor, want economic growth, and are not going to accept being told by rich countries that they cannot enjoy the same sort of wealth because of a hypothetical catastrophe that might happen in the next hundred years when they have bigger problems to deal with from being poor in the present.
  • They incorrectly assume that humans are lazy. Just because you can deliver nuclear energy to all of the primitive humans on Earth everywhere, doesn't mean that those primitives will not continue to research other forms of energy eventually and use them. The secrets of the universe still exist to be found regardless of what toys we have to play with, and in the time travel context of this problem, we already know that humans are capable and have already figured out these secrets. Your time traveler needs to somehow prevent that also, and that is nearly impossible for one person to do.
  • They incorrectly assume that a small number of human activities cause the problem, and that we fully understand the consequences of changing those activities. The assumptions are often that power generation or cars are killing everybody by emitting carbon, and if we magically make that go away, everything will be fine. Maybe, but if we don't have cars and electricity, human beings will do other activities that will have a different environmental impact, not just whatever today is minus the bad thing as though it never happened. If we replace internal combustion engine cars with electric cars, we'll need to generate more electricity, which could increase pollution and increase carbon emissions. If we somehow prevent the industrial revolution, then most people will work in agriculture, which will require a lot more land than it does today, which could require clearcutting forests and trees and reduce the planets' carbon capture capacity. It's hard to figure this sort of stuff out. People fail at this all the time.

How could you prevent Climate Change with Time Travel?

Assuming that the catastrophe has already happened (e.g. you can't say it's not going to be a real catastrophic problem after all), you need a lot more than a single individual and their time machine to fix it. What you need to be able to do, is have perfect knowledge of what caused it, and a dedicated effort to prevent it from happening again. You essentially need to have a time travelling totalitarian government that is omniscient, does not cause itself to cease to exist through its time travelling efforts (e.g. grandfather paradox), and can fix any mistakes it makes (e.g. if they ban the car, and it turns out this leads to too much grazing which makes carbon emissions worse, then they have to be able to undo that and try again until they get things right).

I think this is impossible; the level of control and knowledge required to get this sort of thing to work has never been demonstrated to be within the reach of human beings, and any similar effort to centrally plan in this way on smaller scales has almost always led to economic and environmental disasters. But, time travel is impossible too, so if you can conjure up the a time machine, you can conjure up some sort of perfect planning information machine too that maybe works around those sorts of problems.

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The time traveler could kindly kamikaze with his shuttle against the Chicxulub asteroid, thus preventing (or postponing) dinosaur extinction.

Rampaging dinosaurs would keep apes in checks, preventing their spread. Also, provided that the human species happens anyway, competition for resources between humans and dinosaurs would leave less surplus to invest in technological development.

Now, the interesting thing is that it is possible that dinosaurs might have enjoyed a slightly warmer planet, and millions of years later, they could find themselves wondering what they could have done to cause a global warming.

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  • $\begingroup$ I considered this for other reasons, but since there is only one "top dog" niche in the environment (as far as I know), I don't think there'd be much room for both dinosaurs (evolved to be humanly intelligent or otherwise) and human beings. $\endgroup$ – James Feb 11 at 21:38
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    $\begingroup$ @James thinking of it, the lack of the mass extinction would deprive the world of a lot of oil, and possibly coal. $\endgroup$ – NofP Feb 11 at 21:49
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    $\begingroup$ I looked into that, but it's not just the animals, it's a whole bunch of plant life. $\endgroup$ – James Feb 11 at 22:06
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    $\begingroup$ @James dinosaurs were not the only ones that went missing ;-) $\endgroup$ – NofP Feb 11 at 22:15
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    $\begingroup$ @NofP Oil doesn't come from dinosaurs. Neither does coal. And it's not thought that the mass extinction had significant impact on the production of either. $\endgroup$ – Luaan Feb 13 at 12:59
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Apparently the main sources of greenhouse gasses are

  • Transportation (car, rail, ship, aviation)
  • Electricity (power stations)
  • Industry

So a good "event" to change in the past would be "steam engines" powered by fossil fuels -- 18th century.

Maybe she could do that in two ways:

  • Tell the truth -- i.e./ warn of what will happen if fossil fuels are adopted world-wide
  • Provide alternatives -- solar panels, better batteries than today's, safe nuclear power -- also medicine and telecommunications, because once you have those do you really need heavy industry and long-range shipping too? -- whatever other technologies she can carry from the future in that shuttle of hers
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    $\begingroup$ your basic solution was suggested by way of nuclear power being introduced in the 18th century (plan B, no one would believe plan A without proof and an alternative power source). It seems clear reading everyone's responses so far that there would be no single event or small group of events that could be changed to prevent climate change. If fossil fuels are the ultimate bad guy, would there be a way to prevent them from forming in the first place? $\endgroup$ – James Feb 10 at 20:45
  • $\begingroup$ I can't imagine a man-made (human-scale) intervention that could stop the formation of fossil fuels -- changing geology -- except by preventing biomass somehow (which isn't what you wanted), or inventing a different planet with some other geology. I thought of trying to change human nature, perhaps introduce or promote a religion which doesn't give Man dominion over Nature; but I thought that's hand-wavey, a soft psychology/sociology solution. I've preferred hard science fiction. so I thought maybe use that Shuttle. $\endgroup$ – ChrisW Feb 10 at 20:54
  • $\begingroup$ "Tell the truth"- The truth about global warming has been known by scientists for about a century; these scientists would have had much better credentials than the outsider time traveler. And these scientists had a pretty good story - Global Warming was a risk, but not that urgent in the 18th century. $\endgroup$ – MSalters Feb 12 at 11:00
  • $\begingroup$ @MSalters They didn't have a shuttle though, evidence of being a time-traveller, nor alternative "greener" technologies to offer. $\endgroup$ – ChrisW Feb 12 at 11:05
  • $\begingroup$ @James Well, it'd be quite easy to prevent climate change; don't make fossil fuels attractive. The steam engine allowed cheap coal (it was used to pump water from the deep coal mines). Coking allowed coal to be used to produce cheap steel. This was the central positive feedback loop that got the industrial revolution going. Before that, we used charcoal for steel production - and both were very expensive as a result. Prevent those developments (over and over again), and you prevent the carbon dioxide problem. You also keep the humans in their pre-industrial circle of starvation, though. $\endgroup$ – Luaan Feb 13 at 12:03
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  1. Take a self-replicating robot and send it back in time.
  2. Have it build a mining colony in the asteroid belt.
  3. Have it drop a space elevator down to the surface of the Earth.
  4. Engage in communications with leading scientists (around 1800).
  5. When they are ready, give them thorium reactors or fusion reactors as black boxes.
  6. In return, the deal is that fossil fuels will not be used except to maintain the current climate at acceptable levels.
  7. When the timeline catches up with when you originally sent the robot back in time, the robot sends a brand new robot back in time to do all these things.

This does not involve a mass extinction event (although it's possible that the robot might also supply superior birth control). It relies on substituting easy electricity generation for fossil fuel use.

This is sustainable. While you (as the person with time travel) may cease to exist, the robot can continue to exist. Causality is maintained because the robot creates a closed loop. It sends its younger self back because that is what it was always programmed to do.

Another alternative is that instead of sending reactors down, it might bring people up. So if someone wants the joys of modern technology, they can ride up the space elevator and live in a space station. Meanwhile, the people who stay on Earth can stick with the tried and true. Because the robot controls the transport, it can draw people who want technology off the Earth. Those who remain can stick to more sustainable practices.

Want the internet, television, and self-maintaining toilets? A space station is the place for you. Want to ride horses and grow your own food? Stick to the Earth.

It's possible that the robot might provide modern medical care to the people on Earth. Why? Because prior to modern medical care, people had many children. Wealthy westerners in Japan, the United States, and Europe have negative population growth modernly.

If the 1800s isn't soon enough, the robot might try an earlier period. For example, the robot could try to stabilize the period of the Roman empire by luring Julius Caesar away. Or it could take more active measures, possibly with the help of those on the space stations.

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I once read an article that stated that in the 19th century, the first thermodynamic theories were received with skepticism by the scientific community of the time.
That article wondered that, had those theories been well received since the beginning, the story of science and technology would have been very different.
This is because (Warning: very bad and rough explanation ahead), while the then-predominant Newtonian physics used to see every phenomenon as reversible (just invert in the appropriate way the direction of the forces, and a body will follow the same path in reverse), the principles of thermodynamics (particularly the second one) stated that at every action, something is lost and can't be recovered.
So it was hinted that an early adoption of the thermodynamics (and the concept of irreversibility) in the mainstream scientific and economic culture could have geared the society toward a more conservative and prudent usage of energy sources and natural resources.

Probably it would be an optimistic approach, but a time traveller could use his knowledge of advanced physics to support Boltzmann's theories, or even discover them some tens years before, so that the industrial revolution would also be driven by the awareness of the risks of limited resources and pollution.

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    $\begingroup$ That presupposes that everyone would accept those theories and be driven by motives that would be best for the environment as opposed to their profits. $\endgroup$ – James Feb 10 at 22:09
  • $\begingroup$ @James Profits and the environment aren't in such stark contrast as you might think. Most of the profit-increasing practices also helped the environment - per capita. The problem is they also tended to help support a larger population. Another huge impact was the nationalization of resources - when you no longer own the oil underground, you have every incentive to get it out of the ground as fast as possible before the government revokes your permit. And finally, carbon dioxide isn't everything. "Fixing" it will have its costs; maybe lower, maybe higher. $\endgroup$ – Luaan Feb 13 at 11:59
  • $\begingroup$ I find this highly dubious. People may not have understood physics, but they also had a poor understanding of economics, and one of the economic myths of that era that still is prevent today is the idea that we will run out of resources because there will be too many people and not enough stuff. Thomas Malthus was a prevalent figure during the 18th and 19th century and probably already did provide the insights that you think would be lacking. $\endgroup$ – Joe Feb 13 at 15:26
  • $\begingroup$ Dear @Joe Do you conceed there is some limit to Earth's carrying capacity or are you of the school that tribal cultures follow that when the topsoil is eroded away it is acceptable to invade neighbouring territories to claim resources. Could the earth sustainably feed 100 billion people, because if not then you do have some limit in mind, perhaps just not a realistic one. Global fish stock cannot keep up with 7.5 billion people, how will it cope with infinitely more? $\endgroup$ – KalleMP Feb 14 at 18:57
  • $\begingroup$ @KalleMP The earth's carrying capacity is likely far larger than we can comprehend considering that human societies have means of making many physically limited resources economically infinite, most notable among them being the free market price system (which, doesn't work so well for fish). But... that really is irrelevant to my point, which is that thinkers in the time period in this answer already had the idea of a "carrying capacity" in mind and assumed that it was far lower than 7.5 billion people. This thinking was mainstream then. $\endgroup$ – Joe Feb 14 at 21:39
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The time traveller needs to go back and find the person who invented fire (before she invented it) and give her a solar cooker and a down jacket.

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    $\begingroup$ So that fire will be invented a few years later and a few (thousand) kilometers further? $\endgroup$ – zovits Feb 11 at 11:27
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    $\begingroup$ Hi, Keith McClary. Perhaps you could elaborate your answer to explain how the technology could be spread in such a way as to answer the question's concern. You can edit your answer easily. $\endgroup$ – Agrajag Feb 12 at 23:16
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The tragedy of human damage caused to the biosphere is in direct proportion to the number of humans doing the damage. If we take the carrying capacity of the earth to be 1 billion comfortable humans when living exclusively with renewable resources then all we have to do is prevent humans from increasing beyond 1 billion and they will have less need to fight over resources and be happy with a global Demogarchy (each person responsible for their own regions rules).

As mentioned in my comment your means of messaging may be limited if you cannot take your shuttle in the time machine or the time machine in your shuttle:

Given you are currently on the Moon. If you cannot take your shuttle with you you can only go back to when a shuttle first arrived. If you want to pass on a gift you can visit with Neil Armstrong. If you want to send a message you can go back to the advent of radio or the telescope. Best would be to take time machine back to earth in your shuttle and then work from there.

If we take as given that you can go back then you just go back to before mines are opened and buy up all mineral and fossil resources. You provide them in unlimited amounts to any countries that have self sufficient populations and some form of sustainable government with thousand year plans but prevent over populated countries from ever thriving until they join the club.

You might as well take a copy of the various patent office databases and Archive.org to make lots of money to help you purchase the land.

If you have to send the message via radio from the moon you will have a harder time as the world was divided up already into major superpowers by the time of radio and they would rather shoot you on the moon than listen to world saving propaganda. Sending plans for all solar cells and contraceptives to all who will hear may save the day but odds are you will see the world race towards where we are now. Unless you have the final balance sheet of the US Reserve System and other Central Banks showing the scam they have pulled and are able to nip the economic policy of continuous growth before it is permanently entrenched.

If you have to work with Morse code and lasers pointed at famous Victorian observatories you will have to send the message and task your disciple to start a religion to achieve these same goals.

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Building on Pelenore's answer - about culling a large portion of the population to reduce pollution.

Release self-replicated and repairing killer robots, programmed to attack anything that produces pollution, while simultaneously broadcasting a message along the lines of "pollution source detected, eradicating".

To prevent these robots being thought of as simply a calamity, make them peaceful under normal circumstances, and maybe even helpful. e.g. they could impart advice on how to achieve the same tasks without causing as much pollution, and only eradicate those that consistently/significantly fail to.

This would however, require a lot of collaboration and planning with a large number of people to design such a robot so that it doesn't just become an environmentally concerned Dalek...

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    $\begingroup$ I don't think my time traveler will be taking back a lot of killer robots with sophisticated sensors and programming. $\endgroup$ – James Feb 12 at 17:43
  • $\begingroup$ I think that's a shame! But thanks for still reading my post despite having so many good answers already :) $\endgroup$ – Azrantha Feb 13 at 9:13
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Problems With Time And Incentives

One of the problems with changing the past is, that you don't know where you will end. Killing all people might stop the warming, but not solve your problem. The alteration of the earth climate might be going on now for 10.000 years with the first rice fields and cow breeders. You don't want to change that or any other thing which might hinder humans to go to the moon forever. And I don't see, how nuclear bombs could help humanity survive longer on earth if they were introduced earlier, like let's say as a westener 1618, 1775, 1803 or 1914. And all these people will not believe your time traveler, he might be burned on a stake or end up in a sanatorium.

Solution: Create Incentives And Time

Travel back to a time where everyone can see what global warming does and that it is a good thing to do something against it, but a lot of plants and animals are still there and the people have still the money to invest in their future, some years from our point of view. Introduce all the technology you like, e.g. instant-solar-panel-nanobot-factories or clean and safe nuclear fission plant technology.

If the time runs out too fast, do something to slow down global warming until the other plan works. You don't have to ignite all the forests, tickle yellowstone or start a nuclear war. Just drop a rock on earth, somewhere in the desert, big enough to put up a decent amount of dust in the sky, and you have your cooling. Without (m)any deaths. With your future knowledge you will know exactly how much rock you need, or you just try it out.

Our problem is (or might be) not, that global warming is already irreversible. At least if your main concern is to save most of the humans, but not the dodos, or the mammoths, or some other species which will die out in the next couple of decades or is already dead. The problem might be, that no one wants to pay for the solution until no one has any resources left to do anything, because of all the economic crises, riots, epidemics, floodings etc.

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  • $\begingroup$ This all has to happen within the lifetime of the time traveler, and I'm assuming a person who is not all powerful and infinitely wealthy. Sure, if the time traveler could experiment with different timelines and observe the results, it would be terrific, but as you've alluded to already, there are so many variables, that outcomes are hard to predict. $\endgroup$ – James Feb 18 at 14:11
  • $\begingroup$ Then your time traveler has to introduce better technology. There will be no single bullet solution to this problem. $\endgroup$ – Henning M. Feb 18 at 22:44
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I'll keep it simple. The underlying issue is population. It's already quadrupled in my own lifetime. Actually kind of shocking, thinking back on it.

The rest, the conversion of complex ecologies into monocultures, the bleaching and destruction of corals, the depletion of fisheries, stratospheric ozone depletion, the quilting and destruction of forest systems (well studied by Dr. Lovejoy in the early 1980s) and the ability of those systems to sustain diverse life forms, the 6th extinction event and the environment generally, is just a long list of various symptoms resulting from human population growth, hastened by the advent of "nearly free energy" through the discovery of coal and oil and engineering resulting from science knowledge (such as the Haber process, first used to help the Germans deal with the blockade of South American salt peter, but subsequently used to make nitrogen fertilizers.)

So focus on ideas that might lead to a self-limiting mechanism of human population growth in the tale. We are already eating into the "capital" of Earth, by destroying/consuming 50% more than is generated each year through photosynthesis.

(If you sum up the entire meat mass of humans and their domesticated animals, you'll find something close to 99% of the mass of all land-based vertebrates on Earth. In short, we now occupy very nearly the entire land-based vertebrate niche. This wasn't true when I was born. And it's not sustainable into the future.)

If you can limit human population sufficiently that we no longer consume more than is produced each year, you may find an equilibrium that arguably might work. How you achieve that? I don't know. That creativity is left to you to work out, I suppose.

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You have time travel.

If you allow that, you can allow is being associated with some new inexhaustible power source.

Start a business exploiting that and provide with world with power cheaper than fossil fuel.

Ideally the power source can be adapted for small scale use and provide transport power.

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You need someone who doesn't know about Climate Change or doesn't exist in(side) time itself

So, one big problem with time traveling is the fact than causality can bite you in the balls, but you have some ways to do a work around it:

A) Send a traveler who doesn't know about Climate Change but travels with a mission give it by some otherworldy over-temporal entity who happend to exist beyond time.

So that way the traveler doesn't affect the events than lead him to be sended in the first time, because the entity who send him exist outside and over time.

-And/Or-

B) Make him have an artifact than make him (the traveler) exist beyond time like in the X-men animated series, where time travelers use some bracelets than allow them to exist beyond the impact of time, allowing them to modify time without being afected for causality.

After you fix that problem, i would argue than:

Send a group of time independant traveler to make an intervention in the time before the napoleonic wars

You see, before the napoleonic wars, there was france and england having capitalistic economics reforms than opose to the agrarian concept of richness very common in nation back in the day.

This reforms actualy help to make both nations industrial (or pre-industrial if you want) change them shift to investing resources in production as a way to generate more wealth.

So, what you need is actualy reeplace both economical characters by time travelers, being themselfs second hand of the arguibly most powerfull "kings" on earth in that time they will propose somewhat different economical policys than make a turn for the enviroment, without anyone ever knew them.

If the changes are right, then, the napoleonic wars can happend as it should, but your agents probably already make the change in the timeline just enough to make sure than the oil and coal industry never see his zenith. because you gave that zenith to another, cleaner industry before.

Like, i don't know, send your travelers with vastly electric independant railroads or send very efficient solar panels together with very powerfull Electric engines to reeplace the steam machines much earlier than the moment they become the stars of the movie, and make your main characters make investment policies and recomendation than clearly benefit that tecnologys instead of the steam and coal tecnologys developed back then.

To prevent a technological causality problem just make sure than research is still done about this steam engines, but doesn't give them any chance of being used in industrial production, maybie make them work in ships or some other masive transportation method, than because of massivity is not so frecuent like a car or a train.

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A pandemic.

Humans account for a substantial bulk of all global warming, plastic pollution, acidification, etc. that is currently ongoing. A substantial reduction in the past human population (probably at least 85% reduction), anytime before the Industrial Revolution, could avert any perceivable effects of climate change. If the pace of scientific progress did not slow down too badly, then humanity might be able to out-innovate climate change much faster than they are doomed by it.

Regarding the pace of scientific progress, there is also the option of leaving behind scientific accelerators to help humanity fight climate change faster. This may include publishing in the 19th century the theory of global warming, the greenhouse effect, how to measure the greenhouse effect in a laboratory setting, and various scientific / engineering principles for how to avert climate change (windmills, precursors to solar cells, precursors to hydroelectric and all turbine-powered energy sources).

Since there are minimal diseases realistically capable of wiping out 85% of Earth's population by themselves, it might require that the time traveler release a handful of different diseases at different points of the globe, specifically aimed at the immune systems of the region (i.e. each region is targeted by the infection that historically devastated it the most). This might require that the time traveler have access to a vault of infectious agents. Access to an biological vault AND a time machine would be more realistic if the time traveler was an agent of a coordinated government operation, rather than a lone wolf actor.

Note that in time travel fiction, and even in actual physics, there are different opinions on whether a time traveler is traveling back into their own past (and therefore at risk of erasing themselves or creating causality paradoxes) or traveling along a closed time-like curve to a past on a different line (in which changing past events will not erase your own existence).

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    $\begingroup$ So the answer would be in significantly reducing the human population at a critical time period, say the 18th century. Of course, people being who we are, would eventually continue to reproduce, so wouldn't this just delay the inevitable? $\endgroup$ – James Feb 12 at 12:14
  • $\begingroup$ @James Well, I considered the issue that re-population would undo the effects of the pandemic. But there's a paragraph addressing accelerating the pace of scientific progress -- i.e. by publishing research that would make the society aware of climate change sooner, and able to develop clean energy technology faster. Arguably, that might eliminate the need for a pandemic. But anyways, it's sort of a two-part solution where climate devastation is delayed by 100-200 years while clean energy is accelerated by 50 years. $\endgroup$ – John Feb 12 at 18:47
  • $\begingroup$ The pandemic has other climate benefits, since it slows deforestation, slows habitat loss, reduces over-hunting, reduces # of wood-burning fires (releases black carbon), reduces quantity of chemical pollutants and landfill consumption, etc. It basically de-extincts many species that would have been lost in the 19th-21st centuries. Although, a pandemic is sort of a tool of someone who doesn't mind using mass murder for a greater good. It would also totally re-write 20th century history, and that could be interesting, as a fiction writer to make-up the events that would replace WW2. $\endgroup$ – John Feb 12 at 18:53
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Perhaps I'm too naive but my suggestion is for the time traveller to get as much documentation of the disaster as possible, newspapers, videos, how ever many years worth of everything on the internet and come back to right now. The information should be seen as basically unfakeable (1000s of hours of video from 100s of different sources of New York being swept away) and so comprehensive that it's impossible to believe one person or even a large organisation could create it all (petabytes of future news papers, Facebook posts, Twitter feeds, etc, etc, etc). This is highly likely to contain information such as revelations that certain fossil fuel company executives knew of studies that indicated approaching disaster and covered it up and political leaders that claimed the science was all fake because their financial supporters paid them to.

Given absolute proof that the path we are on leads to disaster I have to think that we, as a species, would change path. This is where the 'perhaps I'm too naive' comes in. My assumption is that we're not already doomed but as other comments and answers indicated it isn't a binary fine/extinct situation. Corrective action at any point not near the conclusion should lessen the disaster.

It would be someone elses problem to deal with the inevitable cries of "the time traveller told us a billion people would starve by this year but that never happened" if the corrective actions actually work.

The main difficulty would probably be avoiding assassination long enough to get your message out. Perhaps I'm too cynical in this case but I have no problem believing some people will choose vast wealth in their lifetime (and a nice cave in the wastelands for their descendants) over the good of humanity as a whole and would not think twice about killing an individual to achieve that.

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  • $\begingroup$ Who would the time traveler show that evidence to? It would have to be someone or some group that had the ability to affect the course of nations. If you're talking about releasing the data to the world press, the vast majority of people aren't going to go through hundreds or thousands of hours of data. Believers will believe and naysayers will say nay. As far as assassination goes, no one has tried to kill Al Gore yet. $\endgroup$ – James Feb 15 at 20:14
  • $\begingroup$ Release it to everyone. Scientists and investigators will dig through the data to find things that are relevant to the world in general. They certainly wil be interested in seeing their own future papers. Regular people will read their own social media content to see what happens to them. All but the most willfully ignorant will accept that it would be all but impossible to produce all this personal information. Of course how many people fall in to that category is open to debate, certainly there seems to be a lot. $\endgroup$ – Eric Nolan Feb 18 at 10:11
  • $\begingroup$ It's an interesting thought, but basically, the evidence already has been released to everyone IRL in one form or the other, and the response isn't universal acceptance, especially by corporations and other entities that have something to lose. $\endgroup$ – James Feb 18 at 14:08
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Simplest fix would be to try and intervene in the works of Thomas Midgley Jr.

He was (at least partially) responsible to adding Lead to petrol (gasoline) and the industrial use of CFCs

the "Legacy" section of his Wikipedia entry states :

Midgley's legacy has been scarred by the negative environmental impact of leaded gasoline and Freon. Environmental historian J. R. McNeill opined that Midgley "had more impact on the atmosphere than any other single organism in Earth's history", and Bill Bryson remarked that Midgley possessed "an instinct for the regrettable that was almost uncanny". Use of leaded gasoline, which he invented, released large quantities of lead into the atmosphere all over the world. High atmospheric lead levels have been linked with serious long-term health problems from childhood, including neurological impairment,and with increased levels of violence and criminality in cities. Time magazine included both leaded gasoline and CFCs on its list of "The 50 Worst Inventions". Midgley died three decades before the ozone-depleting and greenhouse gas effects of CFCs in the atmosphere became widely known. In 1987, the Montreal Protocol phased out the use of CFCs like Freon.

https://en.wikipedia.org/wiki/Thomas_Midgley_Jr.

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    $\begingroup$ While CFC is a big factor, lead is more a residue in the ground. $\endgroup$ – Trish Feb 11 at 13:23
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    $\begingroup$ 'Runaway climate change' as mentioned in the question has nothing to do with either lead or CFCs, only with CO2, methane, and some other gasses. $\endgroup$ – Jan Doggen Feb 11 at 13:36
  • $\begingroup$ @Trish - Lead additives in petrol were a massive contributor to climate change, hence why they were banned and replaced with "unleaded" fuel $\endgroup$ – jimP76 Feb 11 at 15:06
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    $\begingroup$ @jimP76 lead was banned for it accumulated in the areas next to the roads. $\endgroup$ – Trish Feb 11 at 15:24
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    $\begingroup$ @jimP76 You are misremembering. Global warming was contributing to the ozone hole, but not vice versa. The concern about the ozone hole was that it lets more UV rays through. $\endgroup$ – Davislor Feb 14 at 14:43
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I hate time travel stories, because they always end up in a contradiction.

Except, this is a time travel story I might actually be able to appreciate. My answer would not only be ironic, but it would also unavoidably and obviously set up an infinite multivibrator or oscillation in time.

You see, the reality is that around 70,000 years ago there was an extinction event that wiped out all but about 10,000 human predecessors. These remaining 10,000 went on to produce the five billion humans on earth today. (That is why the genetic diversity of humans is only 15,000 or so. That is, it only takes a random sample of 15,000 humans to encompass the entire genetic diversity of the human race). This extinction event is hypothesized by some to have been caused by climate change (the irony).

So if this time traveller went back to this time period, and completed the extinction, the root cause of human-moderated climate change is eliminated. Ten thousand people killed is not an insurmountable goal for one human with modern technology at his disposal, especially when he can keep flipping around from place to place and be everywhere at the same time, only at a different 'same time' every time. That is, he could take his time killing them all, but it could all happen at the same time. Fuel for the shuttle? Every time he came back to his starting point on the moon, if he arrived just before he left the time before, he goes back with a full fuel load. That is why time travel is so contradictory.

But as soon as he kills the last remaining human, he himself would cease to exist. Whereupon he would not have been able to kill all the humans, and he would exist again. But then he would come back to kill all the humans, and... Well, you get the oscillation. Climate change - no climate change - climate change - no climate change - climate change....

This would be an ending to a time travel story that I could really get into.

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  • $\begingroup$ Interesting, but over what area are these 10,000 humans spread. If they were in one small town, a nuke would do it, but extended around various parts of the planet, it would be more difficult to eliminate them all. Of course, the time traveler is trying to stop climate change to prevent human extinction, and if the only way is to cause people to become extinct, he might as well save himself the effort. $\endgroup$ – James Feb 10 at 20:35
  • $\begingroup$ @James The area over which they are spread is irrelevant. The person has access to a shuttle and infinite fuel and infinite time to do it in. But I am glad you see the irony - to eliminate human moderated climate change to prevent the extinction of humans, requires the extinction of humans in a long ago climate change extinction event that almost caused the extinction of humans. $\endgroup$ – Justin Thyme the Second Feb 10 at 20:51
  • $\begingroup$ It's specified in the question that the goal of the time traveler is to avert human extinction. So this isn't an answer. $\endgroup$ – eyeballfrog Feb 11 at 4:24
  • $\begingroup$ Your answer plays wreck havoc with causality: if he takes the fuel he would have loaded in an hour, his shuttle would have never made it to earth for the first (now second) trip, resulting in the first target of his personal timeline not to be eradicated at all in the planet's timeline. $\endgroup$ – Trish Feb 11 at 12:00
  • $\begingroup$ @Trish And that is the paradox. Time travel itself plays havoc with causality. There is just no way that any story involving time travel can pas even the remotest test of causality. That is the essence of my answer - it blatantly rubs causality in the face of the reader and makes a mockery of it. Nothing subtle about it at all. But in that sense, it makes a very good commentary on time travel stories. The inevitable conclusion of any time travel story is an oscillation in time.How many infinite dimensions/universes can you 'consume' in an infinite oscillation? $\endgroup$ – Justin Thyme the Second Feb 11 at 19:52
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A time traveler can't do what you are asking her to do. Sorry.

The climate of Earth has changed many times over the course of the planet's history from ice ball to hundreds of feet deeper oceans. To think it won't change in the future is folly. If she goes back and kills Eli Whitney, so what? Or Watt? Or Carnegie or Ford or any of a thousand others involved in the technological explosion?

Politics aside, as science and technology progress, the effects of technology use in the environment are a surety. Your time traveler's best bet is probably to go way back in time to the Toba Volcanic event, destroy the ten or fifteen thousand humans left on Earth and have done with it.

But then, she'd probably just make it worse. Scientists also say we should be due for an ice age any day now. Destroying the distant ancestors of the people that would eventually come up with all this anthropogenic global warming, she just ends up shooting herself in the foot by creating an arctic wasteland.

Lesson to be learned: you can't mess around with Mother Nature. She's does what she's going to do when she's good and ready to do it.

Earth 700 million years ago or so Earth 100 million years ago and now

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    $\begingroup$ xkcd.com/1732 $\endgroup$ – ths Feb 11 at 7:48
  • $\begingroup$ @ths -- That, only worse. $\endgroup$ – elemtilas Feb 11 at 7:55
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    $\begingroup$ it seems you don't get the point of the xkcd: human action does have a huge impact. $\endgroup$ – ths Feb 11 at 12:50
  • $\begingroup$ I understand Xkcd. Fact remains: one person, even with a time machine, can not stop, reverse, prevent or fundamentally change the climactic shifts that Earth undergoes (and regardless of the agency in question). $\endgroup$ – elemtilas Feb 11 at 18:37
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    $\begingroup$ your original version sounded even more AGW denying. $\endgroup$ – ths Feb 11 at 22:19

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