Argthtjtr

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 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 it's 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.

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 traveller 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 unpacked Magic, so anything goes.

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.

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.

It 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 to 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.

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.

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.

I obviously already did, since it doesn't exist.

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.

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.

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.

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.

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

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.

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.