Jtdylktuy

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.

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.

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.

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.

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.

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.

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.

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.

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

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.

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.

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.

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

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.

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.

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.

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.

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...

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.

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.

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.

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.

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).

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.

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.