Why Don't they fix Global Climate change problems by Altering time?

In order to do this in reality and not just in a TV program or streaming program you might have to go back in time and stop anyone from burning anything like slash and burn techniques for enriching soil which might go back 10,000 to 25,000 years. Then you might have to cause more people to die or prevent more births. 

Because the whole point is if people live on the surface of the earth (and not underground) the maximum sustainable amount of people would actually be around 1 billion people for thousands and thousands of years.

So, getting everyone to agree to do any of that isn't likely going to happen from a Galactic Level to a nation or city level.

You just aren't going to get enough agreement for doing any of this to actually be successful at preventing human extinction on ANY time line in ANY Country and this includes the United Nations.

So, other methods will have to be used than any listed above here to reduce populations down below 1 billion people by 2100 AD which appears to be an unfortunate but necessary Galactic Plan which has been instituted because we are considered so "Childish" as a planet that we cannot figure out what to do to prevent human extinction all by ourselves. 

In the Meantime the Sentient Planet Earth will increase exponentially the deaths in catastrophes caused by Global warming of the oceans because of Greenhouse Gases which will increase the amount of rain falling some places by up to 500% of normal especially in Temperatures (or heat indexes above 100 degrees Fahrenheit when it rains. Because moisture can go  up in the air as much as 6% for every one degree of Celsius which is maybe 60%? for every 10 degrees of Celsius.

So, where it rains (especially anywhere near flood plains) people are going to drown more and more.

So, moving out of all flood plains or nearby Flood plains will become more and more a matter of life and death for everyone worldwide.

For example, every 150 to 300 years the Sacramento River in California is 20 miles wide from Redding to Sacramento and likely further. The last time this happened was 1862 and then they had to move the STate Capital from Sacrmento to San Francisco by Boat for several years because Sacramento was at least 6 feet deep or more in water then likely untli 1863 or 1864.

This 150 to 300 year thing goes back thousands of years and even native American Tribes of California have it in their oral teachings which are memorized by elders of the tribes who lived in the regions of the repeating floods every 150 to 300 years. 

Also, people think that they can swim out of a flood.

This usually doesn't work because of obstacles under the water hidden from you like trees, roots, sign posts cars, trucks, pieces of metal and also in the water will be diesel or gasoline or chemicals used on plants and anything else from anyone's yard.

So, though you might be great in a swimming pool the best bet to actually survive a flood might be on top of a building or in a tree rather than trying to swim for it only to get gutted by a tree branch or sign post or anything else you cannot see in the flood that is just under the surface. Also, the currents might be very fierce too and suck you under the water. Remember this isn't a lake or a pool this is usually muddy polluted water going over unknown obstacles that you cannot see usually at all.