It's easy to be pessimistic about the future of Earth, and humanity, in the face of all the problems that plague us. The idea that the world will ever be made pristine again is almost laughable, so total is the damage that's been done. Rivers and lakes have dried up, there are garbage patches the size of Texas floating in our oceans, the world is warming at an alarming rate, species are vanishing just a swiftly, forests are being exterminated, and pollution is making many waterways toxic and cities miserable to live in. Until the World Revolution, things will only get worse. So long as rich individuals and powerful corporations hold sway over humanity, the world's suffering will continue to accelerate.
After the war, however, humanity will have the opportunity to build a better civilization than the one which led to the conflict. Green cities and arcologies will replace vast sprawling webs of steel and concrete, roads will be replaced by elevated rails and eventually teleportation, and recycling 3D printer/disassembler hybrids will greatly reduce trash. The end of oil, coal, and natural gas will put a stop to air pollution, while much more efficient waste management systems will eliminate human and animal waste. Urban farms will replace rural ones for the most part, with only a few stubborn holdouts. Residential neighborhoods will be far more consolidated, beginning with something like shared backyards, and then evolving into mini-arcologies such as Cahokia-style mounds and pyramids. Eventually, I see many cities floating through the air via antigravity generators, which will lessen humanity's impact on the environment even more.
As humanity reduces its negative impact on the environment, it will step up its positive impact. Robots designed for consuming and disintegrating trash will roam the waterways and seas, making them pristine. Dams will be disassembled, allowing rivers to return to their preindustrial state. Blimps will float through the skies sucking in air, sequestering carbon, methane, and other pollutants within their hulls and releasing harmless nitrogen and oxygen. Swarms of ice-generating robots will suck in seawater, separate the salt from the water, and rebuild melted glaciers in places such as Mount Kilimanjaro and Glacier National Park. Dried up rivers and lakes will be refilled, their evaporation rates closely monitored. Landfills will be consumed by machines designed to break down the waste into its simplest components, using the matter as fodder for picoassemblers/disassemblers.
Utilizing seed vaults and stem cell vaults, humanity will reseed destroyed forests and bring back extinct species. Polar bears, pandas, frogs, lions, tigers, rhinos, elephants, zebras, giraffes, apes, whales, dolphins, sharks, crustaceans, insects...all will be brought back through cloning technology and reintroduced to the wild through an intensive rewilding program. Animals will start out in a controlled simulation of their natural environment, most likely in a structure within the habitat itself, and then released simply by opening a door. There will be many false starts and failures, and it may take centuries, but the rewilding effort will succeed.
It will take centuries to repair Earth. And even once it is restored, Earth won't be as it was. Random mutations and quirks of weather patterns will force the restorers to adapt and improvise. But they won't give up, and with an entire populace raised to respect Earth instead of seeing it as just a big resource for industry and wealth, our planet will become healthy and bountiful again.
Of course, by then humanity will have truly moved out to that most mysterious realm: space.