Suppose we have a world that saw a sudden collapse of civilization, not so violent that humans are suddenly struggling to survive but severe enough to cause humans to revert to a mix of city-state, village and nomadic living. My proposal is that world tension and political dissent caused the US to implode and the resulting economic and political void caused the rest of the world's powers to implode. How severe would this collapse have to be, what kind of population loss (in terms of an order of magnitude) would this kind of collapse result in, and what kind of climate change would this population loss cause?
Our modern technological society is a lot more fragile than people think and is largely possible thanks to modern advances in large scale farming practices and transport technologies.
It's been discussed on this forum before in a number of different contexts, but the reality is that an event that cuts most cities off from the rural communities for more than a couple of days could easily see the collapse of the rule of law there and wide scale deaths from violence and eventually famine. Cities simply don't produce (inside the city itself) food on the scales required to feed its populace. It wouldn't take long without (say) an electrical grid for society in a developed nation to collapse.
Now, I know that your question specifically states a scenario without such a sudden impact, but the point that should be clear is that modern cities are only possible because such a small percentage of human production has to be dedicated to food production to feed everyone, allowing us to entertain a much more diverse range of interests. The moment we go back to city-state, village and nomadic living, the economies of scale in food production are gone, as are many of the technical research and development capabilities, and much more of our production capability (as a percentage) has to be dedicated to food production for the society to survive. In such a scenario, you're dealing with a catastrophic loss of the majority of city dwellers whichever way you cut it.
As for Climate Change, the assumption is that we lose technologies like cars, coal burning power stations, etc. In the scenario described above, that may well be the case in which instance the survivors won't be contributing anywhere near the amount of man-made carbon into the atmosphere. What that would do is effectively arrest the human contribution to climate change which is a good thing, but assuming that such technologies are lost, we've already lost the vast majority of the human population at the same time and the agrarian lifestyle without such tech would put an upper limit on our ability to grow beyond a certain size of population over the earth again. At this point, I'd estimate that without coal or oil you're looking at approx 70% population losses in the first year and the inability to grow much beyond that without alternate tech. I must stress that these figures are best case scenario figures; the losses would probably be much greater but I'm assuming best case because of your statement that it is a 'gentle' change, not cataclysmic.
In terms of climate itself, this doesn't necessarily mean we'd revert to (say) 1900 climate conditions. There is some evidence that the earth is going through a warming cycle at the moment and that human civilisation was just amplifying that in a manner that would be destructive. That said, one would expect the climate to stabilise a bit and that any additional warming would happen at a much more gradual rate.
Collapse of civilization = worse environmental consequences.
Have you even considered unmanned superfund sites?
Lack of direct CO2 emissions are completely irrelevant.
Don't forget about superfund sites that need to be maintained by humans. In reality it will do more harm than good if nobody is there to protect them from spreading their nasty chemicals everywhere.
For example, remember what happened this past year with all the hurricanes in the US? https://www.epa.gov/newsreleases/epa-securing-florida-superfund-sites-prior-hurricane-irma
Who's going to be there to consider this stuff? Nobody. So in reality the environment will get a lot worse! Forget about emissions! Emissions could get even worse with nobody around to maintain various superfund sites. Instead consider aging nuclear plants that aren't properly decommissioned. I'm sure theres more examples.
Reverting to an agricultural society may very well harm the environment more than it helps. A great deal of our food output today is due to systems and technologies developed during the Green Revolution and and the development of high-yield crops in the 20th century.
If we were to revert to pre-20th century agricultural techniques, food production efficiency would likely be less than 38% of what it is today, and assuming you’re not deploying death squads everywhere, or releasing an Armageddon virus to trim the world population down to size, humanity would simply expand the land area it uses for farming.
The idea that stages of human development are accompanied by a monotonic trend of deforestation and ecological destruction is a myth, mostly advanced by the neo-environmentalist movement in the 1970s. Between 1900 and 2000, forest cover in the U.S. actually expanded slightly, despite the population growing fivefold.
As a general rule, as a society industrializes, the land resources it consumes shrinks, as its population retreats to highly dense cities, and farms becomes more efficient and compact. The surrounding environment slowly but consistently recovers through natural processes like reforestation. Currently, worldwide, forest area increases by about 1.6% a year. This figure is despite large amounts of deforestation currently taking place in developing countries like Brazil. According to a study done in Finland, this is in some part due to increased urbanization and farming efficiency.
Increased human migration from rural to urban areas and higher agricultural yields may also have aided regeneration, the authors say.
In terms of carbon emissions, it’s not clear what the net effect would be. The loss of our delicately-recovering global forests would take away a major carbon sink, while the collapse of energy-intensive human activities would also take away a major carbon source. However, we do know that “civilization” in the western sense is not a prerequisite to massive ecological and climatic damage. Contrary to popular depictions in cartoons, it is believed that the Native Americans were so destructive to the environment, that when Columbian diseases decimated their population, the absence of their emissions and the recovery of their environment plunged the Earth into the Little Ice Age of the 16th century. It’s also important to note that The Environment™ is also much, much more than simple carbon dioxide concentrations in the atmosphere. Things like biodiversity and healthy ecosystems are all critically harmed by human land use.