Soylent Green is one of those movies that once you see it, you'll never unsee it ever again. By this I mean beyond the story and basic plot, it's one of those movies that have an underlying, if not outright exposed, message of impending doom. It was kind of common for films of that era to portray on the big screen a cautionary tale juxtaposing the familiar images of mid to late 20th century life and positing a not too distant dystopian future. We see it in films like Planet of the Apes (nuclear war and eons of devolution), Logan's Run (unyielding population and resource control), A Clockwork Orange (collapse of the social order). But Soylent Green has probably the most prescient of looming disasters that were increasingly on the minds of people back then, namely overpopulation, worldwide shortages and eco-disaster at the hands of global warming and pollution.
I remember at roughly the same time this movie came out, I saw an exhibit at a science museum which was simply a mirrored box that you stepped into and the way the mirrors were angled it gave the impression that there were an infinite number of images of you cascading into the background. It was a cool optical illusion enhanced by the chilling placard nearby which boldly stated that if population growth continued throughout the remainder of the 20th century, the global population would be so high that people would be standing in a never-ending sea of other people similar to the way your image was portrayed in the mirrored box.
Here's the opening montage of Soylent Green which I think does a superb job not only in the imagery but the score as well matching the tone of progression from idyllic old-fashioned life to the chaotic and problematic modern world.
The movie's "future," by the way, is set in the year 2022. And some of the images you'll note, towards the end of the montage, hint towards a reality of life during that time...including people wearing face masks. Little could they know how real that all became.