It's hard to pick a single issue because they're all intertwined. I'd say it's a web made up of strings if you pull one you're guaranteed to affect the others.
Let's pick, for example: Pollution, which leads to carbon emissions levels going through the roof, the greenhouse effect goes crazy, the planet heats upo, the ice caps melt, sea levels rise. Farmland is reduced and with overpopulation there's no way we can feed 7.7 billion people, not to mention this figure has no intention of stopping. What good can come from focusing on a string, when we should be looking at the web?
