If we treat microplastics as a symptom of business-as-usual, then they are a marker, and we can think of them as the canary in the coal mine. Entropically, we dug up a bunch of dead stuff and either burned it into the air or refined it into plastic, which we are now mixing into the biosphere at all scales. What other effluents are we dispersing that will have unwanted effects?

Structurally, we concentrate exotic materials and then disperse them. Nothing like that has ever happened at global scale to life before. The economic changes to achieve a system that doesn't make this its default trend are almost unthinkable. That's what I see as the real #4 (or whatever) on the list.