> When I was pursuing environmental studies, there was a cartoon in one of my textbooks depicting microbes giving off oxygen and some of the microbes protesting that they were destroying their environment and needed to stop. Microbes that off-gassed oxygen are how we got an oxygen atmosphere on earth.

Yes, that is called the Great Oxidation Event [1] and it was the Earth's first major extinction event.

The microbes were absolutely correct that their environment was being destroyed. The Earth then shifted to a new atmospheric makeup and new life blossomed but it was still a termination event for most life that existed prior to the event.

> The truth is we don't really know as much as some people like to pretend we do

Yes. Which means we shouldn't run a giant experiment with the atmosphere and the oceans. This is called the precautionary principle: if you're doing something that may result in systemic disaster then it's better to not do it. People who say that this giant global experiment won't do any harm (or go further and say it will be beneficial) are pretending knowledge that they don't actually have.

[1] https://en.wikipedia.org/wiki/Great_Oxidation_Event