>Additionally, the decomposition of dead marine organisms in these zones can release harmful gases such as hydrogen sulfide, which can be toxic to marine life and humans.
If human extinction occurs due to climate change, this will be how. Anoxic ocean events can lead to massive emissions of hydrogen sulfide (H2S) into the atmosphere. This is the speculated mechanism for the end-Permian extinction event, also known as the Great Dying, and several other mass extinction events in Earth's history. H2S is immediately dangerous to human life at concentrations as low as 100ppm. It's so dangerous that humans have evolved to smell it in concentrations as low as 100 parts per billion.
UW-Seattle professor Peter Ward has spoken about this at length: