My opinion is largely colored by the IPCC Ocean/Cryosphere report. I closely followed each IPCC report and this was by far the most "doomer" of the special reports released by the IPCC. Some highlights:

  - ocean acidification and deoxygination (imo the most alarming and underdiscussed doomsday scenarios of climate change): About 0.5 to 3.3% of dissolved oxygen had been lost between 1970 and 2010 which has led to the rapid expansion of Oxygen Minimum Zones ("dead zones"). Acidity has increased by 30 to 40% and at a clearly accelerating rate. That's 10 times faster acidification than at any other point in the last 300 million years
  - they estimated oceans had absorbed about 90% of "excess global heat" and their ability to absorb is slowing rapidly
  - up to 99% of permafrost could melt by 2100. Permafrost melting was completely unanalyzed in the 1.5C report due to lack of good data

Forests on the other hand have shown surprising resilience. And, if they survive the impending destabilization, they might in the long term (I mean looong term) even benefit from increased CO2 concentrations as they historically have.

PS the IPCC reports are more accessible than people realize. Even the technical reports are often not much more difficult to parse than the policymaker reports meant for the general public. I'd quite recommend giving them a try

https://science.nasa.gov/earth/explore/the-ocean-and-climate...

Has a pretty good overview. But in brief: Changes in biosphere due to heat and decreasing pH balance due to increased CO2 can leave species unable to adapt fast enough causing deaths.

wholesale destruction of almost every biome. Overfishing (where 'fish' means anything that swims) with methods that destroy both the fish and the ecosystem-- have you ever considered what a trawler net destroys as it is dragged over the bottom of the seabed?

And now deep-sea mining.