I'd like to chime in on a few things:
cini care a lot about these issues but hothouse feels like such a buzzword
It's a buzzword I haven't really seen used before, but it's just another name for an already heavily considered possibility. The problem (in regards to world heating) is not so much pollution like plastic in the oceans, but rather, the release of greenhouse gases into the air. Earth used to be a "hothouse" as described. There was far more carbon dioxide in the air, and as a result, earth's atmosphere trapped more heat. As plants perform photosynthesis, they take carbon dioxide from the air and trap carbon, releasing oxygen. Then they die, and that carbon is trapped underground. After hundreds of millions of years, some is trapped in the form of coal and oil... Which happens to make great fuel. Burning fossil fuels is essentially undoing hundreds of millions of years worth of natural carbon sequestering. Other effects, like polar ice caps melting, reinforce the trend- blue ocean absorbs more light than white ice, thus heating the earth more. Processes like these are what could in theory make the warming irreversible (at least within a fathomable time frame). Without a good solution, the planet can go back to the old "hothouse" it used to be. This process would be (and already has been) harmful to civilization, as well as harmful to existing species of plants and animals.
Climate change won't directly end civilization, but if it continues in its current trend it will cause untold harm, and future generations may look back and wonder why we were so selfish and irresponsible.
variousdiscussion about aggregate impact of individual actions, in comparison to large scale corporate and governmental actions
The big picture is that humans like material things, comfort, and ease of life. This is all well and good, but currently the way we as a society provide that isn't sustainable in the long term. We can through widespread and painful regulation force changes in how we produce those things (ie more renewable energy, less wasteful packaging, etc etc) and we can make sacrifices in our lives (use public transportation more rather than driving, etc etc) to reduce what we use. The current reality isn't conducive to either of those types of change happening enough, especially as more of the world advances and consumes more. We'll probably continue down this path until the whole world really starts "feeling the heat", pun intended. If we're determined, and lucky, enough policy change and lifestyle changes will be made to change this trend- but it doesn't seem likely right now.