I would hazard a guess that most people are complacent for two reasons: a) lack of actual buy-in from the public and b) far too many conflated issues under the "environmental catastrophe" umbrella, which makes it seem like it's an impossibly huge task.
a) will change over time. Buy-in needs convincing arguments, especially when we are all being asked to significantly change our lifestyles towards less comfort and potentially more work. But, of course, as we witness the actual effects on a larger scale and once we become directy affected, buy-in changes. Let's face it, the question of "what's in it for me" is naturally built into our survival instincts.
b) is much easier to tackle (yes, it actually is). Huge issues need to be broken down into their smaller chunks. It is not one massive environmental disaster, it is an accumulation of many issues, which will result in one.
Non-biodegradable materials.
Mostly, that means plastics. There is a place for them in our lives (for example, when containing highly corrosive substances), but it is the over-use, which causes the issues. Will become far less of an issue over time once our crude oil reserves are sufficiently depleted, but in the meantime we can all do our bit through large-scale refusal to buy unnecessary plastic.
By that I mean switching to anything biodegradable over plastic, any time it is feasible. How feasible that is will be determined by one's ease of access (which is cosumer-driven, even environmental consciousness is increasingly being made into profitable business) and capital, or lack thereof. The latter of which is where buy-in is necessary and maybe making minimalism vogue again.
Reduce, re-use, recyle, in that order, needs to be imprinted into our subconscious. The former can be influenced financially if need be - I don't need my 30th pair of heels and would have far fewer shoes if I didn't have the money to buy them. Yes, it does hit the poorest of society the hardest, but so will a change in resources. Of course, we can hope for buy-in, but that will take far longer to take effect.
Re-using and recycling relies on other skills, which we need to educate our children (and ourselves) for. The Right to Repair bill is a great step in the right direction, but useless if people don't know how to - we need to bring back metal- and woodwork, textiles, arts and crafts, cookery and science at the heart of our education system, rather than focusing on academia, which large swathes of the population cannot access anyway.
The greenhouse effect.
What always astonishes me is how little people understand about greenhouse gases. We focus on carbon dioxide while failing to mention that water vapour is a far bigger issue (and that other gases, such as methane, oxygen and nitrogen are also greenhouse gases - and, to an extent, necessary ones, too). A PP mentioned that a 0.1 degree increase in temperature is insignificant, but fails to see that even a tiny increase will result in greater absorbence of carbon dioxide into the oceans, in itself a concern more evaporation from our oceans, therefore resulting in an ever-accelerating greenhouse effect.
Perhaps it's because we can directly influence our carbon dioxide and methane emissions, as opposed to water vapour, but as has been pointed out on a few threads, any volcano eruption is going to undo our efforts to curb this - therefore, our focus should be less on our emissions as such and more on direct absorbtion, such as re-planting forests.
Decreasing biodiversity
Now this is where the shit really hits the fan. Biology is always seen as the softer of the three main sciences, but it is also where we have the least knowledge.
Ecosystems are finely balanced - they can shift and re-adapt over time, if left to their own devices. And this is where the issue is: while we focus on saving cute panda bears (which really should have become naturally extinct by now) we fail to see that killing off yet another ant species has a far greater effect on our ecosystem. While trying to save fluffy Siberian tigers (which are far less fluffy in real life when they try to bite your face off) we squash a few spiders in our path, which actually reduce our ability to cope with other insect pests. We treat soil to reduce soil bacteria and fungi to save our crops and make our gardens look pretty, but do not see that they actually re-introduce nutrients into our soil, which we then have to artificially recreate through Haber, then mis-judge how much fertiliser is actually needed and kill off a couple of sea life creatures in the process, because we don't realise what eutrophication is.
What we need to do instead of trying to interfere (intelligent as we are, we will never fully understand the complexities of biological ecosystems - far too many life forms and interactions involved) is actually create more large nature reserves, which are left to their own devices. Not re-plant lots of trees of the same species and congratulate ourselves on reducing biodiversity further in the process by creating monohabitats.
I know I am preaching to the choir here, but education is needed in so many directions and a far bigger importance should be placed on scientific education if we want to survive as a species. Then maybe buy-in will come about far quicker, too.
/rant