I look at it this way.
If I knew I or my entire family was going to die in 1 year, would I still cook my own food or eat tasty McDonald, that I have been withholding from myself, nope I would still cook my own food because I really enjoy it and I am not a huge fan of McDonald. Likewise I really enjoy spending time in my garden and I would not try and cut it back, in fact if my thoughts because super morbid, I would unconsciously spend more time in my garden and nature.
To me, Permaculture is about caring for the Earth. So, given the subject of the thread, I don't see a reason to absolve my responsibility to try as best as I can to live in accordance with the Earth's systems. Why, if the end is so dire and near for a human habitable planet, would I not at least try to have as little impact on those non-human beings which must inherit the planet in the wake of our destruction? Earth care, Earth stewardship and connecting our lives as deeply to the Earths systens and cycles as possible must always prevail, regardless of our human centered world potentially going down the drain. The Earth will be around a lot longer than us. We need to always keep that in mind. That's the way I see it anyway.
But I'm not of the opinion that all hope is lost. I believe that we can steer the course of humanity towards a beneficial paradigm. I believe in the fundamental goodness within people and in our unbelievable levels of adaption and perseverance in the face of the seemingly insurmountable. We simply need to change our focus, our direction... we need a different target and to start to aim toward it... pick a direction and head toward it.
Right now, the planetary choices for humanity do indeed look grim. Our current human paradigm, in general is shitty. Change those choices, and we see green horizons, we see healing, we see communities of interaction blossoming in holistic synergistic patterns. It's only a small step away.
I'm also not of (and never have been of) the opinion that there is actually going to be an end point to life on the planet (barring a massive comet strike shattering the sphere). Things will get increasingly harsh if we do not change direction, but our adaptability will preserve some of us into that time at least for a while. We should go down, if we must go down, in a way that shows that some of us tried. And some of the planet's systems will heal, and the Earth will renew in new and previously unimaginable ways, just as it has done before with massive extinction events.
My two cents.
"Never doubt that a small group of thoughtful, committed citizens can change the world; indeed, it's the only thing that ever has."-Margaret Mead "The only thing worse than being blind, is having sight but no vision."-Helen Keller
If I knew that the world would end on Jan 1 2050, I would definitely still do permaculture. I'd rather spend the last 30 years of my life living in a food forest garden, with very little sickness, great quality of life, and peace despite a world gone south. Instead of the stress, sickness, and strife of partying, keeping up with tech and fashion trends, and having a bad conscience for plundering to the end.
I live in a desert and this last year I lost my irrigation water. I still have a well, but will have to let alot of trees die. It made zone 4 much bigger. Looking at my relatives I'm sure everything I've built will be abandoned. Just too much work. That includes my strawbale house. Too small and strange. So I am looking futile straight in the eye.
I've switched to all natives or plants that i hope will survive without me. Started some biscuit root, net leaf hackberry, sagebrush, pinion pines, elder berry, service berry, and misc. wildflowers. I hope to gather and learn much more about native plants. Specifically native food plants. It has stopped being about feeding myself, but more about feeding this piece of land.
A paraphrase from The Lion in Winter (about Henry II, his wife and sons relationships) "When the fall is all you have left, how you fall matters!"
In the end, I will die, my family will die and eventually folks will forget I ever lived. We do what we do for more immediate reasons and, sometimes, just because we think it's right and we won't like what we see in the mirror if we don't act as we think we should.
I have a geology degree. The world has been through way more extreme situations than we are currently facing and things just kind of chug along. Some groups have a harder time, some groups have a better time. Some places become uninhabitable, others become more habitable. I think most of this is hype. You can't sell news or move whatever your agenda is with headlines like "Everythings fine, people are doing ok!" To really get people to do what you want, you need to create great concern or even panic.
Yes, the polar regions are warming. The tropics don't seem to warming much at all. There is quite a bit of evidence that it has been warmer within the last 8,000 years. (Evidence in mostly local, because our models are flawed and we keep finding out what we don't know or didn't take into account. (Did you know that the time when the most land in the British Isles was under the plow appears to have been in the bronze age, then it got cold). If all of the ice melted and sea levels rose, major cities would either be abandoned or we'ld emulate the dutch as long as we could. If ' your world ' is New Orleans or Manhatten the that would be the end. If you include Kansas, Siberia, central Africa etc. things might be ok or even improved (shallow seas extending inland might bring more rainfall). We're finding cities (roman, greek, egyptian) underwater every year or even more often than that (usually due to earthquakes, so pretty quick). They've found neolithic villages on the bottom of the Black Sea. Tough for the locals when the water comes in. Some other guy a few miles inland wakes up and says, 'wow, I have beach front property'. Global warming, at it's worse would be a gradual rise of sea level (way better than a sudden drop from 10 feet above sea level to 30, or even 3 feet below sea level.
Historically, the killer has been mostly the climate getting suddenly colder rather than getting warmer (admittedly, the info is coming mostly from Europe, China and more recently, the US, other areas may be different). If the crops you are used to growing suddenly can't grow in your climate, that's bad. If it's a few degrees warmer, I can mostly still grow my accustomed plants (as long as I have water, which encroaching seas may help), maybe even add to more that my neighbors down south grew.
I think we have many threats to our environment that are more real and more pressing than global warming. I sometimes feel it is being put out there as a red herring to distract people from real, solvable problems. I see our current global society as a house of cards, built higher and higher. Eventually we'll get just the right gust of wind (natural disaster, assassination, war) that will bring it tumbling down. There's an old country song with the line "somebody told us Wall Street fell, but we were so poor that we couldn't tell". The species will still survive (we're kind of like cockroaches), especially the traditional rural poor, and eventually we'll build again, hopefully better this time.
(sorry, I just realized I wandered way off topic)
If I knew everything was going to end in a couple of years, the only thing I think I would do differently is I would retire now, rather than keep going a few years more so my widow will be more comfortable when I eventually check out.