If our civilisation collapses, how is the collapse likely to start?

Actually, none of what you have mentioned is a real threat to continuity of our civilization. 1. End of fossil fuels. Since we have discovered nuclear energy, we know that we are safe. Breeding reactors can supply us with enough power to replace fossil fuels for thousands of years to come. And in fact they are not more expensive than petroleum and coal, if we count all capital investments (offshore platforms, refineries, pipelines etc.). If we switch completely to nuclear power we might experience more nuclear disasters, but it will not end our civilization. 2. Climate change. Changing climate might cause some currently populated areas to become inhabitable, but at the same time some other currently harsh areas will become more friendly. People might have to migrate, but they will adapt. Such change might be huge problem for some countries - some might even disappear, some might start a wars, some new might emerge. But it is nothing new from the point of view of the whole civilization. 3. World food production Food production has not peaked and it is not expected to peak soon. Green revolution made Europe and North America produce much more food than they need. Similar process is happening now in Asia, and number of starving people in Asia decreased four times in the last 20 years. Only Africa is still waiting for its green revolution, and hunger is still a large problem there. But costs of crops modification are dropping, and following years might bring changes also there. 4. Population growth has stopped This is actually great news. Exponential population growth would eventually lead to catastrophe. If population is stabilizing, then we are on the road to stable future. If number of people between age 20 and 60 (working) will be the same as the number of people below age 20 and over 60 combined (not working), then each one of us will have to work only to support one additional person. This is entirely doable. 5. Increasing gap between rich and poor This gap might lead to demise of democracy, riots and civil wars, but again - it is not an end of civilization. Emerging caste of billionaires might become new nobility, and society might return to feudalism and caste system. We might not like it, but democracy was tried before (in ancient Greece), and was forgotten. Why do we assume that this time it will remain forever? Future will not be completely dark and will not be completely bright. We will face many problems and there will be many changes. But there is really no reason to expect that civilization is ending.

http://en.wikipedia.org/wiki/Risks_to_civilization,_humans,_and_planet_Earth Wikipedia actually has a whole article about this.  Here are some risks they point out:  Human-caused self-destruction (nuclear war and other WMD's, biological/chemical weapons, nanotechnology, some kind of experimental disaster, etc.), rebellious Artificial Intelligence, climate change (either manmade, natural, or both), a global pandemic, overpopulation, a massive volcano, alien invasion, a huge meteor (or other cosmic threats), peak oil, and antibiotic resistance.  If you don't think that any of these are actual threats, please read the relevant Wikipedia section before commenting, since they aren't actually my arguments. Personally, I think a civilizational collapse in the medium-term is likely to come from the mix of wars, global warming, overpopulation, famine, and resource wars (particularly related to energy).  Of course, all of these factors feed into each other.  Wars could cause the use of weapons of mass destruction, but they also hurt our ability to respond to civilizational threats and challenges.  Global warming is likely to cause massive hunger, displacement, and turmoil; while in the long-term we may adapt, we can't just write off how much damage global warming could do to civilization.  Overpopulation and famine obviously go together, leading to turmoil, and, potentially, massive amounts of destabilizing migrations.  A lack of resources could be the greatest problem of them all, since the world running out of power (and other resources) could absolutely lead to huge amounts of domestic and international turmoil.  It's possible we'll find some way to get around the inevitable depletion of fossil fuels, but it's not inevitable. Of course, just because that can happen doesn't mean it will happen, but I'm merely answering the question.

I think the premise of the question is presumptuous, and merely reiterates a set of beliefs and attitudes that are all too common today. Namely, they define a worldview that sees all human beings as characterized by greed, stupidity and short-term thinking, and predict that we are going to bring about our own extinction - and that we will deserve that fate, collectively, as a species. I do not subscribe to this view, because it is overly pessimistic, defeatist and based on a kind of misanthropic self-loathing of our species that, frankly, I find silly and contemptible in the extreme. Yes, we will face some great difficulties in this century, but if we tackle them with optimism, self-belief and ingenuity, we can surmount them, as the human race has surmounted numerous challenges it has faced in the past. And the society that will emerge from this will be stronger, wiser, more just and empowered, and capable of achieving things that all the self-loathing misanthropes of the doom-and-gloom movement (such as, I suspect, the author of this question) can scarcely imagine. But then, that has always been their main problem - a lack of imagination. James Martin's excellent book 'The Meaning of the 21st Century' presents a clear-sighted and positive philosophy for meeting the challenges we will face in this century and creating a bright and compelling future for our species. He has also produced a film on the same (see below). Please watch it. And then the next time you meet a doom-monger, set them right on a few points.... http://www.jamesmartin.com/film/watch.cfm

I don't believe your premise (see Strojnowski above), but I'll play along. When it does start, we will be looking the wrong way. Maybe we will be concerned about some other issue that ironically causes a collapse due to ignoring the real danger. When it does start, we will think it's a blip. Things have been improving for a long time, with blips, so well act as if it's just a blip.