... we are now living in a phase of history which is destined never to be repeated. For the fifth of the world's population that lives in regions of machine culture it is a period of unprecedented abundance. And most of us who are a part of that fortunate one-fifth are so enamored with the achievements of the last century and with the abundance which has been created that we believe the pace of achievement will continue uninterrupted in the future. However, only a cursory investigation of the present position of machine civilization is needed to uncover the fact that it is indeed in a very precarious position. A cosmic gambler. looking at us from afar, would in all likelihood give substantial odds in favor of the probability that it will soon disappear, never again to come into existence.

.....within a period of time which is very short compared with the total span of human history, supplies of fossil fuels will almost certainly be exhausted.  This loss will make man completely dependent upon water power, atomic energy, and solar energy-including that available by burning vegetation-for driving his machines. There are no fundamental physical laws which prevent such a transition and it is quite possible that society will be able to make the change smoothly. But it is a transition that will happen only once during the lifetime of the human species. We are quickly approaching the point where, if machine civilization should, because of some catastrophe, stop functioning, it will probably never again come into existence...

As our dependence shifts to such resources as low-grade ores, rock, seawater, and the sun, the conversion of energy into useful work will require ever more intricate technical activity, which would be impossible in the absence of a variety of complex machines and their products-all of which are the result of our intricate industrial civilization, and which would be impossible without it. Thus, if a machine civilization were to stop functioning as the result of some catastrophe, it is difficult to see how man would be able to start along the path of industrialization with the resources that would then be available to him...

Our present industrialization, itself the result of a combination of no longer existent circumstances, is the only foundation on which it seems possible that a future civilization capable of utilizing the vast resources of energy now hidden in rocks and seawater. and unutilized in the sun, can be built. If this foundation is destroyed, in all probability the human race has "had it." Perhaps there is possible a sort of halfway station, in which retrogression stops short of a complete extinction of civilization, but even this is not pleasant to contemplate.

Once a machine civilization has been in operation for some time, the lives of the people within the society become dependent upon the machines. The vast interlocking industrial network provides them with food, vaccines, antibiotics, and hospitals. If such a population should suddenly be deprived of a substantial fraction of its machines and forced to revert to an agrarian society, the resultant havoc would be enormous. Indeed, it is quite possible that a society within which there has been little natural selection based upon disease resistance for several generations, a society in which the people have come to depend increasingly upon surgery for repairs during early life ...such a society could easily become extinct in a relatively short time following the disruption of the machine network.
Harrison Brown, The Challenge of Man’s Future 
There must come a day when humans cannot use fossil fuels, uranium or metals, the ultimate limit for fuels being where the amount of energy required to extract them exceeds the amount of energy they provide, i.e. when the net energy ratio falls below one. At that time, if the human species lasts long enough to bring it about, we shall have to make do without them; and to the extent that technology still exists, it will have to be created out of and work upon renewables such as wood (fire, and rubber and cellulose-based plastics) and other vegetable matter (food and drugs), the skins, bones and sinews of animals, and such non-renewables as exist in a virtually limitless quantity (such as salt, and sand to make glass). In this sense, then, We will have to revert to the early Stone Age, i.e. to the human situation before we began mining metals some 6000 years ago. These are the preconditions for a sustainable human society and a steady-state economy, and when one speaks of sustainable development, such development must either be towards this state, or presuppose its existence.
Craig Dilworth, Too Smart for our Own Good
Before discussing what we can do about the precarious situation in which we now find ourselves, it is essential that we ask what we wish of life beyond the primitive, narrow, and unsatisfactory goal of simple survival.  Survial for what? What do we want to be? If we had the power and could use it differently, what would our goals be?
Harrison Brown, The Human Future Revisited
Then what is the answer---Not to be deluded by dreams.
To know that great civilizations have broken down into violence,
  and the tyrants come, many times before.
When open violence appears, to avoid it with honor or choose
  the least ugly faction; these evils are essential.
To keep one's own integrity, be merciful and uncorrupted and
  not wish for evil; and not be duped
By dreams of universal justice or happiness. These dreams will
  not be fulfilled.
To know this, and know that however ugly the parts appear
  the whole remains beautiful. A severed hand
Is an ugly thing, and man dissevered from the earth and stars
  and history ... for contemplation or in fact ...
Often appears atrociously ugly. Integrity is wholeness,
  the greatest beauty is
Organic wholeness, the wholeness of life and things, the divine
  beauty of the universe. Love that, not man
Apart from that, or else you will share man's pitiful confusions,
  or drown in despair when his days darken. 
 Robison Jeffers, "The Answer"
Your place in the scheme of nature may be large or small, depending on how you perform during your short life. Whether or not your existence will be significant to the future is a matter which you can decide at this crucial time. The strata of the earth are full of the remains of biological dead-ends -- of organic forms that perished instead of advancing into the patterns of the future.
Wilton Ivie, Man and the Nature of Things
In the long run saving yourself requires saving the whole world... We don't know what's coming. We do know we're in it together.
Stewart Brand, The Clock of the Long Now