If a civilization outstrips the available resources and does not manage to stop growing, it will use up its reserves (raw materials, ecosystems, biodiversity, land, water, the ability of the environment (including the atmosphere) to take up its waste products, sources of energy) and then it will be too big for the resources that could be available in a sustainable way. It will then collapse, i.e. it will be destroyed by the entropy accumulating inside it, and fall apart. This can be compared to the starvation of an organism. For our industrial, technical civilization, this end is gradually appearing above the horizon. I don’t know when the final collapse is going to happen; I think it is a matter of a few decades, maybe even a hundred years but hardly more. I may not experience it myself (I am in my 50s now) but it will probably happen during the lifetime of the generation of our children or grandchildren, during the lifetime of people who are now young.
This collapse will not only cause the death of billions of people, part of it is the mass extinction of other life forms as well that we are causing by using these life forms as resources, by polluting them and by competing with them for resources like land and water. This is happening already and we will continue witnessing the accelerating destruction of ecosystems and biodiversity in our lifetime.
I don not know exactly how the final collaps is going to happen. I would expect an increase in “natural” disasters like droughts, storms and floods over the next decades (this is happening already, but we haven’t seen anything yet). I would expect deforestation, soil desctruction, desertification, land erosion, destruction of groundwater reserves and the destruction of aquatic ecosystems. I would expect increasing numbers of cases of unrest and of wars and large and ever increasing numbers of refugees. I would expect deepening economic crises and turmoil. Eventually, in more and more places, technical civilization is going to break down. When the electricity systems collapse, supplies of water, fuel and food will also collapse. Those of us who depend on these systems are going to die except for a few. Those of us living in poorer areas might survive this collapse, but many on these areas are hardest hit by environmental degradation. There are estimates that the population of the world will peak at about 12 billion people in a few decades. After the collapse, I would expect less than a billion to remain, on a devastated planet.
Our system of living is doomed. It is not sustainable. If something is not sustainable, it will not be sustained. That is the simple meaning of the word. There are only two possibilities then: either, the system will be transformed into a sustainable one or it will collapse. As long as most people are only trying to become as rich as possible, i.e. to increase their consumption of resources as much as they can (because that is the actual meaning of the word “rich”), and as long as politicians still think or proclaim that growth is the solution of problems, instead of the problem, everything points towards collapse.
Civilizations are inherently unstable. For example, while the societies of our hunter-gatherer ancestors seem to have been relatively stable and existed for tens of thousands of years, there was a slow growth leading to crises. They spread all over the world and the destruction began already back then, with the extinction of a number of large animals probably caused by our ancestors. Once the climatic conditions became stable enough, agriculture was independently invented about a dozen times. Writing was invented at least 5 times. And had the industrial revolution not taken place in 18th century Britain, it would have happened somewhere else. This course of events was, it seems, almost inevitable.
It is likely that wherever in the universe civilizations, i.e. creative dissipative systems, occur, they will start growing. The question is: will this always lead to a catastrophic collapse in the end, destroying the civilization and leading to a mass extinction of other life forms? Is the destructive development we are witnessing at the moment inevitable?
(The first picture, from http://commons.wikimedia.org/wiki/File:Pripyat_CentralSquare.jpg, showing a view of the abandoned city of Prypat, gives an impression how cities might look after the collapse. In some places, we have to expect violent events to take place during the collapse. This kind of events is exemplified here by the second picture, from http://commons.wikimedia.org/wiki/File:Destruction_in_Homs_(4).jpg, showing a view of the destroyed city of Homs.)