Alternatively phrased, will we run out of sources or sinks?
Before answering the question, it is important to understand why we ask such a question in the first place. It has been a long-standing view of a school of natural philosophy that nature has no intrinsic value, and all value ascribed to it is by humans. In other words, what nature can do for us is the important thing to be considered.
Thus, we take things from nature (say, a rock), convert it to something that is valued by society (say, an iPod) and exchange it for something of equivalent value (say, toilet paper). This process of interaction with other people to determine the value of something and exchanging it for something of equivalent value is what is called the market mechanism. Since exchanging stuff is usually cumbersome, we use a common medium to signify value, and we call this money. Money in itself has no value is indicated that you can find it in so many forms, from solid gold to bits in a database maintained by MasterCard.
To increase the amount of money in circulation, one can do two things: print more money, which makes it lose its value, and this we call inflation (which can happen due to other factors as well), or increase the value of stuff we produce, either by value addition (This instead of this) or simply producing more of it. At the end of the day, the value of an economy (what is called the GDP) reflects the value that society places on the stuff we produce.
Producing things essentially means taking things from a natural source, modifying it to a form useful to humans and then sending it to a sink when its utility is over. Therefore, the throughput of resources from source to sink is what (roughly) determines the size of an economy. The rate of change of this throughput is what is rate of change of GDP (8%, in our case).
A source is a stock of some resource, generated by natural processes, like coal. A sink is what breaks down what we dump into a form that is absorbed by natural processes (like decomposition for organic matter, and oceans for CO2). These processes are not usually under our control, and this puts fundamental physical limits to the throughput of material or equivalently, the size of world GDP. We cannot use a stock of material faster than nature can replenish it, for obvious reasons, neither can we dump stuff ‘somewhere’ faster than natural processes can break it down. These natural processes are known as biogeochemical cycles. If we do change the throughput to greater that what can be sustained, we get effects like eutrophication and global warming.
Many of the early environmentalists stressed on the finite sizes of the sources like coal, oil, metals which made up our economy and said that we will run out of it soon. Unfortunately, the were proved wrong. Newer and larger stocks were found, and all environmental concerns were brushed off as alarmism. However, it is becoming increasingly clear that what ever the size of the sources may be, we may actually be running out of sinks to dump our garbage in. Over and above this, many artificially manufactured substances (especially from chemistry, and nuclear waste) have no known natural sink, and therefore they persist in our ecosystems for large amounts of time, gradually increasing in size of stock. The problem lies in the fact that not many toxicology studies have been done on such chemicals, and we have absolutely no idea how they work in conjunction with each other.
Sources of raw materials have developed over millenia, and will in all likelihood last us atleast another century (in case of fossil fuels). Sinks, however, are usually biological systems which have slow rates of flow. The chain is only as strong as its weakest link, material throughput is usually limited by sink flows. Thus, we find the classic sign of sink overflow everywhere: pollution.
Can humanity conform with natural processes ? Yes. One way is to find technological advances which make it possible to increase value without increasing throughput, which is happening everywhere. The main problems with this approach is how fast will humanity react to physical limits – will it be too little too late ? Another is to reduce consumption, accepting a smaller GDP with more equitable distribution of wealth so that all may live better. The problem with this is that it is politically impossible (well, almost!). Ignoring limits that are put on us will result in devastation, like what is happening to the fisheries of the world today. Hope we can garner enough moral strength to preserve our physical strength.