Sustainable development refers to the practice of adjusting our use of resources in such a way that the resources will still be available to future generations. For some resources this means reducing the amount we use, as with water and woodlands. In many cases we are 'using' resources in ways that are not obvious; we are using the air to dump waste in, reducing its purity, but not its quantity. Thus we must also look at keeping our resources healthy, to keep the Earth in a condition that is pleasant for future generations to live on.
Sustainable development is very dependent on the environment. Our most important resources, water, food, and energy, are all linked to the environment, and can all be affected in dramatic ways by changes in the environment. Cutting down a rainforest destroys the watershed, making a river seasonal rather than continuously flowing. Grazing too many animals on a piece of land kills the scrub grasses, allowing the top soil to blow away in the wind, and changing useful grazing land into a desert. Burning too much fuel spikes the carbon levels in the atmosphere, causing the Earth to warm, ice caps to melt, and sea levels to rise. All of these reduce the resources that are available to us.
The Economics of Sustainable Development.
The writeups above bring up a common misconception about sustainable development: 'because we clearly need to consume less to become sustainable, sustainable development measures must harm the economy.' This is supported by the 'off-the-grid' mentality we hear from many environmentalists, and seems to be common sense. But it stands in stark contrast to most realistic sustainable development plans, which require that people buy into new technologies, rather than simply discard the old ones. If there aren't strong markets selling new, environmentally friendly goods, there is no way to feed, house, and care for the 7 billion humans we expect to have by 2010.
We do not need to consume less. We need to consume different. As a planet, we are critically short on fresh water, soil nutrients, wood, hydrogen, elephants, and fossil fuels. In the coming decades we will find ourselves short on many other things, quite possibly including food. This is really bad for us humans, but even taking all these shortages as fact, we cannot make any real predictions on the state of the economy in the coming years or decades. The economy really doesn't care what we can or cannot buy, all that matters to it is that we buy something.
There are different ways to illustrate this idea in practice. It may help to think of a new market, potentially worth trillions of dollars, the carbon credits market. This is a new market that sells... a planted tree? If there is a high demand for saving the Earth, industries based on organic farming, ecotourism, and yes, planting trees can both stimulate the economy and provide sustainable development.
Of course, basing your hopes of the future on the altruism of mankind is really a very bad idea. You might instead focus on the people spending money on iGoods. Offices are going paperless, fewer paper books, magazines, and newspapers are being published each year, mp3s are killing off the CD industry, and people who once went out cruising the strip are now staying home and playing MMORPGs. Technology is constantly making our old way of life obsolete, replacing material goods with a series of 1s and 0s, and this is often a good thing. When humans are finally uploaded, the Earth will be free.
You might even envision the upcoming crunch as a wartime economy, where employment is at an all time high, everyone is motivated to make sacrifices for the good of our cause, and profits are high. This could happen, and is an example of how sustainable development and a healthy economy could go hand in hand in the short term, but it is isn't actually very likely.
In reality, we are relying on all three of these, in combination and in addition to other factors, to help pull us through the coming decades. We hope that we can telecommute to an Earth-friendly job, eat organic soy beans, and bask in the glory of our own personal rainforest (out back, behind the biosphere, see?) Whether or not this will be achieved before the end arrives is still an open question. But it very well could happen.
Sustainable Development and Other People
Tens of thousands of people are already starving every day in Third World countries. It is clearly not that big a deal to the average American or Australian if countries like Rwanda, Sudan, and Burma are having trouble keeping themselves fed. (Sadly, I am not being sarcastic, merely stating a observed fact). One resource that is generally a focus of sustainable development plans is the elusive resource of 'human rights'. It is generally assumed that as long as we are sorting out the world we should go ahead and get all world governments to treat their citizens in a humane and ethical manner. While the First World doesn't technically need Somalia to be healthy and happy in order to obtain its own sustainable development, it's always a goal that Somalia and the like should be provided for.
This puts sustainable development programs in an awkward position; do you focus on the thousands of people who are dying daily from easily preventable causes, or do you focus on that ass with the air-conditioned Hummer? Both are critical, but maybe the starving baby should get precedence... Unfortunately, there will always be starving babies, and trying to save them all will only distract you from the real task.
As peak oil comes and we are forced to find other energy sources, one of the first sources that many countries will look to (and are already planning on) are ethanol and biodiesel. Unfortunately, these are made from grain alcohol and vegetable oils, meaning that our farmland will have to be used to produce both food and fuel. This will in effect tie food prices to fuel prices, and since there is not enough viable cropland to provide for our current levels of consumption in both of these areas, prices for both will be very high.
If the price of food on the world market goes higher than it is now, even more people will starve. Hunger leads to rioting and failed states; it also leads to refugees, worsening the problem in the wealthier countries. Matters are even worse if things come to outright war. And Pakistan, a country at high risk for becoming a failed state, already has nuclear weapons.
We currently have the technology available that would prevent the 'peak food' catastrophe from happening in First World countries. These countries will surely adopt these technologies, although they may be slow in doing so. But the world economy is very much threatened by countries like China and India, who are major players in the world market and are also facing major problems in the near future, as their food production falls below the level needed to feed their growing populations (even without using food crops for fuel). If major grain exporters (like America) were to stop exporting, the consequences would be dire.
This is an acute emergency, not a chronic one. We will have to make some major changes or face the consequences, but these changes will start moving us in the direction of a long-term sustainable economy/ecology. We are now passing peak oil, but we could reverse that by simply using less oil. It would not be a long term (i.e. sustainable) solution to our fuel problems, but we aren't in a position to scoff at short-term solutions at the moment. Making very fuel efficient cars will slow the coming crises, even if they are burning fossil fuels. Buying more energy efficient appliances will help stabilize carbon emissions, even while we still rely on carbon producing energy sources. Taking shorter showers will slow the draining of aquifers, even if you are living off of a fossil aquifer. Changes like these can give us time to adapt at a more comfortable speed, and put off disaster until we find better ways of preventing it.