Even if the future is a quieter place, with more restricted movement and lower standards of material living, there will still be a requirement for resources and synthetic materials, along with the whole host of chemicals used in modern industry. While there will be a much-reduced demand for fertilizers and pesticides, with most requirements in that area taken up by natural substances, there will be some required for particularly fragile or valuable crops.
Many of the chemicals in modern usage are derived from petroleum stock, whether oil or natural gas. These materials can be replaced in chemical synthesis by naturally-derived materials. Methane from biodigesters can provide some of what is required, but the quantities will likely be insufficient. Whole crops might be raised on marginal land to provide feedstock for the digesters to produce methane.
Another source could come from deep-water integrated farms, and “fields” of rapidly-growing kelp to provide the grist for the bio-digester mill. Another method uses large algae-based bio-reactors, which can theoretically produce up to 400,000 cubic metres of methane per hectare per year.
Much of the world’s plastic requirements will be met from both the small oil reserves remaining, and from bio-plastic materials. Bio-plastics can be derived from a variety of sources, but corn is the most useful one. As meat consumption declines, the acreage that was devoted to cattle feed will be converted to bio-plastic production, along with the high-yield bio-fuels required by the aerospace industries and the military.
Many common items will be made of easily recycled metal and spun glass. Light, strong, and readily available, glass will be an important material for many household uses. Metals will be as well, though increasing amounts of metals will be mined from landfills and abandoned buildings as readily-accessible surface deposits become scarcer.
When commercially-necessary, important metals can be obtained from deep-mine sources, though such mines will require largely automated systems and heavy thermal protection for the few actual human beings in the mines. The rest of the miners will be remote-operated drones, controlled from comfortable surface locations. The few workers on site are for emergencies only, not as actual miners.
Other metals can be obtained from seawater, and are a natural byproduct of flash desalination, which will become much more commonly used as surface fresh water dries up in many areas.
Any of these recovery methods require reliable power, which would be either generated on site from small wind or solar installations, or, if the power requirements are high enough, from small-scale nuclear reactors. In any case, power generation and energy resources will be decentralized.
One dream of futurists is the idea of the extraction of resources from space, from the Moon or asteroids. This would require much cheaper surface-to-orbit transportation, and could only likely be fulfilled by something like a space elevator. The actual mining and extraction would have to be done by robots, as the support costs for human space-flight would be too high in a resource-depleted world.
Despite all of these new methods of material production, the stark fact remains that serious changes in material lifestyles were necessary to ensure survival. The use of scarce bio-fuels for aerospace put jet air travel out of the reach of most, as an example, while expensive plastics and structural materials mean that personal items are more often repaired than replaced. Conservation and reduced demands are the key to life in the future.