Peak oil is a major (and justified) concern that I have come across more and more as of late. For those who are not familiar with this, peak oil is the point where production of petroleum peaks and begins to steadily decline, eventually leading to a scenario where the energy required to extract petroleum is greater than the energy content in the extracted petroleum -- at this point, continuing to produce oil simply drains energy reserves even faster (1). Fortunately, due wholly to the scientific and engineering capabilities of humanity, this scenario need never manifest.
The US is, by far, the biggest consumer of petroleum -- we consume almost as much petroleum as the next 5 nations and 40% more than the nations of the European Union (2). Most petroleum (as well as other fossil fuels) are used in the transportation, electricity-generation, and petrochemical industries -- other major uses of petroleum are in the manufacturing industry to power heavy machinery and as fuel for residential and commercial heating systems, however these uses are easily replaced by machines and heating systems that operate on electricity (3)(4)(5). Thus it is my opinion that the main uses of fossil fuels are as fuel for transportation (there are currently far too many cars, trucks, bikes, trains, planes, ships, etc. to easily or quickly replace with electric vehicles (6), so I do not exclude it as I do the manufacturing and heating uses), generating electricity by heating water to produce steam, and processing and refining into various hydrocarbons to produce various materials such as plastics, synthetic rubbers, paints and dyes, etc.
Generating electricity by burning fossil fuels has been obsolete and unnecessary for years. The most abundant sources of clean, renewable energy are solar, geothermal, and wind, with ~35,000*, ~1,400, and ~15 times the amount of electricity consumed globally in 2008, respectively (7)(8)(9). A global, decentralized, load-bearing energy grid emphasizing these three sources -- as well as hydro and fusion, when it becomes viable -- while simultaneously continuing the development of energy-efficient electrical systems will result in a society that produces far more electricity than it could possibly use and whose main problem is figuring out how to store the excess.20
Transportation is also an issue that is easily solved, though solving this would require a radical restructuring of public intra- and inter-city transit systems as well as significant reduction in the demand for privately-owned transportation. Fortunately, the most promising systems, ET3 and Skytran (10)(11)(12), are both fully-automated, electrically-driven, hyper-efficient, fast, less expensive to install and operate than any existing mass-transit system, including highways, and carry the potential of allowing any individual to travel to any place on Earth that has access to these systems at no or negligible cost to the passenger, greatly reducing the need -- and demand -- of inefficient and polluting private transportation.
The final aspect to cover is the petrochemical industry -- fortunately this, too, has been conquered. Hydrocarbons -- molecules based wholly or mostly on hydrogen and carbon atoms -- are critical to the production of the most ubiquitous materials currently used such as plastic, rubber, paints, adhesives, asphalt, etc. and, until we discover or develop new materials to replace the ones currently in-use, will remain critical to the production of these materials (13)(14)(15)(16)(17). Hydrocarbons have traditionally been produced by refining and processing fossil fuels in energy-intensive facilities. However, a new method developed by the Huber Biofuel Research Group in Dec. 2010 would allow the production of every synthetic hydrocarbon currently used in the petrochemical industry "with no infrastructure changes required" (18)(19).
While most concerned with the prospect of peak oil argue that the global peak has either already passed or is quickly-approaching (20)(21)(22)(23), the technologies that exist today, if utilized properly and to their full potential, will completely eliminate any threat the global peak oil scenario poses to humanity.
* solar energy available calculated by multiplying the Solar Constant (1.366kW/m^2) by the Earth's cross-section (Earth's mean radius squared times pi -- 127.45 * 10^12 m^2) over a period of 1 year.