You wrote: If one is going to start writing off hydrocarbons, and assuming no technological miracle comes down the pike, there is no feasible way to maintain the energy requirements of the modern world without a large-scale concentrated power source like nuclear.

Harvestable biomass yields already exceed 30 tons (dry weight) per hectare:
http://www.feasta.org/documents/wells/contents.html?six/doran.html

ALL of this can be converted into oil suitable for refining:
http://www.gasandoil.com/goc/news/ntn42480.htm
The process can convert any carbon-based form, essentially by speeding up the method the earth uses to break down dead plants and animals into petroleum hydrocarbons. Using specific heat, pressure and water, the feedstock's long molecular chains are broken into gas that is recycled to run the plant, water that is returned to municipal water streams and the other products that are sold.

The US has 915,940,000 hectares of land area, nearly one half of it available as agricultural or forested land for biomass production. Taking round numbers, consider 400,000,000 hectare at only ten tons of biomass per year per hectare. This is 4,000,000,000 tons of biomass, which at a conversion rate of 50% into wouild yield some 2,000,000,000 tons of oil, far more than we now consume.

In addition, fuel efficiency can be significantly increased. Hybrid vehicles already exist that get 60 MPG. Not everybody really needs an SUV that get 12 MPG.

These forests need not be chopped down to harvest the biomass. Rather the brush, leaves, needles and other combustables from the forest floor can be collected. In addition, the biomass of food crops is only a small part of the total yield.

http://egov.oregon.gov/ENERGY/RENEW/Biomass/BiomassHome.shtml

By the way, 1 Acre = .4047 hectare

Finally, Sea Solar Power is coming on stream later this year or early next year. This will produce electricity so cheaply that oil and coal will no longer be needed for power production. It can produce electricity so cheaply that it can be sold ANYWHERE on Earth at a lower price than nuclear generated electricity, even when cost of transmission is included.

I have been involved in the technology for some 30 years. I know it quite well. You have my personal word of honor that the power costs of large plants will be shockingly low. I can not reveal the details until all of our 250 patents are in, but it includes many big surprises.