Burning buried sunlight

From Tim Flannery's The Weather Makers:

Jeffrey Dukes has proferred an equation as to how humans are supplying the demand [for fossil fuels]. He starts with the observation that all the carbon and hydrogren in fossil fuels was gaterhed together through the power of sunlight, captured by long-ago plants. By calculating the efficiency with which plant matter is preserved in sediment, the efficiency with which it is converted into fossil fuels, and the efficiency with which we are able to retrieve that fuel, Dukes has concluded that approximately 100 tonnes of ancient plant life is required to create four litres of petrol.

Given the vast amount of sunlight needed to grow 100 tonnes of plant matter, and the prodigious rate at which we are using petrol, coal and gas, it should come as no surprise that over each year of our industrial age, humans have required several centuries' worth of ancient sunlight to keep the economy going. The figure for 1997--around 422 years of fossil sunlight--was typical.

And from the original University of Utah press release on this study:

In another calculation, Dukes determined that “the amount of plants that went into the fossil fuels we burned since the Industrial Revolution began [in 1751] is equal to all the plants grown on Earth over 13,300 years.”

That's all the plant matter--including the "vast amounts of microscopic plant life in the oceans."