[ Oil is solar power brewed for free by mother nature over hundreds of millions of years. Nothing on earth, not even plants, can scale up to replace the oil we burn — one gallon of oil was once 196,000 pounds of plants. The amount of fossil fuels burned in 1997 is 400 times all the plant matter that grows in the world in a year, even including the microscopic plants in the ocean.
Alice Friedemann www.energyskeptic.com author of “When Trucks Stop Running: Energy and the Future of Transportation”, 2015, Springer]
Dukes, J.S. 2003. Burning buried sunshine: human consumption of ancient solar energy. Climatic Change, 61(1-2): 31-44.
Bad Mileage: 98 tons of plants per gallon. Study shows vast amounts of ‘buried sunshine’ needed to fuel society.
A staggering 98 tons of prehistoric, buried plant material – that’s 196,000 pounds – is required to produce each gallon of gasoline we burn in our cars, SUVs, trucks and other vehicles, according to a study conducted at the University of Utah.
“Can you imagine loading 40 acres worth of wheat – stalks, roots and all – into the tank of your car or SUV every 20 miles?” asks ecologist Jeff Dukes.
But that’s how much ancient plant matter had to be buried millions of years ago and converted by pressure, heat and time into oil to produce one gallon of gas, Dukes concluded.
Dukes also calculated that the amount of fossil fuel burned in a single year – 1997 was used in the study – totals 97 million billion pounds of carbon, which is equivalent to more than 400 times “all the plant matter that grows in the world in a year,” including vast amounts of microscopic plant life in the oceans.
“Every day, people are using the fossil fuel equivalent of all the plant matter that grows on land and in the oceans over the course of a whole year,” he adds.
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.”
Explaining why he conducted the study, Dukes wrote: “Fossil fuel consumption is widely recognized as unsustainable. However, there has been no attempt to calculate the amount of energy that was required to generate fossil fuels, (one way to quantify the ‘unsustainability’ of societal energy use).”
The study is titled “Burning Buried Sunshine: Human Consumption of Ancient Solar Energy.” In it, Dukes conducted numerous calculations to determine how much plant matter buried millions of years ago was required to produce the oil, natural gas and coal consumed by modern society, which obtains 83% of its energy needs from fossil fuels.
“Fossil fuels developed from ancient deposits of organic material, and thus can be thought of as a vast store of solar energy” that was converted into plant matter by photosynthesis, he explains. “Using published biological, geochemical and industrial data, I estimated the amount of photosynthetically fixed and stored [by ancient plants] carbon that was required to form the coal, oil and gas that we are burning today.”
Dukes conducted the study while working as a postdoctoral fellow in biology at the University of Utah. He now works for the Carnegie Institution of Washington’s Department of Global Ecology on the campus of Stanford University in California.
How the calculations were done
To determine how much ancient plant matter it took to eventually produce modern fossil fuels, Dukes calculated how much of the carbon in the original vegetation was lost during each stage of the multiple-step processes that create oil, gas and coal.
He looked at the proportion of fossil fuel reserves derived from different ancient environments: coal that formed when ancient plants rotted in peat swamps; oil from tiny floating plants called phytoplankton that were deposited on ancient seafloors, river deltas and lakebeds; and natural gas from those and other prehistoric environments. Then he examined the efficiency at which prehistoric plants were converted by heat, pressure and time into peat or other carbon-rich sediments.
Next, Dukes analyzed the efficiency with which carbon-rich sediments were converted to coal, oil and natural gas. Then he studied the efficiency of extracting such deposits. During each of the above steps, he based his calculations on previously published studies.
The calculations showed that roughly one-eleventh of the carbon in the plants deposited in peat bogs ends up as coal, and that only one-10,750th of the carbon in plants deposited on ancient seafloors, deltas and lakebeds ends up as oil and natural gas.
Dukes then used these “recovery factors” to estimate how much ancient plant matter was needed to produce a given amount of fossil fuel. Dukes considers his calculations good estimates based on available data, but says that because fossil fuels were formed under a wide range of environmental conditions, each estimate is subject to a wide range of uncertainty.
Plants in your tank?
Dukes calculated ancient plant matter needed for a gallon of gasoline in metric units:
- One gallon of oil weighs 3.26 kilograms. A gallon of oil produces up to 0.67 gallons of gasoline. So 3.26 kilograms for a gallon of oil divided by 0.67 gallons means that at least 4.87 kilograms of oil are needed to make a gallon of gasoline.
- Oil is 85 percent carbon, so 0.85 times 4.87 kilograms equals 4.14 kilograms of carbon in the oil used to make a gallon of gasoline.
- Since only about one-10,750th of the original carbon in ancient plant material actually ends up as oil, multiply 4.14 kilograms by 10,750 to get roughly 44,500 kilograms of carbon in ancient plant matter to make a gallon of gas.
- About half of plant matter is carbon, so double the 44,500 kilograms to get 89,000 kilograms – or 89 metric tons – of ancient plant matter to make a gallon of gas. In U.S. units, that is equal to a bit more than 196,000 pounds or 98 tons.
Dukes made similar calculations for oil, natural gas and coal to determine that it took 44 million billion kilograms (97 million billion pounds) of carbon in ancient plant matter to produce all the fossil fuel used in 1997. That includes 29 million billion kilograms of prehistoric plants to produce a year’s worth of oil (including gasoline), almost 15 million billion kilograms of buried plant matter to make all the natural gas used in 1997, and 27,000 billion kilograms of dead plants to produce all the coal used in the same year.
“It took an incredible amount of plant matter to generate the fossil fuels we are using today,” says Dukes. “The new contribution of this research is to enable us to picture just how inefficient and unsustainable fossil fuels are – inefficient in terms of the conversion of the original solar energy to fossil fuels. Fortunately, it is much more efficient to use modern energy sources like wind and solar. As the reasons keep piling up to switch away from fossil fuels, it is important that we develop these modern power sources as quickly as possible.”
What about modern plant biomass?
Unlike the inefficiency of converting ancient plants to oil, natural gas and coal, modern plant “biomass” can provide energy more efficiently, either by burning it or converting into fuels like ethanol. So Dukes analyzed how much modern plant matter it would take to replace society’s current consumption of fossil fuels.
He began with a United Nations estimate that the total energy content of all coal, oil and natural gas used worldwide in 1997 equaled 315,271 million billion joules (a unit of energy). He divided that by the typical value of heat produced when wood is burned: 20,000 joules per gram of dry wood. The result is that fossil fuel consumption in 1997 equaled the energy in 15.8 trillion kilograms of wood. Dukes multiplied that by 45% – the proportion of carbon in plant material – to calculate that fossil fuel consumption in 1997 equaled the energy in 7.1 trillion kilograms of carbon in plant matter.
Studies have estimated that all land plants today contain 56.4 trillion kilograms of carbon, but only 56% of that is above ground and could be harvested. So excluding roots, land plants thus contain 56% times 56.4, or 31.6 trillion kilograms of carbon.
Dukes then divided the 1997 fossil fuel use equivalent of 7.1 trillion kilograms of carbon in plant matter by 31.6 trillion kilograms now available in plants. He found we would need to harvest 22% of all land plants just to equal the fossil fuel energy used in 1997 – about a 50% increase over the amount of plants now removed or paved over each year.
“Relying totally on biomass for our power – using crop residues and quick-growing forests as fuel sources – would force us to dedicate a huge part of the landscape to growing these fuels,” Dukes says. “It would have major environmental consequences. We would have to choose between our rain forests and our vehicles and appliances. Biomass burning can be part of the solution if we use agricultural wastes, but other technologies have to be a major part of the solution as well – things like wind and solar power.”
University of Utah Public Relations 201 S Presidents Circle, Room 308 Salt Lake City, Utah 84112-9017 (801) 581-6773 fax: 585-3350 www.utah.edu/unews
Calculations illustrate fossil-fuel crisis
Plant-to-oil equations point up unsustainable profligacy.
29 October 2003 BETSY MASON
In 1997 we burned fossil fuels equivalent to 400 times the plant matter produced that year.
If you burned a liter of petrol on the way to work this morning, consider this: it took 23.5 tonnes of ancient, buried plants to produce.
That’s the equivalent of 16,200 square meters of wheat, roots and stalks included. So says new research that aims to raise awareness about the need to change our energy-consumption habits.
The long, slow process that converts plant matter into oil is extremely inefficient, says ecologist Jeff Dukes of Carnegie Institution of Washington, Stanford, who did the calculations. Less than one part in 10,000 of the organic matter becomes oil.
“So much carbon is lost back to the atmosphere through decomposition, it’s only the residues that are turned into fossil fuels,” says Dukes. He warns that less than a tenth of the carbon in plants buried in peat bogs was turned into coal.
In 1997, he points out, we burned fossil fuels equivalent to more than 400 times the amount of plant matter produced on Earth in the same year.
Despite these inefficiencies, fossil fuels created over the past 500 million years have given us a relatively inexpensive fuel source for the past 250 years. “It is fantastic stored free energy from the past, but it’s not sustainable,” Dukes says.
Modern ways to convert biomass into fuels such as ethanol are far more efficient. But it would still take nearly a quarter of all the plants on Earth to replace the fuel used in 1997. That’s 50% more than humans already remove or pave over each year, says Dukes. “Hopefully we’ll use more wind and solar power,” he suggests.
When you start multiplying uncertainties the numbers start to become meaningless. Sandra Neuzil, USGS
It’s a valid point, says geologist Sandra Neuzil of the US Geological Survey in Reston, Virginia, who studies peat decomposition. But she is cautious about the many unknowns in such equations, saying: “When you start multiplying uncertainties the numbers start to become meaningless.”
Dukes acknowledges that his calculations have a large degree of uncertainty, but believes he has captured the essence of the process. “I’m hoping that it will make people think,” he says.