How much plant material does it take to make a gallon of gasoline?
Well, according to an expert - it takes 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, whose study was published in the November 2003 of the
journal Climatic Change.
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 calcultation, 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 percent 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
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
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
Plants in your tank?
Dukes calculated ancient plant matter needed for a gallon of gasoline in
- 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."
Reprinted from the:
University of Utah Public Relations
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