, 2003 -- 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, whose study will be published
in the November issue 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 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 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 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 percent –
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 percent of that is above
ground and could be harvested. So excluding roots, land plants
thus contain 56 percent 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 percent of all land plants just to equal the fossil fuel energy
used in 1997 – about a 50 percent 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.”
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