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fossil plants come back to haunt us
Digging around some climate change stuff (for teaching) lead me to this.
It's from 2003, so some of you may have seen it before, but it's an
interesting take on the situation. I have the link to the pdf if anyone
Bad Mileage: 98 Tons of Plants per Gallon
Study Shows Vast Amounts of 'Buried Sunshine' Needed to Fuel Society
October 27, 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
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
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.”
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
“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.”
School of Environmental and Life Sciences (Geology)
University of Newcastle
Callaghan NSW 2308
Tel: +61 2 4921 5404
Fax: + 61 2 4921 6925