Using Crop Plants to Reduce Atmospheric Carbon Dioxide Levels
Barry J. Micallef
Department of Plant Agriculture, University of Guelph, Guelph, Ontario, N1G 2W1
The burning of fossil fuels and land clearing has caused significant
emissions of carbon dioxide (CO2) globally (Fig. 1A). Correspondingly,
global atmospheric CO2 levels started to rise significantly with the
onset of the Industrial Revolution in the mid 1800’s (Fig. 1B). Since
1958, CO2 levels have been monitored at the Mauna Loa volcano
observatory in Hawaii owing to its lack of vegetation (Fig. 1C).
Of the 6.5 Petagrams of C released per year through the burning of fossil fuels (Fig. 3A),
approximately ½ remains in the atmosphere and the remaining ½ is stored in the oceans and land (Fig.
3B). Much of the fossil-fuel-derived CO2 that is stored in the oceans and land is believed to have been
converted into organic material by photosynthetic organisms including plants, showing that plants play a
natural role in reducing atmospheric CO2 levels.
B
A
A
Carbon dioxide is involved in trapping longwave (low energy) heat
radiation emitted by the earth, and thus it plays a natural role in global
temperature control (Fig. 5). Elevation of the atmospheric CO2 level
increases the potential for adsorption of this heat radiation.
=elevated
ocean
temperature
High energy,
Shortwave radiation
emitted from sun
Low energy,
Longwave heat radiation
emitted from earth
e.g. CO2,
Methane,
Water vapour
Accumulation rate in the atmosphere
B
C
Fig. 1. Global annual emissions of carbon dioxide (1
PgC=1 billion metric tons carbon) due to burning of
fossil fuels and land clearing (A), and changes in
atmospheric CO2 levels between 1000 to 2000 A.D. (B).
The measurements shown in the inset of Fig. 1B (1958
to present) were made at the Mauna Loa volcano
observatory in Hawaii (Mauna Loa volcano is shown in
(C)). Data are taken from Keeling et al. (2002).
Fig. 3. Global stores and flows of carbon shown schematically (also see Fig. 2) (A), and global
accumulation of carbon in the ocean/land and the atmosphere (B). When El Niňo occurs ocean
temperatures rise, and CO2 emissions from the oceans increase (Fig. 3B). Units of Petagrams of Carbon
/Year (PgC yr-1) = 1 billion metric tons. Figures are taken from the Woods Hole Research Center (2007).
It is possible to replace some of our fossil-fuel demand with fuel derived from renewable
plant-derived material (Fig. 4). The total proportion of our energy needs that can be derived
from plants is a matter of debate. If all cropland on earth (~1.5 billion hectares) was used solely
for energy production we would still only supply 50% of our present global energy demand. The
United States has recently developed an initiative to replace 30% of their imported oil with
domestic plant-derived fuels by 2030. Calculations show that this initiative will require ~10
million hectares of farmland, which is equivalent to 0.7% of global cropland area and ~1.2% of
our present global CO2 emissions derived from the burning of fossil fuels. Keep in mind that
replacement of only 1% of our fossil fuel consumption with plant-derived fuel can reduce global
CO2 emissions per year by up to 65 million metric tons (see Figs. 1, 2, & 3).
Fig. 5. Role of atmospheric gases in global temperature
control (i.e. The Greenhouse Effect).
There has been a rise in global temperatures, particularly over the last 30
years (Fig. 6A). This temperature rise correlates with elevated atmospheric
CO2 (Fig. 1). There is still some debate on whether rising CO2 levels are
responsible since CO2 levels can also rise in response to elevated ocean
temperatures; the solubility of CO2 in water decreases as the temperature
rises (see effect of El Niňo in Fig. 3B). Thus, cause and effect can be difficult
to prove conclusively. Severe weather is also occurring more frequently (Fig.
6B); the causes for this are still not clear, but it is possibly related to
increased global temperatures. To conclude, the use of plant-derived
fuels is one strategy to reduce CO2 emissions into the atmosphere, and
to potentially reduce the rate of global warming.
A
Global
Examination of the global carbon cycle (Fig. 2) shows that movement
of carbon as CO2 between the atmosphere and the vegetation/soil and
ocean components are closed cycles with no net emission of CO2 into
the atmosphere, whereas burning of fossil fuels and cement production
is not closed, causing a net release of CO2 into the atmosphere.
Northern Hemisphere
Renewable
“Closed
Loop”
Non-renewable
“Open
Loop”
CO2
Release
When
Burned
CO2
Release
When
Burned
Fuel (e.g. ethanol, biodiesel)
Polymers (e.g. starch, cellulose fiber)
Solvents & Lubricants (e.g. plant oils)
Fig. 2. Carbon cycle showing both the amount of global
carbon stored in different components (in black) and the
yearly global fluxes of carbon between different storage
components (in blue). Units are billion metric tons. Figure
is taken from UNEP (1996).
Southern Hemisphere
B
Fuel (e.g. gasoline)
Polymer plastics
Solvents & Lubricants
Fig. 4. Can green plants replace “plants”? In green plants the inputs are
carbon dioxide (CO2), solar energy, water, and nutrients such as nitrogen.
In chemical plants the input presently is petroleum. Use of green plants
forms a closed CO2 loop, and therefore atmospheric CO2 does not rise
when plant-derived fuel is burned. Figure modified from Chrispeels &
Sadava (2000).
Fig. 6. Changes in yearly average temperature (i.e. anomalies)
over time relative to the average temperature between 1951-1975
(A), and changes in weather patterns over time (B).