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1 Biogéochimie et Géochimie
Expérimentale
LMTG-Université de Toulouse-CNRS-IRD-OMP
14
av. Edouard Belin, 31400 Toulouse, France
E-mail:
oelkers{at}lmtg.obs-mip.fr
2 Chemical Science Division, Oak Ridge National Laboratory
Oak Ridge, TN
37831, USA
E-mail:
coledr{at}ornl.gov
| ABSTRACT |
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KEYWORDS: global carbon cycle, CO2 sequestration, global warming, ocean acidification
| CO2 AND GLOBAL CLIMATE CHANGE |
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Many attribute this recent global warming to human influence on atmospheric composition (e.g. Crowley 2000; Karl and Trenberth 2003). Global circulation models suggest that much of the observed global temperature increase stems from an increase in atmospheric CO2 (Manabe and Stouffer 1994; Johns et al. 2003). These models are apparently confirmed by correlations between historic CO2 concentration and temperature. An example of one such correlation is shown in FIGURE 1. A strong connection exists between temperature and atmospheric CO2 content, as shown by data covering the past 400,000 years from Antarctic ice cores. Perhaps most disconcerting is that there may be a positive feedback between increasing atmospheric CO2 concentration and climate. This positive feedback results from a decreasing ability of the terrestrial biosphere to act as a carbon sink as temperature increases (Cox et al. 2000). As a result of the combined effects of human CO2 emissions and this positive feedback, global climate models predict average temperature increases of 2 to 5°C by 2100 (Johns et al. 2003). Reducing the impact of CO2 emissions on the atmosphere and global climate change is thus considered one of the main challenges of this century (e.g. Gunter et al. 1996; Lackner 2003; Pacala and Socolow 2004; Oelkers and Schott 2005; Broecker 2005; Schrag 2007).
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| THE GLOBAL CARBON CYCLE |
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385 parts per million (ppm) by volume or 582 ppm
by mass. Pre-industrial levels are estimated at
280 ppm by volume. The
variation in CO2 concentration measured at the Mauna Loa, Hawai'i,
observatory is shown in FIGURE
2. This concentration has increased from roughly 325 ppm in 1970
to 380 ppm at the beginning of this century. The smaller seasonal variation
stems from biological activity: CO2 is consumed by biota during the
summer and released in the winter. In urban areas, CO2 is generally
higher, and indoors it can reach 10 times the background outdoor
concentration. The mass of the Earth's atmosphere is 5.14 x
1018 kg (Trenberth et al.
1988), so the total mass of carbon dioxide can be estimated to be
3.0 x 1015 kg, which is 3000 gigatons (Gt) of
CO2 or 800 Gt of carbon (1 Gt = 109 metric tons).
The atmosphere, however, is one of the smallest global CO2
reservoirs. The world's oceans contain
39,000 Gt of carbon, while soils,
vegetation and detritus contain
2000 Gt C and carbonate rocks (limestone,
marble, chalk)
65,000,000 Gt C. So, in total, the atmosphere only
contains roughly 0.001% of the carbon present in the atmosphere-ocean-upper
crust system. Moreover, the mass of carbon in the lower crust and mantle far
exceeds that found in these near-surface reservoirs
(Holland 1978). There are huge
exchanges among these reservoirs
(FIG. 3). Each year,
the atmosphere exchanges an estimated 90 Gt C with the surface ocean and 110
Gt C with vegetation (Houghton
2007). These numbers imply that the residence time of
CO2 in the atmosphere is no more than
4 years. CO2
exchange currently sequesters roughly half the annual anthropogenic global
CO2 emissions into the oceans and soils. This large-scale natural
sequestration provides confidence that it may be feasible to remove
CO2 from the atmosphere and sequester it in other reservoirs in
quantities sufficient to moderate the effects of anthropogenic CO2
emission (Lal 2008).
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| CO2 AND OCEAN ACIDIFICATION |
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| THE HUMAN IMPACT ON THE GLOBAL CARBON CYCLE |
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The molecular weight of CH2 is 14 g/mole, and that of
CO2 is 44 g/mole, so
3.1 kg CO2 is produced for
every kilogram of gasoline burned. It follows that each 50 litre tank of
gasoline produces about 113 kg CO2, which, for 24 annual fill-ups,
translates to roughly 2.7 metric tons of CO2 per year! If this
CO2 were stored mineralogically by creation of calcite
(CaCO3), in accord with the reaction:
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By this simple example, it is easy to imagine how the world's 6.6 billion people can contribute so much CO2 annually to the Earth's atmosphere. Manufacturing, transport of goods, and production of cement all add to the total. Human industrial CO2 emissions, primarily from the use of coal, oil and natural gas, and from the production of cement, currently contribute about 8 Gt C (29 Gt CO2) per year. The evolution of global CO2 emissions over the past 250 years is shown in FIGURE 4. Total human addition to the atmosphere since 1751 is estimated to be 315 Gt C (Marland et al. 2007). Of the current CO2 emissions, 18% originate from burning natural gas, 42% from oil, 36% from coal, and 4% from making cement. In addition, the human population produces an estimated 0.6 gigatons of CO2 per year just by exhaling.
Biofuels have been proposed as a solution, because the CO2 produced by burning came originally from the atmosphere and was fixed by plants via photosynthesis. However, the production and burning of biofuels (1) requires substantial energy for farming, transport and processing, thus producing substantial CO2; (2) requires the use of land, water and fertilizer that could otherwise be used to produce food; and (3) generates aldehydes and other compounds that are dangerous to humans, animals and the Earth's ozone layer.
The global average per capita CO2 emissions over the past 50 years are presented in FIGURE 5. Emissions roughly doubled over two decades, from 0.65 tons in 1950 to 1.2 tons in 1970, and have remained relatively stable since. Per capita emissions vary greatly from country to country. The top 10 emitters for 2004, on a per capita basis, are listed in TABLE 1. Oil-producing countries dominate. The United States and Canada are high on the list, with an average of >5 t C per person per year. Most European countries emit between 1.5 and 3 t C per person per year while less-industrialized Asian and African countries average <0.03 t C per person per year. The top overall CO2 emitters are listed in TABLE 2. Industrialized countries currently lead, and those in a phase of active growth, such as China and India, will increase emissions dramatically as their economies grow.
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| CO2 SEQUESTRATION EFFORTS |
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Three major types of carbon storage have been proposed: geological storage, ocean storage, and mineral carbonation. In this issue, each of them is explored in detail in separate articles (Benson and Cole 2008; Adams and Caldeira 2008; Oelkers et al. 2008). In addition, there may be some potential for increasing the amount of CO2 that is stored as biomass in forests and soils.
Geological Storage
Geological storage relies on the injection of CO2 into porous
rock formations (Holloway
2001; Friedmann
2007; Benson and Cole
2008). CO2 storage reservoirs include sedimentary
basins, depleted oil reservoirs and non-economic coal beds. An impermeable cap
rock is essential because CO2 density is generally less than that
of water, so buoyancy tends to drive CO2 upwards, back to the
surface. Several industrial-scale geologic CO2 storage programs are
already underway, including the Norwegian Sleipner project in the North Sea
(Korbøl and Kaddour
1995) and the Weyburn project in Canada
(Emberley et al. 2004); at
these sites, a million tons or more of CO2 is injected into the
subsurface each year. CO2 can also be injected into oil field
reservoirs in an attempt to enhance petroleum recovery (known as EOR). Despite
the apparently large annual injected volume, these projects, and other similar
efforts, currently store less than one-ten-thousandth, 0.01%, of the global
annual anthropogenic CO2 production.
Ocean Storage
Ocean storage means the injection of captured CO2 into the
ocean, usually at depths greater than 1000 metres, where it would be isolated
from the atmosphere (Adams and Caldeira
2008). CO2 would subsequently dissolve into the ocean
and become part of the global carbon cycle. This storage method has yet to be
attempted at a pilot scale. Ocean storage capacity may be enhanced by the
formation of CO2 hydrates or by the creation of liquid
CO2 lakes on the ocean floor.
Mineral Carbonation
Mineral carbonation aims to create stable carbonate minerals such as
magnesite (MgCO3) and calcite (CaCO3) by reacting
CO2 with silicate minerals containing magnesium and calcium
(Oelkers et al. 2008). Such
minerals are stable over geologic timescales, so sequestration by this method
would minimise risk of later leakage back to the atmosphere. Mineral
carbonation mimics natural weathering, but an industrial-scale operation may
require the mining and grinding of suitable Mg- and Ca-bearing silicate
minerals and the disposal of vast quantities of end-product carbonate
minerals. On the positive side, the resulting material could be used as a
building material, as an additive to concrete or paper, or as a soil amendment
to improve texture, pH and fertility of low-productivity soils. This enhances
overall carbon sequestration by increasing below- and above-ground biomass and
soil organic-matter content.
Biomass and Soil
Partial alternatives to these industrial solutions may be to store
CO2 in forests and soils. The biomass of forests is both a sink and
a source for atmospheric CO2. Vegetation absorbs carbon through
photosynthesis. Although some is emitted again during respiration, there is
net CO2 storage. The stored carbon eventually returns to the
atmosphere when the biomass decays or is burned. Forests can be managed to
increase their stored carbon, thus reducing atmospheric CO2
concentrations. The use of forests for CO2 storage will require (1)
that forests be managed to grow continuously and (2) that the carbon harvested
from forests not be returned to the atmosphere. Management practices to
maintain, restore, and increase carbon storage in forest soil include: the use
of fertilizer; increased density of agriculture and decreased slash-and-burn
practices; the preservation of wetlands, peatlands, and old-growth forest; and
the forestation of degraded and nondegraded sites, marginal agricultural
lands, and lands subject to severe erosion
(Johnson 1992). One solution
for storing carbon fixed by, then harvested from, forests is its addition to
soils in the form of biochar, which also has the potential to greatly enhance
soil fertility (SEE BOX). The potential for forests to sequester
CO2, however, is limited. Nilsson and Schopfhauser
(1995) estimated that only 345
million hectares are available worldwide for afforestation (planting trees on
land that has been without forest cover for more than 50 years); an
afforestation program of this scale would fix only 1.5 Gt C/year, which is
less than 20% of the current anthropogenic carbon input to the atmosphere.
Moreover, even attaining this limited land area for afforestation is
challenging due to pressure to use land for other purposes, in particular
agriculture and development.
| PERSPECTIVES |
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| BOX 1 CO2 Sequestration in Soils Carbon is a major component of soils. Globally the mass of soil organic carbon is more than that of carbon in living matter and in the atmosphere combined (Batjes 1996). Approximately 2 gigatonnes (Gt) of carbon is sequestered in soil organic matter annually (Lal 2003). This amounts to a quarter of the 8 Gt of anthropogenic carbon emitted to the atmosphere per year, making soil a substantial CO2 sink. Carbon is added to soils naturally, by the cycling and burial of organic material from plants, animals and microbes, and agriculturally, by spreading manure. Organic carbon is rapidly released to the atmosphere again, though erosion and oxidation.
A way to increase the residence time of carbon in soils is to add it as
biochar, which is charcoal produced by smoldering biomass in an oxygen-poor
environment. Some heat is produced, but the low concentration of oxygen
prevents burning, thus decreasing the amount of CO2 released and
stabilising the remaining carbon against further oxidation by bacteria.
Evidence for long-term stability of biochar in soils is found in the
carbon-rich Terra Preta de Indio black soils of the Amazon region. Glaser
(2007) concluded that at least some of the carbon of the Terra Preta stems
from biochar added to these soils over 500 years ago. The charcoal improves
soil fertility by adding an adsorptive surface for retention of plant
nutrients. The Terra Preta is more than twice as productive as nearby soils in
the Amazon, and biochar is sold as a soil amendment. This suggests that, if
properly managed, biochar addition to soil could sequester substantial
quantities of CO2, in a form that would make it stable over
hundreds of years. Lehmann (2007) argued that the addition of biochar to soils
could sequester REFERENCES Batjes NH (1996) Total carbon and nitrogen in the soils of the world. European Journal of Soil Science 47: 151-163 Gaunt JL, Lehmann J (2008) Energy balance and emissions associated with biochar sequestration and pyrolysis bioenergy production. Environmental Science & Technology 42: 4152-4158 Glaser B (2007) Prehistorically modified soils of central Amazonia: a model for sustainable agriculture in the twenty-first century. Philosophical Transactions of the Royal Society B362: 187-196 Lal R (2003) Global potential of soil carbon sequestration to mitigate the greenhouse effect. Critical Reviews in Plant Science 22: 151-184 Lehmann J (2007) A handful of carbon. Nature 447: 143-144 The authors are grateful to Vala K. Ragnarsdottir for telling us about the Terra Preta and for providing the references.
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