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MRS Energy & Sustainability : A Review Journal
page 1 of 16
© Materials Research Society, 2015
doi:10.1557/mre.2015.10
Introduction: Climate change and carbon cycle(s)
Discussion of climate change usually focuses on the rise of
Earth's global average surface temperature. Because of the
variability in temperature—daily, weekly, seasonal, annual,
decadal—and the seemingly small temperature rise
15 (about 1 °C)
recorded so far, global surface temperature increase is diffi cult
to appreciate for many people.
61 The more dramatic effects of
warming—melting glaciers and sea ice—are happening in high
latitude regions where few people live. The observed tempera-
ture increase is itself misleading; although seemingly modest, it
is an underestimate of the effect, because the temperature will
continue to rise based on the current g reenhouse gas content of
the atmosphere and the projected continued emissions. The
temperature rise to date is only a fraction of that already pro-
grammed into the Earth system.
15 , 59
The root cause of climate change is what could be called
“carbon cycle change.” To change climate, the amount of car-
bon dioxide and other so-called greenhouse gases in the atmos-
phere needs to change.
7 , 61 , 67 , 71 To change the amount of CO
2 in
the atmosphere, there must be a change in the way carbon is
transferred among the various forms and places it exists in
and on the Earth. The movement of carbon between storage
“reservoirs” on the Earth, including the atmosphere, is com-
plicated and still under investigation.
15 , 34 , 77 This study is an
attempt to present a simplifi ed version of the carbon cycle, to
place the current discussions of climate change in a geological
perspective and provide an entry point for those wishing to
ABSTRACT
Current issues with carbon emissions need to be understood in terms of natural geologic processes that move carbon on the Earth.
Comparison of modern emissions with the norms and extremes of natural processes emphasizes the enormity of the current challenge,
and also the reason there are uncertainties about the future effects. Reaching sustainable emissions in the future can be viewed as a
need to systematically reduce the carbon intensity of energy production.
Achieving sustainable carbon emissions requires understanding of Earth's natural carbon cycles. Geologic processes move carbon in large
quantities between Earth reservoirs, including in and out of the deeper reaches of the planet, and regulate Earth's surface temperature within
a narrow range suitable for life for the past 3–4 billion years. There have been large changes in atmospheric CO
2 in the geologic past; the largest
to offset changes in the brightness of the Sun. Atmospheric CO
2 has been much higher in the past, but not since humans evolved. Geologic
processes act slowly, even during times in the geologic past regarded as examples of catastrophic climate change. In contrast, over the past
100 years, Earth's carbon cycles have undergone revolutionary change as a result of a greatly accelerated transfer of carbon from geologic
storage to the atmosphere . Today, about 98% of the movement of carbon out of geologic reservoirs (coal-, oil-, and gas-bearing sedimentary
rocks and limestone) into the atmosphere is due to human activities; the total carbon fl ux is 40–50 times the geologic fl ux. The extremely large
modern carbon fl ux is unprecedented in Earth history. Returning to a sustainable carbon cycle requires systematic lowering of the carbon
emission intensity of energy production over the next century.
Keywords : carbon dioxide ; geologic ; sustainability
REVIEW
DISCUSSION POINTS
• The current rate of carbon emissions is 10 times larger than the
worst “climate catastrophes” known in Earth history.
• There is no uncertainty in the contribution of fossil fuel
emissions to the current carbon problem—they are ALL of it!
• Earth's carbon cycles work in an extraordinary way to maintain
its habitability; perturbing them to the extent we are doing is a
serious matter.
Sustainable carbon emissions:
The geologic perspective
Donald J. DePaolo , Earth Sciences Division , Lawrence Berkeley National
Laboratory , and Department of Earth and Planetary Science , University of
California , Berkeley , CA 94720 , USA
Address all correspondence to Donald J. DePaolo at djdepaolo@lbl.gov
(Received 24 March 2015 ; accepted 25 June 2015 )
2 Q MRS ENERGY & SUSTAINABILITY // VOLUME 2 // e9 // www.mrs.org/energy-sustainability-journal
understand more about carbon and climate. Most parts of this
story have been explained at various levels of detail and depth,
and several insightful recent reviews have been published,
2 , 80 , 88
including the most recent update of the Intergovernmental
Panel on Climate Change (IPCC) reports, which constitute
comprehensive, although quite technical, reviews.
15
Unlike the global temperature signal, the changes in the car-
bon cycle in the last 100 years are not subtle ( Fig. 1 ). These
changes have been produced almost entirely by burning of fossil
fuel, with a smaller (decreasing and less problematical in the long
term) contribution from destruction of forests.
44 Discussion
of the human-induced changes has in some cases been mud-
died by comparison to the large rates of carbon exchange between
atmosphere, biosphere, and oceans. These large exchange
fluxes are neither the ones that have changed drastically nor
are they particularly significant for understanding what is
happening due to burning of fossil fuel. The main change,
which is the focus of this study, is the rate that carbon is
moved from deep Earth storage—in rocks—to the atmosphere .
This transfer does happen naturally and is responsible for
many familiar aspects of Earth, including the fact that the
planet has maintained a hospitable climate that has allowed
life to flourish for billions of years.
38 , 58 , 60 , 78 However, in the
absence of human actions, the transfer is done mainly by volca-
noes, and at a small rate.
11 , 29 , 52 Fossil fuel burning has increased
this transfer rate by at least 40–50 times ( Fig. 2 ), which is not some-
thing that can argued about—the change is so huge that no likely
level of uncertainty about the numbers can change the conclu-
sion that virtually all the transfer of deep Earth carbon to the
atmosphere is currently a result of fossil fuel burning and
cement production.
29 This radical change represents some-
thing that has never before been done on Earth, even if we
look back hundreds of millions of years. It is the magnitude of
fossil carbon emissions that is the problem, and this can
be understood in terms of relatively simple concepts and
bookkeeping.
Over the past 50 years we have developed a good, although
certainly not complete, understanding of how climate has
changed on the Earth in the geologic past, and how it is related
to the amount of atmospheric CO
2 (National Research Council
2012; Ref. 61 ). The primary means of maintaining the surface
temperature of the Earth is by capture and radiation of solar
energy, and the relationship between greenhouse gas concen-
trations in the atmosphere and global surface temperatures is
straightforward,
4 even if there are a number of issues worth
continuing exploration.
43 , 67 Research has also led to better and
better estimates of the rates of carbon movement on the modern
Earth and the size of various reservoirs where carbon is
stored.
15 , 35 If one looks backward millions of years into deep
geologic time, there are examples of times when the atmos-
phere contained higher amounts of CO
2 than it has now, and
this has occurred as a result of natural processes that can change
the amount of atmospheric CO
2 by large amounts, but only
slowly over thousands or millions of years.
7 , 50 , 66 , 87 However,
there were no humans on Earth during any of the previous
times the atmospheric CO
2 concentrations were high. Although
it is uncertain whether high CO
2 levels would have prevented
Figure 1. Transfer rate of carbon from geologic storage as coal, oil,
natural gas, and limestone to the atmosphere in units of GtC/yr. The major
increases have occurred since 1850 and most of the increase has occurred
since 1950.
Figure 2. Rates of transfer of carbon from geologic storage in rocks to
the atmosphere from 1750 to 2014, plotted on a logarithmic scale and
compared to natural geologic rates estimated for the modern Earth. Data on
combustion and cement are from ORNL database. Geologic rates are from
Ref. 11 . Total geologic is the rate of carbon emission, mostly as CO
2 , from
volcanic areas plus metamorphism in nonvolcanic mountain ranges.
Estimates of total geologic emissions are close to the estimates of carbon
removal from the atmosphere-ocean-biosphere system by sedimentation in
the oceans
56 and hence are likely to be reasonably accurate.
MRS ENERGY & SUSTAINABILITY // VOLUME 2 // e9 // www.mrs.org/energy-sustainability-journal Q 3
humans from thriving at earlier times in Earth history, recent
studies have shown that moderately elevated CO
2 concentra-
tions can affect human behavior.
75 And whether or not we
fully understand the implications, it is nevertheless true that
prior to about 1950 humans had not experienced atmos-
pheric CO
2 concentrations higher than about 300 ppm.
The natural carbon cycle(s)
The term carbon cycle refers to natural processes, such as
volcanic eruptions, photosynthesis and respiration, weathering
of rock to form soil, forest fi res and forest growth, transport of
dissolved chemicals in rivers, and the growth and death of
marine organisms and their shells. These processes and others
act in concert to move carbon around in the environment. The
plural form “cycles” is used here mainly to call attention to the
fact that there are rapid cycles of carbon embedded within
slower, larger cycles. The term “cycle” implies that carbon is
moved in and out of different storage sites, and over an extended
period can circulate among those sites. The growth and fl ower-
ing of trees and plants in Spring, and their death and loss of foli-
age in Autumn, are one of the most familiar parts of the carbon
cycle. Plants grow by photosynthesis, which involves removing
CO
2 from the air, converting it to organic compounds that con-
stitute the living things, and returning O
2 to the atmosphere.
When plants die they decompose, and this process takes O
2
from the air and returns methane and CO
2 , and the methane
soon gets converted to CO
2 . Every year, large amounts of car-
bon are transferred to living plants and animals and then
returned to the atmosphere later.
35 Every year, the amount of
CO
2 in the atmosphere decreases a bit in the Northern hemi-
sphere Spring and Summer, and then increases again in Fall and
Winter.
31 , 47 , 64
The annual cycle is an example of a rapid and large transfer of
carbon between two “reservoirs,” in this case between the
atmosphere and the land biosphere. Another example is trans-
fer of atmospheric CO
2 to and from the oceans. Because CO
2 is
soluble in water to some degree, and because gas transfer
from air to the oceans occurs rapidly, there is a large exchange
of carbon between the atmosphere and the oceans every
year.
39 , 51 , 72 , 74 Because the concentration of CO
2 in the atmos-
phere has been rising, there is a net tendency for atmospheric
CO
2 to dissolve in the oceans although the total amount of
transfer into and out of the oceans is about 30 times larger than
the net transfer to the oceans. The ocean-atmosphere boundary
is dynamic and CO
2 , water vapor, and other constituents are
always evaporating from the ocean surface as well as being
transferred from the atmosphere to the oceans. Living plant
matter also is transformed into dead litter and stored in soil.
Soil organic carbon is not static but tends to decompose
through microbial activity and be returned to the atmosphere
as CO
2 or CH
4 . 20 , 28
The four major surface reservoirs of carbon are depicted in
“box model” form in Fig. 3 along with the next larger reservoir
of carbon—the deep ocean. The four surfi cial storage sites for
carbon (atmosphere, land biosphere, soils, surface ocean),
which together contain roughly 4000 GtC (GtC signifi es 10
9
metric tons of carbon), are the focal point for understanding
what is happening with carbon in the modern world. The deep
ocean also exchanges carbon with the four surface reservoirs,
but we will begin the analysis fi rst without considering the role
of the deep ocean. The surface reservoirs exchange carbon so
rapidly that the fi rst step in understanding carbon cycle change
is to think of the four surface reservoirs as a single Earth carbon
reservoir . The rates of carbon transfer between these reservoirs
are 60–120 GtC/yr.
34 Hence, from these exchange rates and the
size of the reservoirs, it can be inferred that carbon does not
remain in any of the four reservoirs much longer than a few
decades.
77 The transfer of carbon into and out of the surface res-
ervoir box is much slower, more than 1000 years with the deep
ocean and much more slowly with the geologic reservoirs.
These other parts of the carbon cycle will be examined in more
detail in the next section.
A reasonable question might be—how do we know the rates
of carbon transfer between the surface reservoirs? There are
several approaches to measuring and estimating the rates of
carbon exchange between the four surface reservoirs,
39 but
a demonstration of the order of magnitude of these rates is
Figure 3. Simplifi ed “box model” representation of where carbon is stored
in near-surface reservoirs and the rates at which carbon is exchanged
annually between the reservoirs. Fluxes, shown on the arrows, are in units of
GtC/yr and are approximate to 10 Gt/yr. The soil carbon mass of 1800 GtC is
in the lower range of recent estimates.
15 The overall storage in the surface
reservoirs is roughly 4000 GtC. The atmospheric mass represents the
preindustrial amount; the current value as of 2014 is about 830 GtC. The
terrestrial biosphere mass has changed over the past 200 years but not by
a large amount (approximately 50 GtC). Solid arrows depict the normal
geologic fl ux of about 0.23 Gt/yr carbon from deep geologic reservoirs to the
atmosphere. To a good approximation, the reservoirs within the gray box are
the ones that are most affected, over the next few hundred years, by the
increased fl ux of carbon from deep reservoirs caused by fossil fuel burning
and cement production.
4 Q MRS ENERGY & SUSTAINABILITY // VOLUME 2 // e9 // www.mrs.org/energy-sustainability-journal
provided by the record of atmospheric radiocarbon over the
past 60 years ( Fig. 4 ). An excess of the carbon isotope
14 C was
produced in the atmosphere as a result of nuclear weapons test-
ing in the 1950s and 1960s. During the period from 1957 to
1966, the proportion of carbon present as the isotope
14 C
increased by about 60–80% (Δ 14 C = 600–800‰). Atmospheric
testing was all but stopped at that time. Since then the
14 C
excess in the atmosphere, denoted as Δ 14 C, has decreased by
about 50% every 10–12 years. The decrease occurred because
the excess
14 C, initially contained solely in the atmosphere, was
gradually redistributed to the land biomass and the ocean
(including to some degree the deep ocean) and mixed with car-
bon in those other reservoirs that contained much less
14 C.
Although
14 C is radioactive, it decays much too slowly (half life
≈ 5700 years) to account for any of this decrease (in 60 years,
less than 1% of the
14 C excess was removed by radioactive
decay). In more recent years, a larger fraction of the Δ 14 C
decrease is attributable to
14 C-free carbon (Δ 14 C = −1000) com-
ing from fossil fuel combustion.
55 Nevertheless, a simple model
in which atmospheric carbon is exchanged with the ocean and
land biomass requires transfer rates of about 50–60 GtC/yr to
account for the rapid disappearance of the excess
14 C in the
atmosphere and its appearance in the terrestrial biosphere and
the surface ocean.
58 , 55 Thus, although this calculation is simpli-
fi ed, the
14 C data provide a clear demonstration that there are
large exchange fl uxes between the surface reservoirs, and hence
that these four surface reservoirs can be considered as one well-
mixed reservoir for understanding carbon cycle variations on
time scales longer than 100 years.
Where else is the carbon in and on the earth?
The four major surface reservoirs are the starting point for
discussing carbon on the Earth, especially if one is concerned
about the next 100–300 years, but to get a more comprehensive
picture, other reservoirs of carbon need to be considered. We
start with a summar y of where the Earth's deeper carbon is
stored ( Fig. 5 ).
Most of the Earth's carbon is thought to be in Earth's metal-
lic core, buried some 2900 km below the surface and not acces-
sible by any process we know about. The core is made mostly of
iron and nickel but has some amount of lower atomic number
alloying elements in it. It is not precisely known how much of
which elements the core contains besides Fe and Ni, but the
most likely suspects are C, S, O, and Si, because they are com-
mon and abundant elements in the Sun and in planets and are
known to form alloys with Fe at high pressure and temperature.
83
A conservative estimate is that the Fe–Ni alloy in the core con-
tains about 0.2% by weight carbon. Because the mass of the core
is 2 × 10
12 Gt, it therefore contains about 4 billion GtC, which
constitutes about 90% of the Earth's carbon (and 1 million
Figure 5. Distribution of carbon in the Earth. A large proportion of
Earth's carbon is stored in the core, where it is isolated and not available
to signifi cantly affect the surface environment. The Mantle and Continents
are large reservoirs of carbon that exchange with the surface slowly over
millions and billions of years. The surface reservoirs, shown in blue type,
are much smaller but determine the amount of carbon in the atmosphere
and hence climate. The deep ocean is an intermediate storage reservoir—it
exchanges carbon with the surface reservoirs on a time scale of 1000 years,
much shorter than the mantle and continents, but much slower than the
exchange among the surface reservoirs. The mass of the remaining fossil
fuel stores (5000 GtC) that could still be extracted and burned are not
precisely known, but are believed to be roughly equal to the amount of
carbon stored currently in the surface reservoirs, and 5–10 times the
amount currently in the atmosphere.
Figure 4. The radiocarbon content of the atmosphere from 1950 to 2010,
expressed in units of permil deviation (Δ
14 C) from the approximately 1950
value of
14 C/ 12 C in the atmosphere (Figure from Ref. 45 ). Between the late
1950s and the mid-1960s, the
14 C content of the atmosphere increased by
about 60–80% (600–800‰) due to testing of nuclear weapons. The
14 C is
produced from nuclear blasts in much the same way it is produced
naturally, by neutrons released from the explosions reacting with atmos-
pheric nitrogen. The gradual return of the atmospheric Δ
14 C value to zero is
a measure of the rate at which atmospheric C is exchanged with the oceans
and biosphere. For a simplifi ed model where this effect is caused by
exchange between the atmosphere and surface ocean (1000 GtC) and land
biosphere (600 GtC), the annual exchange rates needed to account for the
approximately 10–15 year time scale of Δ
14 C decay are about 50–60 GtC/yr,
55
which is evidence for the large magnitude of exchange fl uxes shown in
Fig. 3 . In the last 25 years, the decreasing Δ
14 C is signifi cantly infl uenced
by fossil fuel carbon emissions, since fossil fuel carbon contains no
14 C
(Δ
14 C = −1000) and hence tends to accelerate the decrease of Δ
14 C.
MRS ENERGY & SUSTAINABILITY // VOLUME 2 // e9 // www.mrs.org/energy-sustainability-journal Q 5
times more than is in the surface reser voirs). The amount of car-
bon contained in the core is interesting but not relevant for
understanding climate change except during the earliest stages
of formation of the Earth 4.5 billion years ago.
18
The next biggest repository of carbon is the Earth's mantle.
The rocks of the mantle, which are largely silicate compounds
of Mg, Fe, Ca, and Al, contain about 0.01% (100 ppm) carbon by
weight (probably between 60 and 100 ppm). The carbon is con-
tained in the mantle as diamond and as impurities in the major
mineral phases.
19 The mass of the Mantle is almost exactly twice
that of the Core (4 × 10
12 Gt), and hence the Mantle contains
about 240–400 million GtC; about 8–9% of the total, and about
100,000 times more than is in the surface reservoirs. Together,
the mantle and core contain about 98–99% of the carbon in the
Earth. Although the carbon contained in the Earth's core
cannot be transferred to the atmosphere by any known geologic
process, the carbon contained in the Earth's mantle can, and is,
regularly transferred to the atmosphere through volcanoes and
other means of leakage through the Earth's crust. This is an
important part of Earth's carbon story.
The so-called “crust” of the Earth comprises the continents,
which are slightly silica-enriched rocks forming a layer about
40 km thick and that largely is exposed above sea level, and the
ocean fl oor, which is about 6 km thick and largely submerged
below sea level by an average of almost 5 km. Most of the carbon
stored in the crust is stored as limestone and dolomite (Ca,Mg)
CO
3 , and it's metamorphic equivalent—marble—and most of it is
in the continental crust. The rest, and a much smaller amount,
of the carbon is stored as organic material (petroleum, peat, and
coal) or as graphite. The amount of carbon stored in the crust is
estimated to be about 60–70 million Gt, about 20–25% of the
amount in the Earth's mantle. The carbon stored in the crust
might seem to be inert and irrelevant for climate change, but
there are geologic processes that release CO
2 from limestone
and marble and allow it to enter the atmosphere. Minerals like
calcite (chemical formula CaCO
3 ) break down and release CO
2 ,
when they are heated to high temperature in the Earth.
26 , 41 The
released CO
2 fi nds its way to the surface and is transfer red to the
atmosphere by springs, geysers, and seepage through soils and
rocks. A similar process is involved in the making of cement.
Limestone is heated until it breaks down and releases CO
2 . This
calcining process leaves calcium oxide, which is needed for
cement. The global cement-making industry produces CO
2 at
the rate of about 0.5 GtC/yr,
15 which is roughly 5 times more
than the entire Earth normally makes naturally by metamor-
phism ( Fig. 2 ).
The next biggest repository of carbon not accounted for in
the surface reservoirs, and one that is quite important for
understanding carbon and climate, is the deep ocean, which
contains almost 38,000 GtC (0.001% of the Earth's carbon),
most of it dissolved in the form of the bicarbonate ion (HCO
3 − )
but with some dissolved CO
2 and organic carbon as well. The
deep ocean carbon reservoir is about 10 times larger than the
sum of the four surface reservoirs, and hence quite large, but
still much smaller than the geologic reservoirs. The deep ocean,
however, unlike the geologic reser voirs, is directly connected to
the surface reservoirs, and there is a large amount of carbon
exchanged between the surface ocean and the deep ocean.
15 , 34
The only way that the surface reservoirs can lose carbon over
the next few thousand years is by transport through the surface
ocean to the deep ocean, and ultimately to ocean sediments. As
discussed further below, the limited number of mechanisms
that can take carbon out of the surface reservoirs is the reason
that adding carbon to the atmosphere in large amounts is
problematical.
Connecting the surface reservoirs to the deep earth
To understand climate change on the different time scales in
which it is known to occur naturally, from thousands of years to
billions of years, we need to place the four surface reservoirs
into the broader context of deeper Earth carbon cycles. The
longer the time scale considered the more and deeper the car-
bon that is involved.
The surface reservoirs exchange carbon with the much
larger deep ocean at a substantial rate, but because the deep
ocean is so large, the time necessary for the surface reservoirs
to mix their carbon with the deep ocean is more than one
thousand years.
35 This exchange rate can also be estimated
from radiocarbon data. The radiocarbon “age” of deep waters
in the ocean is in the range of 1000–2000 years.
24 This age is
a measure of the amount of time since the deep ocean carbon
was last in the atmosphere. Transfer of carbon between the
deep ocean and the surface reservoirs is important for under-
standing climate change. Current models for the last ice age,
e.g., suggest that compared to the present, the deep ocean
contained about 600 Gt more carbon at the expense of the
atmosphere (200 Gt less C) and the terrestrial biosphere
(400 Gt less C).
77 , 85 Hence, climate can change on a several
thousand-year time scale by shifting carbon between the
surface reservoirs and the deep ocean. The exact causes and
mechanisms of the glacial–interglacial shift are not agreed
upon,
77 but the time scale is right for the carbon shift to
involve the deep ocean reservoir. The climate and atmos-
pheric CO
2 transition at the end of the last glacial period
took roughly 6000 years.
53 The 1000-year time scale of the
deep ocean-surface reservoir exchange is also important
because it defines the amount of time needed for the deep
ocean to contribute to absorbing anthropogenic carbon
emissions.
2
Planetary carbon cycle and long-term climate change
The amount of carbon in the four active surface reservoirs
plus the deep ocean effectively determines Earth's global cli-
mate and can change slowly due to geologic processes. As
implied above, one source of carbon for the atmosphere is vol-
canoes. Volcanic eruptions involve the release of gases (CO
2 ,
but also H
2 O, SO 2 , and others) as well as the eruption of lava
and ash. The CO
2 that comes from volcanoes is extracted
from Earth's mantle and deep continental crust, so it is
“new” carbon for the surface reservoirs that is transferred
from long-term geologic storage to the atmosphere. It is
6 Q MRS ENERGY & SUSTAINABILITY // VOLUME 2 // e9 // www.mrs.org/energy-sustainability-journal
challenging to estimate how much carbon comes from volca-
noes each year, but the most recent estimates yield a number
of about 0.15 GtC/yr,
11 which is somewhat higher than previ-
ous estimates.
29 In addition to volcanic emissions, there is
another 0.08 GtC/yr that comes from metamorphism that
takes place in largely nonvolcanic mountain building zones
like the Alps and Himalaya.
41 , 52 The total carbon emitted
from the Earth's crust and mantle therefore is estimated to be
about 0.23 Gt/yr, which is close to being compatible with car-
bon removal rates of about 0.18–0.24 GtC/yr by sedimentation
in the oceans.
56 This amount of carbon emitted each year, if
not compensated by removal mechanisms, would be enough
to increase the CO
2 concentration in the atmosphere by about
0.1 ppm per year. However, the natural processes that remove
this added carbon from the atmosphere into marine sediments
compensate for the volcanic and metamorphic additions,
so that the natural atmospheric CO
2 concentration changes
are typically much less than ±0.1 ppm/yr over thousands and
even millions of years.
Under normal circumstances the primar y planetary fl ow of
carbon is from the solid Earth to the atmosphere and then into
the oceans. The extra carbon that is added to the oceans does
not stay in the oceans for long but is turned into calcium car-
bonate by marine organisms making their shells, and then bur-
ied in sediment on the ocean fl oor, both in the deep ocean as
well as on continental shelves (e.g., coral reefs) and other near-
short environments. Ultimately, this planetary carbon “cycle”
takes carbon from the solid rock of the Earth, puts it into the
atmosphere via volcanoes and metamorphism, the atmosphere
shares it with the biosphere and the ocean, and then it is put
back into rock on the ocean fl oor. The existence of a mechanism
for removing carbon from the atmosphere and surface reser-
voirs and returning it to geologic storage is what makes the
Earth a habitable planet.
37 , 82
Carbon cannot be returned to solid rock storage unless the
continents and the hydrologic cycle get involved ( Fig. 6 ). The
CO
2 in the atmosphere dissolves in the ocean and in atmos-
pheric water vapor to create a weak acid—carbonic acid. When
rainwater and groundwater are in contact with rocks of the
Earth's crust, they slowly dissolve the silicate minerals, a pro-
cess referred to as “weathering,” which effectively neutralizes
the acid. The water that is returned to the oceans by rivers thus
is more alkaline than rainwater and tends to offset the acidity of
the oceans while also supplying cations like Ca
2+ and Mg
2+ that
have been leached from the rocks. The divalent cations com-
bine with carbonate ions (CO
3 2− ) in the ocean to produce
Ca,Mg-carbonate minerals. The formation of the solid car-
bonate minerals on the modern Earth is mostly mediated by
microorganisms growing carbonate shells
46 , 73 rather than just
by inorganic precipitation, although earlier in Earth histor y
most of the carbonate precipitation was inorganic.
79 The car-
bonate shell material accumulates on the ocean fl oor fi rst as a
soft carbonate mud, and later by gradual heating and compac-
tion is turned into chalk and then limestone. Additional car-
bonate forms inorganically fi lling fractures within the volcanic
oceanic crust.
81
The carbon content of the surface reservoirs and deep
ocean is maintained within a narrow range by a dynamic bal-
ance between volcanic and metamorphic carbon supply, and a
removal process involving weathering of continental rocks
exposed at the Earth's surface. Because adding CO
2 to the
ocean-atmosphere system has the effect of both acidifying nat-
ural waters and raising the air temperature, increasing CO
2
tends to accelerate weathering rates, which acts to counterbal-
ance the increase of atmospheric CO
2 . 7 , 9 , 12 , 82 This weather-
ing feedback cycle ( Fig. 7 ) is a key feature of the Earth that
allows it to maintain an equitable surface temperature over
millions to billions of years.
37 There are aspects of this cycle
that are not completely understood. The key idea, that weath-
ering rate increases when atmospheric CO
2 increases, has
not been proven but is regarded as inescapable (see Ref. 23 ),
although the mechanism is debated.
48
Figure 6. Diagram of the geologic carbon cycle that regulates the
amount of CO
2 in the atmosphere and surface reservoirs over long time
scales of millions of years and has kept the Earth “habitable” over most
of its 4.5 billion year lifetime. The key feature of the Earth is that carbon
(from the Mantle) released to the atmosphere by volcanic emissions can
be returned to the deep Earth rather than retained in the atmosphere
where it would accumulate to exceedingly high levels over geologic time.
The CO
2 released from geologic storage by volcanoes tends to acidify
rainwater (and the oceans), which slowly dissolves rocks in a process
referred to as “weathering.” Weathering generates alkalinity, including
aqueous Ca
2+ (and Mg
2+ ) ions, that when added to the oceans can
combine with dissolved carbonate ions to form solid carbonate (mostly as
shelled organisms on the modern Earth), which accumulate on the sea
fl oor and form limestone by compaction and chemical processing over
millions of years. Limestone can be added to the continents and stored as
sedimentary or metamorphic rock for hundreds of millions of years, or it
can be “subducted” back into the deep mantle where it can be retained
for one or two billion years. The strength of the feedback between volcanic
CO
2 emissions, which tend to acidify the oceans and increase atmos-
pheric CO
2 , and weathering which tends to make the ocean more alkaline
and decrease atmospheric CO
2 , can change slowly over millions of years,
resulting in slow swings in global climate between hotter and cooler
states (see following fi gure).
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The climate feedback cycle is effective. There is evidence
that the surface temperature of the Earth has been kept in a
fairly narrow range, probably between about 5 and 35 °C over
the past 3.5 billion years.
60 However, the cycle is also sluggish.
This can be illustrated by considering the amount of carbon in
the surface reservoirs and ocean, which is roughly 40,000 Gt at
present and probably somewhat more at various times in the
geologic past. Using a volcanic and metamorphic emission rate
of 0.23 GtC/yr, it can be inferred that the time scale over which
the geologic climate feedback operates is a little more than
150,000 years (≈40,000/0.23). This calculation means that any
perturbation, such as a sudden increase in volcanic emissions,
will be compensated by an increase in weathering, but the
system won't reach a new equilibrium for about two to three
times this inferred time scale, roughly 300–400,000 years.
This time scale is important because it defi nes the time needed
for the geologic cycle (rock weathering in particular) to contrib-
ute to compensating anthropogenic carbon emissions.
2
The effect of the Sun on the Earth's carbon cycle is strong
but constant when considering time scales even as long as
10 million years. But over the history of the Earth, the Sun is
believed to have become gradually hotter and brighter as the
internal composition evolved.
30 The standard model has the
Sun's luminosity increasing gradually and systematically start-
ing from about 70% of the modern value at 4.4 billion years ago.
Relative to the modern situation, where about 280 ppm CO
2 in
the atmosphere is adequate to keep the average Earth surface
temperature at about 15 °C, when the Sun was only 70% as
bright, about 300,000 ppm CO
2 would have been needed
38
( Fig. 8 ). The atmosphere composition could, e.g., have been
about 70% N
2 and 30% CO
2 (there was little or no O
2 in the
atmosphere at that time). The required greenhouse warming
could have been partly supplied by methane rather than CO
2 ,
(or potentially by other mechanisms) which would mean
somewhat lower concentrations of CO
2 were needed.
36 , 37 , 65 , 84
Nevertheless, it is clear that the biggest perturbation to cli-
mate over the history of the Earth is the brightening of the
Sun. This point is shown schematically as a fulcrum in Fig. 7
to emphasize the large role that solar luminosity plays in
driving the climate system and by inference the atmospheric
CO
2 concentration. The effectiveness of the weathering feed-
back is demonstrated by the fact that Earth's surface temper-
ature has remained within a few 10's of degrees above the
freezing point of water over most of the last 4 billion years
even though the luminosity of the Sun has changed by a large
amount.
37
Moving carbon in and out of the deep earth
Although the natural, geologic rate of carbon release from
rocks to the atmosphere seems quite small, over the long extent
of geologic time, a huge amount of carbon is moved. At the esti-
mated modern rate of 0.23 GtC/yr, over just the limited time
between Pleistocene ice ages—100,000 years—natural processes
release 23,000 Gt carbon to the atmosphere from the solid
Earth, about half the amount needed to completely replace the
carbon in the oceans, atmosphere, and biosphere. Over the past
1 million years, again assuming the same rate, about 230,000 Gt
carbon would have been naturally released to the atmosphere
by geologic processes, which is about 5 times more than is con-
tained in the surface reservoirs. These numbers show that car-
bon is indeed “cycled” through the surface reservoirs and
originates in the geologic reservoirs. Over the past million
years, we have reasonable estimates of the CO
2 concentration
of the atmosphere from ice cores,
25 and the amount of carbon
in the atmosphere has remained between 400 and 600 Gt.
Even though huge amounts of carbon are injected into the
atmosphere at a slow rate, Earth's carbon cycling system regu-
lates the amount of carbon in the atmosphere and keeps it
Figure 7. Diagrammatic representation of the CO
2 -temperature feedback
that regulates Earth's surface temperature. Atmospheric CO
2 is supplied
at a small but signifi cant rate, currently estimated at about 0.23 GtC/yr
(probably larger in the geologic past) by volcanic emissions and metamor-
phism. Weathering has the net effect of removing CO
2 from the atmosphere-
ocean system and converting it to solid carbonate minerals. Weathering
reactions are accelerated by higher temperature and lower pH, so it is
inferred that increased atmospheric CO
2 and temperature will accelerate
weathering. Hence there is feedback, where any increase in volcanic
emissions, which will tend to increase atmospheric CO
2 and tempera-
ture, will be counterbalanced by an increase in weathering, with a lag
time of roughly 100,000 years.
82 The rate of volcanic emissions can
change due to the strength of the “carbon short circuit,” and the rate
of weathering can change as a result of continent–continent collisions,
which accelerate weathering by creating high mountain ranges where
erosion is rapid, making more mineral surface area available for weathering
reactions. Over millions to hundreds of millions of years, Earth's climate
can drift between warmer and cooler regimes. Ultimately, the strength
of the feedback is determined by the absorption of solar energy by the
atmosphere, and because the brightness of the Sun has increased
gradually over the past 4.5 billion years, there has been a tendency for
the CO
2 content of the atmosphere to decrease gradually during Earth
history ( Fig. 9 ). As the Sun's luminosity increases, less CO
2 is required
to balance weathering.
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almost constant. If the time scale is stretched to 50 million years,
the volcanic emission rates are less certain (probably some-
what higher), but nevertheless the same calculation suggests
that geologic processes have injected more than 12 million Gt
carbon into the atmosphere. Over this longer time period, the
atmosphere has been more variable in carbon content, having
changed from about 2000 Gt carbon at about 50 million years
ago to about 600 Gt carbon today
49 , 66 (see Fig. 10 ). Over this
50 million year time span the carbon in the atmosphere, bio-
sphere, and oceans has been completely replaced more than
200 times.
If we use the volcanic emission rate of 0.15 GtC/yr and con-
sider the entire 4.5 billion year history of the Earth, the total
transfer of carbon from the Earth's mantle to the atmosphere
would be 6.75 × 10
9 Gt carbon. This number is about twice the
estimated amount of carbon in the mantle and crust and would
imply even the carbon in the mantle has been replaced at least
twice during Eart h history. One problem with this calculation is
that much of the 0.15 GtC/yr modern volcanic emission rate is
thought to be due to the “carbon short circuit” that affects vol-
canoes associated with subduction zones
23 , 26 ( Fig. 7 ). A more
likely number for the modern fl ux of deep mantle carbon to the
atmosphere is much smaller—about 0.03–0.05 GtC/yr, although
the rate might have been higher in the Earth's past when the
mantle was hotter.
18 The outgassing of carbon from the mantle
is believed to have decreased gradually as the Earth lost some of
its initial heat and mantle convection slowed
89 ( Fig. 8 ). An aver-
age value for the transfer rate, inferred from Fig. 8 , might for
example be 0.05 GtC/yr over the last 4 billion years. At this
rate, about 2 × 10
8 Gt carbon would have been transferred from
the mantle to the atmosphere over 4 billion years, still a large
fraction of Earth's internal carbon inventor y.
There are two interesting points related to this number of
2 × 10
8 Gt carbon transferred from the mantle to surface reser-
voirs. Venus, which is a planet similar in size to the Earth, but
that does not have liquid water at its surface and hence has no
mechanism for removing carbon from its atmosphere,
35 cur-
rently has 1.26 × 10
8 Gt carbon in its atmosphere , almost all of it
as CO
2 . Hence, Venus can be viewed as evidence that it is possi-
ble to transfer large amounts of carbon from a planet's mantle
to its atmosphere. Another point is that, of the 2 × 10
8 Gt carbon
or more that has been transferred out of the Earth's mantle, we
can currently account for only about 1/3 of it (about 6–7 × 10
7
Gt carbon) in the continental crust plus the surface reservoirs
( Fig. 5 ). Hence, t here is evidence that a large fraction of the car-
bon that is expelled from the deep mantle is returned to the
Figure 8. (a) Plausible rate of outgassing of deep mantle carbon to the atmosphere over the course of Earth history (from Ref. 89 ). The inferred modern
rate is 0.03 GtC/yr and the rate at 3.0 billion years ago is about 0.06 GtC/yr. Other estimates are as much as twice these numbers (e.g., Ref. 19 ). At the
rates shown, approximately all the original carbon in the mantle would have been released to the atmosphere over the past 4+ billion years. In this case,
and even more so if the rates were higher, the fact that the Earth's mantle still contains a lot of carbon would suggest that there is an effi cient mechanism
to return carbon from the atmosphere back to the mantle. (b) Results of a simple calculation showing the partial pressure of CO
2 needed in the atmosphere
to maintain the Earth's surface temperature between 0 and 15 °C to compensate for the changing luminosity of the Sun.
MRS ENERGY & SUSTAINABILITY // VOLUME 2 // e9 // www.mrs.org/energy-sustainability-journal Q 9
mantle, presumably by subduction as shown in Fig. 6 (see
Ref. 18 ). Thus, the deep carbon reservoirs of the Earth are
part of a carbon “cycle” that operates on a time scale of a bil-
lion years or above more.
Water on the earth—no water, no carbon cycle
Implicit in the discussion above is the presence on the
Earth's surface of a large amount of liquid water. Atmospheric
CO
2 cannot be removed without the acidifi cation of rainwater
and groundwater, and the consequent rock weathering that
occurs.
82 Flowing rivers constitute the means of returning the
alkalinity to the oceans, and the ocean provides the environ-
ment within which carbonate rocks can form and provide long-
term storage for carbon. It is also believed that the small amount
of water dissolved in minerals in Earth's Mantle changes the
rock viscosity and facilitates plate tectonics and certain aspects
of the solid state convection in Earth's interior that provide a
means to return carbon from the surface reser voirs and the con-
tinental crust into the deeper parts of the Mantle.
69 The pres-
ence of surface water on Earth means that most of the carbon
continuously supplied by volcanism does not accumulate in the
atmosphere but instead is returned to geologic storage deep in
the Earth.
Maintaining the surface water inventory of Earth requires
that the surface temperature of the Earth be above the freezing
point of water but not too close to the boiling point. If at any
time in the Earth's past, the atmospheric temperature was high
enough that there had been a large fraction of the H
2 O in the
atmosphere, then it may have been possible, through H
2 O dis-
sociation and loss of hydrogen from the top of the atmosphere,
to have greatly depleted the Eart h's store of water.
37 The funda-
mental difference between Venus and Earth in this regard is
attributed to the fact that Venus is suffi ciently close to the Sun
that water cannot be kept in the liquid state, and hence through
hydrogen loss, water is nowhere to be found at or above the sur-
face and there is an accumulation of about half a planet's worth
of C in the atmosphere. This accumulated carbon, which
enhances the greenhouse effect, makes it impossible to escape
this state once it is reached. Earth currently occupies the
“Habitable Zone” in terms of distance from the Sun in the
Solar System,
37 being neither too close to allow complete
vaporization of water at the surface, but not so far as to allow
all the water to freeze into ice.
“Extreme” carbon events in the earth’s past
There are examples in geologic history where the amount of
CO
2 in the atmosphere was changing “rapidly,” before humans
were around to have an effect. A recent example is the period
between 18,000 and 12,000 years ago, the end of the last Ice
Age, when glaciers were melting in the Northern hemisphere
[ Fig. 9(a) ]. During this time, about 150 Gt carbon was added to
the atmosphere by natural processes.
53 , 85 The rate of addition,
150 Gt carbon per 6000 years, equates to a transfer rate of
0.025 GtC/yr. Considering the relatively rapid climate changes
in this time period, this carbon transfer rate must be considered
extreme by geologic standards. Nevertheless, this “rapid” rate
is almost 200 times smaller than the rate that CO
2 is currently
accumulating in the atmosphere (4–5 GtC/yr, or about half the
total emissions). During the 6000-year deglaciation per iod, and
analogous to the present, there was more carbon being moved
into and out of the atmosphere than was accumulating in the
atmosphere. Overall, it is estimated that there was about 600 Gt
carbon transferred from the deep ocean through the atmos-
phere and into the land biomass during the 6000-year
Figure 9. (a) Atmospheric CO 2 during the climate transition from the last glacial maximum, from 22,000 years ago to 9000 years ago.
53 During the period
from 17,000 to 11,000 years ago, atmospheric carbon concentration changed by about 70 ppm, corresponding to an atmospheric carbon increase of 150 GtC
in 6000 years or about 0.025 GtC/yr. (b) Carbon isotope record and estimated deep ocean temperature during the Paleocene-Eocene Thermal maximum
(PETM) about 50 million years ago. These signals correspond to the addition of 3000–7000 GtC in 6000 years, a rate of 0.5–1.2 GtC/yr (fi gures from Ref. 86 ).
10 Q MRS ENERGY & SUSTAINABILITY // VOLUME 2 // e9 // www.mrs.org/energy-sustainability-journal
deglaciation interval. This rate (600 Gt in 6000 years) is about
0.1 GtC/yr, still 100 times smaller than the modern rate of
10 GtC/yr emissions by burning of fossil fuels ( Figs. 1 and 2 ).
The prime example from the geologic past of a rapid addition
of CO
2 to the atmosphere happened about 55 million years ago
in an event referred to as the “Paleocene-Eocene Thermal
Maximum” or PETM.
50 , 86 , 87 At that time, the Earth already had
a high atmospheric CO
2 concentration (1000 ppm or more;
Fig. 10 ) and was overall much warmer than it is at present.
Due to some kind of “catastrophic” geologic process, possibly
involving release of methane from the seafl oor,
22 , 40 in a “rela-
tively short” period of about 6000 years, an estimated 3000 to
7000 Gt carbon was transferred from geologic storage to the
atmosphere.
50 , 87 This “catastrophic” event involved carbon
transfer at a maximum rate of about 0.5–1.4 GtC/yr,
17 still about
10–20 times slower than the present rate of 10 GtC/yr due to
combustion and cement production. The addition of atmos-
pheric carbon 55 million years ago produced roughly a doubling
(possibly more) of the amount of carbon in the atmosphere over
this 6000-year time period, from about 1000 to about 2000 ppm
or more ( Figs. 9 and 10 ). Currently, we are in the process of dou-
bling atmospheric CO
2 over a period of about 150 years, which
is 40 times faster and apparently unprecedented in Earth his-
tory. Climate models indicate that a doubling of atmospheric
CO
2 concentration produces an increase in Earth surface tem-
perature of 2–5 °C.
43 At 55 million years ago, the doubling of
atmospheric CO
2 caused an already warm Earth with no polar
ice caps to become even warmer. At present, we are starting
from a relatively cold Earth with polar ice caps and rapidly
headed for an atmosphere with high CO
2 concentration while
the polar ice caps are still present. If this situation ever happened
before on Earth, the last time was probably about 700 million
years ago, long before any kind of complex life was present, and
the conditions that would have prevailed are so unlike the pres-
ent that it strains the best minds to create models that might
describe the Earth's climates at that time.
32 , 33 , 42 , 68 , 76
How to understand what is happening now
Since the beginning of the industrial revolution, human
activity has increased the amount of carbon being transferred
from geologic storage (as limestone, coal, oil, and natural gas) to
the atmosphere . The rate of this transfer is 10 GtC/yr in 2014
( Fig. 1 ), and the rate is increasing by 1 GtC/yr every 4 years. The
current transfer rate is roughly 40–50 times higher than what
is normal for the Earth ( Fig. 2 ). This carbon is effectively
dumped into the surface reservoir “box,” where it is distributed
between the atmosphere, the ocean, soils, and the biosphere in
a matter of decades. The problem is that this carbon cannot be
returned to geologic storage at anywhere near the rate it is being
added because there is no natural mechanism to do it .
Most of the 10 GtC/yr being injected into the atmosphere by
fossil fuel burning and cement production is staying in the sur-
face reservoirs. However, the deeper ocean plays a signifi cant
mitigating role. The net transfers of carbon between the various
reservoirs at present are shown in Fig. 12 (based on Refs. 15
and 34 ). The net transfers are determined by a combination of
means that are described in detail in the IPCC reports and also
summarized on an annual basis by the Global Carbon Project
( www.globalcarbonproject.org ). Currently, the transfer of 10
GtC/yr from geologic storage to the atmosphere is partly com-
pensated by transfer of about 2 GtC/yr from the surface reser-
voirs to the deep ocean. Consequently, 8 GtC/yr is accumulating
in the surface reservoirs. The normal geologic transfers are small
in comparison, and the tiny transfer of carbon into geologic stor-
age as carbonate and buried organic material (0.23 GtC/yr)
just compensates for the continuing volcanic and metamorphic
emissions.
The overall, but simplifi ed, picture is that there is no way for
the ocean system to keep up with the 10 GtC/yr anthropogenic
input. Predictions are that the ocean sink will continue to oper-
ate as it is for the foreseeable future.
2 , 15 , 51 , 72 The ocean has been
taking up a consistent fraction of total emissions for the past
40 years, about 30% of annual fossil emissions. The other 70%
is distributed between the atmosphere and the biosphere. The
atmosphere is currently accumulating about 50% of the fossil
emissions, and the biosphere about 20%. Optimistic projec-
tions for the future are that the biosphere proportion will
Figure 10. Representation of atmospheric CO 2 history from 5 to 65 million
years ago based on oxygen isotopes in marine benthic foraminifera.
Oxygen isotope data are from references given in Ref. 86 . The formula
for CO
2 concentration is modified slightly from Ref. 49 : (CO
2 ) = 280 ×
1.5^(2.6-δ
18 O). Other estimates of CO
2 history are described in Refs. 5 ,
63 , and 66 , but show generally similar values. The spike to high CO
2
concentrations at 55 million years is associated with the so-called PETM
where a large release of carbon to the atmosphere over about 6000 years
temporarily increased atmospheric CO
2 concentration by as much as
1000 ppm above the ambient (already high) level of about 1000 ppm
87
( Fig. 9 ). The Antarctic ice cap began to form at about 34–38 Ma, after
atmospheric CO
2 had dropped below 600–700 ppm.
21 Current atmospheric CO
2 ,
at 400 ppm, is higher than it has been for about the last 10 million
years. If continued fossil carbon release drives CO
2 much above 1000 ppm,
it will then be higher than it has been for roughly 100 million years,
with the exception of the geologically brief PETM event.
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remain constant or increase somewhat, but many models sug-
gest that after the mid-21st century, the terrestrial carbon sink
will start to shrink and by the end of the century there will be no
more capacity for the biosphere to absorb carbon.
1 4 , 1 6 , 2 7
As of 2014, about 400 Gt carbon has been transferred from
geologic storage to the surface reservoirs,
15 an amount that is
about 10% of the original inventory in these reservoirs. Of this
amount, 250 Gt carbon has been added to the atmosphere and
about 150 Gt carbon has been added to the oceans. Much of the
150 Gt carbon added to the oceans has been transferred to the
deep ocean. The terrestrial biosphere and soils have been
approximately carbon neutral overall because the loss of carbon
due to land use change starting before 1750 has been largely bal-
anced by increased carbon uptake over the past 50 years.
Three assumed (approximate) limiting carbon emissions
curves for the next 300 years are shown in Fig. 11 . At the current
(and increasing) rate of carbon emissions, the 400 Gt carbon
total transfer will increase to about 1200 Gt carbon by the end of
the century. The total available amount of coal, oil, and gas that
could be combusted is estimated at roughly 5000 Gt carbon,
1
and if the annual rate were to reach 20 GtC/yr, there could be
2000 GtC transferred just between 2100 and 2200 AD. If an
additional 4500 Gt carbon is emitted over the next 250 years,
and the oceans continue to take up 30%, the ocean inventory
will increase by 1350 Gt carbon. If the biosphere were to con-
tinue to take up 20%, then its carbon mass would increase by
900 Gt. The biosphere mass is currently only about 600 Gt, so it
is probably unlikely that it will be able to take up as much as an
additional 900 Gt carbon
14 , 27 unless the transfer of this carbon
to soils becomes much faster and the soils can somehow retain
that carbon in the face of increasing temperatures.
20 Neverthe-
less, in this scenario, the remaining 2250 Gt carbon is retained
Figure 11. (a) Three possible emission scenarios for the next 200+ years and the total integrated emissions to which they correspond. Black line is
historical emissions to 2014. Redline is the IPCC “worst case” scenario to 2100. Dashed lines are other arbitrary possibilities with lower emissions. (b) Graph
depicting where 5000 Gt of fossil CO
2 emissions is likely to be stored over the next several thousand years, if only “natural” carbon removal mechanisms are
at work (fi gure adapted from Ref. 15 . This model assumes that 5000 GtC is added to the atmosphere at one time at time zero, which would correspond
approximately to the year 2100 in the graph in Fig. 11(a) and the scenario where a total of 4700 GtC is emitted. The atmospheric amount shown is the
amount in addition to the 600 GtC that was in the preindustrial atmosphere. After 2000 years (which would be the year 2100 + 2000 = 4100), there would
still be 1800 + 600 = 2400 GtC in the atmosphere, equivalent to about 1100 ppm. By the year 12,000, the concentration would have decreased only to about
800 ppm and would drift down slowly after that over the subsequent 200–300 thousand years. In the next 50 years, we will choose the emission scenario
that will determine the Earth's climate over the next 100,000 years or more.
12 Q MRS ENERGY & SUSTAINABILITY // VOLUME 2 // e9 // www.mrs.org/energy-sustainability-journal
in the atmosphere. When added to the 840 Gt carbon currently
in the atmosphere, the total would be about 3100 GtC, which
translates to a concentration of 1470 ppm (a little more than
5 times the preindustrial value of 280 ppm). If the terrestrial
biosphere were to take up no more carbon over the next
250 years, then the atmospheric inventor y would reach 4000 Gt
carbon, and the concentration would be 1900 ppm (almost
7 times the preindustrial value). The 1470–1900 ppm range of
concentration values is similar to that produced by the various
more detailed models that have been used.
2 , 70 If, alternatively,
total emissions can be kept to about 1200 Gt carbon, the impacts
would be much less. Peak atmospheric concentration might be
about 680 ppm, and this would require that the terrestrial bio-
sphere take up about 160 Gt carbon, an amount that may be
possible.
Undoing the deed using the Earth system
In most projections of the future of carbon emissions, it is
assumed that in the worst case we will burn all the accessible
combustible carbon (coal, oil, and natural gas). The total
amount is estimated to be 5000 Gt carbon, and this incorpo-
rates the assumption that we will continue to get better at fi nd-
ing and extracting carbon fuels from the Earth. The expect ation,
therefore, is that if we continue on our current trajectory, we
will use up all the carbon fuels over the next 300 years ( Fig. 11 ).
Since we know that the Earth system cycles and recycles carbon,
it might be expected that, after all the carbon is combusted, the
carbon buildup in the atmosphere and other surface reservoirs
will decrease as natural Earth processes operate, and the whole
system will return to the preindustrial state. This is likely to be
true, but because of the sluggishness of Earth processes, the
amount of time it takes is long.
2 , 15 , 70 Once atmospheric CO
2
concentrations get as high as 1470–1900 ppm, it is estimated
that it will take several thousand years to come back down to
1000 ppm, and then more than 100,000 years to get close to the
preindustrial value of 280 ppm ( Fig. 11 ). This is a key point—as
a consequence of the time scales discussed above, even though
we add 5000 Gt carbon to the atmosphere and surface reser-
voirs in 300 years, it will take 100–1000 times as long to get that
carbon out of the surface reservoirs and back into geologic stor-
age through natural mechanisms. Returning the atmosphere to
the preindustrial 280 ppm CO
2 concentration will require that
most of the extra 5000 Gt carbon added to the surface reser voirs
and deep ocean be turned into carbonate sediments on the
seafl oor.
The slow return to background levels of atmospheric carbon
is due to the normal sluggishness of the natural cycles as dis-
cussed above and to complications with the chemistry of the
oceans. The oceans can take up a substantial fraction of the
atmospheric CO
2 , but only if the deep ocean is involved. Hence,
the rate at which the oceans can take up carbon is limited by the
rate at which the surface ocean can mix with the deep ocean;
this time scale is 1000 years as noted above. Even if the absorbed
atmospheric CO
2 is distributed uniformly throughout the shal-
low and deep ocean, the uptake of carbon by the ocean will even-
tually slow down by a large factor. The slowdown occurs because
the oceans become too acidic to absorb more CO
2 . They
reach a new equilibrium with the atmosphere when the ocean
has absorbed about half the excess carbon—about 2500 Gt
carbon.
3 , 70 At that point the atmospheric CO
2 concentration
would still be above 1000 ppm ( Fig. 11 ).
When the oceans cannot take up more CO
2 by simply allow-
ing it to dissolve in ocean water, a much slower process comes
into play. That process is the dissolution of limestone (calcium
carbonate) from the seafl oor.
2 , 3 , 10 The dissolution would occur
because the oceans had become acidic enough that calcite
would no longer be stable on the ocean fl oor. The models of this
process are not particularly certain, but estimates are that the
time scale for dissolution is about 10,000 years. So the projec-
tions are that after 1000 years, while the atmospheric con-
centration will continue to decrease, it will take another
10,000 years to bring the atmospheric concentration down
to 600–700 ppm ( Fig. 11 ).
The 600–700 ppm level of atmospheric CO
2 is a signifi cant
number, because that is the value above which it is believed that
the great East Antarctic ice sheets are not stable. The East Ant-
arctic ice sheets contain an amount of water that is equal to 65 m
of sea level rise. A plausible history of atmospher ic CO
2 over the
past 65 million years is shown in Fig. 10 . This particular CO
2
history is inferred from the record of deep-sea temperature
derived from the oxygen isotope records of bottom-dwelling
foraminifera
86 ; it is generally consistent with other measure-
ments and estimates.
5 , 63 , 66 The record shows that atmospheric
Figure 12. Carbon cycle approximately 2014. The transfer of rate of
10 GtC/yr from geologic reservoirs now dwarfs the natural rate from
volcanoes and metamorphism, and the fl ux of carbon into sediments as
carbonate shells and buried organic material is similarly dwarfed. The only
relief from accumulation of the 10 GtC/yr fl ux in the surface reservoirs is the
transfer to the deep ocean, which is signifi cant at about 2 GtC/yr, but much
smaller than 10 GtC/yr.
MRS ENERGY & SUSTAINABILITY // VOLUME 2 // e9 // www.mrs.org/energy-sustainability-journal Q 1 3
CO
2 was high about 50 million years ago ( ≥ 1000 ppm) and
decreased systematically between 50 and about 35 million
years ago. Prior to about 34–38 Ma, there were no Antarctic ice
sheets. Studies suggest that the Antarctic glaciers did not start
to form until the atmospheric CO
2 concentration decreased to
below about 600–700 ppm.
21 Hence, if atmospheric CO
2
remains above this level for 10,000 years or more, it will mean
that the conditions may be suitable to cause melting of the
East Antarctic ice sheet. Ten thousand years is probably not
enough time to melt a large fraction of the East Antarctic ice
sheet because it is so large, but exactly how much could melt
is uncertain.
During the time that ocean acidifi cation and deep sea car-
bonate dissolution are helping to lower atmospheric CO
2 , the
weathering cycle will also be contributing to reducing atmos-
pheric CO
2 . However, because the time scale for that process is
order 100,000 years, it will not be doing much in comparison to
the other processes. Nevertheless, once the ef fect of dissolution
of deep-sea calcite is exhausted, then only the weathering pro-
cess will be operating to lower atmospheric CO
2 . Consequently,
it would take another 100,000 years or more to get the atmos-
pheric CO
2 concentration from 600–700 ppm back to 280 ppm
solely by natural processes.
The curves shown in Fig. 11 suggest that the next
50–100 years is a critical time. Whatever we decide to do
may determine the Earth's climate for the next 10,000 to
100,000 years. In terms of climate and carbon cycles, the 21st is
the most important century in Earth's histor y at least since the
end of the last Ice Age. We apparently have the option to return
the atmosphere to a state that it was last in 50–150 million years
ago. This is an exciting, if dangerous, experiment, and one for
which we cannot precisely predict the outcome because, as far
as we know, the Earth's atmosphere has never before gone
from 280 to 2000 ppm CO
2 in 200 years.
Undoing the deed with engineering and discovery
There are of course options for limiting emissions of fossil
carbon to the atmosphere. One obvious one is to stop burning
carbon as an energy source, replacing carbon fuel combustion
with other forms of energy production such as solar, wind, and
nuclear.
13 , 62 Getting to this point is expected to take many dec-
ades, and the problem globally is that the demand for energy is
growing faster than the production rate of these alternative
energy sources, especially in countries outside of Europe and
North America.
44 Solar, wind, nuclear, and hydroelectric can
provide for electricity production but will not be suitable for air or
sea transportation, so there is still a need for liquid fuels. The
hope is that biofuels or fuels from artifi cial photosynthesis can be
used for these transportation applications.
57 Biofuel and fuels
made by artifi cial photosynthesis, at least ideally, could be near-
zero net carbon sources to the atmosphere since the carbon in the
fuel comes from the atmosphere through photosynthesis.
An alternative technology that would allow for some contin-
ued carbon combustion is carbon capture and storage (CCS).
In CCS, carbon released from large stationary sources like
coal- and natural gas-powered electricity plants would be cap-
tured, compressed into a denser supercritical state, and
injected underground into deep porous rock formations situ-
ated more than 1 km below the Earth's surface.
6 Ideally, the
injected CO
2 would be permanently retained underground, and
in the long term could be returned to permanent geologic stor-
age as carbonate minerals by the same process (weathering of
silicate minerals to neutralize the acidifying effect of CO
2 dis-
solved in subsurface water, followed by precipitation of car-
bonate minerals) that normally regulates CO
2 levels in the
atmosphere. If broadly deployed globally this technology could
probably intercept about 3–5 GtC/yr from being released to the
atmosphere, which would allow for some continued combustion
of carbon-based fossil fuels. If applied to burning of biomass
(formed with carbon from the atmosphere via photosynthesis)
it could also allow for gradual extraction of CO
2 from the
atmosphere.
One measure of the effi cacy of the global energy system is the
carbon emission intensity of energy production—the amount of
carbon released to the atmosphere per unit energy produced.
57
Although this measure is not discussed frequently, it provides a
way to measure progress toward the ultimate energy goals. Cur-
rently, global primary energy production (the amount of energy
released from the energy generation systems; not the amount
actually available for use) is roughly 160 Petawatt-hour (PWh)
per year. This energy generation is associated with the release
of 10 GtC/yr, so the intensity is about 63 MtC/PWh (MtC = 10
6
metric tons of carbon; Fig. 13 ). Globally, this number has
decreased since about 1900 as coal- and wood combustion have
gradually been replaced fi rst by oil, and then by natural gas,
hydroelectric, and other noncarbon energy sources. However,
prior to about 1850, most of the combusted carbon fuel was not
fossil carbon. In terms of fossil carbon combusted per unit
energy produced, the carbon intensity increased from near zero
in 1800 (prior to widespread coal use) to a maximum of about
65 MtC/PWh in 1960. Subsequently, in the latter part of the
20th century this number was decreasing, but starting in 2000
it started to rise again due to extensive construction of coal-
fi red power plants in China and India. Ultimately, the intensity
needs to decrease to less than 5 MtC/PWh by next century to
keep integrated global carbon emissions below about 1200–
1500 Gt carbon and still meet projected energy demand. This
number can only be achieved by a combination of noncarbon
energy sources and combustion with CCS. In the interim,
switching from coal to natural gas, increasing the efficiency
of coal-fired power generation, and replacing oil and its
by-products with fuels produced from biomass or artifi cial pho-
tosynthesis will help. To reach the target, the carbon intensity of
energy generation needs to decrease by about 5 MtC/PWh each
decade until the middle of next century ( Fig. 13 ).
Summary and conclusions
The Earth's natural systems move carbon among the various
forms (minerals, living and dead organic material, CO
2 and
methane, dissolved carbon, etc.) and reservoirs (atmosphere,
14 Q MRS ENERGY & SUSTAINABILITY // VOLUME 2 // e9 // www.mrs.org/energy-sustainability-journal
oceans, biosphere, soils, crust, and mantle) it exists in and on
the Earth. Atmospheric CO
2 concentration has varied between
the recent ice age values of about 170 ppm, to levels that may
have been more than a thousand times higher billions of years
ago. However, natural Earth processes always move carbon very
slowly, and the previous high atmospheric carbon concentra-
tions occurred long before humans had evolved. Since humans
have existed—the last 1–2 million years—the Earth's atmosphere
has never had more than about 300 ppm CO
2 until 65 years ago.
The only way that the atmospheric CO
2 concentration can
increase by a large factor, as it is doing now, is by transfer of
carbon from geologic storage—as coal, oil, gas, limestone—to
the atmosphere, where it is then distributed between the sur-
face reservoirs (atmosphere, oceans, biosphere, soils) within a
matter of decades. The normal transfer rate of geologic carbon
to the atmosphere is about 0.2–0.25 GtC/yr, and in extreme and
rare cases far in the geologic past that are considered to have
been catastrophic to climate and life, this rate may have been
temporarily as high as 0.5 to 1.4 GtC/yr. The modern rate due to
fossil fuel combustion and cement production reached 10 GtC/yr
in 2014, which is 40–50 times higher than the normal rate and
about 10 times higher than the highest rate documentable in
the geologic record. The current rate of transfer of carbon from
geologic storage to the atmosphere is unprecedented in Earth
history, especially as it is occurring dur ing a time of polar glaci-
ation and low atmospheric CO
2 concentration, and conse-
quently there are uncertainties in predicting the Earth system
response. Ancient examples of periods of exceptional carbon
transfer rates are inadequate analogs because they occurred at
times when climates were warmer, there were no polar ice caps,
and atmospheric CO
2 concentrations were already much higher.
Until now (2014) about 400 Gt carbon has been transferred
from geologic storage to the atmosphere by combustion of fossil
fuel and production of cement.
15 With major decreases over the
next several decades in the fraction of global energy production
coming from fossil fuel combustion, the total amount trans-
ferred might be held to 1200–1500 Gt carbon. In the absence of
such drastic changes, the current increasing rate of emissions
suggests that up to 5000 Gt carbon could be transferred to the
atmosphere over the next 200 years or so. There is a huge differ-
ence in outcomes for these two scenarios. If “natural” processes
are left to remove and redistribute the excess atmospheric car-
bon, their natural sluggish operation will take tens to hundreds of
thousands of years to return the atmospheric CO
2 concentra-
tions to values below 300 ppm that are normal for human habita-
tion. Although the current uptake of carbon by both the oceans
and the land biosphere is a “natural” process in that the process
happens without any purposeful intervention by man, the rates
of uptake are unnatural . The current and anticipated uptake
rates are larger than the normal rates by about 50 times for the
oceans, and even more for the land biosphere. If a goal of limiting
atmospheric CO
2 concentrations to less than about 600 ppm
(still worrisomely high) is to be addressed with new types of
energy production combined with CO
2 mitigation technologies
such as CCS, the objective must be to decrease the carbon inten-
sity of energy production from its current value of 65 MtC/PWh
to about 5 MtC/PWh by 2150. To achieve this objective, the car-
bon intensity needs to decrease by 5 MtC/PWh each decade start-
ing now. Delays will mean that the rate of decrease will need to be
faster to achieve the same limit on total integrated emissions.
Acknowledgments
The author thanks A.P. Alivisatos and S.M. Benson for
encouragement to prepare this review, and to the Offi ce of Sci-
ence, Department of Energy, for its support of an Energy Fron-
tier Research Center in carbon storage science.
Figure 13. Carbon intensity of global primary energy production (squares
and light line) from 1800 to 2010. Prior to the early 1900s, coal and
biomass were the main sources of energy, so the carbon intensity of energy
production reflects their relatively low efficiency (90–100 MtC/PWh).
Subsequent to about 1920, the gradual addition of petroleum, then natural
gas, nuclear, hydro, and renewables, caused a systematic decrease in
carbon intensity through 2000. Since 2000, the large number of new
coal-fi red power plants in China and India has reversed the trend.
44 The
carbon intensity for fossil carbon (circles and bold line) shows a different
trend. From 1800 to 1920, the proportion of energy generated from coal
rather than biomass increased until about half of energy production was
from coal. Fossil carbon intensity continued to increase through 1960
but at a slower rate as a larger proportion of energy was generated from
petroleum. Subsequent to 1960, the trend was downward as natural gas,
nuclear, and hydropower played a larger role. The ultimate objective for
next century is to get the fossil intensity down to below 5 MtC/PWh. This
objective will require that carbon intensity decrease by 4–5 MtC/PWh each
decade, which means systematically shifting to nuclear and renewables
plus possibly CCS-aided coal and natural gas combustion. The recent
announcement by China that it will not reach peak carbon emissions until
2030, means that the necessary steep trajectory toward low intensity will
have been set back by 30–40 years. The difference between the two arrows
shown represents approximately an additional 1200 GtC of total emissions.
This graph also shows that while natural gas combustion is preferable to
coal and petroleum, its carbon intensity is still far too high to get to the
needed levels by next century. The carbon intensity numbers for the various
energy sources are adapted from Ref. 58 and other sources.
MRS ENERGY & SUSTAINABILITY // VOLUME 2 // e9 // www.mrs.org/energy-sustainability-journal Q 1 5
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