The carbon cycle and carbon sequestration
In this section, we focus on the modern carbon cycle – particularly the humaninduced changes in flux of carbon dioxide to the atmosphere and a consideration
of the options for ameliorating that change.
This chapter is not intended to substitute for a basic knowledge of the carbon
cycle. Interested readers who would like an overview or a refresher on the
carbon cycle are referred to Schlesinger’s book Biochemical cycles or Turco’s
Earth under Siege.
Click here http://abacus.bates.edu/~raustin//FluxFlow/CCycle.jpg for a sketch of
the major compartments and estimates of fluxes for the contemporary carbon
cycle. As a check on your understanding of stocks and flows, take a moment to
calculate the residence time of each reservoir (recall that for a system at steady
state that is determined by the stock divided by the flux in or out – at steady state
these are the same).
There are three points to make about our reoccurring themes in this text. First, in
moving from the two previous topics to this topic, the range of scales that we are
concerned with is larger. On the one hand, the scale is much larger. We are
interested in the distribution of carbon all over the world and throughout the
atmosphere. On the other hand, the scale is smaller. We will be concerned with
the roles that iron atoms play in specific enzymes. Time also becomes a less
intuitive factor – something that we need to explicitly consider. The processes
we are most interested are predicted to occur over the next 20-100 years but
they are overlaid on top of processes that have been occurring over millions of
years. This brings us to the final point. Uncertainty plays a more significant role
in this section than it has in prior sections. At times, the order of magnitude of
what we don’t know is about the same as the order of magnitude of the question
we need to answer.
In the simplest terms, humans are currently putting about 3.3 gigatons more of
carbon in the form of carbon dioxide into the atmosphere than the earth appears
capable of reabsorbing. Consequently, the atmospheric concentration of carbon
dioxide is increasing by 1 – 2 ppm per year. In addition, we are unsure where
another approximately 1.8 gigatoms of carbon, again in the form of carbon
dioxide, are going each year. We have a pretty good estimate of how much
carbon dioxide we are producing from combustion and we think we know how
much carbon dioxide is effectively being put into the atmosphere from changes in
land use (primarily deforestation). We have an idea how much “excess” carbon
dioxide the earth (especially oceans) is soaking up. When we add everything up,
we find that there are about 1.8 gigatons of carbon we can’t account for. We can
1
tell that it is not ended up in the atmosphere (because it is pretty easy to measure
atmospheric CO2 concentrations). In some respects, this missing “sink” would
not matter so much except for the fact that if we do not know where it is, we do
not know what sustains it and it could become exhausted quickly, which would
lead to even more carbon dioxide in the atmosphere. 1
There is no question that carbon dioxide in the atmosphere absorbs infrared
radiation and then reradiates some of it back towards earth, thereby keeping the
surface of the earth warm. We know that without carbon dioxide in the
atmosphere, the earth would be way to cold to sustain life. We know that more
carbon dioxide (up to a point at least) will lead to warmer temperatures on earth.
We are not certain just how warm the earth will get. We are not certain how high
atmospheric carbon dioxide concentrations will get before they begin to diminish.
We are not certain about regional differences in average temperatures or
weather. Some of these uncertainties come from limits on our knowledge and
some of these uncertainties come from being unable to predict human behavior
and economic development. We do know that the majority of carbon dioxide that
is currently being produced by humans (and the majority that has been produced
since the industrial revolution) has been produced by a minority of the population.
As countries become more industrialized, populous and developed, especially
large countries like China, it is reasonable to assume that they will produce more
carbon dioxide.
Very few people disagree that some sort of solution is needed. People do
disagree on what that solution should be.
In this section we want to explore different possible solutions to the problem of
excess anthropogenic carbon dioxide. We want to think about the various
options and, most importantly, consider the “order of magnitude” effects that they
1
From the book Consider a Spherical Cow, we find that there are approximately 1.8 x
1020 moles of molecules in the atmosphere. The atmospheric concentration of COe is
currently approximately 370 ppm. That means that out of a million molecules of gas in
the atmosphere, 370 of them will be CO2 (note that this is a different definition of ppm
for the one employed in aqueous environments where ppm is defined as 1 mg of solute
per kg of water and often roughly translated as 1 mg per liter). So, we can calculate that
there are
! 360 $
1.8x10 20 moles #
= 4.48x1016 moles of CO 2
" 1x10 6 &%
! 44 g $
4.48x1016 moles of CO 2 #
= 2.85x1018 gCO 2
" 1 mole &%
! 12 g C $
2.85x1018 g CO 2 #
= 770 x 1015g C or 770 G tons Carbon in the atmosphere
&
" 44 g CO 2 %
2
are likely to have and then see how they compare to the “order of magnitude” of
the problem (currently about 3.3 excess gigatons of carbon per year).
Many college students believe that the solution to carbon problem must lie with
conservation. Many people have an ethical commitment to using less energy,
especially when they recognize the relative gluttony of modern western life.
Many people also believe that it is important that other people are “converted” to
their ethical position and that they come to see how wrong it is to consume so
much energy “needlessly”. 2
Let’s begin with a “back of the envelope” problem. How much carbon could we
save if all rental car companies in the US used high fuel efficiency vehicles?
Take a moment to tackle this problem on your own before continuing to read
because below we’ve spelled out some hints to help you through the problem.
How many miles are driver per year by rental cars?
There are a number of ways to estimate this. A little web searching might give
you an answer. We took an internet-less approach and asked “how many major
airports in the US?” How many cars does each one rent per day? How many
miles does each rented rental car go per day? How long is each rental car
rented?
If we assume that the average rental car fleet value is 20 mpg (another number
that could be looked up) then we can estimate how many gallons of fuel rental
cars in the US currently consume.
Once we know how many gallons of fuel they consume, then if we know that fuel
is primarily octane (C8H18) and that is has a density of 0.726 g/ml, we can
calculate how many grams of octane are burned a year. If we assume 100%
efficient combustion
C8H18 + O2  CO2 + H2O (unbalanced) … you can test your high school
chemistry knowledge and quickly try to balance the equation (recall that you need
the same number of atoms of each element on the right and left hand side of the
equation).
If that knowledge is too rusty, then
2
Please note, it is not our intention to make ethical judgments. We are deliberately trying
to provoke the reader into critically thinking through these issues.
3
C8H18 +
25
O2  8CO2 + 9H2O
2
So for every molecule of octane that is burned, 8 molecules of carbon dioxide are
produced.
Alternatively, and equivalently, for every 1 mole of octane that is burned, 8 moles
of carbon dioxide are produced. A mole of molecules is 6.02 x 1023. To find out
how much a mole of any atom weighs, look on the periodic table.
6
12.011
C
Here is an example of carbon. 1 mole of carbon weighs 12.011 grams. To figure
out how much a mole of any molecule weighs, simply add up the atomic weights
of the atoms in the molecule. So, 1 mole of carbon dioxide (CO2) would weigh
12.011 + 15.9994 + 15.9994 = 44.01 grams.
So, convert gallons to grams using density (as well as some English units to
metric units conversion). Then convert grams of octane to moles of octane.
Using the balanced equation for octane combustion (which you either figured out
or used the one above), convert moles of octane to moles of carbon dioxide and
then convert moles of carbon dioxide back to grams of carbon dioxide.
Next you will need to make an estimate of how fuel efficient a rental car fleet
could be and then rework the calculations with that estimate and then compare
the two scenarios to determine how much carbon dioxide could be saved with
this conservation step.
Another “back of the envelope” question for you – “How much carbon dioxide is
emitted through students taken advantage of your college’s study abroad
program?” Focus your answer on travel to and from the study site, although if
there are other aspects of the problem that you think are worth including, feel
free.
The Kyoto Protocol represented another possible solution to the problem of
excess carbon dioxide in the atmosphere. In this international treaty, developed
4
countries were asked to reduce carbon dioxide emissions by at least 5% below
1990 levels between 2008 and 2012. In the United States, had we chosen to
sign the agreement, our target would have been 93% of the 1990 levels. In
1990, we were emitting 5.96 x 103 teragrams of carbon dioxide equivalents (this
term takes into account other gases that cause global warming). One teragram
is 1 x 1012 grams, so this number represents 6 x 1015grams of CO2 which is the
same as 1.6 x 1015 grams of carbon or 1.6 gigatons of C (one gigaton is 1 x 1015
grams). In 1999, the US was emitting 6746 teragrams of carbon dioxide (1.84
Gtons C). Our Kyoto target would have been 5.6 gigatons carbon dioxide or 1.5
gigatons of carbon, so we are currently over our Kyoto goal by about 0.3 gigatons
of Carbon or 1+ gigatons of carbon dioxide.
Most people agree that the Kyoto Protocol alone would not be enough to reverse
the predicted effects of global warming. It was considered, by its supporters, to
be a “step in the right direction”, which, once implemented, would pave the way
for further action in the future. Still, we can begin by considering various
strategies for meeting the goals that we would have tried to attain had we signed
the Kyoto Protocol.
One approach, which President Bush suggested in 2002 (February 12th) was a
voluntary reduction in greenhouse gas intensity by encouraging industries to
reduce the amount of carbon dioxide that they emit per dollar of GDP. The idea
behind his proposal is that rather than stifle economic development, as critics of
the Kyoto Protocol feared that agreement would do, his proposal encourages
economic development. It simply encourages industries to find ways to make
money while emitting less greenhouse gases. His proposal asked that we take
our current ratio of emissions to GDP (183 metric tons of carbon/million dollars of
GDP 2002) and reduce is to 151 metric tons of Carbon/million dollars of GDP.
Let’s take a moment and calculate what this will mean in terms of reducing our
carbon emissions. Our current GDP is around $10 trillion. If we are releasing
183 metric tons of carbon per million dollars of GDP and we have a total GDP is
10 trillion dollars, then we are currently emitting about 1.83 Gt C (note that this is
for the US only and yet it represents a relatively large percentage of the 3.3 Gt C
that we have been worrying about.
How much CO2 will we be emitting in the future if we follow the administration’s
plan? To determine this, we need to predict the growth of GDP using the
equation for logarithmic growth
N (t) = N (0)ert
where t = time, r = growth rate, N(0) = initial amount and N(t) = final amount.
5
If we assume a growth rate of 2.3% for GDP, GDP in ten years would be 12.2
trillion. If we are able to reach our goal of 151 tonnes/$million GDP that will
mean that we are emitting 1.84 Gton of C in ten years.
How much is that per person? If we have 225 million people, that is
gC
approximately 8 ! 10 6
. What if everyone in the world consumed in the
person year
same way we do and so this ratio of grams of C per person held for all people?
How much carbon would be emitted into the atmosphere?
The section above should give you a sense for how to “back-of-the-envelope”
questions related to the possible effectiveness of conservation in addressing the
modern carbon problem. Click here ____________ for a selection of additional
problems or get together with some friends and talk about the conservation
strategies you think would be most effective and then estimate the order of
magnitude effect these strategies are likely to affect.
For many reasons, people are considering solutions to the carbon problem other
than conservation alone. These other solutions fall into the category of methods
to increase the drawn down of carbon dioxide from the atmosphere (as opposed
to decreasing the amount of carbon dioxide put into the atmosphere). One can
consider them “carbon mitigation strategies” and there are two primary types of
strategies we want to consider. One type of strategy involves increasing the
sequestering capacity of existing biological carbon sinks – primarily terrestrial
sinks (trees, biomass) and marine sinks (phytoplankton). The other type of
strategy involves engineered solutions. The two most often discussed
engineered solutions are deep well injection of carbon dioxide and conversion of
carbon dioxide to carbonate minerals. We’ll discuss each of these options in
sufficient detail for you to get a sense of how they function as well as their
potential capacity for carbon sequestration.
For any potential carbon sink, we need to ask the following questions.
1. how much carbon can be taken out of the atmosphere?
2. How long can the carbon be stored?
3. What are both the likely and unlikely but potentially catastrophic impacts
(and how “unlikely” are the unlikely ones)?
4. What are the costs – both directly financial and environmental? What are
the “opportunity costs”? What else could be done with the resources
being used for mitigation? If land is devoted to planting trees that are then
buried, that land can’t be used to grow food to feed people.
Let’s begin by considering land-based sinks. To many people they seem the
most “benign” of the various options and they have been incorporated into the
6
Kyoto Protocol. How much carbon can be stored in biomass in land? There are
a number of different estimates available because the amount of carbon stored
will depend a great deal on what kinds of plants or trees are planted, how often
they are harvested, how densely they are planted, etc. One project in central
American that involved 12,00 ha community woodlots, 2800 km live fencing (this
is where trees or bushes serve as fences), management of forest fires, and
protection of 2000 ha of vulnerable land was estimated to sequester 16.3 x 106
metric tons of C over the next 40 years. The IPCC estimates that the
technological maximum for forest sequestration is 1.53 to 2.47 Gt C y-1 above
what they are sequestering now (this assumes widespread planting). A general
rule of thumb you can use to estimate the efficiency of plant-based carbon
sequestration is that one half of the biomass of a tree comes from carbon in the
air. The residence time of carbon in biomass is something you should be able to
estimate.
As you estimate it, a concern should immediately pop into your mind. The
residence time of plants is extraordinarily short – certainly too short to offset the
amount of carbon we assume we will burn over the next 100 years UNLESS
those plants are continuously planted and replanted (what concerns from the last
section come to mind as you consider this possibility?). One could consider
extended the residence time of carbon in biomass by burying the biomass where
it would not be vulnerable to aerobic oxidation (anaerobic oxidation is much
slower).
In general, arable land is in short supply and is likely to come under increasing
pressure as human population continues to increase.
So, what about using the oceans to sequester carbon? How would that be done?
One can’t plant trees in the ocean. No, one can’t. But the ocean has lots of plant
life in it in the form of small single celled organisms called phytoplankton. These
microscopic plants are thought to contribute about 25-40 Gt C y-1 to annual
primary productivity. Phytoplankton forms the basis of the marine food web. Are
there ways in which we could increase this number and grow more plants in the
ocean?
Think back to the last section on nutrients in lakes. What was the key issue we
considered in that section? In that section we saw that nitrogen and particularly
phosphorous are limit growth in those aquatic environments. Run off from farms
and lawns bring those nutrients to lakes, where they stimulate growth that in
those environments seems “excessive” or “undesirable” (because it looks ugly
and ultimately leads to oxygen depletion). However, how does that same
reasoning inform our thoughts about deliberately increasing the primarily
productivity in the oceans to increase the amount of carbon stored in the ocean?
7
The oceans are characterized by very low concentrations of a number of
elements that are essential for life. In comparison to lakes, concentrations of
trace and macronutrients are often thousands of times lower. Scientists consider
open oceans to be “desert-like” because they are so bereft of the elements
organisms need to grow and live. In the 1980’s, the late John Martin
hypothesized that there are vast regions of the ocean where iron is the limiting
nutrient. For people who don’t study such topics, the idea that iron could limit
growth seems odd. Certainly most of us don’t think of ourselves as “made of
iron”. Yet iron is one of the most essential trace elements. It plays critical roles
in many enzymes that catalyze essential life processes and in proteins required
for the transport of small molecules (like oxygen) and for moving energy through
our bodies (and the bodies of all living organisms). Given oddities in the
chemical evolution of our earth (specifically the introduction of oxygen into the
earth’s atmosphere about 2 billion years ago), iron, which is one of the most
abundant elements in the earth’s crust and which formed a key ingredient in the
development of primordial life, became extraordinarily unavailable (insoluble) in
the open oceans. See relevant section of website for an overview of the
chemical and physical factors that influence the flow of materials. There have
now been a number of mesoscale experiments that have demonstrated that the
addition of iron to the open ocean results in a striking bloom of phytoplankton
with a 10-20 fold increase in the concentration of phytoplankton as measured by
the concentration of chloropyll A3. The lab associated with this chapter explores
this same idea and gives you an opportunity to measure chlorophyll A yourself.
So, what about this observation that fertilizing the oceans with iron can lead to a
10-20 fold increase in phytoplankton concentration? What might the significance
of this observation be for the idea of using the oceans to store carbon? What if
we could increase oceanic primary productivity by a factor of 10? Could we go
from drawing down 25 Gt of carbon per year to 250 Gt? Wouldn’t that more than
take care of the excess three Gt of carbon we’re currently putting into the
atmosphere?
Hopefully by now you are saying “Stop! What is the residence time of carbon in
phytoplankton? If you just said that iron is very insoluble in the ocean, how long
does the iron fertilizer persist in the surface ocean (remember from the last
chapter that it is only on the surface of water that there is abundant light for
photosynthesis)? And how is this iron getting to the oceans? Are fossil fuel
consuming boats taking it there? Is primarily productivity in all of the oceans
limited by iron?
3
These experiments include IRONEX 1 (1993) and II (1995) in the equatorial south
Pacific and SOIREE (1999) and EISENEX (2000) in the southern ocean. See Nature
2000, 407, 695-702 for a peer reviewed report.
8
Take some time to think about these questions that you have just raised. Most of
them are significant shortcomings with fertilizing the oceans as a carbon
mitigation strategy. Phytoplankton generally live less than a month. When they
die, perhaps as much as 90% of their biomass are remineralized immediately.
No more than 10% is estimated to sink to the deep ocean where residence times
are significant, although fast growing phytoplankton (like those that are fertilized)
may be more buoyant and sink less readily. Only 16% of the world’s oceans are
thought to be iron limited and many of these are in the southern ocean, where
year round travel is difficult. Some scientists are concerned about changes in
communities resulting from fertilization (see the Policy Forum by leading
oceanographers Chisholm et al in Science4) while other people argue that the
side benefits of fertilization – more phytoplankton = more fish – make iron
fertilization beneficial regardless of its ultimately ability to draw down carbon.
Iron can be formulated in a more soluble form that often contains nitrogen as
well.
How much would iron fertilization cost? One of the initial iron fertilization
experiments fertilized 30 square miles of open ocean with 3.5 tons of soluble
iron. The resulting bloom lasted one month. Ship costs can be significant
although some people have relied on sail boats to distribute iron and other
people have suggested adding iron “sprinklers” to ocean going boats. Given all
this information, estimate the cost to sequester 1 Gt of carbon per year. What
are the opportunity costs associated with this technology?
The other two options for carbon sequestration require us to capture carbon
dioxide before it is emitted into the atmosphere. What immediately comes to
your mind as a challenge that would need to be solved before this technology
could be maximally effective? Yes, you are right – transportation. This
technology, to be maximally effective, would require transportation that does not
burn fossil fuel. That is not an insurmountable challenge. We already know how
to make hydrogen from fossil fuels and could concert fossil fuels to hydrogen
(albeit without 100% efficiency5) and then burn hydrogen in fuel cells. Facilities
would have to be developed that could scrub carbon dioxide prior to emission.
The primary technology that is being developed at the moment involves deep
well injection of liquefied CO2. Interestingly, this is a technology that is already
being used to help extract fossil fuels from select oil fields. It is a technology that
also builds on the well-established technology of deep well injection of hazardous
wastes, which (as one might imagine) has been subjected to extraordinarily
rigorous testing and scrutiny. To be permitted in the US, a hazardous waste
4
“Dis-Crediting Ocean Fertilization”, Sallie W. Chishol, Paul G. Falkowski, John J.
Cullen, Science, 2001, 309-310.
5
It is estimated that the conversion of gas to H2 can occur with 72% efficiency, oil to H2
with 76% efficiency, and coal to H2 with 55-60% efficiency
9
injection site must demonstrate that it can safely sequester hazardous waste for
10,000 years. The basic idea is that liquefied carbon dioxide is injected into
saline aquifers deep under the earth’s surface. The assumption is that the
carbon dioxide will migrate out very very slowly, taking thousands of years.
Hence deep well injection of liquefied carbon dioxide is seen as a carbon
mitigation technology with a sufficiently long residence time.
The first commercial scale carbon dioxide reinjection project took place at
Sleipner Project in the North Sea off the coast of Norway. One millions tons of
carbon dioxide were injected per year since 1996. There has been extensive
monitoring and modeling of the Sleipner site and it is from this data that scientists
estimate that deep well injection of carbon dioxide could safely sequester carbon
for thousands of years. Another project is the Weyburn oil recovery in
Saskatchewan, Canada, where carbon dioxide from a coal gasification plant in
North Dakota is being injected into an active oil field.
The last technology to consider is mineral carbonation where rocks are formed
from CO2. This is a process that occurs very slowly in nature. The idea is to
expedite the process by developing it into an industry. The byproduct would be
rock, which weathers slowly and is essentially energetically stable and thus also
sequesters carbon for thousands of years, by which time we will either have
annihilated ourselves or developed alternative fuels.
(Mg,Ca)xSiyOx+2y+zH2z + xCO2
x(Mg,Ca)CO3 + ySiO2 + zH2O
e.g. Mg3Si2O5 + 3CO2 --> 3MgCO3 + 2SIO2 + 2H2O
serpentine
to magnesite, releases 64 kJ per mole of CO2
It is estimated that up to 10 x 1012 tons of carbon can be sequestered using this
method. In the ideal case, one ton of serpentine can one half ton of carbon
dioxide. Since the reaction is energy releasing (exothermic), the overall process
has the possibility of being economically viable. The starting materials must be
mined, which has associated costs, and current technology has not developed
methods to adequately speed up the reaction or to optimize the thermodynamics
of the reaction.
Estimate how much rock would be produced if this method were the sole method
used to enable the US to meet the Kyoto Protocol. Mining similar rocks as those
used for starting materials is estimated to cost about $9 per ton of ore in a mine
that is generating about 20,000 tons of ore per day.
10
Let’s take some time to summarize the key issues in this section.
Below is a slide that I was graciously given by the Princeton University Carbon
Mitigation Institute (CMI). It shows one estimate of how much carbon the world is
estimated to release per year over the next one hundred years in a “business as
usual” scenario (solid line) and how much carbon the world must release per year
if atmospheric carbon dioxide concentrations are going to stabilize at 550 ppm
(almost 200 ppm above where they are today).
This gives you a good sense for the magnitude of the problem. The scenario
above assumes no decrease in carbon emissions until about 2012. However that
scenario assumes that carbon emissions will increase significantly in the
developing world, an increase that would need to be offset in part by a decrease
in emission in the developed world. Take the problem you did earlier in this
section on how much carbon would be emitted if all the world’s population
11
emitted carbon with the carbon per person ratio emitted in the US and consider
that number in light of the reductions ultimately necessary to stabilize CO2 at 550
ppm.
At the beginning of the section you played around with conservation strategies.
The one strategy we have not explored is the development of renewable fuels,
which would largely obviate the problems of global warming. At the moment, we
don’t have a suite of renewable energies ready to replace fossil fuels. It is
important to envision when they might be available, but there is much uncertainty
associated with these estimates. After exploring conservation strategies, you
considered various engineered solutions. Many environmentalists reject these
strategies – at least the ones that require humans to do anything besides plant
trees. At the same time, the republican administration of George W. Bush has
embraced carbon sequestration. From the DOE web page in 2003
While many countries, including the United States, are committed to
substantial efforts to enhance the deployment of renewable energy
sources and to developing energy efficient technologies for the longer
term, fossil energy use for power generation worldwide is projected to
double by 2030. Many nations are also advancing new technologies for
nuclear energy, which emits no greenhouse gasses. These measures will
help in reducing emissions of greenhouse gases, but most scientists
believe that they alone will not be sufficient to meet the goals of stabilizing
atmospheric concentrations of greenhouse gases at acceptable levels.
In fact, global emissions of carbon dioxide from human activities are
projected to increase 60 percent by 2020 as many nations consider to rely
on coal, oil and natural gas to fuel economic growth. Fossil energy is too
large a part of the global economy and too inherently cost-effective to
realistically eliminate from the world’s energy mix.
Carbon sequestration, however, offers the potential for countries to
achieve large-scale reductions of greenhouse gases without necessitating
massive and economically disruptive changes to their energy
infrastructures.
What do you think? Do engineered solutions seem necessary? Are they
fundamentally problematic because they relieve the necessity to make profound
ethical and cultural changes? Is that a sensible question to ask?
A final alternative would be to simply adapt to the changes global warming will
bring. We could try to neutralize excess atmospheric carbon by painting roads
white, putting reflectors in space, maybe even putting aerosols in space to cool
12
the earth and increasing hurricane insurance, moving people to higher ground,
and developing agriculture in northern climates.
Hopefully by the time you have finished working through this section you have
more questions than you did when you started. This is a complex subject about
which much has been written. At the end of this chapter, we have compiled a list
of references for further study,
http://abacus.bates.edu/~raustin//FluxFlow/RefCarbon.pdf
trying to limit our list to peer reviewed publications, government publications or
publications from scientifically well respected non-governmental organizations.
We apologize in advance for links that will have broken by the time you check
them. Please let us know if you have suggestions for changes.
13