The Global Carbon Cycle
Contents
I. Introduction to the Carbon Cycle
Diagram of Global Carbon Cycle
III.
Short-Term Carbon Cycle Models
V. Modeling
the Long-Term Carbon Cycle
Introduction and History
Carbon is unquestionably one of the most
important elements on Earth. It is the principal building block for the organic
compounds that make up life. Carbon's electron structure gives it a plus 4
charge, which means that it can readily form bonds with itself, leading to a
great diversity in the chemical compounds that can be formed around carbon;
hence the diversity and complexity of life. Carbon occurs in many other forms
and places on Earth; it is a major constituent of limestones, occurring as
calcium carbonate; it is dissolved in ocean water and fresh water; and it is
present in the atmosphere as carbon dioxide, the second most important
greenhouse gas.
The flow of carbon throughout the biosphere,
atmosphere, hydrosphere, and geosphere is one of the most complex, interesting,
and important of the global cycles. More than any other global cycle, the
carbon cycle challenges us to draw together information from biology,
chemistry, oceanography, and geology in order to understand how it works and
what causes it to change. The major reservoirs for carbon and the processes
that move carbon from reservoir to reservoir are shown in Figure 7.00 below. We
will discuss these processes in more detail below and then we will construct
and experiment with various renditions of the carbon cycle, but first, we will
explore some of the history of carbon cycle studies.
The global carbon cycle is currently the
topic of great interest because of its importance in the global climate system
and also because human activities are altering the carbon cycle to a
significant degree. The potential effects of human activities on the carbon
cycle, and the implications for climate change were first noticed and studied
by the Swedish chemist, S. Arrhenius, in 1896. He realized that CO2
in the atmosphere was an important greenhouse gas and that it was a by-product
of burning fossil fuels (coal, gas, oil). He even calculated that a doubling of
CO2 in the atmosphere would lead to a temperature rise of 4-5°C --
amazingly close to the current estimates obtained with global, 3-D climate
models that run on supercomputers. This early recognition of human
perturbations to the carbon cycle and the climatic implications did not raise
many eyebrows at the time, but the experiment was just beginning then.
About 60 years later, Roger Revelle, an
American oceanographer, pointed out that we were effectively conducting a giant
experiment with our climate system by emitting more and more CO2
into the atmosphere. One of the problems, he realized, was that we were
relatively ignorant about the possible results of this experiment and so he
decided that it would be wise to begin monitoring atmospheric concentrations of
CO2. In the late 1950's, Revelle and a colleague, Charles Keeling,
began monitoring atmospheric CO2 at an observatory on Mauna Loa, on
the big island of Hawaii. The record from Mauna Loa, shown in Figure 7.01
below, is a dramatic sign of global change that captured the attention of the
whole world because it shows that this "experiment" we are conducting
is apparently having a significant effect on the global carbon cycle. The
climatological consequences of this change are potentially of great importance
to the future of the global population.
As the Mauna Loa record and others like it
from around the world accumulated, a diverse group of scientists began to
realize that we really did not know too much about the global carbon cycle that
ultimately regulates how much of our emissions stay in the atmosphere.
Consider, for instance, the present best estimate of what happens to all of the
carbon dioxide emitted to the atmosphere through human activities, summarized
in the table below.
Carbon Cycle Budget for Anthropogenic Effects
·
Sources:
Fossil Fuel Burning & Cement Production 5.5±0.5 GtC/yr
Forest Burning & Soil Disruption 1.6±1.0 GtC/yr
Total Anthropogenic 7.1±1.1 GtC/yr
Sinks:
Storage in Atmosphere 3.3±0.2 GtC/yr
Oceanic Uptake 2.0±0.8 GtC/yr
Boreal Forest Regrowth 0.5±0.5 GtC/yr
Missing Sink 1.3±1.5 GtC/yr
GtC = Gigatons of carbon = 109tons data from IPCC, 1996
The primary problem in our understanding of
the current state of the global carbon cycle is reflected by the "missing
sink" -- we do not know where all of the anthropogenic CO2 is
going. We will explore this question of the missing sink in several of the
modeling exercises in this chapter.
The importance of present-day changes in the
carbon cycle, and the potential implications for climate change became much
more apparent when people began to get results from studies of gas bubbles
trapped in glacial ice. The bubbles are effectively samples of ancient
atmospheres, and we can measure the concentration of CO2 and other
trace gases like methane in these bubbles, and then by counting the annual layers
preserved in glacial ice, we can date these atmospheric samples, providing a
record of how CO2 changed over time in the past. Figure 7.02 (below)
shows the results of some of the ice core studies, relevant for the recent past
-- back to the year 900 A.D.
The striking feature of these data is that there is an exponential rise in atmospheric CO2 (and methane, another greenhouse gas) that connects with the more recent Mauna Loa record to produce a rather frightening trend. Also shown in Figure 7.02 is the record of fossil fuel emissions from around the world, which show a very similar exponential trend. Notice that these two data sets shown an exponential rise that seems to begin at about the same time. What does this mean? Does it mean that there is a cause-and-effect relationship between emissions of CO2 and atmospheric CO2 levels? Although we should remember that science cannot prove things to be true beyond all doubt, it is highly likely that there is a cause-and-effect relationship -- it would be an extremely bizarre coincidence if the observed rise in atmospheric CO2 and the emissions of CO2 were unrelated.
It is always worth considering if we can test
an hypothesis. Here, the hypothesis is that anthropogenic emissions of CO2
are the cause of the rise in atmospheric CO2.
Can we test this? The answer is yes; there are in fact several ways of testing
this hypothesis. One involves analyzing the ratios of carbon isotopes in CO2
molecules found in the atmosphere. (A brief aside on carbon isotopes -- carbon
atoms don't always have the same number of neutrons in them, so they occur with
different atomic weights 14, 13, and 12, with 12C making up around
98.9%, 13C making up about 1.1%, radioactive 14C, the
radioactive one making up a tiny fraction. 12C and 13C
are stable isotopes meaning they do not decay, while 14C has a
half-life of 5270 years and is continually being produced when 14N
in the atmosphere interacts with high-energy solar radiation.) The CO2
produced by burning fossil fuels has a much
lower ratio of 13C /12C than normal atmospheric CO2.
If we were adding new CO2 that had the same ratio as the rest of the
carbon in the atmosphere, the total amount of carbon will increase, but the 13C
/12C ratio will stay the same; so by adding new with a much lower
ratio of 13C /12C, we are diluting the atmospheric ratio
of 13C /12C.
We therefore predict that if our hypothesis
is correct, there should be a decline in the carbon isotope ratio in the
atmosphere that should match the history of fossil fuel burning. But in order
to test our hypothesis in a meaningful way, we would need to have some record
of the carbon isotope ratio of the atmosphere far enough back to understand the
significance of recent changes. We can get this information from bubbles of air
trapped in glacial ice, and also from tree rings, which of course contain a lot
of carbon and preserve a record of the atmosphere at the time they form. As
shown in Figure 7.02 above, these data do show an exponential decrease
beginning at about the same time as the onset of massive fossil fuel burning,
so our hypothesis has passed the test, increasing our confidence in it.
Another test takes advantage of the fact that
burning fossil fuels consumes an average of 15 oxygen molecules for every 10
molecules of CO2; this means that we predict a decline in the
overall concentration of oxygen in the atmosphere. This turns out to be a very
difficult thing to measure, but Ralph Keeling (son of Charles mentioned above)
has begun to make measurements and has already observed a decline in oxygen.
Thus our hypothesis passes a second test, further increasing our confidence. A
final test comes from the fact that the carbon released from burning fossil
fuels is essentially devoid of 14C, which has a short (5270 years)
half-life; much of the fossil fuel we burn is on the order of 50 to 100 million
years old, hence its depletion in 14C. So, if our hypothesis is
correct, then there should be a measurable decline in the concentration of 14C
in the atmosphere, beginning at about the time when we started to burn fossil
fuels to fuel our industrial revolution. Indeed, this decline in atmospheric 14C
is observed, further strengthening our hypothesis that the rise in atmospheric
CO2 is caused by our burning of fossil fuels.
How serious is our modification of the
natural carbon cycle? Here, we need a slightly longer perspective from which to
view our recent changes, so we return to the records from ice cores and look
deeper and further back in time than we did in Figure 7.02. In addition to
providing a record of the past concentration of CO2 in the
atmosphere, the ice cores also give us a temperature record. By studying the
ratios of stable isotopes of oxygen that make up the glacial ice, we can
estimate the temperature (in the region of the ice) at the time the snow fell
(glacial ice is formed by the compression of snow as it gets buried to greater
and greater depths). From these data, shown in Figure 7.03 below, we can see
the natural variations in atmospheric CO2 and temperature that have
occurred over the past 200,000 years (200 kyr).
Looking at this much longer span of time enables us to clearly see that the present CO2 concentration of the atmosphere is unprecedented in the last 200 Kyr. To find atmospheric CO2 levels equivalent to the present, we have to go back millions of years. This means that, to the extent that the state of the carbon cycle is closely linked to the condition of the global climate, we are pushing the system toward a climate that has not occurred anytime within the last several million years -- not something to be taken lightly.
From this brief look at the record of fossil
fuel emissions and atmospheric CO2 concentrations, it is clear that
we have cause for concern about the effects of this global experiment. Because
of this concern, there is a tremendous effort underway to better understand the
global carbon cycle. In this chapter, we will explore the global carbon cycle
by first examining the components and processes involved and then by
constructing and experimenting with a variety of models. The first set of
models will be relevant to the dynamics of the carbon cycle over a period of
several hundred years -- these will enable us to explore a variety of questions
about how the system will behave in our lifetimes. The second set of models
will provide an introduction to modeling the changes in the isotopic ratios of
carbon alluded to earlier. A third model will be relevant for much longer
periods of time, allowing us to explore a different set of questions.
GO ON TO CARBON CYCLE PROCESSES