The greenhouseeffect allowed both the organic
and the geochemical carbon cycles to begin. The
carbon cycle most familiar to Earth readers is the
organic cycle: we participate in it whenever we
breathe.We inhale oxygen and exhalecarbon dioxide. Then green plants return the favor by taking in
our carbon dioxide and returning oxygen.
But the Earth has its own breath, breathing in
CO2,then returning it. This geochemicalcarbon cycle is so gradual that only now are we beginning to
appreciate its power. Between the two cycles, the
Earth has achieved a balance, taking in just as
much CO2as it releases.
A calcite crystal forms from a drop of water in this underground
cave. This slow process has locked our ancient atmosphere in rock.
Photo by Chris Anderson.
Mars
and Venus formed at the same time as
Earth and from similar elements,yet neither one of
them learned to breathe in and out: Venus exhaled
all its CO2while Mars still holds its breath.
Only Earth is still breathing.
Just as in the story of Goldilocks and the three
bears,Venus is way too hot, Mars is way too cold,
and Earth is just right. Venus never learned to inhale CO2 into its rocks and overheatedearly in its
history, boiling away all its oceans. Mars never
learned to breathe CO2back out of its crust and is
freezing without a warm blanket of atmosphere.
As mentioned,while Earth'soriginal atmosphere
had roughly as much CO2 as Venus' atmosphere
today (96.5percent), Earth has removed this huge
burden of CO2and locked it up in the rock beneath
our feet, giving us a pleasantworld.
The story behind why our atmosphere turned
pleasant takes 4.5 billion years - the entire history
of Earth as a planet. By examining a few chapters
in that story, we will better understand the greenhouse effect as well as two important Earth cycles:
the geochemicalcarbon cycle and the organic carbon cycle.
28 EARTH
T he carbon cycle started in the earliest years of
our planet, so let's examine what was happening
in the atmosphereand on the surfacethen.
It appearsthat even during Earth's early years,
from 4.5 to 3 billion years ago,~the planet had all
the essential ingredients for the carbon cycle:
oceans,atmosphere, continents, and plate tectonics. It also had the Sun, but with an important difference:the Sun was only 70 percent as bright as it
is today. If the Sun were only that bright now, icebergs would clog the PanamaCanal.
And therein lies the first mystery. With such a
dim Sun, Earth might have remained a solid ball of
rock and ice until about 2 billion yearsago, waiting
for the Sun to warm. Yet geologiststell us that our
oceans have always been liquid, that they have
never been a solid pack of ice stretching from the
North Pole to the South Pole,dim Sun or no.
The answer to this enigma is the greenhouse.
Carbon dioxide is nature's thermostat, which
controls the temperature in the greenhouse.Acting
like glass in a greenhouse, CO2 controls the balanceof radiant energy Earth receivesfrom the Sun
and radiatesback into space.The greenhouseeffect
explains why on a sunny sub-zero day, we can
stand inside an unheated glass structure and be
warm, even get a suntan. Light enters through the
clear glass and warms our skin. Just as hot charcoal glows red, our skin glows with infrared radiation as it is warmed. This wavelength is invisible to
our eyes, but we can feel it on our skin. But infrared cannot pass back through the glass, so the
room warms. In the atmosphere,sunlight streams
through CO2like x rays cut through paper, but the
infrared finds it difficult to do the same.
Carbon dioxide is a necessarypart of today's
atmosphereas well as the primordial sky. If all of
the CO2 were stripped out of our heavens,the radiant energy balance would change and global
temperatureswould drop some 700F (350C). Conversely, if no radiant energy escaped,EartK would
roast as Venus does now. Carbon dioxide, along
with water vapor, methane, and several other
greenhousegases,allows just enough energy to be
radiated back into space, keeping Earth the way
we know and like it best.
During the first few billion years of Earth's history, the greenhousethermostat was turned all the
way up for maximum warming. This early warming system allowed life to develop much earlier
than it might have otherwise. This premier greenhouse was well-timed becauseit allowed the primordial oceansto stay liquid. In stark contrast,the
oceansof Venus boiled away into their dense atmosphere,then escapedinto space.Geologistsare
still puzzling over what Mars did with its seas.
But Earth enjoys warm oceans,the only planet
in the solar system to do so. One reason liquid
oceans are .critical to life is that they give us
weather, especially the wet kind. Freed by the
greenhouseto roam the world, oceanswere able to
evaporateprodigious amounts of moisture into the
skies, generating clouds, hurricanes, storms, and
rivers, in a processcalled the hydrological, or water, cycle. A thundering water cycle allowed the
weather to begin in earnest.
What weather it must have been! Winds blasting acrossopen plains with no trees to slow them
and no vegetationto prevent erosion.Little oxygen
except for a smattering on the face of the oceans
that the unfiltered radiation of the Sun would ionize from the water. With so little oxygen - nothing would bum. With so little oxygen, there was
no ozone layer, so the planet was scorchedby dangerous, purplish, ultraviolet light, its high-energy
beams so powerful that they tend to change the
chemicalcontent of cells,killing them.
Our planet, in short, was a world void of life
but full of potential.
Life had plenty of excusesnot to start up at all
under theseconditions, but it did anyway. Its strategy for survival was to run away, hiding from the
burning ultraviolet light by putting a layer of
oceanbetween itself and the Sun. It used about 30
feet (10 meters) of ocean as sun screen. The first
cells were not too fancy (they did not even have a
nucleus),and were dependent on whatever minerals were floating by for food.
Nevertheless, life continued peacefully under
the waves for about a billion years. Then something happened that would change Earth forever:
chlorophyll. The immediate benefit of this green
pigment was that plants could take whatever meager sunlight existed at those depths and use it to
manufacture their own food right on the spot. The
organic cycle began when, through this process,
called photosynthesis, individual cells began to
make their own food from sunlight, water, and
CO2 rather than try to get by on low-energy flotsam. (In microfossils,scientistshave found the imprints of primitive chlorophyll-based organisms
We are standing on, and mining, Earth's ancient atmosphere.
This limestonequarry was once in a marine environment.
Photo by StevenA. Zaburunov.
similar to bacteria that we know today. The Warrawoona Formation, located in Western Australia,
is known for thesefossilsbelieved to be more than
3 billion yearsold.)
The teeming oceansled a tranquil life for years
on end. Then something remarkable started to
happen. Ocean life changed the environment.
Green organismsgave off oxygen as a waste product, just as they do today. It took this green life
lounging in the ocean depths a couple of billion
years to add significant amounts of oxygen to °\lr
skies,but they did.
/
These higher levels of oxygen must have
wreaked havoc on the landscape.Rocks and minerals that were resistant to weathering in a CO2atmosphere now were being ravaged by oxygen's
terrible activity. Even in the ocean,the oxygen reacted quickly with just about everything. Iron, for
JANUARY 1992 29
TheEarth breathesout through volcanoes,as with Mt. Pinatubo
in the Philippines. Photo by Daniel V. Dungla.
example, soon oxidized (rusted) and settled out
into massivebeds. Someof these beds, created as
the oxygen atmosphereformed, were raised by tectonics and are now important sourcesof iron ore.
The increasing levels of oxygen not only ravaged the oceansand land, they created the ozone
layer that blocks harmful ultraviolet radiation. This
provided a marvelous benefit for life: ozone
blocked much of the raw ultraviolet light from the
Sun. With the dangerousultraviolet light problem
solved, the green plants found they could live
closer to the surface of the ocean.The production
of oxygen snowballed: the more ultraviolet light
that was shut ~ut, the more life increased,which in
turn produced more oxygen. In a few billion years
it was safe to go to the beach,so to speak. Exactly
when our atmospherebecameoxygen-rich is estimated to be a brief 500million yearsago.
This, then, is the chapter on the origins of the
organic cycle.
32 EARTH
Meanwhile,
as the greenhousewas keeping the
oceansliquid so the organic cycle could build up,
the geochemicalcycle was finding the water cycle
convenient for its own ends. Long before the organic cycle began, all the necessaryingredients for
the geochemical cycle were found in abundance:
liquid oceans,a bit of weather to move the clouds
around, plenty of exposed rock, and a seemingly
unlimited amount of CO2 inthe atmosphere.
A single geochemical cycle removes one
molecule of CO2from the atmosphereand locks it
in rock. To do this, water containing CO2 (raindrops) dissolves silicate mineral (usually feldspar,
a silicate mineral found in basalts and granites
and a regular ingredient of mountains). The dissolved stone is then free to flow off the mountain,
into caves,rivers, lakes, or the ocean.Somewhere
along the way, the carbon is locked up into a new
variety of rock, calcium carbonate.These carbonates often end up on the bottom of the oceansbut
can be found in limestone caverns as stalactites
and stalagmites.
The silica-rock weathering process proved to
be the key to the successof the geochemicalcycle.
The task of removing CO2 from the skies and
building rock was enormous. The original atmosphere was not only loaded with CO2,it was jampacked. By adding up all the CO2 locked in the
crust and mantle of Earth, scientistsproject that the
original atmospherehad so much CO2 that the air
pressurewas 60 timesgreater than today. It makes
our presentday atmosphereappearthin.
If all the carbon in our present-dayatmosphere
is counted as one unit, the measure locked up in
rock is 100,000units. The amount of carbon tied up
in recoverablefossil fuels is about 5.5 units, while
the oceans In a planetary perspective, there is
roughly as much carbon in the rocks of Earth as
there is in the atmosphereof Venus (which has an
air pressure90 times as great as ours).
Besides locking CO2in rock, the geochemicalcycle restoresCO2back to the atmosphere,primarily
in subduction zones where the ocean's crustal
plates dive under the edges of continents. In the
middle of the ocean, upwelling molten magma
drives the renewal process, pushing the plates
apart at the mid-oceanridg~s.
The ocean floor is on the move, speeding
along at almost two inches per year in places.
That's not too fast on the Autobahn, but it is
Mach nine on the geologic speedometer. It is
so fast, that the enormous geochemical engine
takes only 200 million years to turn over. The
young ocean floors are only a mere few hundred million, while the mature continental
rock is often billions of years old. The oldest
surface rocks found so far are 20 times older
than the oldest rock on the ocean floor.
The fact that ocean plates slide means that all
the carbon locked up in oceanfloors has a chance
to get free again. The subducted plates grow hot
from the incredible friction generated by sliding
under a continent. The subducted plates begin to
melt, forming magma. This magma becomes
lighter than the cooler rock and starts making its
way toward the surfacewhere it releasesCO2back
to the atmospherethrough volcanos.If the red-hot
lava appearsto be boiling, that's becauseit is boiling off CO2and other gases.
If heat and pressure are applied to carbonate
rocks, there is a possibility that CO2 will be released.As plates collide becauseof tectonic forces,
the rock can fold and bend. Called metamorphosis,
this process of bending and twisting rock generatesheat and pressure,enough to produce a chemical reaction. After the gas escapes,the rock may
return to what it was to begin with, feldspar. The
geochemicalprocesscan begin again if the metamorphosedrock is exposedto weathering.
As the gas rises toward the surface,it dissolve
easily with groundwater. This is why some spring
water, even though it is not as hot as the deep
molten magma,is naturally carbonated.
T wo hundred years ago around the start of the
Forestsproduceoxygen in the organic carbon cycle.
Industrial Revolution, CO2 levels were at an alltime low, about 285 parts per million (ppm). The
following 200years of industry have increasedlevels of CO2 by 25 percent to present levels of 360
ppm, as recently measured by the National
Oceanicand Atmospheric Administration's Mauna
Loa Observatory in Hawaii. Yet CO2 continues to
be releasedby the geochemicalcycle and the burning of fossil fuels.
Studying thesepast levels and processesis useful becausethe information gained may help predict future climate changes. Although computer
simulations are powerful tools to predict increasing levels of CO2, the ideal solution would be to
find a planet somewhere,jack up the levels of CO2,
and seewhat happens.Although there are no planets handy to play with, we do have a perfect laboratory: Earth through the last 4 billion years. The
only limitation is that the experiments have all
beenconducted;now all that's left is to find the ancient lab books and look through them.
Geologists and climatologists are doing just
that. Tracesof the ancient atmosphereare literally
written in stone. Eachlayer of sedimentary rock is
like a page in the lab book, revealing the climate of
that particular age. The pages of this ancient lab
book are still being found and deciphered, with
many discoveries remaining to be made. But we
have already confirmed that when the temperature
Photoby WilliamR. Edgar.
went up, the atmospheric CO2 levels were higher
as well. Likewise, when global temperatures fell,
so did levels of CO2,
One page of this lab book suggests that unusual volcanic activity in the mid-Cretaceous
caused the increased temperatures of that time.
During the Cretaceousperiod, when the dinosaurs
thrived, global temperatures were significantly
warmer by about 10° C. Thesewarm temperatures
allowed life to boom, with the green belt extending
from the equator to the Antarctica. We are still using the coal and oil that originated in this fertile
environment. High levels of volcanic CO2 (3,500
ppm) appear to have maintained the global temperaturesduring these100million years.
Will an increasein CO2causeglobal warming?
All other factors remaining\constant,yes. But how
fast, how soon, and at what threshold - these
questionsremain locked in the ancient lab book.
Nevertheless,even though we don't know all
the scientific language theselab books use, we can
easily read someparts in any language.Simply by
skimming through the chapters, as we have done
here, it is easy to read between the lines that becauseof the actions of two great natural cycles,we
are living in a paradise.
JANUARY 1992 33