The greenhouse effect allowed both the organicand the geochemical carbon cycles to begin. Thecarbon cycle most familiar to Earth readers is theorganic cycle: we participate in it whenever webreathe. We inhale oxygen and exhale carbon diox-ide. Then green plants return the favor by taking inour carbon dioxide and returning oxygen.

But the Earth has its own breath, breathing inCO2, then returning it. This geochemical carbon cy-cle is so gradual that only now are we beginning toappreciate its power. Between the two cycles, theEarth has achieved a balance, taking in just asmuch CO2 as it releases.

A calcite crystal forms from a drop of water in this undergroundcave. This slow process has locked our ancient atmosphere in rock.

Photo by Chris Anderson.

T he carbon cycle started in the earliest years ofour planet, so let's examine what was happeningin the atmosphere and on the surface then.

It appears that even during Earth's early years,from 4.5 to 3 billion years ago,~ the planet had allthe essential ingredients for the carbon cycle:oceans, atmosphere, continents, and plate tecton-ics. It also had the Sun, but with an important dif-ference: the Sun was only 70 percent as bright as itis today. If the Sun were only that bright now, ice-bergs would clog the Panama Canal.

And therein lies the first mystery. With such adim Sun, Earth might have remained a solid ball ofrock and ice until about 2 billion years ago, waitingfor the Sun to warm. Yet geologists tell us that ouroceans have always been liquid, that they havenever been a solid pack of ice stretching from theNorth 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. Actinglike glass in a greenhouse, CO2 controls the bal-ance of radiant energy Earth receives from the Sunand radiates back into space. The greenhouse effectexplains why on a sunny sub-zero day, we canstand inside an unheated glass structure and bewarm, even get a suntan. Light enters through theclear glass and warms our skin. Just as hot char-coal glows red, our skin glows with infrared radia-tion as it is warmed. This wavelength is invisible toour eyes, but we can feel it on our skin. But in-frared cannot pass back through the glass, so theroom warms. In the atmosphere, sunlight streamsthrough CO2 like x rays cut through paper, but theinfrared finds it difficult to do the same.

Carbon dioxide is a necessary part of today'satmosphere as well as the primordial sky. If all ofthe CO2 were stripped out of our heavens, the ra-diant energy balance would change and globaltemperatures would drop some 700 F (350 C). Con-versely, if no radiant energy escaped, EartK wouldroast as Venus does now. Carbon dioxide, alongwith water vapor, methane, and several othergreenhouse gases, allows just enough energy to be

Mars and Venus formed at the same time asEarth and from similar elements, yet neither one ofthem learned to breathe in and out: Venus exhaledall its CO2 while 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 in-hale CO2 into its rocks and overheated early in itshistory, boiling away all its oceans. Mars neverlearned to breathe CO2 back out of its crust and isfreezing without a warm blanket of atmosphere.

As mentioned, while Earth's original atmospherehad roughly as much CO2 as Venus' atmospheretoday (96.5 percent), Earth has removed this hugeburden of CO2 and locked it up in the rock beneathour feet, giving us a pleasant world.

The story behind why our atmosphere turnedpleasant takes 4.5 billion years - the entire history

of Earth as a planet. By examining a few chaptersin that story, we will better understand the green-house effect as well as two important Earth cycles:the geochemical carbon cycle and the organic car-bon cycle.

28 EARTH

radiated back into space, keeping Earth the waywe know and like it best.

During the first few billion years of Earth's his-tory, the greenhouse thermostat was turned all theway up for maximum warming. This early warm-ing system allowed life to develop much earlierthan it might have otherwise. This premier green-house was well-timed because it allowed the pri-mordial oceans to stay liquid. In stark contrast, theoceans of Venus boiled away into their dense at-mosphere, then escaped into space. Geologists arestill puzzling over what Mars did with its seas.

But Earth enjoys warm oceans, the only planetin the solar system to do so. One reason liquidoceans are .critical to life is that they give usweather, especially the wet kind. Freed by thegreenhouse to roam the world, oceans were able toevaporate prodigious amounts of moisture into theskies, generating clouds, hurricanes, storms, andrivers, in a process called the hydrological, or wa-ter, cycle. A thundering water cycle allowed theweather to begin in earnest.

What weather it must have been! Winds blast-ing across open plains with no trees to slow themand no vegetation to prevent erosion. Little oxygenexcept for a smattering on the face of the oceansthat the unfiltered radiation of the Sun would ion-ize from the water. With so little oxygen - noth-ing would bum. With so little oxygen, there wasno ozone layer, so the planet was scorched by dan-gerous, purplish, ultraviolet light, its high-energybeams so powerful that they tend to change thechemical content of cells, killing them.

Our planet, in short, was a world void of lifebut full of potential.

We are standing on, and mining, Earth's ancient atmosphere.This limestone quarry was once in a marine environment.Photo by Steven A. Zaburunov.

Life had plenty of excuses not to start up at allunder these conditions, but it did anyway. Its strat-egy for survival was to run away, hiding from theburning ultraviolet light by putting a layer ofocean between itself and the Sun. It used about 30feet (10 meters) of ocean as sun screen. The firstcells were not too fancy (they did not even have anucleus), and were dependent on whatever miner-als were floating by for food.

Nevertheless, life continued peacefully underthe waves for about a billion years. Then some-thing happened that would change Earth forever:chlorophyll. The immediate benefit of this greenpigment was that plants could take whatever mea-ger sunlight existed at those depths and use it tomanufacture their own food right on the spot. Theorganic cycle began when, through this process,called photosynthesis, individual cells began tomake their own food from sunlight, water, andCO2 rather than try to get by on low-energy flot-sam. (In microfossils, scientists have found the im-prints of primitive chlorophyll-based organisms

similar to bacteria that we know today. The War-rawoona Formation, located in Western Australia,is known for these fossils believed to be more than3 billion years old.)

The teeming oceans led a tranquil life for yearson end. Then something remarkable started tohappen. Ocean life changed the environment.Green organisms gave off oxygen as a waste prod-uct, just as they do today. It took this green lifelounging in the ocean depths a couple of billionyears to add significant amounts of oxygen to °\lrskies, but they did. /

These higher levels of oxygen must havewreaked havoc on the landscape. Rocks and min-erals that were resistant to weathering in a CO2 at-mosphere now were being ravaged by oxygen'sterrible activity. Even in the ocean, the oxygen re-acted quickly with just about everything. Iron, for

JANUARY 1992 29

Meanwhile, as the greenhouse was keeping theoceans liquid so the organic cycle could build up,the geochemical cycle was finding the water cycleconvenient for its own ends. Long before the or-ganic cycle began, all the necessary ingredients forthe geochemical cycle were found in abundance:liquid oceans, a bit of weather to move the cloudsaround, plenty of exposed rock, and a seeminglyunlimited amount of CO2 inthe atmosphere.

A single geochemical cycle removes onemolecule of CO2 from the atmosphere and locks itin rock. To do this, water containing CO2 (rain-drops) dissolves silicate mineral (usually feldspar,a silicate mineral found in basalts and granitesand a regular ingredient of mountains). The dis-solved stone is then free to flow off the mountain,into caves, rivers, lakes, or the ocean. Somewherealong the way, the carbon is locked up into a newvariety of rock, calcium carbonate. These carbon-ates often end up on the bottom of the oceans butcan be found in limestone caverns as stalactitesand stalagmites.

The silica-rock weathering process proved tobe the key to the success of the geochemical cycle.The task of removing CO2 from the skies andbuilding rock was enormous. The original atmo-sphere was not only loaded with CO2, it was jam-packed. By adding up all the CO2 locked in thecrust and mantle of Earth, scientists project that theoriginal atmosphere had so much CO2 that the airpressure was 60 times greater than today. It makesour present day atmosphere appear thin.

If all the carbon in our present-day atmosphereis counted as one unit, the measure locked up inrock is 100,000 units. The amount of carbon tied upin recoverable fossil fuels is about 5.5 units, whilethe oceans In a planetary perspective, there isroughly as much carbon in the rocks of Earth asthere is in the atmosphere of Venus (which has anair pressure 90 times as great as ours).

The Earth breathes out through volcanoes, as with Mt. Pinatuboin the Philippines. Photo by Daniel V. Dungla.

Besides locking CO2 in rock, the geochemical cy-cle restores CO2 back to the atmosphere, primarilyin subduction zones where the ocean's crustalplates dive under the edges of continents. In themiddle of the ocean, upwelling molten magmadrives the renewal process, pushing the platesapart at the mid-ocean ridg~s.

The ocean floor is on the move, speedingalong at almost two inches per year in places.That's not too fast on the Autobahn, but it isMach nine on the geologic speedometer. It isso fast, that the enormous geochemical enginetakes only 200 million years to turn over. Theyoung ocean floors are only a mere few hun-dred million, while the mature continentalrock is often billions of years old. The oldest

example, soon oxidized (rusted) and settled outinto massive beds. Some of these beds, created asthe oxygen atmosphere formed, were raised by tec-tonics and are now important sources of iron ore.

The increasing levels of oxygen not only rav-aged the oceans and land, they created the ozonelayer that blocks harmful ultraviolet radiation. Thisprovided a marvelous benefit for life: ozoneblocked much of the raw ultraviolet light from theSun. With the dangerous ultraviolet light problemsolved, the green plants found they could livecloser to the surface of the ocean. The productionof oxygen snowballed: the more ultraviolet lightthat was shut ~ut, the more life increased, which inturn produced more oxygen. In a few billion yearsit was safe to go to the beach, so to speak. Exactlywhen our atmosphere became oxygen-rich is esti-mated to be a brief 500 million years ago.

This, then, is the chapter on the origins of theorganic cycle.

32 EARTH

surface rocks found so far are 20 times olderthan the oldest rock on the ocean floor.

The fact that ocean plates slide means that allthe carbon locked up in ocean floors has a chanceto get free again. The subducted plates grow hotfrom the incredible friction generated by slidingunder a continent. The subducted plates begin tomelt, forming magma. This magma becomeslighter than the cooler rock and starts making itsway toward the surface where it releases CO2 backto the atmosphere through volcanos. If the red-hotlava appears to be boiling, that's because it is boil-ing off CO2 and other gases.

If heat and pressure are applied to carbonaterocks, there is a possibility that CO2 will be re-leased. As plates collide because of tectonic forces,the rock can fold and bend. Called metamorphosis,this process of bending and twisting rock gener-ates heat and pressure, enough to produce a chem-ical reaction. After the gas escapes, the rock mayreturn to what it was to begin with, feldspar. Thegeochemical process can begin again if the meta-morphosed rock is exposed to weathering.

As the gas rises toward the surface, it dissolveeasily with groundwater. This is why some springwater, even though it is not as hot as the deepmolten magma, is naturally carbonated.

Forests produce oxygen in the organic carbon cycle.Photo by William R. Edgar.

went up, the atmospheric CO2 levels were higheras well. Likewise, when global temperatures fell,so did levels of CO2,

One page of this lab book suggests that un-usual volcanic activity in the mid-Cretaceouscaused the increased temperatures of that time.During the Cretaceous period, when the dinosaursthrived, global temperatures were significantlywarmer by about 10° C. These warm temperaturesallowed life to boom, with the green belt extendingfrom the equator to the Antarctica. We are still us-ing the coal and oil that originated in this fertileenvironment. High levels of volcanic CO2 (3,500ppm) appear to have maintained the global tem-peratures during these 100 million years.

Will an increase in CO2 cause global warming?All other factors remaining\ constant, yes. But howfast, how soon, and at what threshold - thesequestions remain locked in the ancient lab book.

Nevertheless, even though we don't know allthe scientific language these lab books use, we caneasily read some parts in any language. Simply byskimming through the chapters, as we have donehere, it is easy to read between the lines that be-cause of the actions of two great natural cycles, weare living in a paradise.

T wo hundred years ago around the start of theIndustrial Revolution, CO2 levels were at an all-time low, about 285 parts per million (ppm). Thefollowing 200 years of industry have increased lev-els of CO2 by 25 percent to present levels of 360ppm, as recently measured by the NationalOceanic and Atmospheric Administration's MaunaLoa Observatory in Hawaii. Yet CO2 continues tobe released by the geochemical cycle and the burn-ing of fossil fuels.

Studying these past levels and processes is use-ful because the information gained may help pre-dict future climate changes. Although computersimulations are powerful tools to predict increas-ing levels of CO2, the ideal solution would be tofind a planet somewhere, jack up the levels of CO2,and see what happens. Although there are no plan-ets handy to play with, we do have a perfect labo-ratory: Earth through the last 4 billion years. Theonly limitation is that the experiments have allbeen conducted; now all that's left is to find the an-cient lab books and look through them.

Geologists and climatologists are doing justthat. Traces of the ancient atmosphere are literallywritten in stone. Each layer of sedimentary rock islike a page in the lab book, revealing the climate ofthat particular age. The pages of this ancient labbook are still being found and deciphered, withmany discoveries remaining to be made. But wehave already confirmed that when the temperature

JANUARY 1992 33