"Life and the Evolution of Earth's Atmosphere." Earth

More documents

Recommendations

Info

LIFE AND THE EVOLUTION OF EARTH’S ATMOSPHEREphotosynthesis. The high concentration ofoxygen needed to keep the contemporarybiosphere going, including all of the humansbreathing it, is maintained despite its continualremoval through oxidative processes likeweathering of rocks (which tend to give them areddish color as in the Grand Canyon), rusting,burning, and so on. In fact, if all photosynthesiswere to cease, which would be a catastrophicevent indeed, the oxygen in the atmosphere ofthe Earth would be mostly gone in less than amillion years, along with the protective ozonelayer. In the absence of photosynthesis, theoxygen present in the atmosphere wouldcombine with carbon in organic matter at thesurface to make carbon dioxide, or react withiron in minerals, to soon be removed from theatmosphere altogether. In 1979, in his bookGaia: A New Look at Life on Earth, JamesLovelock emphasized that the atmosphericcomposition of the Earth is far from achievingchemical equilibrium with the surface and mustbe sustained by biological processes. Accordingto Lovelock’s Gaia Hypothesis, life isresponsible for making the unique conditions forhabitability on Earth; life regulates the globalenvironment. The other planets near the Earthprovide a useful measure of how different ourplanet is from places that are in equilibrium.Again, take a look at the lifeless planets of Marsand Venus in Table 1. They have negligibleamounts of oxygen in their atmospheres, andare dominated by carbon dioxide. The samewould be true for the Earth if life had neverappeared and changed the atmosphere to keepthe planet habitable.The Build-up of OxygenWhen oxygen began to first accumulate onEarth billions of years ago, it caused a crisis forprimitive anaerobic bacterial life, which isactually poisoned by excess oxygen in theenvironment. Such life still exists today inorganic-rich soils, swamps, deep in sediments,and lake bottom muds where levels of oxygenare low. This early oxygen crisis, however,created an opportunity and led to importantnew evolutionary changes as life responded tothis challenge with metabolic innovations thatmade use of the newly-available oxygen.Organisms evolved which used oxygen in theirmetabolic cycles to get more energy out oftheir food—twenty times more than anaerobicorganisms get.A coupled biological and geological mechanismtermed the “carbon cycle” began to manifestitself early in the history of life on Earth, as lifetook over the production of organic matter andoxygen and began to regulate the balance ofcarbon between the atmosphere and theoceans. This cycle has helped to regulate theplanet’s surface temperature by balancing CO2output from volcanoes with weathering, burial oforganic matter in sediments, and other meansof removing CO2 from the atmosphere. (Tolearn more details about how this works, readRachel Oxburgh’s essay on the carbon cycle inSection Six). In a sense, industrialization andthe burning of fossil fuels, which have releasedvast amounts of carbon dioxide into theatmosphere, represent a small reversal of thegeneral trend toward higher oxygenconcentrations and lower carbon dioxideconcentrations with geologic time. Humanactivity, the long-term actions of plate tectonics,and variations in Earth’s orbit will inevitablycause large fluctuations in the amount of carbondioxide in the atmosphere again, and perhapschanges in oxygen concentration as well.When Did These Changes Occur?Life originated more than 4 billion years ago.How soon after the origin of life did oxygenlevels begin to rise? This is a question thatscientists have been exploring for some time(Figure 2). Data gleaned from ancient soils
(paleosols) and a variety of other geologicalevidence indicates that starting about 2.5 billionyears ago, sedimentary rocks began to becomeincreasingly affected by higher amounts ofoxygen in the atmosphere, and appear red, asthough “rusted.” Scientists believe that oxygenconcentration began to rapidly increase at thattime because more oxygen began to beproduced by photosynthesis than could bewhisked away by reactions with theatmosphere, oceans, and rocks. A thresholdhad been reached, and the rise of oxygen hadbegun. At around that same time, morecomplex bacterial life evolved in response to theoxygen-rich conditions, including organisms witha true cell nucleus (Eukaryotes) from which allplants, animals, and fungi are descended.The final significant section in the emergence ofan oxygen-rich atmosphere on Earth beganabout 700 million years ago, when the fossilrecord documents the emergence ofmulticellular organisms. When oxygen levelsbecame high enough, a little more than onepercent of the present level, a powerful ozonescreen started to form in the upper atmospherethat protected the first land-dwelling creaturesfrom the Sun’s harmful ultraviolet radiation.Abundant oxygen in the atmosphere wasessential to the rise of animal and plant lifeon land.A Hot FutureKnowing what we do about the past trends inatmospheric evolution of the Earth, what can wesay about the distant future for life on thisplanet? The Sun will continue to grow brighterover the 5 billion years or so that remain in itsstellar life span. Its luminosity has beenincreasing on a near-linear path for more than4.5 billion years because hydrogen iscontinually converted into the heavier element,helium, by nuclear fusion. This conversionreleases tremendous amounts of energy everysecond and is responsible for sunlight. Ashelium accumulates, the average density of theSun increases over time, which in turn causespressures and temperatures in the Sun’s interiorto increase. These higher pressures andtemperatures increase the rate of fusion, andthe extra energy that is generated leads toincreased luminosity. As this positive feedbackcontinues, the Sun will continue to increase inintensity by about six percent or more in thenext billion years.One of two things needs to happen to keep theEarth from slowly going the way of deadly hotVenus. Either almost all of the greenhousegases have to be removed from theatmosphere, or the planet’s albedo—theamount of sunlight the surface can reflect backinto space—must increase. Unless new plantsevolve a type of photosynthesis that requiresvery little carbon dioxide, or some future culturesomehow protects the Earth from a brighterSun, life on land will have to be abandoned. Inthe end, some 5 billion years from now, muchof the Sun’s fuel will be exhausted. The Sun willbegin to grow as it approaches the Red Giantphase of its stellar life cycle. By then, theoceans on Earth will have long since boiledaway. The last life, primitive bacteria much likethe first that appeared, residing in deep crustalrocks and sediments, will have perished. As thesurface temperature rises, all of the volatilegases stored over many eons in the Earth’scrust will be released to form a thick, hot,blanketing atmosphere. Finally, there will betwin Venuses from that time until the end of thesolar system.SummaryA great deal can be learned by approachingthe history of life on Earth from the standpointof geology. The rocks that make up thegeologic record preserve changes in theplanet’s crust, oceans, and atmosphere thathave accompanied the history of theevolution of life. Studies of the geologic record
Page 1 and 2: Life and the Evolution ofEarth’s
Page 3 and 4: illions of years, the product of ph
Page 5: that were more than enough to vapor

"Life and the Evolution of Earth's Atmosphere." Earth

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?