Contents2TheEarthSystem 12.1 ComponentsoftheEarthSystem.................. 12.1.1 Theoceans .......................... 22.1.2 Thecryosphere........................ 132.1.3 Theterrestrialbiosphere................... 212.1.4 Theearth’scrustandmantle ................ 232.1.5 Roles of various components of the Earth system in climate 252.2 Thehydrologiccycle ......................... 262.3 Thecarboncycle........................... 302.3.1 Carbonintheatmosphere.................. 312.3.2 Carboninthebiosphere................... 332.3.3 Carbonintheoceans .................... 352.3.4 Carbonintheearth’scrust ................. 362.4 OxygenintheEarthsystem..................... 382.4.1 Sourcesoffreeoxygen .................... 392.5 AbriefhistoryofclimateandtheEarthsystem.......... 432.5.1 FormationandevolutionoftheEarthsystem ....... 432.5.2 The past 100 million years .................. 482.5.3 Thepastmillionyears.................... 492.5.4 Thepast20,000years .................... 542.6 Earth:thehabitableplanet ..................... 562.7 Exercises ............................... 58iii CONTENTSChapter 2The Earth SystemClimate depends not only upon atmospheric processes but also on physical,chemical and biological processes involving other components of the Earth sys-tem. In this chapter we review the structure and behavior of those other com-ponents. We show how the cycling of water, carbon and oxygen among thecomponents of the Earth system has affected the evolution of the atmosphere.Drawing on this background, we summarize the history of climate over the life-time of the earth, with emphasis on causal mechanisms. In the final section wediscuss why Earth is so muc h more habitable than its neighbors in the SolarSystem. Chapter 10 revisits some of these same topics in the context of climatedynamics, with a quantitative discussion of feedbacks and climate sensitivity.2.1 Componen ts of the Earth SystemThis section introduces the cast of characters and briefly describes their rolesand interrelations in the ongoing drama of climate. The atmosphere, which insome sense plays the starring role, has already been introduced in Chapter 1.The interplay between atmospheric radiation and convection regulates the tem-perature at the earth’s surface, setting the limits for snow and ice cover and forthe various life-zones in the biosphere. The stratospheric ozone layer protectsthe biosphere from the lethal effects of solar ultraviolet radiation. Atmosphericwind patterns regulate the patterns of oceanic upwelling that supplies nutrientsto the marine biosphere, they deter mine the distribution of water that sustainsthe terrestrial (land) biosphere, and they transport trace gases, smoke, dust,insects, seeds and spores over long distances. Rain, frost and wind erode theearth’s crust, wearing down mountain ranges, reshaping the landscape and re-plenishing the soils and the supply of metallic ions needed to sustain life.Other components of the Earth system also play important roles in climate.The oceans are notable for their large "thermal inertia" and their central role inthe cycling of carbon, which controls atmospheric carbon dioxide concentrations.Extensive snow and ice covered surfaces render the earth more reflective, and12 CHAPTER 2. THE EARTH SYSTEMconsequently cooler, than it would be in their absence. By evaporating largequantities of water through their lea ves, land plants exert a strong moderatinginfluence on tropical and extratropical summer climate. Living organisms onland and in the sea have been instrumental in liberating oxygen and sequesteringcarbon in the earth’s crust, thereby reducing the atmospheric concen trationcarbon dioxide. On time scales of millions of years or longer, plate tectonicsexerts an influence on climate through continental drift, mountain building,and volcanism. This section describes these processes and the media in whichthey occur.2.1.1 The oceansThe oceans cover 72% of the area of the earth’s surface and they reach anextreme depth of nearly 11 km. Their total volume is equivalent to that of alayer 2.6 km deep, covering the entire surface of the earth. The mass of theoceans is ∼250 times as large as that of the atmosphere.a. Composition and vertical structureThe density of sea water is linearly dependent on the concentration of dissolvedsalt. On average, sea water in the open oceans contains ∼35 g of dissolvedsalts per kg of fresh water, with values typically ranging from 34 to 36 g kg−1(or parts per thousand by mass, abbreviated as o/oo). Due to the presence ofthese dissolved salts, sea water is ∼2.4% denser than fresh water at the sametemperature.The density σ of sea water (expressed as the departure from 1 in g kg−1or o/oo) typically ranges from 1.02 to 1.03. It is a rather complicated functionof temperature T , salinity s and pressure p; i.e., σ = σ(T,s,p). The pressuredependence of density in liquids is much weaker than in gases and, for purposesof this qualitative discussion, will be ignored1. As in fresh water, ∂σ/∂T is tem-perature dependent, but the fact that sea water is saline makes the relationshipsomewhat different: in fresh water, density increases with increasing tempera-turebetween0and4◦C, whereas in sea water density decreases monotonicallywith increasing temperature.2In both fresh water and sea water ∂σ/∂T issmaller near the freezing point than at higher temperatures. Hence, a salinitychange of a prescribed magnitude δs is equivalent, in terms of its effect on den-sit y, to a larger temperature change δT in the polar oceans than in the tropical1The small effect of pressure u pon density is taken into acco unt throug h th e use of potentialdensity, the density that a submerged water parcel would exhibit if it were brought up to sea-level, conserving temp erature and salinity. (See E xercise 3.54.)2Ice floats on lakes because d ensity of fresh water decrea ses with tempera ture from 0 to4◦C. In contrast, sea ice floats because water rejects salt as it freezes.2.1. COMPONENTS OF THE EARTH SYSTEM 3oceans, as illustrated in Fig. 2.1.Fig. 2.1 The change in temperature of a water parcel required to raise thedensity of sea water at sea level as m uch as a salinity increase of 1 g kg−1,plottedas a function of the temperature of the parcel. For example, for sea water ata temperature of 10◦C, a salinity increase of 1 g kg−1would raise the densityas much as a temperature decrease of ∼5◦C, whereas for sea water