Understanding the Planetary Climate System

This is going to be a brief intro­duc­tion to the cli­mate mod­el­ing beyond those atmos­pher­ic mod­els I did ear­li­er. Glob­al-scale cli­mate mod­els, which sim­u­late the cli­mate of the entire plan­et, are of a par­tic­u­lar inter­est here.

Cli­mate is the long-term sta­tis­ti­cal expres­sion of short-term weath­er. It varies over dif­fer­ent time scales rang­ing from years to hun­dreds of mil­lions of years, and each peri­od­ic­i­ty is a man­i­fes­ta­tion of sep­a­rate forc­ing mech­a­nisms. The over­all state of the glob­al cli­mate is deter­mined by the bal­ance of solar and ter­res­tri­al radi­a­tion bud­gets.

When con­struct­ing or select­ing a cli­mate mod­el, one must con­sid­er three groups of process­es: the trans­fer of radi­a­tion through the cli­mate sys­tem (e.g. absorp­tion, reflec­tion); the hor­i­zon­tal and ver­ti­cal trans­fer of ener­gy (e.g. advec­tion, con­vec­tion, dif­fu­sion); and the inclu­sion of process­es involv­ing land/ocean/ice, and the effects of albe­do, emis­siv­i­ty and sur­face-atmos­phere ener­gy exchanges. And, of course, due the lim­i­ta­tions imposed by incom­plete under­stand­ing of the cli­mate sys­tem and com­pu­ta­tion­al con­straints, cli­mate mod­els are usu­al­ly sim­pli­fied ver­sions of this very com­plex cli­mate sys­tem.

There are four main cat­e­gories of cli­mate mod­els: ener­gy bal­ance mod­els (EBMs); one dimen­sion­al radia­tive-con­vec­tive mod­els (RCMs); two-dimen­sion­al sta­tis­ti­cal-dynam­i­cal mod­els (SDMs); three-dimen­sion­al gen­er­al cir­cu­la­tion mod­els (GCMs). The sim­plest mod­els per­mit lit­tle inter­ac­tion between the pri­ma­ry process­es, radi­a­tion, dynam­ics and sur­face process­es, where­as the most com­plex mod­els are ful­ly inter­ac­tive.

Com­pu­ta­tion­al cost is always an impor­tant fac­tor to con­sid­er when choos­ing a cli­mate mod­el, and cer­tain process­es may be omit­ted from the mod­el if their con­tri­bu­tion is neg­li­gi­ble on the time scale of inter­est.

The NCAR-based Community Earth System Model (CESM) is one of the world’s most sophisticated models of global climate. Created by scientists at NCAR, the Department of Energy, and collaborators, this powerful model simulates the many processes in our climate system, ranging from clouds and atmospheric chemicals to ice to marine ecosystems. ©UCAR.
The NCAR-based Com­mu­ni­ty Earth Sys­tem Mod­el (CESM) is one of the world’s most sophis­ti­cat­ed mod­els of glob­al cli­mate. Cre­at­ed by sci­en­tists at NCAR, the Depart­ment of Ener­gy, and col­lab­o­ra­tors, this pow­er­ful mod­el sim­u­lates the many process­es in our cli­mate sys­tem, rang­ing from clouds and atmos­pher­ic chem­i­cals to ice to marine ecosys­tems. ©UCAR.

The Big Five of the Glob­al Cli­mate Sys­tem

The glob­al cli­mate sys­tem is a con­se­quence of and a link between the atmos­phere, oceans, the ice sheets (cryos­phere), liv­ing organ­isms (bios­phere) and the soils, sed­i­ments and rocks (geosphere). In this post the Earth’s cli­mate sys­tem is dis­cussed. On extra­so­lar plan­ets with dif­fer­ent obliq­ui­ties, inso­la­tion rates, and oth­er ele­ments not like of the Earth this whole sys­tem will be quite dif­fer­ent sto­ry (I will speak about it in details in future posts.)

The Atmos­phere

The atmos­phere is a mix­ture of gas­es and aerosols that forms an inte­grat­ed envi­ron­men­tal sys­tem with all the Earth’s com­po­nents. It is not phys­i­cal­ly uni­form and has sig­nif­i­cant vari­a­tions in tem­per­a­ture and pres­sure with alti­tude. The atmos­phere does not respond as an iso­lat­ed sys­tem. It influ­ences the ener­gy bud­get of the glob­al cli­mate sys­tem and the ener­gy trans­fers of the Earth-atmos­phere sys­tem are in equi­lib­ri­um. The flux­es of ener­gy, mois­ture momen­tum and mass deter­mine the state of the cli­mate.

The Oceans

The ther­mo­dy­nam­ic state of the oceans, like the atmos­phere, is deter­mined by the trans­fer of heat, momen­tum and mois­ture to and from the atmos­phere. These flux­es with­in this cou­pled ocean-atmos­phere sys­tem also exist in equi­lib­ri­um (the oth­er com­po­nents of the cli­mate sys­tem are ignored here.)

Sur­face winds trans­fer momen­tum is trans­ferred to the oceans, mobi­liz­ing the glob­al sur­face ocean cur­rents. Sur­face ocean cur­rents assist in the lat­i­tu­di­nal trans­fer of heat sim­i­lar to the process occur­ring in the atmos­phere. Warm water moves towards the poles whilst cold water returns towards the equa­tor. Mois­ture is anoth­er mech­a­nism to trans­fer ener­gy. Water evap­o­rat­ing from the sur­face of the oceans stores latent heat which is sub­se­quent­ly released when the vapor con­dens­es to form clouds and pre­cip­i­ta­tion.

Ocean stores much greater quan­ti­ty of ener­gy than the atmos­phere because of its larg­er heat capac­i­ty (4.2 times that of the atmos­phere) and its much greater den­si­ty (1000 times that of air). The ver­ti­cal struc­ture of the ocean can be divid­ed into two lay­ers which dif­fer in the scale of their inter­ac­tion with the over­ly­ing atmos­phere. The low­er lay­er com­pris­es the cold deep water sphere, mak­ing up 80% of the oceans’ vol­ume. The upper lay­er, which has clos­est con­tact with the atmos­phere, is the sea­son­al bound­ary lay­er, a mixed water sphere extend­ing down only 100 m in the trop­ics but sev­er­al kilo­me­ters in polar regions. The sea­son­al bound­ary lay­er alone stores approx­i­mate­ly 30 times as much heat as the atmos­phere (Hen­der­son-Sell­ers & Robin­son, 1986). Thus for a giv­en change in heat con­tent of the ocean-atmos­phere sys­tem, the tem­per­a­ture change in the atmos­phere will be around 30 times greater than that in the ocean. There­fore small changes to the ener­gy con­tent of the oceans could have con­sid­er­able effects on glob­al cli­mate.

Ener­gy exchanges also occur ver­ti­cal­ly with­in the oceans, between the mixed bound­ary lay­er and the deep water sphere. Sea salt remains in the water dur­ing the for­ma­tion of sea ice in the polar regions, with the effect of increased salin­i­ty of the ocean. This cold, saline water is par­tic­u­lar­ly dense and sinks, trans­port­ing with it a con­sid­er­able quan­ti­ty of ener­gy. To main­tain the equi­lib­ri­um of water (mass) flux­es, a glob­al ther­mo­ha­line cir­cu­la­tion exists, which plays an impor­tant role in the reg­u­la­tion of the glob­al cli­mate. Broeck­er & Den­ton (1990) have pro­posed that changes in this ther­mo­ha­line cir­cu­la­tion influ­ence cli­mate changes over mil­len­nia time scales.

Addi­tion­al read­ing: The Ther­mo­ha­line Ocean Cir­cu­la­tion

The Cryos­phere

These are the regions of the plan­et, both land and sea, cov­ered by snow and ice, includ­ing high moun­tain ranges through­out the world, where sub-zero tem­per­a­tures per­sist through­out the year. With­out the cryos­phere, the glob­al albe­do would be con­sid­er­ably low­er. More ener­gy would be absorbed at the Earth’s sur­face rather than reflect­ed, and con­se­quent­ly the tem­per­a­ture of the atmos­phere would be high­er.

The cryos­phere also acts to decou­ple the atmos­phere and oceans, reduc­ing the trans­fer of mois­ture and momen­tum, sta­bi­liz­ing the ener­gy trans­fers with­in the atmos­phere (Hen­der­son-Sell­ers & Robin­son, 1986). And as stat­ed in the Oceans sec­tion, the for­ma­tion of sea ice in polar regions can ini­ti­ate glob­al ther­mo­ha­line cir­cu­la­tion pat­terns in the oceans.

The pres­ence of the cryos­phere itself affects the vol­ume of the oceans and glob­al sea lev­els, changes to which can affect the ener­gy bud­get of the cli­mate sys­tem.

The Bios­phere

The bios­phere, both on land and in the oceans, affects the albe­do of the Earth’s sur­face. Large areas of con­ti­nen­tal for­est have rel­a­tive­ly low albe­dos com­pared to bar­ren regions such as deserts. The pres­ence of the con­ti­nen­tal forests influ­ence the ener­gy bud­get of the cli­mate sys­tem.

The bios­phere also influ­ences the flux­es of cer­tain green­house gas­es such as car­bon diox­ide and methane. The oceans effec­tive­ly absorb the gas from the atmos­phere cre­at­ing the flux of car­bon diox­ide. Plank­ton in the sur­face oceans uti­lizes the dis­solved car­bon diox­ide for pho­to­syn­the­sis. On death, the plank­ton sink, trans­port­ing the car­bon diox­ide to the deep ocean. Such pri­ma­ry pro­duc­tiv­i­ty reduces the atmos­pher­ic con­cen­tra­tion of CO2 by at least four-fold, weak­en­ing sig­nif­i­cant­ly the Earth’s nat­ur­al green­house effect.

The bios­phere also influ­ences the amount of aerosols in the atmos­phere. Mil­lions of spores, virus­es, bac­te­ria, pollen and oth­er minute organ­ic species are trans­port­ed into the atmos­phere by winds, where they can scat­ter incom­ing solar radi­a­tion, and so influ­ence the glob­al ener­gy bud­get.

Pri­ma­ry pro­duc­tiv­i­ty in the oceans results in the emis­sion of com­pounds known as dimethyl sul­phides (DMSs). In the marine atmos­phere those are oxi­dized to var­i­ous sul­fur-con­tain­ing compounds–sulphate aerosols called marine non-sea-salt (nss) sul­phate (Charl­son et al., 1987). These nss sul­phates act as con­den­sa­tion nuclei for water vapour in the atmos­phere, thus allow­ing the for­ma­tion of clouds. Clouds have a high­ly com­plex effect on the ener­gy bud­get of the cli­mate sys­tem. Thus changes in pri­ma­ry pro­duc­tiv­i­ty in the oceans can affect, indi­rect­ly, the glob­al cli­mate sys­tem.

Besides these major influ­ences there are, of course, many oth­er mech­a­nisms and process­es which cou­ple the bios­phere with the rest of the cli­mate sys­tem.

The Geosphere

The soils, the sed­i­ments and rocks of the Earth’s land mass­es, the con­ti­nen­tal and ocean­ic crust and ulti­mate­ly the inte­ri­or of the Earth itself is anoth­er com­po­nent of the glob­al cli­mate sys­tem.

Long-scale changes in the shape of ocean basins and the size of con­ti­nen­tal moun­tain chains (dri­ven by plate tec­ton­ic process­es) may influ­ence the ener­gy trans­fers with­in and between the cou­pled com­po­nents of the cli­mate sys­tem.

On much short­er time scales phys­i­cal and chem­i­cal process­es affect cer­tain char­ac­ter­is­tics of the soil, such as mois­ture avail­abil­i­ty and water run-off, and the flux­es of green­house gas­es and aerosols into the atmos­phere and oceans (Cubasch & Cess, 1990; McBean & McCarthy, 1990). Vol­can­ism, although dri­ven by the slow move­ment of the tec­ton­ic plates, occurs reg­u­lar­ly on much short­er timescales. Vol­canic erup­tions replen­ish the car­bon diox­ide in the atmos­phere, removed by the bios­phere, and emit con­sid­er­able quan­ti­ties of dust and aerosols. Vol­canic activ­i­ty can there­fore affect the ener­gy bud­get and reg­u­la­tion of the glob­al cli­mate sys­tem (Sear et al., 1987).

Num­ber Six, the Human Fac­tor

Anoth­er com­po­nent of the glob­al cli­mate sys­tem is an anthro­pogenic sys­tem, mankind. In the last 200 years, through increased uti­liza­tion of the world’s resources, humans have begun to influ­ence the glob­al cli­mate sys­tem, pri­mar­i­ly by increas­ing the Earth’s nat­ur­al green­house effect.

Basic Cli­mate Ele­ments

Tem­per­a­ture is a valu­able cli­mate ele­ment in cli­mate obser­va­tion because it direct­ly pro­vides a mea­sure of the ener­gy of the sys­tem under inspec­tion.

The mea­sure­ment of glob­al rain­fall offers an indi­rect or qual­i­ta­tive assess­ment of the ener­gy of the Earth-atmos­phere sys­tem. Increased heat stor­age will increase the rate of evap­o­ra­tion from the oceans (due to high­er sur­face tem­per­a­tures). In turn, the enhanced lev­els of water vapor in the atmos­phere will inten­si­fy glob­al pre­cip­i­ta­tion. Rain­fall is, how­ev­er, sub­ject to sig­nif­i­cant tem­po­ral and spa­tial vari­abil­i­ty, and the occur­rence of extremes.

Humid­i­ty, the amount of water vapor in the air can be described in terms of the water-vapour pres­sure; the rel­a­tive humid­i­ty; the absolute humid­i­ty; the mix­ing ratio; and the dew­point. A detailed account of these def­i­n­i­tions may be found in Linacre (1992).

Anoth­er ele­ment is wind, the bulk move­ment of air of cer­tain strength and direc­tion.

Changes in Cli­mate

Although the cli­mate sys­tem is in bal­ance, that bal­ance is dynam­ic, ever-chang­ing. The sys­tem is con­stant­ly adjust­ing to forc­ing per­tur­ba­tions and, as it adjusts, the cli­mate alters. A change in any one part of the cli­mate sys­tem will have much wider con­se­quences as the ini­tial effect cas­cades through the cou­pled com­po­nents of the sys­tem. As the effect is trans­ferred from one sub-com­po­nent of the sys­tem to anoth­er, it will be mod­i­fied in char­ac­ter or in scale. In some cas­es it will be ampli­fied (pos­i­tive feed­back), in oth­ers, it may be reduced (neg­a­tive feed­back) (Cess & Pot­ter, 1988).

Addi­tion­al read­ing: An Assess­ment of Cli­mate Feed­backs in Cou­pled Ocean–Atmosphere Mod­els

Glob­al cli­mate change results from a com­bi­na­tion of peri­od­ic exter­nal and inter­nal forc­ing mech­a­nisms, and a com­plex series of inter­ac­tive feed­backs with­in the cli­mate sys­tem itself.

Exter­nal forc­ing sets the pace of cli­mate change; but the inter­nal (non-lin­ear) dynam­ics of the cli­mate sys­tem mod­u­late the final response. This is true for all time scales.

Some of the exter­nal forc­ing mech­a­nisms include galac­tic vari­a­tions; orbital vari­a­tions (fac­tors of the Milankovitch cycles), such as obliq­ui­ty, eccen­tric­i­ty, pre­ces­sion; and the solar cycle.

The inter­nal mech­a­nisms for cli­mate change include oroge­ny (the tec­ton­ic process of moun­tain build­ing and con­ti­nen­tal uplift); epeiroge­ny (changes in the glob­al dis­po­si­tion of land mass­es, and like oro­genic process­es, these changes are dri­ven by inter­nal plate tec­ton­ic move­ments); vol­canic activ­i­ty; ocean cir­cu­la­tion; and atmos­pher­ic com­po­si­tion.

A longer per­spec­tive on cli­mate vari­abil­i­ty can be obtained by the study of nat­ur­al phe­nom­e­na which are cli­mate-depen­dent. Such phe­nom­e­na pro­vide a proxy record of the cli­mate.

Jeno Marz
JENO MARZ is a science fiction writer from Latvia, Northern Europe, with background in electronics engineering and computer science. She is the author of two serial novels, Falaha’s Journey: A Spacegirl’s Account in Three Movements and Falaha’s Journey into Pleasure. Marz is current at work on a new SF trilogy. All her fiction is aimed at an adult audience.

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