earth dynamics and climate changes

8/12/2019 Earth Dynamics and Climate Changes

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C. R. Geoscience 335 (2003) 157174

Geodynamics / Godynamique

Earth dynamics and climate changes

La dynamique terrestre et les modifications climatiques

Frdric Fluteau a,b

a UFR des sciences physiques de la Terre, universit Denis-DiderotParis-7, T24-25 E1, case courrier 7011, 2, place Jussieu,

75251 Paris cedex 05, Franceb Laboratoire de palomagntisme, Institut de physique du Globe de Paris, T25-25 E1, case courrier 089, 4, place Jussieu,

75252 Paris cedex 05, France

Received 9 October 2002; accepted 27 November 2002

Written on invitation of the Editorial Board

Abstract

The evolution of the Earths climate over geological time is now relatively well known. Conversely, the causes and feedbackmechanisms involved in these climatic changes are still not well determined. At geological timescales, two factors play aprevailing role: plate tectonics and the chemical composition of the atmosphere. Their climatic effects will be examined using

palaeoclimatic indicators as well as results of climate models. I focus primarily on the influence of continental drift on warmand cold climatic episodes. The consequences of peculiar land sea distributions (amalgamation/dispersal of continental blocks)are discussed. Plate tectonics also drive sea level changes as well as mountain uplift. Marine transgressions during the Mid-Cretaceous favoured warmth within the interiors of continents, although their effect could be very different according to theseason. Mountain uplift is also an important factor, which is able to alter climate at large spatial scales. Experiments relative toclimatic sensitivity to the elevation of the Appalachians during the Late Permian are discussed. To affect the whole Earth, thechemical composition of the atmosphere appears to be a more efficient forcing factor. The carbon dioxide driven by the long-term carbon cycle has influenced the global climate. Geochemical modelling simulates more or less accurately the long-termevolution of pCO2, which corresponds roughly to the icehouse/greenhouse climatic oscillations. However, the uncertaintieson pCO2 are still important because different parameters involved in the long-term carbon cycle (degassing rate, chemicalweathering of silicates, burial of organic matter) are not well constrained throughout the past. The chemical composition of theatmosphere is also altered by the emissions of modern volcanic eruptions leading to weak global cooling. The influence of largeflood basalt provinces on climate is not yet known well enough; this volcanism may have released huge amounts of SO2as well

as CO2. At last, the chemical composition of the atmosphere may have been altered by the release of methane in response tothe dissociation of gas hydrates. This scenario has been proposed to explain the abrupt warming during the Late Palaeocene. 2003 Acadmie des sciences/ditions scientifiques et mdicales Elsevier SAS. All rights reserved.

Rsum

Lvolution du climat de la Terre lchelle des temps gologiques est maintenant relativement bien connue. En revanche,les causes ainsi que les mcanismes de rtroaction impliqus dans ces changements climatiques ne sont pas encore totalementdtermins. lchelle des temps gologiques, deux facteurs semblent avoir jou un rle majeur : la tectonique des plaqueset la composition chimique de latmosphre. Leurs effets climatiques seront examins en utilisant la fois des indicateurspaloclimatiques ainsi que les rsultats de modles climatiques. Je me focaliserai tout dabord sur linfluence de la drive descontinents dans le cadre de priodes globalement chaudes puis froides. Les consquences dune rpartition continentocanparticulire (regroupement/dispersion des blocs continentaux) seront prsentes. La tectonique des plaques pilote aussi bien

1631-0713/03/$ see front matter 2003 Acadmie des sciences/ditions scientifiques et mdicales Elsevier SAS. Tous droits rservs.doi:10.1016/S1631-0713(03)00004-X


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158 F. Fluteau / C. R. Geoscience 335 (2003) 157174

les changements de niveau marin que la surrection de reliefs. Une transgression marine au Crtac moyen a favoris lextrmedouceur qui rgne au cur des continents, mais son influence sur le climat peut diffrer nettement dune saison une autre.Enfin, la surrection des reliefs est un forage climatique important qui peut affecter le climat lchelle dun continent. Desexpriences de sensibilit climatique la hauteur de la chane des Appalaches au Permien suprieur le montrent clairement.Pour affecter la Terre dans son intgralit, la composition chimique de latmosphre semble tre le forage le plus efficace. Ledioxyde de carbone pilot par le cycle du carbone a influenc le climat global. Des modles gochimiques ont simul, avecplus ou moins de prcision, lvolution long terme de la pCO2, laquelle correspond approximativement aux grands cyclesclimatiques chauds et froids que la Terre a connus. Cependant, les incertitudes sur la pCO2restent importantes, car les diffrentsparamtres impliqus dans le cycle du carbone long terme (taux de dgazage, altration chimique des silicates, enfouissementde la matire organique) ne sont pas trs bien contraints dans le pass. La composition chimique de latmosphre est galementperturbe par les missions de gaz au cours druptions volcaniques rcentes. Linfluence des larges traps basaltiques sur leclimat reste mal connue, malgr le fait que ce volcanisme ait pu relcher des quantits normes de SO2 et de CO2. Enfin, lacomposition chimique de latmosphre peut tre galement modifie par les missions de mthane issues de la dissociationdhydrates de gaz. Ce scnario a t propos pour expliquer le rchauffement brutal la fin du Palocne.

2003 Acadmie des sciences/ditions scientifiques et mdicales Elsevier SAS. Tous droits rservs.

Keywords:Palaeoclimate; Forcing factor; Plate tectonics; Sea level; Gas

Mots-cls :Paloclimat ; Forage ; Tectonique des plaques; Niveau marin; Gaz

1. Introduction

Since the middle of the 19th century, geologists andpalaeontologists have collected data and interpretedtheir findings in terms of palaeoclimate. Thanks tothis database, the evolution of the Earths climate hasprogressively been reconstructed. The absence of verycold periods or excessively warm ones, which wouldhave been able to kill off life and completely steriliseEarth, highlights that the Earths climate is relativelystable, since at least the first appearance of life onEarth, despite intense climate oscillations betweenicehouse (about seven cold) and greenhouse (warm)periods over geological time scales. The duration ofwarm and cold periods is highly variable, rangingfroma few million years or less for the Ordovician ice ageto several tens of millions of years for the Triassicgreenhouse age [23]. Moreover, some short but drasticevents, either warm or cold, are likely to punctuate the

long-term climate trend [99].The climate represents an equilibrium state of a

complex system composed of the atmosphere, hy-drosphere (essentially ocean), cryosphere (ice sheets,sea ice), biosphere (vegetation) and Earth surface. De-spite their huge differences in compositions, proces-ses, etc. as well as time responses, each component iscoupled to others through mass (precipitation, evapo-

E-mail address:[email protected] (F. Fluteau).

ration), energy (latent heat, sensible heat) and momen-tum exchanges. Moreover, the complexity of the cli-mate system induces numerous, generally non-linear,feedback mechanisms.

Forcing factors are needed to alter the equilibriumstate of the climatic system [56]. Over geological timescales, the Earths internal processes play a prevailing

role in climate evolution. In the first part of this paper,we will focus on the climatic effect of plate tectonics,and especially continental drift, mountain uplift andsea level changes. In a second part, the climatic impactof degassing of the Earth is discussed.

2. The tools

Except for the early work of Kppen and We-gener [55], the understanding of climate over geologi-cal time scales was largely improved only after plate

tectonic theory had become widely accepted by theEarth science community. Knowledge of past climatehas grown thanks to the rise of the use of palaeon-tological, palaeomagnetic, lithological and geochem-ical data (e.g., [74]) as climate indicators. These dataprovide an estimate of the major key factors such astemperature, precipitation, ice volume or wind as afunction of (palaeo)latitude. Thanks to these data, wehave established the main changes through the Earthsclimate history. Unfortunately, our knowledge is un-even over geological time scales, because of poor data


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F. Fluteau / C. R. Geoscience 335 (2003) 157174 159

distribution in time and space. These data give ac-cess to climate parameters, however they generally donot provide information about the cause(s) of climatechanges.

Since the beginning of the 1980s, climate mod-elling has been largely used to investigate the past cli-mate, thanks to an improved knowledge of the phys-ical laws driving the Earths climate system and tothe development of high-speed computers. Differentmodels from zero (0D) to three dimensions (3D) havebeen elaborated. Energy balance models (0D1D) helpto understand climate stability for example, whereas3D models (such as Atmospheric General CirculationModels=AGCM) are used to simulate palaeoclimatesat a global scale and elucidate their sensitivities to dif-ferent forcing factors. Unfortunately, the more com-plex numerical models remain simplified representa-tions of the climate system. We are unable to dupli-cate all processes in an atmospheric general circula-tion model due to the (too) large diversity of spatialscales acting in the real climate system. While someprocesses may be represented faithfully, others mech-anisms must be parameterised. The differences in pre-dicted climate using two different AGCMs with thesame boundary conditions are largely related to these

differences of parameterisation schemes [94].Moreover, we are unable to simulate the equilib-

rium state of the entire climate system because ofthe difference in time responses of the components.Continental drift and mountain building are very slowwith respect to the time response of the atmosphericcomponent. To take into account its effect, landseadistribution and topography have to be prescribed asboundary conditions in numerical experiments. Theother components of the climate system (except at-mosphere) are prescribed according to the type ofmodels. Because the ocean carries heat from low tohigh latitudes, its role is crucial in the climate sys-tem. Fully coupled atmosphereocean general circu-lation models are little used in pre-Quaternary climatemodelling. Thus the oceanic component is either con-sidered as a boundary condition, prescribing sea sur-face temperature derived from oxygen isotopic data orpresent day values, or represented by a mixed surfacelayer (slab ocean) in which the amount of heat is pre-scribed.

Recent development of fully coupled models (atmo-sphereoceanvegetationice) should help to investi-

gate palaeoclimates. However, the use of basic models(uncoupled models) should remain useful to investi-gate the sensitivity of climate to different forcing fac-tors. Most of the examples cited in this paper are usu-ally based on experimentsmade with atmosphericgen-eral circulation models.

3. The impact of plate tectonics on climate

The Earths surface has undergone successive epi-sodes of formation of continental landmasses throughWilson cycles, which formed supercontinents, suchas Pangea, during the Phanerozoic or Rodinia duringthe Neoproterozoic, followed by their dispersal. As aconsequence, the landsea distribution, as well as itsdistribution with respect to latitude has evolved. Usingoceanic magnetic anomalies and palaeomagnetic data,we are able to reconstruct the locations of continents inthe past until the Late Jurassic. Because of the absenceof constraints in longitude, palaeogeography prior tothe Jurassic is less well constrained by palaeomagneticmethods.

The continental drift hypothesis has been amongthe first mechanisms proposed to explain climate

change [55]. Latitudinal drift has obvious conse-quences on temperature because of the change in in-coming solar radiation. In theory, longitudinal chang-es should not alter climate, because incoming solarradiation remains unchanged. Actually the dispersaland formation of megacontinents such as Pangea dur-ing the Late Palaeozoic deeply affect atmospheric cir-culation and climate. Thus the mean global tempera-ture is sensitive to plate motions, even if there is onlya change in longitudinal distribution of continents.Could changes in landsea distribution be the causeof successive warm periods (such as Mid-Cretaceous)and cold episodes (such as the Early PermianLateCarboniferous)? In the past two decades, the climaticimpact of landsea distribution has been investigatedusing climate models. Numerous experiments have fo-cused on the Mid-Cretaceous warm period. The pole-ward displacements of coral reefs, invertebrates, di-nosaurs and the poleward expansion of floral provinces[23] are evidence for global warming at this time. Bar-ron and Washington [2] and Barron et al. [4] have in-vestigated the consequences of landsea distributionon the Mid-Cretaceous warm climate. However, the


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palaeogeography of this period cannot prevent tem-perature from dropping well below the freezing pointabove continents at mid-latitudes [4].

During the Late Permian, all continents (except fora few small blocks) were associated as a single, hugesupercontinent, called Pangea. This configuration dif-fers drastically from the Mid-Cretaceous one, whichis marked by both large seaways in the northern hemi-sphere and young oceans, formed as a consequence ofthe break-up of Gondwanaland in the southern hemi-sphere.

Based on numerical experiments made with an En-ergy Balance Model, Crowley et al. [25] suggestedthat such a configuration induced summer tempera-tures in the subtropics exceeding the present warmestvalue by 6 to 10 C. In winter, the coldest tempera-ture simulated in the mid-latitudes falls below30 C.This large seasonal thermal variation is due to the sizeof Pangea [25] and a weak influence of the ocean,which remains limited to coastal areas. The improve-ment of palaeogeographic maps has much reduced dis-agreement between simulated climate and data. Ear-lier mismatch was suggested to have been due to atoo coarse representation of the Late Permian palaeo-geography [25,57]. A more detailed Late Permian map

that better defined landsea distribution (including in-land seas and also topography)replaced the earlier ide-alised Pangea, and led to better agreement betweensimulated climate and field observations [34,43,58].

The landsea distribution during the Palaeozoic hasbeen proposed as a cause for the inception of an iceage, as well as its demise. Indeed the presence of conti-nents near the pole was not always associated with thedevelopment of ice sheets [86]. The Cretaceous seemsto be largely ice-free, despite the fact that Antarcticawas at the pole at that time. Furthermore,there were noice sheets between the Early Silurian and the MiddleCarboniferous, when Gondwanaland drifted across theSouth Pole. Using an Energy Balance Model, Crow-ley et al. [24] showed that summer temperatures atthe South Pole were close to the freezing point duringthe Late Ordovician. The drift of the interior of Gond-wanaland across the South Pole induced an increaseof summer temperature, which prevented the accumu-lation of perennial snow. The drift of Gondwanalandappears to drive the glaciation, however this suppo-sition may be challenged because the duration of theLate Ordovician glaciation appears to be very short,

perhaps less than 3 Myr [42]. In this case, we may sug-gest that plate tectonics were not the only explanation.Could the long-term cooling trend during the Cain-

ozoic and the Antarctica glaciation be explained solelyby changes in landsea distribution? According toBarron [3], changes in landsea distribution are notsufficient to trigger the Tertiary climatic changes. Nev-ertheless, the thermal insulation of Antarctica (afterthe rifting away of Australia) and the opening of theDrake Passage were suggested to be the causes of theLate Eocene glaciation [31,53,72]. Data and climatemodelling have recently challenged this explanation.We know that the volume of the Antarctica ice sheethas widely fluctuated during the past 40 million years.Rapid variation in ice volume eliminates the need for aplate tectonic explanation for the glaciation as the onlyforcing factor [99]. This result is confirmed by recentnumerical experiments using an asynchronous cou-pled ice sheet atmospheric general circulation mod-els [26]. These experiments suggest that the openingof Southern Ocean gateways may actually have playedonly a minor role. Conversely, a change in atmosphericchemical composition, such as a lowering of carbondioxide, could induce the Antarctic ice sheet. Indeed,the influence of plate motions on climate varies as a

function of palaeogeographic configuration. The loca-tion of continents is generally not a sufficient factorto explain global warm epochs. Other forcing factorsmust be taken into account. However the role of thelandsea distribution becomes more significant at a re-gional scale.

4. Sea-level fluctuations and climate changes

Besides plate motions, the landsea distributionis greatly influenced by sea level change [46]. Haqet al. [46] have constructed sea levels since theTriassic, based on sequence-stratigraphic depositionalmodels. These curves show rapid sea-level changessuperimposed on a long-term trend. The slow sea-level change reflects a change in geometry of theocean basins. It leads to variation in the global volumeof ocean basins. Larson [63] suggested that the rateof ocean-crust production drives long-term sea-levelchange. However, the rate of ocean-crust productionpeaked during the Aptian [63] and the maximumof marine transgression (leading to the flooding of


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a large part of Europe, Northern Africa and NorthAmerica) was reached during the Turonian, about20 Myr later [46]. A sea-level change of some 100150 m between the Aptian and the Turonian as wellas a slow long-term transgression imply a tectonicprocess. Fluctuations of mantle temperatures beneaththe mid-ocean ridge [50] may contribute to drive long-term sea-level change.

The volume of water has also fluctuated in responseto climate change (thermal dilatation), ice-sheet for-mation or tectonic events (desiccation of isolatedbasins, as occurred during the Messinian crisis [16,49]). These mechanisms are able to change abruptlythe volume of water, whereas tectonic processes act onlonger timescales. However rapid sea-level variationsover geological timescales are not known accuratelyenough. The rapid sea-level fluctuations proposed byHaq et al. [46] were challenged by Rowley and Mark-wick [84], who showed that the oxygen isotopic evo-lution deduced from the sea-level curve of Haq et al.does not fit that measured in sediment cores. The latterprocesses can account for maximum amplitude of sea-level change of about 300 m. Marine transgressionsor regressions alter the surface area of emerged lands,which induces changes in climate and atmospheric and

oceanic circulations.The change in landseadistribution induced by sea-

level fluctuations has been proposed to explain thetemperate climate of continental interiors during theMid-Cretaceous [95]. A marine transgression duringthis period is thought to be responsible for the cre-ation of large epicontinental seas. At its peak, the sealevel was about 200 m higher than at present. A sea-way, called the Western Interior Seaway, connectedthe Arctic basin to the Gulf of Mexico through thewhole of North America; another seaway in north-ern Africa led to episodic marine interchange betweenthe Tethys and South Atlantic Oceans, and a largepart of Eurasia was also flooded. The important sea-level rise and the flooding of North America (InteriorSeaway) and Eurasia prevented winter temperaturesfrom dropping well below the freezing point. Indeeda higher sea level damps the annual thermal ampli-tude. Too cold a winter climate within the interiorsof North America and Siberia was, for a long time,a point of disagreement between models and data [4,95]. However, Fluteau et al. [35] noted some pecu-liar cases, in which the climatic effects of large trans-

gressions are weaker than expected. Using an AGCM,Fluteau et al. [35] performed numerical experimentsin order to investigate the effect of sea-level changeon Mid-Cretaceous climate (Fig. 1). The climate atmid-latitudes is more sensitive to marine transgres-sion than other latitudinal zones. However, the cli-mate response to sea-level change varies also season-ally (Fig. 2). For example, the simulated climate overthe southwestern part of North America in the Aptianexperiment is marked by a warm (>30 C) and dry(


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(a)

(b)

Fig. 1. (a) Aptian (120 Ma) and (b) Cenomanian (95 Ma) landsea distributions represented at model resolution. Grey filled boxes representoceans.

Fig. 1. Rpartition continentocan la rsolution du modle (a) de lAptien (120 Ma) et (b) du Cnomanien (95 Ma).

5. Formation and destruction of oceanic gateways

The opening and closure of oceanic gateways inresponse to local tectonic events may have impor-tant climatic consequences at both local and globalscales [8]. However their influences differ accordingto their width, depth and location. I illustrate this pointthrough two classical examples: the closures of theIsthmus of Panama and the Indonesian seaway. Theclosure of the Isthmus of Panama highlights the com-plexity of the climate system. The narrowing of thisseaway and its final closure at about 3.7 Ma [17,18]intensified the Gulf Stream, bringing more moistureand warmer and more saline waters to the North At-

lantic [47]. At that time, the flow direction though theBering Strait reversed and salinity in the Arctic Oceandecreased [48], which was also predicted by modelsimulations [65]. Although the lower salinity of theArctic Ocean may favour sea-ice formation, a warm-ing due to intensification of the Gulf Stream is inducedover Greenland and northwestern Europe [48]. Thusthe closure of the Panama Isthmus altered both at-mospheric and oceanic circulations. However, the cli-matic consequences induced by this event are not fullyunderstood.

The closure of the Indonesian seaway may alsohave led to a global climate changes [15,82]. Caneand Molnar [15] pointed out that palaeogeographic


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(a)

(b)Fig. 2. Differences in temperature in North America between Aptian (low sea level) and Cenomanian (high sea level) (Cenomanian minusAptian). The black solid line represents the shoreline of North America during the Cenomanian. (a) In winter, a part of the Northern Americancontinent is warmer by about 3 C. The Western Interior Seaway has little influence over the northeastern continent. (b) In summer, a coolingof some 5 C is simulated over North America, the air masses above the Western Interior Seaway cools by more than 15 C. These results areonly due to changes in landsea distribution. In the two experiments, we use the same sea-surface temperature distribution (only adapted tothe palaeogeography), the same level of atmospheric CO2), and the same distribution of vegetation (only adapted to the palaeogeography). Thetopography may be slightly different but it concerns only some grid points between the Western Interior Seaway and the Pacific Ocean.

Fig. 2. Diffrences des tempratures en Amrique du Nord entre lAptien (bas niveau marin) et le Cnomanien (haut niveau marin) Cnomanien moins Aptien. Le trait noir reprsente la ligne de rivage de lAmrique du Nord au Cnomanien. ( a) En hiver, une partie delAmrique du Nord est plus chaude denviron 3 C. Le Western Interior Seaway a peu dinfluence sur le Nord-Est du continent. (b) Ent, un refroidissement moyen denviron 5 C est simul sur lAmrique du Nord, les masses dair au-dessus du Western Interior Seawayse refroidissent denviron 15 C. Ces rsultats ne rsultent que des changements de distribution terremer. Dans ces deux expriences, nousavons utilis la mme distribution de temprature de surface des ocans, le mme taux de CO2, la mme rpartition de vgtation (adapte la

palogographie). La topographie peut changer trs lgrement, mais cela ne concerne que quelques points de grille entre le Western InteriorSeawayet locan Pacifique.

changes in the Indonesian Archipelago alter the con-vective process above the western Pacific Ocean andalter the circulation in the Walker cell. In response tothe closure of the Indonesian seaway, heat transportedtoward northern high latitudes increased, which inhib-ited the possible development of an ice age. The fluxof warm Pacific water mass entering the Indian Ocean

was also reduced, which induced the aridification ofEastern Africa.

Although the opening and destruction of gatewaysare discrete (rapid), geological events in comparisonto mountain building or global plate motions, theycan influence the large-scale oceanic and atmosphericcirculations.


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6. Mountain uplift

The Earths geological history is punctuated bythe formation of high mountain ranges and occasion-ally flat elevated plateaus. The link between mountainbuilding and climate change has been studied for acentury. The development of powerful climate mod-els over the last 25 years has further encouraged sci-entists to study this effect. Through different exam-ples, we highlight below how these geological struc-tures may alter the atmospheric circulation and influ-ence climate. We also show that the influence on cli-mate depends on the location, orientation and shape ofthe mountain ranges. Two different examples, the Hi-malayan range and Tibetan Plateau, and the Variscanrange will illustrate our purpose. Both cases could beconsidered as major geological events in the Earthshistory during the Phanerozoic.

6.1. Climatic consequences of the rises of the

Himalayan and Tibetan Plateau

The IndiaAsia collision began in the Eocene[75]. This continentcontinent collision has producedthe worlds highest mountain ranges (the Himalayas)

and the largest elevated plateau (the Tibetan Plateau,with a mean elevation of 5000 m over an areaexceeding 4 106 km2) [91]. Different models havebeen proposed to explain the formation of the TibetanPlateau. Nevertheless, the numerous data acquiredduring the two last decades highlight the fact that theuplift of the Tibetan Plateau has a complex history(e.g., [20,93]).

One clear climatic consequence of mountain build-ing is cooling. Numerical experiments have been per-formed to investigate the sensitivity of climate to theheight of the Tibetan Plateau. An uplift of about 4to 5 km decreases mean annual surface temperaturesby 1620 C over the Tibetan Plateau [33,79,87,88].This cooling is due to the vertical lapse rate, about6.5 C km1. Nevertheless, additional processes af-fect the surface temperature of high topography. Inwinter, snow induces an additional cooling linked tothe albedo-temperature feedback, which intensifies thedrop of temperature. Because of its uplift, the thick-ness of the troposphere is reduced and thus the roleof the radiative processes becomes more important.The rise of the Tibetan Plateau favours the develop-

ment of a high-pressure cell above the plateau, in-ducing an outflow of cold air over the adjacent low-lands. In summer, a cooling of about 6 C is simulatedover the Tibetan Plateau, due to its uplift. Becauseof the decrease of tropospheric thickness above theplateau, the solar radiation intercepted by the TibetanPlateau surface and emitted as sensible heat rapidlywarms the atmosphere. In response to the tempera-ture change, a low-pressure cell forms over the TibetanPlateau in summer, which advects wet air masses com-ing from the Indian Ocean. The rise of the TibetanPlateau alters the upward and downward air motionsabove the plateau as well as the horizontal motionsof air masses. The advected air masses, coming fromthe ocean, bring moisture over the continent where in-tense precipitation occurs. Sensitivity experiments onthe Tibetan Plateau uplift simulate a precipitation in-crease exceeding 100% over Tibet and southern Asia[5961]. A large part of southern Asia undergoes amonsoon regime, which is strengthened in response toTibetan Plateau uplift. These results agree well withfield data. Indeed a major change attributed to the in-tensification in the monsoon regime during the LateMiocene has been observed in both marine and ter-restrial data. This abrupt change supports the hypoth-

esis of a threshold elevation for the Tibetan Plateau.However, most of experiments involve an idealiseduplift of the Tibetan Plateau, which rises as a sin-gle unit in space. This scenario is challenged by re-cent model, which proposes a stepwise rise to explainthe formation of the Tibetan Plateau [69,92,93]. Thisscenario differs largely from most climate simulationsthat have been performed. Using an AGCM, Ramsteinet al. [79] and Fluteau et al. [33] have performed sen-sitivity experiments on the elevation of the TibetanPlateau taking into account a progressive northwardshift of the uplift. Two Late Miocene simulations werecompared both without and with Tibetan Plateau (al-though it was considered smaller and lower than to-day). There is almost no change in summer monsoonprecipitation between the two experiments.How couldwe explainthis absence of a drastic change in the mon-soon regime? The main difference between the pre-vious experiments [5961] with those performed byRamstein et al. [79] and Fluteau et al. [33] is the treat-ment of the Himalayas. Because of the weak spatialresolution of models, the Himalayas and the TibetanPlateau are currently considered as a single unit. Ac-


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tually the Himalayan range represented a significanttopographic feature undergoing rapid erosion since atleast 20 Ma [38], whereas the Tibetan Plateau wasprobably still low. Indeed the absence of an elevatedTibetan Plateau does not inhibit the summer Asianmonsoon. The monsoon is driven by the thermal con-trast between a cool ocean and an overheating con-tinent. The thermal landsea contrast exists with orwithout a Tibetan Plateau. However, monsoon inten-sity will be reduced. Indeed we decreased the eleva-tion of the Tibetan Plateau and kept the Himalayas un-changed (present-day elevation) [33,79]. Although thesensible heat decreases above a lower Tibetan Plateau,the release of latent heat remains constant over theHimalayas. The Himalayas plays an important rolein monsoon evolution (as well as in global climatechanges). The advection of wet air masses by the low-pressure cell over the (flat or high) Tibetan Plateau insummer produces an important release of latent heatdue to moisture condensation and heavy precipitationover the southern edge of the Himalayas [33]. Accord-ing to our experiments, moisture advection and precip-itation over the Himalayas appear to be coupled withtheir elevation but seem to be nearly independent ofthat of the Tibetan Plateau.

Precipitation changes over the Himalayas stronglyinfluence the hydrology of southern Asia, increas-ing runoff by a factor of 10 [61] and heavy rainyevents by a factor of 3 (daily rainfall >10 mm d1 in[33]). Moreover, in response to the palaeogeographicchanges, summer precipitation with respect to the an-nual mean doubles over the Himalayas [33]. Thesehydrological changes influenced both mechanical ero-sion as well as chemical weathering. The enhance-ment of silicate weathering lowers atmospheric CO2and thus provides a possible mechanism for globalcooling during the Late Miocene (see below) [80,81].A strengthening of mechanical erosion may influencethe uplift rate of the Himalayas [70] as well as the sed-imentary burial of organic carbon, also leading to CO2consumption [37].

6.2. Climatic influences of the Appalachians during

the Late Palaeozoic

During the Late Phanerozoic, the convergence be-tween Gondwanaland and Laurasia created the Varis-can mountain range, which was likely the highest

mountain range in the Late Palaeozoic. This mountainrange stretched for about 6000 km along the Equa-tor. Radiogenic isotopes suggest that the uplift wasdiachronous from east to west. The eastern part (theHercynian mountain range) was uplifted during theLate Carboniferous, whereas the western part (the Ap-palachians) was uplifted during the Early Permian,rejuvenated earlier orogens. The elevation of thesetwo mountain ranges remains poorly constrained. Ev-idence for mountain glaciers has been observed inthe French Massif Central, suggesting that the Her-cynian range possibly exceeded 5000 m at the Permo-Carboniferous boundary [7]. The elevation of the Ap-palachians is estimated between 2.5 km [32] and 6 km[64]. The geological context supports the hypothesisof mountain ranges comparable in elevation to the Hi-malayas. The presence of a flat elevated plateau re-mains more speculative, although it has been sug-gested in both cases [28,68].

Contrary to the uplift of the Tibetan Plateau andthe Himalayas, which influenced both the tropical andextra-tropical circulations, the Variscan range affectedboth hemispheres because of its equatorial location.Before discussing its effect, we discuss briefly the cli-matic impact of supercontinents (Fig. 3), which have

been investigated in several studies. The primary workwas performed using an idealised flat Pangea configu-ration [57]. This geographic configuration induces amega-monsoon in the summer hemisphere and pre-cipitation belts oscillate according to the season. Thisexperiment did not account for topography, despitethe fact that the climate change depends strongly onthe size, height and geographic location of mountainranges. The major uncertainty in adding topography,such as the Variscan Mountains, is the estimation ofits palaeoelevation. Different methods based on plantphysiognomy [98], plant physiognomy and enthalpy[36], oxygen isotopic analyses [85], and sensitivity ex-periments to topographic scenarios using AGCM [34]have been developed to constrain altitude.

The atmospheric circulation above Pangea dependsstrongly on the elevation of the Appalachians [34,73]. Using an AGCM, Fluteau et al. [34] have madetwo simulations with respectively moderately elevated(about 2.5 m) and high (4.5 km) Appalachians. Fora moderate elevation, one observes a seasonal shiftof precipitation associated with the intertropical con-vergence zone (ITCZ) in the equatorial realm above


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Fig. 4. Mean precipitation (mmd1) and relief (m) averagedbetween 80W and 70W across the Appalachians mountain range.The grey filled box represents the elevation of the Appalachiansand the adjacent areas. The solid (dotted) line corresponds tothe mean precipitation averaged over June, July, and August(December, January, and February). (a) Using low Appalachians.

The wet climate is located above the Appalachians; a summer-wetclimate is simulated in the adjacent plains and an arid climateto the north/south of 20N/S. (b) Using high Appalachians. TheAppalachians have wet climate. The summer-wetclimate is replacedby a dry climate.

Fig. 4. Les prcipitations moyennes (mmj1) et le relief (m)moyenns entre 80W et 70W travers la chane des Appalaches.La zone grise reprsente le profil des Appalaches et des zonesadjacentes. Le trait continu (pointill) correspond la moyennedes prcipitations moyennes sur les mois de juinjuilletaot(dcembrejanvierfvrier). (a) Cas dune chane des Appalachesbasses. Le climat humide se situe au-dessus des Appalaches ; unclimat humide en t est simul dans les plaines adjacentes et unclimat aride au nord et au sud du 20e parallle. (b) Dans le casdune chane des Appalaches hautes. Les Appalaches connaissent un

climat humide. Le climat humide en t est remplac par un climatsec.

free climate could not exist without a strong green-house effect, which balanced the low incoming solarradiation [12]. Without greenhouse effect feedback,the Earth would have been frozen continuously.

Climate changes could be the result of the imbal-ance between CO2 emissions and uptakes. In orderto estimate the impact of past CO2 atmospheric con-

centration, a better knowledge of the long-term evolu-tions of the sources and sinks of carbon in the pastis strongly required. We recall below the long-termglobal carbon cycle. We show that pCO2evolution atgeological timescales is not known accurately, becausedata are sparse and the respective influences of carbonsources and sinks in geochemical models [9,10,13,19,39,66] differ from one model to the other.

Although the role of CO2 on Earth climate iscrucial, other gases such as sulphur dioxide (emittedduring volcanic eruptions) also affect the chemicalcomposition of atmosphere. We will discuss theireffect on radiative balance using observations frommodern volcanic eruptions and briefly discuss whatclimate effects could have resulted from flood basaltevents. At last, we will discuss the possibility of large-scale methane gas hydrate release and its climaticconsequences.

7.1. Carbon dioxide and climate changes

On geologic time scales, CO2 concentration inthe atmosphere is driven by the long-term carboncycle. Volcanism along mid-ocean ridges, as well asarc and plume-type volcanism, injects CO2 in the

oceanatmosphere reservoir. The global present-dayflux is about 6 1012 molyr1 [67]. Most degassedCO2 appears to be recycled rather than primordial[51]. The CO2 degassing rate is considered to beproportional to volcanic activity. Thus fluctuations inseafloor spreading rate and mantle plume productionmodulate the amount of carbon dioxide injected in theoceanatmosphere reservoir [11,19]. Nevertheless thedegassing rate would also depend on the amount ofcarbonate subducted and metamorphosed to CO2[19].

On the other hand, CO2 is removed from the at-mosphere by chemical weathering of carbonate andsilicate rocks. Rivers carry the weathered products(Ca2+ and HCO3) to the oceans, where they precipi-tate as calcite in marine sediments [11,19].Weatheringof carbonate has no long-term effect on pCO2becauseone mole of CO2 is removed by weathering and thesame amount of CO2 is released by the precipitationof calcium carbonate [11]. The weathering of silicatehas an effect on pCO2since the weathering of silicateremoves 2 mol of CO2 from the atmosphere and onlyreleases one mole of CO2. At last, pCO2 (as well aspO2) is influenced by the organic matter subcycle. The


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canic rocks. Dessert et al. [27] suggested, using ageochemical model, that the emplacement of basalticprovinces (e.g., Courtillot and Renne, this issue) andits subsequent weathering have induced geochemicaland climate changes. The emplacement of the Dec-can basaltic province at the KT boundary increasedpCO2 by about 1000 ppm, inducing an abrupt warm-ing of some 4 C [27]. After this warming episode,weathering of the Deccan basaltic province consumedatmospheric CO2. At the end of the experiment, theatmospheric pCO2was found to have been lower thanin pre-Deccan times by about 50 ppm, and temperatureby about 0.5 C.

There is no doubt about the importance of large,sometimes geologically rather fast changes in CO2concentration on climate, although the long-term evo-lution of pCO2remains uncertain.

7.2. Volcanism

Sulphur dioxide emitted during volcanic eruptionsalters the net radiation budget of the Earth. Our knowl-edge concerning the climatic consequences (mainlytemperature change) of volcanic eruptions has im-proved thanks to the discovery of climatic anom-

alies during the last five decades. Not surprisingly, itappears that temperature changes induced by erup-tions depend on numerous factors. The 1963 explo-sive eruption of the Agung volcano in Indonesia pro-duced 0.3 km3 of magma [78]. While the volume oferupted magma was minor, it was accompanied bya large amount of sulphur dioxide (greenhouse gas)emitted into the atmosphere [78]. The oxidation of sul-phur dioxide produced a thick layer of sulphuric acidaerosol in the stratosphere (estimated between 10 and20 Mt), which scattered and absorbed incoming solarradiation [78] (see review in [97]). Due to the lack ofvertical mixing in the stratosphere, the aerosol cloudspread horizontallyand modifiedthe net radiation bud-get for a few years, by cooling the Earths surface bysome 0.3 C annually, yet warming the stratosphereby 6 C over the three years following the eruption[78]. In order to better understand the radiation ef-fect from eruptions, volcanism over the past 10 kyrhas been studied in Greenland ice cores, through acid-ity profiles and dust anomalies [44,45]. These acidityprofiles revealed that most of the cold events duringthis period followed a volcanic eruption. The climatic

consequences on volcanism depend on the amount ofSO2emitted in the atmosphere, the plume height andthe location of the volcano. The amount of SO2 de-pends on the tectonic context and also on the volumeof magma. The plume height depends on the eruptionvolume rate [90] (Fig. 5). As a consequence of ver-tical mixing in the troposphere, SO2 injected belowthe tropopause has less impact on climate. Conversely,SO2 injected above the tropopause may remain for afew years in the stratosphere.

However, all volcanic eruptions known over thepast millennia are tiny in comparison with floodbasalts. Flood basalts occurred throughout the Earthshistory and coincided temporally with continentalbreak-up[22]. The individuallava flow volumesrangesfrom several hundred to several thousand cubic kilo-metres [22,89,97] (see also Courtillot and Renne, thisissue). The climatic impact of large flood basalts is notclearly established. This uncertainty is due to a lackof evidence on the total duration needed to produceflood basalts (ranging from 0.5 to 1 Ma) and the dura-tion needed for a single lava flow [22,89,97] (see alsoCourtillot and Renne, this issue). The climatic influ-ence may vary according to these durations. The Dec-can traps erupted at the CretaceousTertiary boundary

and lasted less than 1 Myr [21]. Over this time inter-val, the Deccan traps might have injected 1014 kg ofsulphate aerosols once every 10100 ka over a periodof 1 Myr (Widdowson et al., 1997, in [97]) leading toacid rain and a 1 C global cooling. This effect maybe hidden by a global warming induced by the emis-sion of about 5 1017 moles of CO2(McLean, 1985,in [97]): global warming might have not exceeded 1 to2 C. However, Dessert et al. [27] showed that the neteffect of the emplacement of large basaltic provincesmight have ended up being a global cooling.

7.3. Methane release

Carbon dioxide as well as sulphate aerosols are notable to explain the entire Earth climate history, andespecially sudden and brief climatic change. A briefepisode during the Late Palaeocene ( 55 Ma) is su-perimposed on the long-term climate trend [99], asevidenced by an abrupt decrease of 18O values by23, measured on deep-ocean benthic foraminiferaand on high-latitudes planctonic foraminifera [29,30].The negative 18O excursion reflects a warming of


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some 48

C in less than 200 kyr [83]. This warm-ing may not be global, given the weak change ob-served in the tropics [14]. A major faunal turnover aswell as migration of mammalian faunas using high-latitude routes [6] coincided with the Late Palaeocenethermal maximum (LPTM). This event is also associ-ated with a negative 13C excursion in both the deepand surface oceans, as well as in terrestrial sequences[29,30, and references within]. Dickens [29] suggestedthat the current model of the global carbon cycle couldnot explain the Late Palaeocene negative 13 C excur-sion. The solution could be a massive input of carbonfrom an external reservoir enriched in 12C. Dickenset al. [30] and Dickens [29] have attributed this neg-ative 13C excursion to a sudden release of methaneinduced by the dissociation of methane gas hydratestrapped in oceanic sediments. This dissociation of gashydrates would have resulted from a change in tem-perature gradient as well as a pressure change in theoceanic sediment column. The methane released dur-ing this event was partly oxidised to carbon dioxideas it passed through the water column, which wouldexplain the drop in deep-water O2concentration [52].The amount of CO2introduced in the atmosphere andthus its climatic impact depends on the efficiency of

water masses to oxidise methane. Consequently, theclimatic impact of methane release has been chal-lenged by Kvenvolden [62]. Using a modified versionof a global carbon cycle to account for the methanehydrate reservoir, Dickens [29] showed that the mas-sive input of methane in the carbon cycle model duringa short time interval reproduces quite well some LatePalaeocene observations (negative13C excursion anddrop in deep water O2concentration). Conversely, at-mospheric pCO2increases by only 5080 ppm. Thusthe impact of methane release on climate could likelybe weak, as suggested by Kvenvolden [62], especiallyduring the Late Palaeocene, which belongs to a highCO2 period (26 times the present-day concentra-tion). Therefore, the dissociation of methane gas hy-drates must be seen as a positive feedback mecha-nism. Peters and Sloan [76] suggested that high at-mospheric methane concentration (17 times the prein-dustrial value) might have induced the formation ofpolar stratospheric clouds, which can warm high lat-itudes. Prescribing high CH4 atmospheric concentra-tion (14 times the present CH4concentration and dou-blingthe present CO2concentration), stratospheric po-

lar clouds and Late Palaeocene geography, Peters andSloan [76] simulated a winter warming by as muchas 20 C over Ellesmere Island, although winter tem-perature remained below the freezing point. The in-fluence on climate depends on the amount of CH4released in this atmosphere, which is actually poorlyconstrained. This mechanism has been suggested forthe Late Palaeocene thermal maximum as well as foroceanic anoxic events [97] and more recently for theCambrian negative13C excursions (810) [54].

8. Conclusion

Understanding the Earths climate history over ge-ologic time-scales remains a challenge, despite thehuge amount of data accumulated over a century andthe use of numerical models since two decades. Thefirst-order climate history over the Phanerozoic, whichshows a succession of warmer and colder global peri-ods, is now relatively well known. However, we arestill unable to reproduce the long-term trend. Differ-ent forcing factors, most of them resulting from inter-nal Earths processes, have been proposed to accountfor climate evolution over geologic timescales. Two of

them are of primary importance, plate tectonics andgreenhouse gases, both being themselves possibly inpart causally related.

The past position of continents is accurately knownfor the past 200 Ma, but progress is still required forolder periods. Climate modelling has shown that thisfactor cannot account alone for global long-term cli-mate changes. Indeed, the location of continents can-not explain the extreme warmth in the Mid-Cretaceousor the short duration of the Ordovician ice age. Theimpact of plate configurations on climate may haveevolved in the past. Numerical experiments reveal thatthe Late Permian climate was largely driven by a pe-culiar palaeogeographic configuration (Pangea super-continent). All continents are assembled as a singleentity, deeply reducing the effect of oceanic circula-tion on the climatic system. Most recently, a com-bination of break-up of the equatorial supercontinentof Rodinia, emission of a huge flood basalt and en-hanced weathering has been suggested as a possiblecause for the 700-Ma Proterozoic Snowball Earth(Godderis et al., in review). Conversely, the rise of sealevel driven by plate tectonics affects the landsea dis-


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tribution and both alter atmospheric and oceanic circu-lations. Climate modelling reveals that marine trans-gression causes a warming within the interiors of con-tinents. The Mid-Cretaceous warmth within the interi-ors of continents could be explained by such a mech-anism. The coupled effect of plate tectonics and sea-level variations has led to the opening and closure ofmarine gateways. The effects of marine gateways onclimate vary drastically with respect to their location,width and depth. In the case of closure of the Isthmusof Panama, as well as of the closure of Indonesianseaway, changes in oceanic and atmospheric circula-tion may likely have contributed to large-scale climatechanges. Finally, the elevation of mountain rangessuch as the Appalachians during the Late Palaeozoicor the Himalayas and Tibetan Plateau during the LateCainozoic appears to be an important forcing factor,which alters atmospheric circulation and therefore cli-mate at the continental scale.

To influence the climate on a global scale, changesin atmospheric chemical composition, and especiallyCO2 fluctuations appear to be a more efficient mech-anism. Fluctuations of pCO2deduced from geochem-ical models simulate roughly long-term climate evo-lution. An elevated pCO2is required to partly explain

the warmth of the Mid-Cretaceous. The evolutions ofsources and sinks of carbon as well as fluxes betweencarbon reservoirs have to be better constrained overlong-term timescales. The impact of silicate weather-ing on climate has been suggested as a potential mech-anism to balance the input of CO2 from volcanoes(mid-ocean ridges, arc- and plume-type volcanism)and other sources. The Late-Cainozoic global coolinghas been ascribed to an increase in silicate weather-ing resulting from the Himalayan uplift. However, theuptake of CO2 by sedimentary burial of organic mat-ter could have been a more efficient mechanism, be-cause of the high denudation rate during the rise of theHimalayas. The influence of weathering of volcanicrocks may have been significantly underestimated.

Volcanic eruptions are a major potential candidatefor climate change on short timescales, resultingfrom the injection of aerosols such as SO2 in theatmosphere. However, the climatic effects of recentvolcanic eruptions do not exceed several years. Theinfluence of large flood basalts on climate remainsto be better studied. The amounts of SO2 and CO2must be known more accurately, as well as eruptive

fluxes during eruptions of single lava flows. Mostrecently, the emission of methane resulting fromthe dissociation of gas hydrates has been proposedto explain an extreme warm episode in the LatePalaeocene, as well as a negative excursion of 13C.The contribution of the latter mechanism on long-termclimate evolution is still unknown.

Because of the complexity of both the climatic sys-tem and climate changes, there is no single factor thatcan explain all of the Earths climatic evolution. Wemust improve our understanding of climate evolution,of the climatic system, as well as the forcing factorsin order to better reconstruct the history of the Earths

climate.

Acknowledgements

I thank Stuart Gilder, Jean Besse and VincentCourtillot for help in improving the manuscript. I amvery grateful to William Ruddiman and ChristianFrance-Lanord, who made useful and constructivecomments. This is the IPGP contribution No. 1871.

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