continental drift, runoff and weathering feedbacks ...· continental drift, runoff and weathering

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  • Continental drift, runoff and weathering feedbacks:Continental drift, runoff and weathering feedbacks:Continental drift, runoff and weathering feedbacks:Continental drift, runoff and weathering feedbacks:Continental drift, runoff and weathering feedbacks:ImplicaImplicaImplicaImplicaImplications frtions frtions frtions frtions from com com com com climalimalimalimalimate model ete model ete model ete model ete model experxperxperxperxperimentsimentsimentsimentsiments

    Bette L. Otto-BliesnerDepartment of Geology, University of Texas at Arlington

    AbstrAbstrAbstrAbstrAbstract.act.act.act.act. Changes in atmospheric carbon dioxide have been proposed as a major regulator ofclimate during the last 570 million years. Continental weathering and its variation over time arehypothesized to be important for controlling atmospheric carbon dioxide. Continental weatheringis altered by changes in total runoff as well as changes in the size and elevation of the landmasses to be weathered. When paleogeographic information for the Phanerozoic (570 m.y. agoto present) is used in a global climate model, the model exhibits substantial variations in precip-itation, evaporation, and runoff. Even with dramatically different land-ocean distributions, anincrease in global surface temperature leads to an increase in global precipitation but not always anincrease in global runoff. During the early Phanerozoic (514-342 Ma), when the continents arerelatively small and widely distributed, runoff depends on continental evaporation and temperature.As the continents migrate into subtropical latitudes, global runoff decreases as global landtemperatures and evaporation increase. As the continents shift to higher latitudes, global runoffincreases as global land temperatures and evaporation decrease. During the middle to latePhanerozoic (306 Ma-present), when large continental land masses predominate, runoff dependson continental precipitation. Experiments with increased atmospheric CO2 for the middleOrdovician (458 Ma), when the paleocontinent of Gondwana was centered at subtropical latitudes,and the early Silurian (425 Ma), when Gondwana had shifted to middle and high latitudes,also point to a correlation between land mass location and runoff. Global runoff increases 15%with increased CO2 at 425 Ma but remains unchanged for 458 Ma, even though global meantemperature and precipitation increase comparable amounts for the two time periods. These resultsimply that weathering feedback between temperature and runoff may be dependent on land-ocean configuration.

    1. Introduction Only about 10% of the atmospheres carbon dioxide CO

    2 is

    cycled through the Earth system every year. On a scale muchlonger than the human timescale, carbon is transferred betweenrocks and the Earth-atmosphere system. This transfer istermed the geochemical carbon cycle and operates on a multi-million year timescale [Holland, 1978; Berner and Lasaga,1989]. To estimate the atmospheric CO

    2 content at any particular

    time in the Phanerozoic, geochemists model the elementsthat can alter the Earths respiration rate [Walker et al.,1981; Berner et al., 1983; Garrels and Lerman, 1984; Budykoet al., 1987; Berner, 1994] . Outgassing of CO

    2 to the atmo-

    sphere is estimated from rates of seafloor spreading and shallowwater versus deep-sea carbonate deposition [Heller andAngevine, 1985; Gaffin, 1987]. The removal of atmosphericCO

    2 by weathering can include the effects of paleogeography

    [Barron et al., 1980; Bluth and Kemp, 1991], topography[Raymo, 1991; Raymo and Ruddiman, 1992], river runoff[Tardy et al., 1989], and plant evolution [Cawley et al., 1969;Volk, 1989]. One unknown that geochemists include in their models isthe effect of temperature on runoff. Global warming experiments

    with global climate models (GCMs) have shown that aglobally warmer world is a globally wetter world [Mitchell,1989]. From these results, geochemical models have oftenassumed that the rate of chemical weathering will increase asglobal temperatures rise [Berner, 1991]. This increase is based on the temperature dependence of the uptake of dissolvedions and on an assumed increase in runoff as temperature in-creases. The latter is based on results from GCM model simu-lations [Gates, 1976; Manabe and Stouffer, 1980; Manabe andBryan, 1985]. Past studies with snapshot simulations for other periods inEarth history point to a hydrologic cycle that is highly sensitiveto land-sea distribution. Barron and collaborators [Barronet al., 1989], using a version of the National Center forAtmospheric Research (NCAR) Community Climate Model(CCM), compute a 4% increase in global precipitation for a1% increase in global surface temperature. In terms of conti-nental runoff, the correlation is less straightforward. Foridealized geography, where present-day land area is confinedto either tropical (equatorial continent) or polar (polarcontinent) latitudes, the equatorial continent case has a 4.6oChigher global surface temperature than the polar continentcase, yet appreciably less runoff [Hay et al., 1990]. In addition,experiments for a Cretaceous land-ocean configurationfind decreased runoff as surface temperatures warm withincreased loading of the atmosphere with CO

    2 [Barron et al.,

    1989].

    Copyright 1995 by the American Geophysical Union

    Paper number 95JD00591.0148-0227/95/95JD-00591$05.00

    JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 100, NO. D6, PAGES 11,537-11,548, JUNE 20, 1995

  • It is apparent that continental runoff for periods in theEarths past was probably highly sensitive to the land-oceanconfigurations. In this study, a low-resolution model of theglobal climate is forced with the land-ocean configurations for14 time slices from 514 Ma to present. Particular attention ispaid to the components of the hydrologic cycle includingprecipitation, evaporation, and runoff. The correlationbetween global runoff and surface temperature and the effect ofincreased atmospheric CO

    2 on this correlation are also

    examined.

    2. Model A global climate model that includes an atmospheric com-ponent as well as simplified components for the ocean and land surface is used for this study. The atmospheric componentincorporates atmospheric dynamics based on the equationsof motion, together with prognostic equations fortemperature and moisture [Otto-Bliesner et al., 1982;Gallimore et al., 1986]. The equations are formulated inspectral terms, and the model retains longitudinal wavenumbers 1 through 10 and an 11.6o by 11.25o latitude-longitudegrid for spectral-grid transforms. The model includes the sea-sonal cycle of solar radiation and the orographic effects ofmountains as well as radiative and convective processes,condensation, and evaporation. The ocean is represented as a 50-m-deep layer that ishomogeneously mixed [Washington and Meehl, 1984;Gallimore and Houghton, 1987]. Ocean surface temperatureand heat storage are determined solely from atmosphere-oceanenergy exchange. The roles of salinity, ocean currents, upwelling,and energy exchange with deeper layers are omitted.The thermodynamic sea ice model [Semtner, 1976] allows seaice to accumulate when ocean surface temperatures fall below-2oC.

    The land surface component consists of a surface heat budgetfor determining land surface temperatures as well aspredictive equations for snow cover and surface hydrology.The surface hydrology scheme uses model-derived precipitationand evaporation rates to simulate changes in soilmoisture and runoff using a bucket approach [Manabe, 1969;Manabe and Holloway, 1975; Gallimore and Kutzbach, 1989].A minimum soil moisture (1.6 cm) is specified to account foran assumed availability of some soil moisture via lateral flow in rivers and subsurface aquifers. Runoff occurs when soilmoisture exceeds the assumed field capacity of 15 cm. There is fairly good agreement between the simulated andobserved modern climate [Gallimore et al., 1986; Otto-Bliesner and Houghton, 1986; Houghton et al., 1991]. In par-ticular, the model simulation of the components of the surfacehydrologic cycle for present-day geography and topography(Table 1) is in good agreement with the observed estimates ofBaumgartner and Reichel [1975]. The model computes a present-day, global-average precipitation rate of 933 mm yr-1

    and evaporation rate of 912 mm yr-1, both of which arecomparable to observed estimates of 973 mm yr-1. In addition,the proportions of precipitation and evaporation occurringover land and oceans and contrast between land and ocean arewell simulated. Similar to observed, the model predicts a netexcess of precipitation compared to evaporation over the worldspresent-day land areas (280 mm yr-1) and a net deficitover the present-day oceans (-91 mm yr-1), implying a net moisture transport from oceans to land. The model-simulatedrunoff for modern geography of 247 mm yr-1 compares toBaumgartner and Reichels observed estimate of 240 mm yr-1. The utility of low-resolution climate models for understand-ing first-order responses to changes in forcing, in particular,land-ocean configuration and solar radiation, has been clearlyestablished. For present-day continental configuration, Otto-Bliesner and Houghton [1986] found that at midlatitudes in the

    TTTTTaaaaabbbbble 1.le 1.le 1.le 1.le 1. Global and Annual Averages of the Simulated Components of the Surface Climate___________________________________________________________________________________________________

    Age, Mean Latitude, Land Area, Surface Temperature . Precipitation . Evaporation RunoffLand+Ocean, Land, Land+Ocean, Land, Land, Land,

    Ma degrees X108 km2 C C mm yr-1 mm yr-1 mm yr-1 mm yr-1

    ____________________________________________________________________________________________________

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