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Earth, Atmospheric, and Planetary Physics GEOPHYSICAL RESEAIO,CH LETTEI•,S. VOl,. 28, NO. 2, PAGES 2813-286,.IANUARY 15,2001 CO2 levelsrequired for deglaciationof a "Near-Snowball" Earth Thomas J. Crowley, William T. Hyde Dept. of Oceanography,Texas A&M University,College Station,Texas W. Richard Peltier Dept. of Physics,University of Toronto,Toronto, Canada Abstract. Geologic evidence suggeststhat in the Late Hoffman et al. [1988] invoked estimatesby Caldiera and Neoproterozoic,(-600 Ma) almost all land masses were Kasting[1992] that massivelevels of atmosphericCO 2 (--0.i glaciated, with sea-levelglaciation existing even at the bar) were requiredto escapefrom this state,and suchvalues equator. A recentmodeling study has shownthat it is possi- are comparableto thosecalculated by Hydeet al. [2000]. ble to simulate an ice-coveredEarth glaciation with a cou- A furtherresult of the Hyde et al. [2000] study is that pledclimate/ice-sheet model. However,separate general cir- stand-alonegeneral circulation model (GCM) experiments culation model experimentssuggest that a secondsolution with fixed ice sheetsin some casesyielded open water may exist with a substantialarea of ice free ocean in the conditionsin equatorialregions removed from the ice sheet tropics.Although 0.1 to 0.3 of an atmosphereof CO2 (-•300 [cf. Baumand Crowley,2000]. The existenceof openwater to 1000 X) is requiredfor deglaciationof a "SnowballEarth," solutionsreflects standardlength-scale arguments discussed the "exit"CO 2 levelsfor an openwater solution could be in the climate literature[North, 1984; Manabe and Broccoli, significantly less. In this paper we utilize a coupled 1985], in which the thermalinfluence of an ice sheet(or other climate/ice sheet model to demonstratefour points: (1) the perturbation)diminishes by a scale which is determinedfrom openwater solution can be simulatedin the coupledmodel if characteristicradiative damping and heat transportvalues for the sea ice parameter is adjusted slightly; (2) a major the Earth. GCM sensitivityexperiments [Baum and Crowley, reductionin ice volume from the open water/equatorialice 2000] indicate that this open water solution is obtained for solutionoccurs at a CO2 levelof about4X presentvalues - CO2 valuesof about2.5X presentlevels. This studyalso abouttwo ordersof magnitudeless than required for exit from demonstratedthat a strong negativecloud feedbackplays a the "hard"snowball initial state;(3) additionalCO 2 increases role in offsetting the enhancedalbedo from expanded ice are requiredto get fuller meltbackof the ice;and (4) the open cover, as the decreasedair temperaturesare associatedwith water solution exhibits hysteresisproperties, such that lower water vapor levels [cf. Jenkins, 1993; Chandler and climateswith the same level of CO2 mayevolve into either the Sohl, 2000]. snowball,open water, or a warmerworld solution,with the The existenceof the open water solution has important trajectorydepending on initial conditions.These results set implicationsfor climate theory and the survival of metazoans, usefultargets for geochemicalcalculations of CO2 changes and it would provide a valuable constrainton geochemical associatedwith the open-watersolution. modelsof the late Neoproterozoic. The open water may also leadto significantlylower deglaciation CO 2 thresholdlevels than are necessary for a "hard" snowball solution. In this Introduction paper we use the samecoupled climate/ice sheet model as that The possibilityof a late Neoproterozoic(---800-600 Ma, in Hyde et al. [2000] to conduct a range of sensitivity million years ago) "SnowballEarth" has stimulateda great experimentsto determinewhether open water solutionscan be deal of interest. Ice sheetsapparently reached the equatorat obtainedby suchcoupled models and, if so, the CO2 levels thistime [e.g,Hambrey and Hadand, 1985;Eries, 1993], and requiredfor deglaciationfrom sucha state. the first appearanceof multi-celledanimals (metazoans) may havepreceded [e.g, Hofmannet al., 1990;Bromham et al., 1998] the last phaseof the glaciation(-600 Ma). Although Climate Model and Boundary Conditions early modelingwork on this problemfailed to simulateequa- The climate/ice sheet model [Deblonde and Peltier, 1991; torial glaciation,recent studieswith a coupledclimate/ice Deblonde et al., 1991; Tarasov and Peltier, 1997] consistsof sheetmodel [Hyde et aL, 2000] indicatethat a SnowballEarth four submodels,which respectively predict ice flow, mass solutioncan be parsimoniouslyobtained if solar luminosity balance, temperature, and bedrock sinking. The latter is is reducedby the astrophysicallyreasonable value of 6% and assumedto occur with a time constantof 4,000 years [Peltier, CO2 levelsare near present values. Hyde et al. [2000]further 1998]. Ice flows subject to a temperature-independent demonstrated that the existence of the ice-covered Earth rheologybased on the Nye [1959] formulation,and massbal- solution is dependenton inclusionof the ice sheet, because ance is computedaccording to statisticalmodels [Reeh, 1990; the elevation and thermal inertia of the ice-sheet allows it to Huybrechts and T'Siobbel, 1995], which take as input "weather" Milankovitch interglacial orbital configurations. monthly temperatures generated from a nonlinear, two- dimensionaldiffusive seasonalEBM [Hyde et al., 1990]. The Copyright2001 by the American Geophysical Union. EBM has been validated against many different GCMs Papernumber 2000GL011836. [Crowley et al., 1991], while the coupled ice sheet model 0094-8276/01/2000GL011836505.00 reproducesmany featuresof both the last glacial cycle and the 283 284 CROWLEYET AL.; CO 2 LEVELS REQUIRED FOR DEGLACIATION OFA NEAR-SNOWBALLEARTH Carboniferousand Neoproterozoicglaciations [Tarasoy and Ice Sheet Peltier, 1999; Hyde et al., 1999, 2000]. 60' 60' Baselinepaleogeography for the Neoproterozoicis from Dalziel [1997], althoughsensitivity experiments [Hyde et al., 30' 30' 2000] indicate that the principal results are not strongly dependenton detailsof geography.We examineda rangeof elevationsfor the continentsranging from sea level to 500 m. In the extremelycold climatewe studyin this paperour equi- -30' -30' librium results are relatively insensitiveto this parameter. -60' -60' The model is driven with Milankovitch insolation variations appropriateto the Pleistocene[see for example Hyde et. al 1989]. Other boundaryconditions include a solar luminosity I i I 6% belowpresent [Crowley and Baum,1993], CO 2 levels 1 1000 2000 3000 4000 5000 varying from -•0.5-10X present levels and a variable sea ice Mean Annual Temperature albedo. The sea ice albedo of the EBM in Pleistocene simulations [Deblonde and Peltier, 1991] is 0.5. This value 60' 60' is lower than employed in GCMs and reflects the fact that the 30' 30' EBM specificationis for the total surface-atmospherecolumn. Due to atmosphericreflectance and absorption,the column 0' - - 0' albedois about 10% lower than the surface[see Hyde et al., 1989]. Becausecloudiness decreases in our Neoproterozoic -30' -30' GCM runs[Baum and Crowley,2000], we mimic this effect on -60' -60' absorbedsolar radiationby loweringalbedo to 0.45 over sea ice. The net changesin radiationreceipt from the decreased ,,• ! . ! ! I I . albedo(---25 W/m 2) arestill somewhat less than the changes in I I I I I I I ! surface radiation receipt from a 20% decreasein cloudiness -50 -40 -30 -20 -10 0 10 20 [Baum and Crowley, 2000], but given all the uncertaintiesin Figure 2. Example of model open water solution: (a) the albedo calculations, we consider it a reasonableadjust- distribution of ice sheet (thickness in m)' (b) mean annual ment for the model. Further justification for the sea ice temperature. albedo reduction is basedon the suppositionthat the albedo should be lower at low zenith angles in the tropics [for example, the albedo of water varies from 0.02 to 0.06 as the zenith angle changesfrom 0 to 60 degrees,Peixoto and Oort, northern hemisphere mid/high-latitute land areas. Previous 1992, p. 103]. work [Hyde et al., 1999] indicates only a modest sensitivity Precipitation is highly parametertized;we employ a spa- to variations in this term in very cold climates, and for the tially uniform value of 0.6 mm/day, which declines with work presentedhere we considerthe choice satisfactoryfor an height and temperature. This value is characteristicof present initial examinationof the "openwater" deglaciationproblem. The numerical experimentswere conductedin two ways. In the first class of experimentsthe model was initialized as ice NeoproterozoicIce Age Phase Space free, and run until equilibrium under prescribed input parameters. Most such runs came to equilibrium after "-:•, o i is "- 100,000 years of integration, though some were extended to 10•.. " 4- O'O' .-O...... ' 'O'i-': O O O • +.... + i++ --. ..+ 200,000 years to determine the stability of the resulting climate against Milankovitch forcing. A secondset of runs e !%o .' ....... was started at the end of a 200,000 year simulation that E 4o- eo yielded a stable open water solution. In this set of (D ' : : experimentswe linearlyincreased the CO 2 concentrationfrom • 4e. ..-: years 220,000 to 270,000 and ran the model to equilibrium (•,- ::o.+.o. o . > o .... with the new forcing, generally until about year 350,000. In •' : -" '"' -(9 • i--' : * ..... this way we are able to test the stability of the open water (D .... ,• ....... oO. o solution to changesin forcing. •-- ..: .,... .-o . ..,., .-..... .... .--..._ ..............+ . ...... .'" :" . :" '. o.-' ' .... .. "" :' -~' ,. ' ..... -50,,. --"*•" ..:'" '- •.,: "** '*
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