JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 100, NO. D7, PAGES 13,755-13,765, JULY 20, 1995 Hypercanes: A possible link in global extinction scenarios Kerry A. Emanuel,• Kevin Speer,2 Richard Rotunno,3 Ramesh Srivastava,4 and Mario Molina • Abstract. Bolide impactsand large-scalevolcanic eruptions have been proposedas possiblecauses of the massiveextinction of life that has occurredepisodically in Earth's history.In spite of the catastrophicdisruption of the local environmentthat accompanies bolide impactsand volcaniceruptions, it has been difficultto explainwhy these events sometimeslead to global extinctionof species.We proposethat, in some cases,the missinglink may be providedby hypercanes,runaway hurricanes that are capableof injectingmassive amounts of water and aerosolsinto the middle and upper stratosphere, where they may have profound effectson atmosphericchemistry and radiative transfer. Hypercanesare theorized to occurwhen the sea surfacetemperature exceedsa critical threshold,which may occurwhen sea water is locally heated by bolide impact, shallow-sea volcanism,or possibly,by overturningof superheatedbrine pools formed by underwater volcanicactivity. Simulationsusing a convection-resolvingnonhydrostatic, axisymmetric numericalmodel show that hypercanescan indeed developwhen the sea surface temperature is high, and that they inject large amountsof massinto the stratosphere. 1. Introduction [Luck and Turekian,1983]. Many Cretaceous-Tertiary(K/T) boundary sequencesalso contain abundant shocked mineral Evolution of life has been punctuated by events during grains that suggestan impact, although it has also been pro- which large numbers of specieshave become extinct. Largest posed that this may result from explosivevolcanism. Recent among the known events was the Late Permian extinction, attention has focused on the Chixslub feature on the northern about 245 m. y. ago, during which about 96% of all species Yucatan coastas a possibleimpact crater, thoughseveral other disappeared.The most recent major extinctionoccurred at the craterscan also be dated to the K/T transition, suggestingthat end of the Cretaceous,between 64 and 66 m. y. ago, and was multiple impactsmay have occurredin a short time. There is marked by the disappearanceof about three fourths of all also evidencein K/T sedimentsof a large tsunamievent in the speciesthat existedat that time. Two hypotheseshave emerged Caribbean region. to explain the end-Cretaceousextinction. Alvarez et al. [1980] Although the evidencefor the impact of at least one large proposedthat the impact of a large asteroidled to the massive bolide at the end of the Cretaceousis mounting,there remains extinction,while Officerand Drake [1983] and othersargue that considerablecontroversy about just how such an event might the extinction was related to massivevolcanic eruptions that lead to massiveextinction of life. The original Alvarez et al. resultedin the formation of the DeccanTraps. It has alsobeen [1980] paper suggestedthat the hypothesizedasteroid impact suggestedthat these two geophysicalevents are related [Altet would have thrown up a large quantity of dust into the strato- al., 1988]. sphere,where it would have reduced substantiallythe incom- Considerableevidence has accumulatedin recent years in ing solar radiation, leading to a dramatic decreaseof photo- support of the idea that the Earth was struck by one or more synthesisand concomitantdisruption of the food chain. bolides at the end of the Cretaceous.Foremost among such Other means by which the impact might have led to mass evidenceis the finding byAlvarez e! al. [1980] that iridium and extinctionsinclude: (1) dramatic,direct warmingof the atmo- sphere and/or ocean by the impact [Hst2, 1980]; (2) global other platinum group metalsknown to be more concentrated coolingowing to sootproduced by globalwildfires [Wolbachet in extraterrestrial bodies than on Earth, are enriched in a thin al., 1985]; (3) chemical contamination of the atmosphere clay layer at the Cretaceous-Tertiary boundary worldwide. and/or upper ocean [Hst2,1980]; (4) global warming by large Other evidenceincludes the findingthat this claylayer contains increasesin atmosphericCO2 owingto the impact on carbon- extraterrestrialamino acids[Zahnle and Grinspoon,1990], soot ate terrain [O'Keefeand.4hrens, 1989]; and (5) globalacid rain attributed to global wildfires [Wolbache! al., 1985], and an owing to large increasesin atmosphericsulfur and nitrates Osmium isotopic ratio with a distinctlyextraterrestrial signal [Prinnand Fegley,1987]. As an alternativeto the bolide impact hypothesis,large-scale volcanism has alsobeen cited as a pos- •Centerfor Meteorologyand PhysicalOceanography, Massachu- settsInstitute of Technology,Cambridge. siblecause of massextinctions. Of particularinterest are flood 2InstitutFrangais de Recherche pour l'Exploitation de la Mer, Brest, basaltfissure eruptions, which are capableof producingindi- France. viduallava flows with volumesgreater than 100 km3 at very -3NationalCenter for AtmosphericResearch, Boulder, Colorado. high eruption rates. It is speculatedthat suchlarge eruptions 4Departmentof GeophysicalSciences, University of Chicago,Chi- would inject massiveamounts of debris into the stratosphere cago, Illinois. where it might lead to reduced sunlight and depletion of Copyright 1995 by the American GeophysicalUnion. ozone, and that immensequantities of acid rain would reduce Paper number 95JD01368. the alkalinity and p H of the upper ocean. 0148-0227/95/95JD-01368505.00 The massextinctions were evidentlyselective. The weight of 13,755 13,756 EMANUEL ET AL.: HYPERCANES AND GLOBAL EXTINCTIONS evidencesuggests that the most dramaticextinctions occurred amongthe planktonic foraminifers [D'Hondt and Keller, 1991] and calcareousnannoplankton [Thierstein, 1981], and a major molluskgroup disappeared entirely [Ward, 1988]. But diatoms, dinoflagellates,and other dwellersof the upper oceansuffered only modest reductions.The planktonicflora and fauna ap- peared to undergo a rather suddendecline. There were also large reductionsin land animals,particularly among the larger species(e.g., dinosaurs),but considerablecontroversy sur- roundsthe questionof the timingof suchreductions, and many smaller animals survived the K/T transition. Many, although not all, of the proposedmechanisms by whichbolide impactsand/or large-scalevolcanism lead to mass extinctionsrely on massiveinjections of material into the stratosphere,where it may remain for manyyears. By contrast, injectionsof large quantitiesof aerosolsinto the troposphere shouldhave only transientand local effects,since excess ma- terial is washedout of the troposphereon a timescaleof weeks. Figure 1. The Carnot cycleof a mature tropical cyclone.Air The original Alvarez et al. [1980] thesis contendsthat large spiralsinward from the environment a to near the center c, amountsof material from the collidingbolide and from the while acquiringentropy from the seasurface. Air then ascends crustwould find its way to the stratosphere,perhaps by being adiabaticallyand flowsout to large radius,where it eventually mechanicallyinjected through the "hole" punchedin the at- losesheat by radiation to spacealong legs 3 and 4. mosphere by the incoming bolide. To our knowledge, this conjecturehas never been subjectedto rigoroustests. Another conjectureholds that significantamounts of mass hydrostaticmodel, lending credenceto the idea that hyper- entered the stratospherethrough hot plumes generated di- canesare physicallypossible. We then discussplausible sce- rectly through the heat of impact or the combustionof surface nariosin which hypercanesmight have occurredas a result of material.The volcanichypothesis also relies heavily on thermal bolide impact or underseavolcanism. We concludeby specu- plumesto loft material into the stratosphere,where it can have latingabout the effectsof hypercaneson the stratosphereand long-term effects.But the altitude to which thermal plumes how theseeffects may have helped causemass extinctions. from areally limited surfacesources ascend in the stratosphere - 1 increasesonly as the • powerof thesurface heat flux [Morton 2. Hypercanes et al., 1956].It is thushighly unlikely that globalwildfires could affect the stratospherein a significantway, even if there were 2.1. Theory many of them, sincethe surfaceheat flux per unit area is vastly The energycycle of a mature hurricane can be characterized insutficientto achievestratospheric altitudes. Moreover, even as that of a Carnot heat engine [Emanuel,1986], as illustrated the very largestof historicaleruptions (e.g., Krakatoa in 1883) in Figure1. In its normalstate, the tropicalatmosphere is quite managedto achieveonly limited penetration into the strato- far from being in thermodynamicequilibrium with the under- sphere,such that the material settled out in a matter of a few lying ocean. This disequilibriumis necessaryto supportthe years (see, for example,Budyko et al., [1988]). Moreover, the large rates of heat lossby evaporationneeded to balancethe settlingtime of aerosolsin the stratospheremay have an upper incomingsolar radiation. (Little heat is lost directlyby infrared bound, owing to the fact that the very smallestparticles have radiation,owing to the opacityof the moistatmosphere above, diameterscomparable to or smallerthan the mean free path at leaving evaporationas the principal means of disposingof those altitudes[Reitmeijer, 1993]. It is not obviousthat either heat.) The disequilibriumis reflectedin the subsaturationof large-scalevolcanism