Actualistic catastrophism and global change

Actualistic catastrophism and global change

Palaeogeography, Palaeoclimatology, Palaeoecology (Global and Planetary Change Section), 89 (1990) 309-313 309 Elsevier Science Publishers B.V., Ams...

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Palaeogeography, Palaeoclimatology, Palaeoecology (Global and Planetary Change Section), 89 (1990) 309-313

309

Elsevier Science Publishers B.V., Amsterdam

Actualistic catastrophism and global change K e n n e t h J. Hsii Geological Institute, ETH, Zurich, Switzerland (Received by publisher July 30, 1990)

Frequency of occurence ol natural processes is inversely proportional to their magnitude. One needs to go back to the biRion-year geological record to study catastrophic global changes

The interactive physical, chemical and biological processes on Earth can be described as fluxes of m a t t e r and energy across the boundaries of atmosphere, hydrosphere, lithosphere, and biosphere. An objective of the Environmental Science Program at E T H is aimed at a better understanding of those interactive processes that regulate the total earth system. This is also the objective of earth scientists who are oriented to understand those processes, recent or ancient. The priority in international programs on global change and on hazard reduction has fallen on those areas t h a t deal with key interactions and significant change on time scales of decades to centuries. It has been said t h a t earth scientists are dealing with ancient rocks, and that they have little to contribute to studies of short-term global changes. Geologists have themselves to blame for the common misconception that geology is irrelevant to problems of short-term global change because of a misunderstanding of the meaning of uniformitarianism. The original principle elucidated by Hutton at the end of the 18th century states that natural laws are constant (uniform) in space and time, namely: Processes now operating to mold the earth's surface should be invoked to explain the events of the past (uniformity of process through time). Charles Lyell made a great leap in 1833 when he proposed t h a t past processes were not only the same kind of pro0921-8181/90/$03.50

cess, but also operating with the same efficacy, i.e., energy-release rate. Uniformitarianism became not only an axioms of uniform physical laws, but also an assumption of uniform rate. Lyell's antagonists were the catastrophists, led by George Cuvier. Reading the geological record, Cuvier concluded in 1817 t h a t revolutionary changes of the past were so stupendous " t h a t the thread of Nature's operations was broken by them, t h a t her progress was altered, and t h a t none of the agents which she employs today could have sufficed for the accomplishment of ancient works". Lyell set up, however, a straw man to defeat his opponent, when he implied t h a t Cuvier's alternative to his substantive uniformitarianism was to invoke the supernatural. In fact, Cuvier was not a creationist and never did invoke the supernatural as a cause; the choice of his word "suffice" implies that the past agents differ from those Nature employs today in efficiency, but not necessarily in kind. He postulated, for example, mass extinction of terrestrial animals by catastrophic flooding of continent, or marine transgression, which is a natural process. Lyell's insistance t h a t the processes of the past have operated at the same energy state as those of the present was a supposition. The assumption of uniform rate could not be falsified because the variable " t i m e " could not be numerically evaluated before the discovery of radioactivity. But the working hypothesis of the last century has been turned into a dogma of today. Substantive uniformitarianism has been adopted as an article of faith, and catastrophists have been labelled fellow travellers of creation-

© 1990 - Elsevier Science Publishers B.V.

310 ists. Even after we had independent means to determine the duration of time and thus the energy-release rate of natural processes, few felt compelled to challenge Charles Lyell, " t h e Father of Geology". In my presidential address to the International Association of Sedimentologists, I pointed out the fallacy of the Lyellian dogma and coined the term a c t u a l i s t i c c a t a s t r o p h i s r n (Hsil, 1983). Statistics have shown t ha t frequency of occurrence of natural processes is inversely related to their magnitude. Dally occurring processes operating at rare intensity are natural catastrophes. Small stones fall, but when a mountain falls, we have a landslide. Small streams of water trickle, but an occasional mudflow ruins a community. Meteors decorate a summer sky, but when a trillion-ton bolide hit the earth, three quarters of the living species became extinct. The statistics relating the magnitude and frequency of earthquakes, river discharges, volcanic eruptions, and meteorite impacts have been given in numerous publications (e.g., Gutenberg and Richter, 1954; Shoemaker, 1966; Scheidegger, 1975; Hsil, 1983, 1989), although we did not know that the empirical relation is a manifestation of the [ r a c t a l geometry of Nature. In a recent breakthrough in mathematics, Mandelbrot (1977) invented the fractal concept as an alternative to determinism and chaos. Originally defined to relate the length of coastline to the length of measuring unit, a fractal relation is N = c/r D

(1)

where N is a number, r is a linear dimension, c is a proportionality constant, and D is the fractal dimension of a phenomenon. Mandelbrot pointed out that the length of natural boundaries is indefinite, it varies inversely with the length of the measuring unit r. The length of the common frontier between Spain and Portugal is in 987 km according to a Spanish encyclopedia, but 1214 km according to a Portugese encyclopedia. The discrepancy results from the choice of measuring unit; Portugal, being a smaller country, chose a smaller r and measures its border more accurately than its big neighbour.

AM (1981) showed t hat the frequency-magnitude relation is entirely equivalent to Eq. 1 if N is the number of earthquakes with magnitude greater than M and r is the size of the rupture zone associated with a magnitude M. This relation has since been confirmed by statistical analyses of all registered earthquakes since the beginning of the century (see Hsti, 1989; Turcotte, 1989). T he smaller the magnitude, the more frequent is the earthquake occurrence. This is, of course, common sense: A laboratory with more senstive instruments could detect more earthquakes. Fractal relations imply t hat the interval elapsed between two events of the same magnitude is proportional to the magnitude of the events. When we speak of the strongest hurricane in a decade, the heaviest storm of the century, the largest earthquake in our memory, or the most violent volcanic eruption on record, we are thinking in terms of fractal mathematics. Of course, Lyell knew t hat the uniformity is not absolute. There are drizzling rains, and there are storms. There are fallen stones, there are landslides. There are tides and there are tidal waves. There are imperceptible earth trembles, and there are devastating earthquakes. What Lyell advocated, ! believe, was t hat we, H o m o s a p i e n s , live long enough, or have lived long enough collectively, to witness all processes at their most energetic manifestation. Every natural phenomenon has a maximum magnitude. Earthquakes energy release is, for example, limited by the strength of earth's crust and the dimensions of the slip surface. The strongest earthquake is probably not much greater than magnitude 10 and almost certainly not more than 12 in the Richter's scale (Fig. 1). Likewise there is a physical limit, or a maximum magnitude for each process, the frequency occurrence of an event of greatest possible energy release, according to Eq. 1 is Fma x = c / r l~

(2)

The time interval elapsed between the events of maximal intensity should thus be Tma x = 1 / I ~ m ~ = 1 / c r '~

(3)

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their most energetic manifestation is longer t h a n t h e Holocene, b u t shorter t h a n the history of t h e earth, or ~-s 12

lO

maximum

11

possible

.... po.,b,e 1

10 4

Earthquakes~~o

probable m a x i m u m

/

probable m ~ .

-4

.~

~0~

F

-3

--:

S

~3

I

I once

in log time

Fig. 1. Fractal relation in nature. An earthquake event with maximum magnitude is likely to take place within historical time, if not in a person's lifetime; we do not have to go back to the geological record to learn of the damages caused by earthquakes. Other events, such as meteorite-impact event with the maximum-energy release took place only a few times in the earth history, and such rare events did result in major environmental degradations. Anthropogenic activities are causing the fluxes of materials and energy at rates comparable to those during the rare catastrophes of the past. We have to examine the geological record to predict the drastic consequences of such environmental crisis; we cannot wait until we reach the point of non return.

Lyell assumes t h a t this T n ~ is the duration of a life time (102 years), of h u m a n history (103 years) or of the Holocene, (104 years). This intuitive assu~nption has no theoretical basis and is not supported b y facts; it has, in fact, been proven wrong. T h e e a r t h history has a span of 109 years, and some agents of the past, such as volcanic eruptions or meteorite impacts, were more, or m u c h more, powerful t h a n those witnessed by us, recorded b y h u m a n history, and manifested b y the Holocene geological record (Fig. 1). On those rare occasions when such an event t o o k place, we h a d a n a t u r a l catastrophe. It is not the kind of process, b u t the intensity of the process which distinguishes a catastrophe: meteorite impact, for example, takes place daily, b u t a trillion-ton long-period comet only collided with e a r t h once in a billion years (Weissman, 1982). T h e f u n d a m e n t a l postulate of actualistic catastrophism is that, for some natural phenomena, the time required for

years < Tm~ < 4 × 104years

However, the anthropogenic disturbance of the fluxes of energy and material has been of such magnitudes, t h a t catastrophes which normally occur only once in a million or a billion years (Tm~ = 106-109 years) could be happening in not too distant a future. This is one reason w h y geology is relevant to predict global changes in near future. A good example is the nuclear winter scenario, which has been designed to s t u d y the consequence of a sudden flux of energy into the terrestrial system after a nuclear war. Was there a n y occurrence of comparable flux in the earth history? Yes, we could go back to the end of the Mesozoic Era, to find such a case history, with T ~ = l0 s years; the interactive physical, chemical, and biological processes at the end of the Cretaceous produced significant environmental changes in 101-102 years. T h e event was probably a large-bolide i m p a c t (Alvarez et al., 1980; Hsii, 1980), and the impact winter scenario has been p o r t r a y e d b y c o m p u t e r modelling. T h e ICSU's project on nuclear winter was inspired b y the investigations of the impact winter b y e a r t h scientists. T h e programs modelling atmospheric, climatic, and geochemical changes in the wake of a nuclear war have been modified from those modelling the impact winter. We hope we would never have to verify experimentally the nuclear winter studies, b u t the impact winter scenario has been tested b y geology. Investigations of the K / T b o u n d a r y event indicated: (1) E n v i r o n m e n t a l catastrophes of darkness and freeze led to mass mortality, (2) Excess NO x produced b y the combination of atmospheric oxygen and nitrogen in the m u s h r o o m cloud rising above the impact site led to stratospheric ozone depletion, which resulted in biocidal damage and genetic mutations, leading to accelerated extinctions and speciations, (3) Acidization of an ocean b y acid rains suppressed fertility of organisms, thus causing drastic reduction of ocean biomass, and the consequent

312 reduction of photosynthesis led to increase of atmospheric carbon oxide and further acidization of the ocean. Thus a large bolide-impact may only have triggered a short-term catastrophic flux, but the long-term environmental degradations, induced in part by feedback mechanisms, may have persisted for many thousands of years after the initial catastrophe. The nuclear winter scenario is irrelevant if mankind is wise enough to avoid such a catastrophe. Yet the catastrophe theory of mathematics states that secular and gradual fluxes could also lead to catastrophes not unlike what might be produced in a few hours by a holocaust. Interactions of biosphere and geosphere in earth history, as indicated by the lessons learned from the geological record, often involve feedback mechanisms. The additive effects could be nonlinear. Often, after the cumulative result reaches a threshold value, there would be the straw that breaks the camel's back. One example is the dramatic climatic changes just before the beginning of the Holocene. Glaciers retreated rapidly in Europe and North America some 15,000 years ago. Steppe vegetation came first and then forests grew in deglaciated regions. Suddenly, at 11,000 B.P., there was a reversal. An arctic flower Dryas octopetala reappeared in Denmark, indicating a return of fully glacial condition to northern Europe. This reversal, called the Younger Dryas, has now been documented as a global change (Berger and Labeyrie, 1985). Just as suddenly, the cold spell ceased at 10,000 B.P. Both the cooling and the warming were extremely abrupt; drastic changes probably took place in centuries if not in decades. A thorough understanding of an event like the Younger Dryas enlightens us on the feedback mechanisms which could amplify secular signals to such an extent as to cause a catastrophe. This is one more reason why geology is relevant to predictions of short-term global changes. Still another reason to delve into the geological record is to investigate the consequence of the irreversible. Extinction of a few species may not be very important, but those which become extinct can never be brought back to life. We do

not know if such extinctions would have grave consequences, and we could not wait and see what happens until we reach the point of no return. As a matter of fact, the biodiversity reduction caused by man is not only irreversible, but also catastrophic. The rate of historical extinction is comparable to that of the "Great Dying" at the end of the Mesozoic (Flessa et al., 1985). By the time the consequences, such as mass extinction caused by destruction of tropical forests, are visible, it is already too late; irrevocable damages will have been done. The lesson must, therefore, be learned from earth history. Recognizing the relevance of geology to IGBP, the Executive Committee of the International Union of Geological Sciences (IUGS), in their February, 1988 meeting at Canton, China, constituted a Task Group on Global Changes, to explore the ways and means to contribute to the IGBP/Global Change of ICSU. The Task Group, chaired by the author, met at Samedan, Switzerland, in April, 1988 and decided that the efforts by the earth science community should be focused on the four following themes: (1) To document the marine and atmospheric records, particularly those of the last 105 years. (12) To document the terrestrial records, particularly those of the last 105 years. (3) To document the anthropogenically induced global changes, including depletion of nonrenewable resources. (4) To explore the consequences of reduction of bio-diversity, with reference to records of past catastrophes. A Workshop on Past Global Changes was at Interlaken, Switzerland, April, 1989, to discuss the implementation of the IUGS program, and the reports of the Interlaken Workshop will be published in a future issue of this Journal. A decision was made at Interlaken to recommend the initiation of a cooperative program between the IUGS and UNESCO to study Earth Processes and Global Changes. The program, if initiated, will promote international and interdisciplinary cooperations to study regional aspects of past global changes. Our recommendation has been accepted by the IUGS

313

Council, and a proposal is being submitted to the UNESCO General Conference by the Swiss UNESCO delegation to initiate the EPGC program.

References Aki, K., 1951. A probabilistic synthesis of precursory phenemona. In: D.W. Simpson and P.G. Richards (Editom), Earthquake Prediction. Am. Geophys. Unions, Washington, DC, pp. 556-574. Alvarez, L., Alvarez, W., Asaro, F. and Michel, H.Y., 1980. Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science, 208: 1095-1108. Anonymous, 1988. The International Geosphere-Biosphere Programme: A Study of Global Change. ICSU/IGBP Rep., 4, 200 pp. Berger, W.H. and Labeyrie, L.D. (Editors), 1987. Abrupt Climatic Change: Evidence and Implications. Reidel, Dordrecht, 425 pp. Cuvier, G., 1817. Discours sur les Rdvolutious du Globe. I~dot, Paris. Flessa, K.W., 1986. Causes and consequences of extinction. In: D.M. Raup and D. Jablonski (Editors), Patterns and Processes in the History of Life. Springer, Heidelberg, pp. 235-257

Gould, S.J., 1965. Is uniformitarianisrn necessary? Am. J. Sci.,263: 223-228. Gutenberg, B. and Richter, C.F., 1954. Seismicity of the Earth and Associated Phenomena. Princeton Univ. Press, Princeton, NJ, 273 pp. Hst~, K.J., 1980. Terrestrial catastrophe caused by cometary impact at the end of Cretaceous. Nature, 285: 210-203. I-IsiS, K.J., 1983. Actualistic catastrophism: Address of retiring president of the International Aseociation of Sedimentologists. Sedimentology, 30: 3-9. I-Is~, K.J., 1989. Catastrophic extinctions and the inevitability of the improbable. J. Geol. Soc. London, 146: 749-734. Lyell, C., 1833. Principles of Geology. Murray, London, 3, 398 pp. Mandelbrot, B.B., 1977. The Fractal Geometry of Nature. Freeman, New York, NY, 468 pp. Scheidgger, A.E., 1975. Physical Aspects of Natural Catastrophes. Elsevier, Amsterdam, 289 pp. Schoemaker, E.M., 1966. Progress in the analysis of the fine structure and geology of the lunar sudace from the Ranger VIII and IX photographs. Jet Propulsion Lab., Pasadena, CA, Tech. Rep., 32-800: 249-337. Turcotte, D.L., 1989. Fractals in geology and geophysics. In: Wegener Conf. Geosphere Fluctuations, Abstr. Riesen, Hamburg. Weissman, P.R., 1983. Terrestrial impact rates for long and short-period comets. Geol. Soc. Am. Spec. Publ., 190: 15-24.