Impact lethality and risks
in today's world:
Lessons for interpreting Earth history
Clark
R. Chapman
Southwest Research Institute
Suite 426,
1050 Walnut Street
Boulder
CO 80302 USA
Author's email address:
cchapman@boulder.swri.edu
Submitted 3 December 2000, revised
23 January 2001
Proceedings of
the Conference on
Catastrophic Events and
Mass Extinctions:
Impacts
and Beyond
Abstract. There is a
modern-day hazard, threatening the existence of civilization, from impacts of
comets and asteroids larger than about 1.5 km diameter. The average annual world fatality rate is
similar to that due to significant accidents (for instance, airliner crashes)
and natural disasters (e.g. floods), although impact events are much
rarer and the deaths per impact event are much greater. (Smaller, more
frequent impacts can cause regional catastrophes from tsunamis of unprecedented
scale at intervals similar to the duration of recorded human history.) As the telescopic Spaceguard Survey census
of Near Earth Asteroids advances, numerical simulations of the dynamical and
collisional evolution of asteroids and comets has also become robust, defining
unambiguously past rates of Earth impact of larger, more dangerous cosmic
bodies. What are very tiny risks for
impacts during a human lifetime become certainties on geological
timescales. Widely reported errors in
predictions of possible impacts during the next century have no bearing on the
certainty that enormous impacts have happened in the past. The magnitudes and qualitative features of
environmental consequences of impacts of objects of various sizes are
increasingly well understood. Prime
attributes of impacts, not duplicated by any other natural processes, are: (a) extreme suddenness, providing little
opportunity for escape and no chance for adaptation, (b) globally pervasive,
and (c) unlimited potential (for K/T-boundary-scale impacts and larger) for overwhelming
destruction of the life-sustaining characteristics of the fragile ecosphere,
notwithstanding the rather puny evidence for impacts in the geological
record. A civilization-ending impact
would be an environmental and human catastrophe of wholly unprecedented
proportions. K/T-scale impacts, of
which there must have been at least several during the Phanerozoic (past 0.5
Gyr), are 1,000 times still more destructive.
No other plausible, known natural (or man-made) processes can approach
such catastrophic potential. The
largest impacts must have caused mass extinctions in the fossil record; other
natural processes could not have done so.
Perspectives concerning both (a) the potential modern-day destructive
potential of impacts and (b) conceivable, almost miraculous refugia in our own
world provide a new gestalt for thinking about past cataclysms.
I.
INTRODUCTION
The
idea that cosmic impacts on the Earth have played a significant, or even
dominant, role in mass extinctions (and subsequent explosive radiation of new
species) has evolved from widespread skepticism to substantial acceptance
during the last two decades since publication of the Alvarez et al.
(1980a) hypothesis concerning the K/T boundary. However, there remain pockets of non-acceptance as well as a wide
spectrum of opinions about the degree to which impacts have influenced
evolution. Even among those who fully
accept a role for impact in the K/T boundary extinctions, views range all the
way from belief (a) that the only substantiated case of an impact playing a
role in mass extinction was the Chicxulub impact 65 Myr ago, which is viewed as
just hastening the demise of already stressed populations, to (b) the
hypothesis of Raup (1991) that all mass extinctions, large and small,
could have been caused by sudden environmental changes due to impacts.
As
the Alvarez hypothesis was researched, awareness grew among scientists and the
public alike of the modern-day risk to civilization from cosmic impacts. Once the purview of science fiction, it has
become widely accepted (for example, the report of an independent Task Force
set up by the British Government [Atkinson, 2000]) that the threat of a
calamitous impact ranks among other hazards meriting national and international
attention and consideration of preventative measures. While the chances of impact with an asteroid larger than 1.5 km
diameter (deemed sufficient to threaten modern civilization; Chapman and
Morrison 1994) are very small, about 1 chance in several hundred thousand per
year, the potential consequences are so enormous (perhaps death of a quarter of
the world's population) that the annualized fatality rate is similar to
fatality rates associated with other natural hazards, like floods and
earthquakes (Fig. 1).
Research
on the impact hazard, especially during the 1990s, has yielded a voluminous
literature on the numbers and physical traits of the impactors, on the physical
and environmental effects of impacts, and even on the potential response of
human society to an imminent impact or to the aftermath of one. With the perspective from modern research on
the impact hazard, the issues faced by historical geologists trying to
understand the role of impacts during Earth history can be viewed from a new
gestalt.
Yet
the modern-day impact hazard, itself, is poorly understood by the public, by
policy makers, and even by most scientists.
Both the extremely low probabilities and the extremely great
consequences of impact tax our intuition and common sense, because they are so
far beyond the realm of our personal, or even historical, experience. Therefore, before applying insights from
studies of the modern impact hazard to the historical record (the subject of
Sect. V), I will first introduce the impactor population (Sect. II) and what is
known about the consequences of impacts (Sect. III). I will then discuss (Sect. IV) the issues of risk perception and
uncertainties in impact prediction in order to demonstrate that they have no
bearing on the certainty that impacts with unparalleled ecological consequences
happened in the past. Finally, I will
turn to the implications for the role of impacts in Earth history from lessons
learned in the study of the impact hazard.
Let
me indicate where I am going by making some bold assertions, to be justified
later, about past mass extinctions:
*
It is virtually certain that several other impacts have occurred during
the Phanerozoic (last 0.5 Gyr) having at least the energy and potential
ecological consequences of the Chicxulub K/T boundary impact, and that many
other impacts have occurred with potential global consequences nearly as
great. This is not a hypothesis: it is
an inescapable fact derived from robust knowledge of asteroids and comets.
*
There is no other plausible, known kind of natural calamity that can
possibly approach asteroid/comet impact in terms of the suddenness of
the onset of devastating global consequences.
(I exclude human devastations like nuclear war, as well as other
conceivable but unlikely disasters like a nearby supernova.) I assert that this suddenness -- ranging
from minutes to months -- greatly magnifies the devastation compared with any
other equally profound geologic/oceanic/meteorological catastrophe.
*
The largest impacts during the Phanerozoic must have caused mass
extinctions and, conversely, no other known, plausible mechanism can approach
the magnitude of consequences of such impacts.
Therefore, the largest mass extinctions must have been caused by
impact. (Only if required evidence of
such impacts is missing from the geological record must one then turn to the
unlikely alternative explanations, like a nearby supernova or explosion of an
unexpectedly stupendous super-volcano.)
Bear
this fact in mind: What is commonly
accepted among impact hazard researchers as the threshold size of asteroid that
could terminate civilization as we know it (1.5 km diameter) is more energetic
than the explosive force of the world's combined arsenals of nuclear weapons by
a factor of ~20. Yet the
magnitude of the K/T boundary impactor (10 to 15 km diameter), and each of the
several other equivalent or larger impacts that must have occurred since the
Phanerozoic, is equivalent to a thousand civilization-ending impacts all
occurring simultaneously! The miracle
is that anything survived at all.
Perhaps the best way to visualize mass extinctions is to try to imagine
the refugia that might exist for us, and for various species of animals and
plants, in our modern world after it has been utterly devastated by an unimaginably
colossal holocaust.
II.
THE IMPACTING POPULATION
Geologists
have traditionally invoked the uniformitarian concept that continuous
geological processes observable today can account for what is observed in the
geological record of the past. In recent
decades, a reasonable balance has been achieved between this two-century-old
tradition and the important role of episodic, even catastrophic, geological
processes. However, when geoscientists
turn their attentions away from their specialties, old habits can
re-emerge. Frequently during this
conference (Catastrophic Events and Mass Extinctions: Impacts and Beyond), speakers from
geological/paleontological backgrounds made statements like, "for this
particular mass extinction, there is no reason to invoke an ad hoc
impact from the heavens." They
apparently miss the point, developed robustly over the last 70 years, that
cosmic impacts -- despite their rare, catastrophic manifestations on Earth that
concern us here -- are part of an ongoing, continuous process that is observable
today and has necessarily operated during the past history of the
Earth.
Impact
hazard researchers currently direct most attention to telescopic searches for
Near Earth Asteroids (NEAs; these are defined as [a] those so-called
Earth-crossing asteroids [including dead comets] whose orbits cross the Earth's
orbit, in the sense that their closest and farthest distances from the Sun
include 1 Astronomical Unit, the mean distance of the Earth from the Sun, plus
[b] the so-called Amors, which get as close to the Sun as 1.3 times the Earth's
distance from the Sun). Depending on
details of counting, currently more than 1,250 are known even though the first
asteroid in an orbit that actually crosses the Earth's orbit was not found
until 1932 and less than 20 NEAs were known as recently as the early 1970s. Currently, numerous telescopes equipped with
modern detectors systematically scan the skies. Astronomers assemble data on detections and calculate orbits for
these bodies. Nearly one-half of all
Earth-approachers larger than 1 km diameter have been cataloged, as well as
large numbers of smaller bodies ranging down to the size of a small house.
These
are just the largest, and potentially most dangerous, of a vast complex of
interplanetary objects and particles in Earth-approaching orbits, ranging from
enormous asteroids like Eros (34 km long; Veverka et al. 2000) and still
larger comets down to rocks and dust particles. The basic physics of how this debris is created (by hypervelocity
collisions among the debris) and how it is lost (by collision with the Sun or
planets, and other loss mechanisms) has been understood for a long time. For instance, the "collisional
cascade" that creates and maintains the population of smaller bodies was
explained by Piotrowski (1953) and Dohnanyi (1969); modern research has made
changes that only specialists could care about. Fundamentally, the asteroids and smaller debris orbit around the
solar system (inside of Jupiter's orbit) in a way that, despite some
regularities, generates essentially random encounters and collisions among
themselves at speeds of many kilometers per second. From the well-known mechanical properties of the common materials
of which the debris is composed (rock, ice, carbonaceous "mud",
metal), the objects inevitably are broken by such collisions into smaller
fragments and dust.
The
resulting size distribution (numbers as a function of size) from multi-hundred
kilometer asteroids and comets down to dust grains is well known and
essentially invariant (cf. Durda et al., 1998). Dust grains are abundant and the Earth's
large cross-section continuously sweeps them up, as anyone can observe on a
clear dark night: a meteor flashes through the upper atmosphere, as viewed from
one location on the ground, every few minutes.
Dust detectors on spacecraft confirm the widespread distribution of such
grains throughout interplanetary space.
Impacts of meter-sized bodies are much rarer, but are routinely observed
by downward-looking satellites searching (primarily) for signs of military
activity (Nemtchinov et al., 1997), and occasionally by ordinary human
beings, as stunningly brilliant fireballs.
For example, a 5-meter diameter impactor shone ten times brighter than
the Sun, as observed from the Yukon, when it struck in January 2000 (Brown et
al., 2000), yielding some precious meteorite fragments.
Impactors
several tens of meters and larger are too uncommon to strike regularly during a
human lifetime, although the 15-Mt-equivalent Tunguska event in 1908 is well
documented (probably caused by an asteroid ~50 meters in diameter). However, objects of these sizes passing
"near" (within a few million km of) the Earth and the Moon are
regularly discovered by the telescopic scanning programs, especially by the
Spacewatch Program on Kitt Peak, Arizona, which is optimized for discovering
smaller bodies. The sampling becomes a
complete census for Earth-approaching bodies larger than about 7 km diameter,
not counting rare comets that can approach from the darkness of the outer solar
system.
It
is purely a matter of random chance, equivalent to rolling dice, about just
when an impact will happen -- whether a faint meteor streaking across the sky
or a dinosaur-killing impact -- but the average frequencies of impacts of
objects of different sizes is well known and has not significantly changed
since Shoemaker's (1983) review. Subtle
regularities cause only slight departures from purely random chance. Specialists do debate the exact numbers of
bodies of specific sizes. But
differences are rarely greater than a factor of a few, and are often less than
a factor of two. For example, it was
long estimated that the number of Earth-approaching asteroids larger than 1 km
diameter might be ~1500. During the
last few years, there has been a well-publicized debate (for example,
Rabinowitz, 2000; Bottke et al. 2000; M.I.T. 2000) about whether that
number is really as low as just 700 or at least as high as 1100. The answer has potentially important
political consequences, such as whether NASA can reach its committed goal to
find 90% of such objects by 2008 (Pilcher, 1998) without building more, larger
search telescopes. Such arguments are
inconsequential, however, in the context of impact catastrophes past or future.
Not
only are the numbers and impact frequencies of interplanetary objects of all
sizes well known today, but today's samplings and census are known to have been
generally unchanged during the past 3.5 Gyr.
As described above, the physics of these bodies and their collisional
evolution is well understood, and must have been as applicable in the past as
today. Furthermore, understanding the
sources and sinks of these bodies and their dynamics (for instance, how they
move through the solar system on time scales ranging up to billions of years)
has developed remarkably in the past decade due to the advent of inexpensive,
very fast computers. Although Kepler's
Laws were never in doubt, the dynamical systematics of the entire complex of
asteroids and comets has become well understood only during the last five years. Furthermore, examination of the cratering
record on the Earth and terrestrial planets -- and especially on the Moon --
has demonstrated the continuity through the last 3.5 Gyr of impact processes. Once again, specialists are interested in
minor variations in impact rates and in the shape of the size
distribution. But since the Late Heavy
Bombardment ended about 3.8 Gyr ago, the impact rates at all sizes have never
varied by more than factors of a few, except (probably) for brief, transient
"showers" of modest magnitude that have made a negligible
contribution to the cumulative record of craters.
Table
1 translates the known, largely invariant size distribution of interplanetary
projectiles, and their rates of colliding with Earth, into some relevant
"chances" of impact by bodies of three interesting sizes: a small asteroid 200 m across capable of
creating a devastating tsunami unprecedented in historical times; a
civilization-ending impactor 2 km in diameter; and a K/T boundary extinctor (10
- 15 km in diameter). I will return to
some interesting attributes of this table, but first we must progress beyond
the impact frequencies to the consequences of such impacts. For instance, how do we know what will
happen if the Earth is struck by a 200 m, 2 km, or >10 km body?
III.
CONSEQUENCES OF IMPACTS
Environmental effects
Studies
of the modern-day impact hazard have greatly augmented our understanding of the
consequences of impacts, probably more so than have analogous studies of the
physical, chemical, environmental, and biological effects of giant impacts in a
K/T context. Studies of the modern-day
hazard (cf. Adushkin and Nemchinov, 1994) have usually focussed on the
dangerous objects that are most likely to strike, those ranging from producers
of giant tsunamis (~200 m diameter, ~1000 Mt explosive yield) up to the
civilization enders (~2 km diameter, 105 Mt), which involve modest
extrapolations from weapons tests and the Tunguska event. A reality check, the impact of Comet
Shoemaker-Levy 9 into Jupiter in 1994 (roughly equivalent to the ~2 km
terrestrial case because of the much higher impact velocity at Jupiter), was
extensively researched and applied to the NEA hazard (cf. Boslough and
Crawford, 1997). Such research has also
provided a guide for extrapolating to the far more energetic case of the K/T
boundary impact. The latest, most
comprehensive review of the environmental consequences of impacts, ranging from
20 m to 20 km diameter (1 to 109 Mt), is that of Toon et al.
(1997).
The
first salient fact is that the impact of a cosmic body with Earth, whether at
15 or 25 km/sec (or sometimes greater speeds for comets), essentially causes an
explosion -- an instantaneous conversion of the kinetic energy of the impactor
into fragmentation/comminution/cratering of the substrate, heating/melting/vaporization
of the projectile and target materials, kinetic energy of cratering ejecta,
seismic shock waves penetrating the planet, and other types of destructive
energy. Precisely how the kinetic energy
is partitioned into the various forms of energy is the subject of continuing
research, but we only have to look at the now many-decades-old sites of nuclear
weapons tests to understand the general idea.
Modern computer codes reliably reproduce the weapons tests and, based on
sound physics, can be extrapolated robustly to the energy scales of
civilization-ending impacts and perhaps even to mass-extinctors.
The
second salient fact is that a very significant fraction of the energy from an
impact is dissipated in the ecosphere, that thin shell of air, water, and
surface rocks and soils whose constancy sustains and nurtures life. The Earth as a planetary body has
been unfazed by any impacts subsequent to the colossal interplanetary collision
that is believed to have formed our Moon.
And even the geological record is only marginally perturbed by even the
largest post-Late-Heavy-Bombardment impacts: witness the general obscurity of
the famous "clay layer" at the K/T boundary; the boundary is readily
recognized by the permanent change in the diversity of species, but it
is not prominent as a geological feature itself (the cm-scale layer is dwarfed
by ordinary sedimentation and erosion and by faulting and other pervasive
effects of tectonism and volcanism).
But our thin ecosphere is exceptionally subject to damage and
instantaneous modification by events of these magnitudes, even if only a tiny
fraction of the kinetic energy of the impactor is partitioned into the
atmosphere during the bolide phase (passage of the impactor through the
atmosphere), during the explosion, and during the subsequent ejecta plume
phase.
A
final fact about consequences of impacts is that those that exceed the relevant
threshold sizes (dependent on the particular consequence) necessarily
distribute their consequences globally: while the greatest damage is
obviously at ground zero, the stratosphere is badly polluted with dust on a
global scale from impacts exceeding 105 Mt (1 km diameter), glowing
ejecta are distributed globally from impacts exceeding 108 Mt (15 km
diameter), and even seismic shock waves may reach moderately damaging
proportions on a global scale for impacts of 108 Mt scale (K/T
level). Even much smaller impacts (for
example, by a 200 m impactor), if into the ocean, can cause devastation
thousands of kilometers away due to the efficient transmission of energy to
great distances by tsunamis (Ward and Asphaug, 2000). In normal times, the distributive character of air and water is
what lubricates our world, maintains chemical balance, and sustains life. But in times of catastrophe, which overwhelm
the modest mass of the atmosphere and ocean and their thermal/chemical
balances, these media distribute poisons, sun-darkening dust and aerosols, and
meteorological/climatological consequences around the entire globe. Rebound from past catastrophes that have
afflicted civilization (for instance World War II) have often depended on some
portions of the planet remaining unaffected by the localized or regional
devastation, thus serving as nuclei of recovery. In the case of a sufficiently large impact, there are essentially
no unaffected refugia where life continues normally.
Consider
the Comet Shoemaker-Levy 9 impact into Jupiter in 1994: with the kinetic
energies roughly that of a civilization-ending impact on Earth, the largest
comet fragments created immense, black patches in Jupiter's stratosphere
(certainly appreciably dimming the sunlight beneath); several of them exceeded
the size of the entire planet Earth and persisted for months (Chapman, 1995)!
Precisely
what dominant environmental consequences arise from impacts is less certain
than the generalizations just listed.
Certainly the vagaries of weather forecasting and of other contemporary
forecasts of environmental scenarios (like global warming), engender an
understandable skepticism among the public about the predictive sciences (cf.
Sarewitz et al., 2000). But the
magnitude of a major impact is so enormous compared with the environmental
perturbations resulting from 20th/21st century civilization, that the kinds of
uncertainties that plague the other predictions are overwhelmed. Furthermore, since there are so many
separate phenomena, the synergies among them, which are difficult to model,
probably lead to conditions appreciably worse than the simple addition of their
separate effects. If one or two of them
turn out to be less effective than initially calculated, there remain numerous
other damaging consequences. For
example, estimates of the production of nitric acid, once thought to be a primary
environmental effect of a K/T-scale impact, have more recently waned even as
sulfuric acid has received greater attention due to the probable anhydrite-rich
substrate near Chicxulub (Pope et al., 1994).
The
complete suite of consequences for a 2 km impactor and for a 10-15 km impactor,
primarily as gleaned from the comprehensive review of Toon et al.
(1997), is summarized in Table 2, supplemented in some cases by insights from
other, still more recent work. I have
left out (a) less significant, more localized damage (for example, blast
effects near ground zero), (b) less well understood effects (general toxicity
of the environment and effects on ocean chemistry), and (c) secondary and
long-lasting effects.
While
there are significant uncertainties in some of these results (some more so than
others), the inevitability of most of them within the range of impactor scales
we are considering is assured. Several
of them seem independently to be capable of global deterioration of the
biosphere (for 2 km impactors) ranging up to massive destruction of the
biosphere (for K/T-scale impacts). Some
of the effects are complementary; for example, the dramatic cooling effects of
impact winter would be moderated near ocean shores due to the ocean's heat
capacity (Covey et al., 1994); however, these are just the regions that
would be inundated and scoured by tsunamis.
The tabulated consequences acting in concert (along with other effects
not yet fully evaluated) and extended by the less-certain, longer-term consequences
for the chemistry and temperature of the atmosphere and the ocean, would make
life on Earth following a big impact horrific, indeed.
Civilization-ending impact
The
consequences of a civilization-ending impact can dwarf the environmental
effects of historical environmental catastrophes such as (a) the "year
without summer" due to the massive Tambora volcanic eruption in 1815 as
well as (b) "nuclear winter" scenarios envisioned to result from
all-out nuclear war (discounting the immediate and long-lasting radiation
effects of the latter). It should be
noted that an impact is far more efficient than nuclear war (or volcanic
explosions) at polluting the stratosphere, despite the fact that other kinds of
damage are far more concentrated in one locality in an impact. As researchers on the impact hazard
contemplate the consequences for modern civilization, the most dramatic effect
seems to be the prospect that all agriculture would be lost for a year. Given ongoing episodes of Third World
starvation that occur even under the optimized international food-distribution
systems in stable times, it seems likely that a sudden impact by a
kilometer-scale comet or 2-km sized asteroid would lead to mass starvation of a
sizeable fraction of the world's population.
The
end-game of such a scenario naturally involves highly uncertain speculation
about the longer-term response of the ecosphere; of corporate, national, and
international infrastructures; of the global economic system; and so on. Some commentators view civilization as
inherently fragile. Human beings have
moved away from nature and lack knowledge about survival in the absence of
manufactured goods and retail stores.
Technology has become highly specialized and is generally inaccessible
to non-specialists. There is a network
of interdependencies among nations which is fragile even absent a global
calamity. A breakdown of the social
order (like that fictionalized in the
aftermath of a comet strike in Lucifer's Hammer, Niven and Pournelle,
1977) is viewed by some as inevitable, probably leading to conflicts and wars
on local to global scales (and modern warfare has become very
dangerous). Such fragility could easily
lead from an impact catastrophe to the death of most of the world's population
and a long-term Dark Ages.
On
the other hand, some commentators believe that civilization is robust. Frequently, the human spirit rises to meet
challenges that seem overwhelming.
Cooperation rather than social disintegration seems more likely to
some. There exist technological refugia
(like bomb shelters) and other forms of mitigating the disaster, especially if
there is some warning (for example, food supplies outlasting the darkness could
be grown and stored, given a decade's warning -- and, thanks to the Spaceguard
Survey, warning of an impact years to decades in advance is increasingly
likely). Human history has demonstrated
society's ability to recover from such holocausts as the Plague and World War
II (though minimally affected peoples and nations contributed to recovery,
which would not be the case in a truly global catastrophe).
Extrapolation to K/T scale impact
With
increasing size of impactor, the magnitude of the catastrophe grows toward a
K/T boundary scale event (with a thousand times the destructive energy of the
civilization-threatening event just discussed), and it becomes certain not only
that civilization would collapse, but probably the human species itself would
be rendered extinct. Who could survive? Even a well-trained survivalist, capable of
living off desert lands in perpetuity, would be overwhelmed by months (not to
mention years) of trying to survive in a burned, denuded, bitterly cold,
perpetually dark, and poisoned environment.
And even to get to the point of trying to live off the land and the
dregs of a destroyed civilization, an individual would have to have survived
(in some deep cave somewhere) the initial calamity of a global firestorm,
global earthquakes, and other immediate traumas of the impact itself. How would landbased animals, or complex
plants, be any more successful at surviving?
Oceanic life would be buffered from the fire, but would still be subject
to changes in chemistry and, eventually, temperature, which would be pervasively
distributed throughout the waters, with adequate refugia even more difficult to
imagine.
From
the perspective of a typical individual (human, animal, plant), survivability
from a K/T-scale event is impossible to imagine. But lessons from the aftermath and recovery of local populations
following the Mt. St. Helen's eruption and evidence that some species have
evolved "accidental" protections from otherwise highly lethal
environments (including extreme temperatures and even high doses of radiation),
suggest why the K/T event didn't doom all larger-than-microbial life. To understand such survivability, one must
concentrate on exceptional environments, most readily imagined by
thinking of the world we know, including its special environmental niches and
microclimates. Presumably analogous
circumstances existed in the past.
An
individual, or small herd, happening to be next to a thermal springs is --
luckily -- in a much better position to survive a multi-year winter than most
individuals of the same species...especially so if the springs happens to be
deep within a cave where the lucky herd avoided being scorched during the
initial post-impact firestorm and is thermally buffered from the multi-year
winter. If the cave-with-springs
happened, also, to be on a far-offshore island which may have been luckily
shielded from the glowing ejecta by a thick overcast at the time of impact, a
small ecosystem of animals and plants might have temporarily survived and could
be harvested. All would be lost if that
island were subsequently submerged by the impact-generated tsunami or by storms
generated by catastrophic meteorological changes. But perhaps not -- perhaps the island is perched high above sea
level and/or is very far from ground zero.
This hypothetical concatenation of lucky circumstances has gotten our
lucky individual or herd a few steps up the ladder of potential
survival...although many more environmental challenges must still be overcome
to assure long-lasting survival and repopulation of the species. Through such fortuitous circumstances in an
exceptional refugium, one can imagine that small reproductive groups might
permit the survival of certain lucky species, even if 99.99999% of individual
species-members have died. That is
presumably what happened 65 Myr ago.
IV.
PERCEPTIONS OF RISK
The
impact hazard has received some bad press in recent years, giving the subject a
certain "Chicken Little" unreality.
In order to underscore the robustness of my central message, I want to
address the issues that affect individual (and society's) perception of the
risks associated with the impact hazard.
Among the most important are:
*
Failure to grasp the meaning of low-level probabilities or of
randomness.
*
The fact that ordinarily negligible errors can overwhelm the
"signal" of a low-probability event, requiring exceptional procedures
for handling calculations and reporting of low-probability events.
*
Failure to understand that scientific research (in this arena,
especially) is an ever-improving process and that retracted predictions of
impacts or near-misses are the usual outcomes of this research, and generally
do not imply that "mistakes" have been made.
In
the literature of the psychology of risk perception (see for example, Cole,
1998), it is a commonplace that the human brain finds it inherently difficult
to grasp the meaning of probabilities outside of the range of our practical
experience. The 1-in-649,739 chance of
being dealt a Royal Flush in poker (not to mention winning a national lottery)
is lower than the chance that the Earth will be struck by a civilization-ending
asteroid next year. Yet few
gamblers could imagine worrying about the end of everything and everyone they
know and love while they still harbor a real hope of "beating the
odds." People also fear that they
may die by several other frightening causes that are also less likely than
being killed by an impact catastrophe, including being killed by a wild animal,
by lightning, or in a tornado.
Companies, governments, and citizens apply great pressure for increased
airline safety, despite the fact that an individual American is more likely to
die as a result of an asteroid impact than by jetliner crash. On the other hand, extremely dangerous
activities (far exceeding dangers from airplanes or asteroids), like smoking,
driving automobiles, or failing to exercise, are readily tolerated and
rationalized.
Another
common confusion involves misunderstanding that a typical "waiting time"
until the next impact (a few hundred thousand years for the end-civilization
impact) justifies inaction at the moment.
(A related, common illusion familiar to geologists is the layperson's
expectation that one can ignore the possibility of a flood since "the
hundred year flood just happened two years ago.") The impact could happen just as readily next
year as in some particular year tens of thousands of years from now.
The
history of widely publicized impact scares during the past decade may be leading
to a "boy who cries 'wolf'" skepticism about the robustness of
astronomers' observations and calculations about impact probabilities. Despite attempts to improve, regularize, and
simplify the reporting of inherently difficult-to-understand results to the
public (for example, through de facto adoption of the Torino Scale [Binzel,
2000], analogous to the Richter Scale for earthquakes, to categorize
predictions of possible future impacts), there continue to be headlines about
dangerous impacts in the next decades, generally immediately followed by what
are perceived as "retractions".
Several factors, beyond the commonplace hyperbole and misreporting by
news media, contribute to these unfortunate perceptions.
Consider
what is happening -- in interplanetary space and in astronomical
observatories. The Earth literally is
in a "cosmic shooting gallery," although space is very big so nothing
consequential hits Earth very often.
During the last three decades, and especially during the last five
years, astronomers have begun to scan the skies for asteroids, especially the
ones more likely to hit (for example, not asteroids in the main asteroid
belt, most of which are safely there "forever" and all of which are
safely there for millennia). NEAs are
"found" as an unknown, uncharted star on a photographic plate or,
more recently, on a CCD image; they are confirmed when, after several
exposures, they are found to be moving at an appropriate rate (not as fast as
an airplane or satellite, but not so slow as a main-belt asteroid or distant
comet) during the course of the night.
After observations over the course of a few weeks (provided skies are
clear and the patch of sky is in the coverage area of one of the photographic
search telescopes), positions of the object are good enough to calculate an
approximate orbit.
While
most such preliminary orbits do not permit the asteroid to come anywhere near
the Earth in the foreseeable future (in which case the future impact
probability is exactly zero), a small fraction of such orbits -- especially
when propagated forward in time a few decades -- do include the Earth in
the large volume of space that is within the very broad error bars associated
with the preliminary orbit. The chances
of impact may even be smaller than the chance of a random,
thus-far-undiscovered object, hitting the Earth, but at least there is now a
known date or dates in the future when such a specific object could conceivably
hit; it thus bears monitoring in the future.
After
some more weeks of additional observations of this still-threatening object, or
possibly after discovery of a pre-existing observation of it in an archive (but
which had not previously been successfully linked with other observations to
compute a preliminary orbit), the preliminary orbit can be refined and the
error bars reduced. In most such cases,
the refined orbits no longer include the Earth within uncertainties, and the
probability of impact goes to zero.
Very occasionally the refined orbit narrows down to a zone that still
includes the Earth, and the probability of impact goes up -- perhaps to better
than 1 chance in a million (for a 1 km diameter asteroid) or 1 chance in 10,000
(for a 100 m body), which merits moving it from "zero" on the Torino
Scale (meaning roughly equivalent to the background chance of unknown asteroids
striking the Earth) to "1" ("events meriting careful
monitoring"). Such cases have been
happening a couple of times a year lately, and they may happen more frequently
as search techniques advance.
A
Torino Scale rating of 1 (or higher) generates considerable interest, in the
media and within the astronomical community.
An automatic review of the calculations by a Working Group of the
International Astronomical Union (IAU) commences, and observers around the
world focus on the potential impactor with urgency, generating new observations
or discoveries of archived observations.
Commonly, within a few days, the refined data shrink the error bars and
an accurate orbit can now be computed.
Almost always, the chance of impact reverts to exactly zero and an
"all clear" is announced, which the media -- having just published
news of an impact possibility a few days earlier -- tend to call a
"retraction". But the
possibility exists, although it has never happened yet and is not likely to,
that the accurate orbit indeed predicts -- now with much higher likelihood,
perhaps certainty -- a future impact.
That, after all, is the purpose of the search. We already know that there is only a 1-in-a-few-thousand chance
of impact of a kilometer-sized body sometime this century, so we expect
refined orbits of new discoveries will continue to swing toward zero
probability impact. But there are bound
to be a few cases a year in the intermediate stage of orbit improvement that
temporarily swing as high as Torino Scale = 1, meriting attention for a while
before finally being found to be safe.
The
normal routine, described above, illustrates why media discussion of
"retractions" and "not going to hit after all"
mis-represents the Spaceguard search process.
On the other hand, there have been unforeseen surprises and even
mistakes. A surprise occurred in
October/November 2000 when an asteroid was calculated to have an astonishingly
high 1-in-500 chance of impacting the Earth 30 years hence. The body was faint, hence small, but
plausibly of Tunguska size, hence meriting a "1" on the Torino
Scale. The IAU, following its mandated
72-hour review process, reported confirmation of the
calculation...unfortunately just hours before an earlier observation was found,
proving that the impact would not happen.
The news media had a field day with the "correction". Further investigation revealed that the
object was, in all probability, a highly reflective old booster rocket from the
early 1970s. Not only is it hollow, but
it is much smaller than had been estimated, and constitutes no danger at all if
it is to hit the Earth...which, indeed, seems likely to happen within some
thousands of years. Its surprisingly
Earth-like orbit would be a strange one for a real asteroid, but typical of
"space junk." In the future,
astronomers are likely to be more aware of the possibility of being confused by
space debris.
Much
of the skepticism about astronomers' predictions is the legacy of an actual
mistake made in 1998 (cf. Chapman, 2000), when an internationally respected
astronomer announced that a civilization-ending asteroid, 1997 XF11, would come
spectacularly close to the Earth in 2028, "virtually certain" to pass
within the orbit of the Moon but nominally only 40,000 km away, implying an
impact probability as high as 0.1%. It
turns out that the calculations were faulty.
Data archived by the astronomer during several previous months were
sufficient to calculate an impact probability of essentially zero (about 1
chance in 1042). But he was
excited and he failed to check his results with colleagues before issuing a
Press Information Statement that generated banner headlines around the
world. Once again, astronomers rushed
to their archived images and found positions for 1997 XF11 that showed it to be
in an orbit such that it could not possibly hit the Earth but would actually
pass 2½ times farther away than the Moon in 2028. But unlike the nominal process described above, this time the
original prediction was just plain wrong.
An
unappreciated reality affecting predictions of very low-probability occurrences
is that the probability of making an error in calculating such a probability is
much larger than the probability itself.
Ordinary human care, resulting perhaps in 99% reliability, doesn't
suffice when trying to reduce the already extremely tiny chances of an airliner
accident, or in assuredly calculating a low-probability asteroid impact. In the operations arena, the engineering
discipline of "surety systems analysis" has been devised to build-in
safeguards against even the extremely low probability concatenation of
improbable events that after-the-fact analysis often shows to be the cause of
rare accidents, like airliner crashes or the Three-Mile Island nuclear
accident. Surety involves
"out-of-the-box" thinking about exceptionally unusual circumstances,
human factors analysis, and multiple closed-loop redundancies.
In asteroid astronomy, similar procedures
must be implemented to avoid cries of "Wolf!". At the time of the 1998 mistaken
announcement, given the known impact probabilities, it was much more likely
that the astronomer had made a mistake than that the newly implemented Spaceguard
Survey had already found an asteroid, large enough to destroy human
civilization, with a significant chance of striking within our lifetimes. Indeed, the astronomer was mistaken. The calculation-checking procedures of the
IAU were subsequently developed, in part, to minimize the chances of future mistakes. Henceforth, we may hope that reported
possibilities of future impacts are at least objective, even if they will
almost certainly quickly evolve to zero.
In
conclusion, the widespread dissension within the astronomical community
concerning issues of impact probabilities and the outright skepticism sometimes
expressed in the media are an inevitable result of misunderstandings over how
to understand and communicate about unfamiliarly tiny probabilities. They in no way should be taken to undercut
the robust understanding of how often the Earth is likely to be struck by
cosmic projectiles of various sizes.
There
is a related analogy relevant to how geologists and paleontologists, facing
rare crises in Earth history, should evaluate evidence in the geological
record. Given the unimaginably
grotesque consequences of large asteroid impacts, which have certainly
happened, as well as the range of lesser but nonetheless dramatic catastrophes
occasionally posed by volcanism, tectonics, and potential climatological
instabilities, one really must get "out-of-the-box" in order to think
realistically about how biological populations and ecosystems might have been
affected by such rare disasters. The
rules are different at such times from anything we have personally witnessed or
can even easily imagine.
V.
UNDERSTANDING CRISES IN EARTH HISTORY
Comparisons of natural hazards
The
first lesson for historical geology from studies of the impact hazard as it
would affect us in the modern world is to understand the almost unfathomable
differences in scale of impacts of various sized asteroids. Even the "small" ones have enormous
consequences beyond our experiences.
The 1908 Tunguska impact unleased an explosive energy equal to more than
a thousand Hiroshima bombs and only a few times less than the largest bomb test
ever. Tunguska devastated about 1,000
km2 of Siberian forest or about 0.001% of the land area of the
Earth. In contrast, the energy of the
K/T boundary impact was 10 million times greater than Tunguska; one could think
of each and every 1,000 km2 land unit on our planet being allocated
500 times the energy that leveled the Tunguska region. Actually, the destructive processes change
with scale of impact and the consequences vary with distance from ground
zero. But, clearly, even if the
comparative destructive efficiencies are extremely low, our fragile ecosphere
has to absorb an enormous amount of destructive energy within an hour or two of
a K/T-scale impact.
[PLACE FIG. 1 NEAR HERE]
Figure
1 is a highly schematic representation of the comparative consequences of
various kinds of accidents and natural disasters, represented by human
lethality. The vertical axis represents
the annualized world fatality rate from various types of accidents and
disasters; the more serious sources of death plot higher on the graph. The horizontal axis (deaths per event)
depicts an important qualitative difference between the various accidents and
disasters. Automobile accidents kill
many people; they happen frequently, but generally kill only a few at a
time. Accidents involving busses,
trains, ocean vessels, and airplanes have the potential for killing many more
people at a time, and occasionally they do, which is why their curves extend
somewhat to the right. While natural
disasters, like a small avalanche or a minor earthquake, can kill just a few
people, many deaths from natural disasters result from rather rare, big events. For example, between 100,000 and 2 million
people died in each of the eleven worst natural disasters (chiefly earthquakes,
floods, and cyclones) during the period 1900 - 1987 (Munich Reinsurance
Company, 1988), even though many years passed with no natural disasters even
approaching these rates of lethality.
The
impact hazard represents still another jump toward extremely high lethality per
event, but extreme rarity. Averaged
over time, the lethality (height on the diagram) is comparable with many other
individual kinds of natural disasters, although less than for some kinds of
accidents. (War, famine, and --
especially -- disease greatly exceed both natural disasters and accidents as
the chief killers.) Qualitatively, the
impact hazard is very different from anything else plotted: it is the only hazard capable of
killing hundreds of millions of people, or even the entire world population, in
one event.
Let
me justify that statement. Of course,
nuclear war has been hypothesized as having the potential to reach this level
of death and destruction. But it
presumably has no relevance for understanding past mass extinctions. Conceivably, some virulent disease could break
out and decimate, or even eradicate, the human species; this also is probably
not relevant to understanding mass extinctions because diseases are normally
species-specific and are not easily spread among numerous species, although
breakdowns of ecological systems could conceivably magnify the consequences of
such an outbreak. A nearby
astrophysical disaster (supernova) cannot be completely ruled out, though it
would be very unlikely.
Most
geophysical natural hazards necessarily have natural upper bounds to their
catastrophic potential. For example, in
the case of earthquakes, Chinnery and North (1975) state: "There are good
reasons for believing that there must be an upper bound to earthquake Mo
values, due to the geometry of seismic zones and the strength of crustal
material." The only possible
competitor for asteroid impacts is volcanism.
It has been argued that monstrous volcanic explosions (cf. Rampino et
al. 1988), dwarfing those recorded during human history but occasionally
recognizable in the geologic record, could approach the magnitude of a
kilometer-scale asteroid impact. This
topic deserves further research (see double-headed arrow in Fig. 1 indicating
conservative and liberal possibilities for the magnitude of large volcanic
events), but it also seems unlikely to apply to mass extinctions. There are inherent limitations, imposed by
the strength of the Earth's crustal rocks, in the possible magnitude to which
pent-up volcanic energy can rise before breaking through. So there must be an upper limit to the
magnitude of a volcanic explosion; the Toba event of ~75,000 yr ago, recorded
in the geological record, may be as big as they get -- and no mass extinction
was associated with that.
The
asteroid/comet size distribution, however, continues to larger sizes without
end. While only a few Earth-approaching
asteroids currently exceed the size of the K/T boundary impactor (none of them
can strike the Earth in the near future, although Earth-approachers are
replenished on timescales of millions to tens of millions of years), an unknown
comet could arrive anytime with only months or a year-or-two warning...and it
could have an immense size. Comet
Hale-Bopp, prominent in the sky in 1997, was estimated to have a diameter of at
least 25 km and perhaps as large as 70 km.
Indeed, it came within the Earth's orbit, although (fortunately) on the
other side of the solar system. Had it
struck, with its energy of tens to hundreds of K/T boundary impactors at once,
it might have sterilized our planet of all but microbial life. Simply put, no hazard other than cosmic
impacts has the possibility of conceivably eradicating humanity in a single
event. Fortunately, the odds are very
small that such an event will happen any time soon.
Some perspectives on the past from today
Looking
to Earth history, however, extremely small odds during a human lifetime become
virtual certainties on a timescale of geological epochs. Refer again to Table 1. The odds of any of the three examples of
impactors (200 m, 2 km, 10-15 km) striking during a year -- the usual temporal
yardstick for measuring human hazards -- are very small, ranging from 10-4
to 10-8. However, all of
them are certain to happen on geological timescales. The huge-tsunami makers have struck
repeatedly during Earth's history, and one may even have struck during human
history (conceivably contributing to one or more of the great flood
myths). A
"civilization-ender" is likely to strike a couple of times every
million years (which means ~100 of them since the K/T boundary). They have necessarily caused "bad
years" for most species dependent, directly or indirectly, on a summer
season. K/T scale impactors have surely
struck several to a dozen times during the Phanerozoic, and it is natural to
try to associate the worst crises in Earth history with those randomly-timed
but irrefutable cataclysmic events of the past. To repeat: what is rare with almost negligible chances on a human
timescale (thus permitting international society to largely ignore this threat
to its very existence) becomes a certain fact in the context of interpreting
the paleontological record. Impacts
can't be ignored: they have happened, and the larger among them were
unimaginably devastating.
There
are some ways of thinking about mass extinction events that can be seen as
unrealistic if viewed from the perspective of a modern-day catastrophe. We must especially heed the variety of
things that can happen within lengthy durations that are unresolvable in the
geologic record. We must not attribute
to the global ecosystem, but rather to exceptional refugia, the characteristics
that permitted some species to get through a mass extinction. Here are a few anecdotes from discussions at
this conference, exemplifying how we must change our thinking:
*
The difference in timescales relevant to the survival of a species in
the face of a sudden, global, environmental catastrophe compared with that
resolvable in the geologic record is profound.
The survivability of animals may depend on migrations over enormous
distances taking just weeks or months, timescales orders-of-magnitude shorter
than the accuracy of dating the stratigraphic age of fossils.
*
A speaker at this conference suggested that sub-freezing temperatures
lasting months would be incompatible with the survival of certain
reptiles. But that would not be true if
a few reptiles survived next to a thermal hot springs in a favorably located
cave. One must guard against
attributing to the environments of a few exceptional refugia the average conditions
of the Earth during a global environmental crisis.
*
There is a tendency to confuse killing or survival of a species with
general death or survival of individuals during a crisis. Thus one speaker discussed the theoretical
possibility that small carnivorous dinosaurs might have been able to survive on
mammals, lizards, and other species that made it through the extinction. In all probability, however, this is not
even theoretically possible: in a devastated world, where virtually every
individual mammal presumably was killed, the survivors that enabled continuance
of some mammal species were probably small groups in totally exceptional
refugia, hardly a findable food source for some carnivores stumbling blindly
through the darkness.
The importance of sudden changes for mass
extinctions
Traditionally,
mass extinctions have been ascribed to various changes in the environment that
evolve extremely slowly compared with the sudden events (impacts, volcanic
explosions) that I have discussed. Sea-level
changes, chemical and thermal changes to the oceans, global warmings,
glaciations, hot-spot volcanic outpourings...all have traditionally been
interpreted to evolve over durations ranging from tens of thousands of years to
millions of years. Even recently
hypothesized runaway geophysical processes commence on timescales long compared
with the characteristic timescales of impact devastation -- minutes to
years. To me, it seems obvious that a
sudden event (happening on a timescale, like months, that is short compared
with the lifetime of an individual animal or plant) would be a far more potent
cause of mass death and a possible mass extinction than changes, almost no
matter how great, that evolve over centuries, millennia, or even millions of
years. Here, a modern-day perspective
is helpful.
A
disaster, in human terms, is necessarily something that happens during a day,
or perhaps over months or a year, but never over decades or centuries. After all, 100% of human beings now alive
will die during the next 120 years or so, but that is considered normal
life-and-death-as-we-know-it, not a catastrophe. A powerful hurricane that strikes Florida can be a major natural
disaster, but if waters rise and flood Florida during the next half-century
(perhaps resulting from global warming), then people and enterprises can calmly
move out of Florida at the rate that they moved in during the past
half-century; it would be one of the usual ebbs-and-flows of economic and
societal change, not a catastrophe.
As
we look at Earth history, we must realize that species will be much more
seriously affected by a catastrophe that is (a) short compared with the
reproductive cycle of individuals and (b) globally pervasive...two unique
attributes of impacts. Some of the most
powerful effects of impacts are over within the first few hours; most of the
others are over within a few years.
While much longer lasting effects will certainly ensue, and are
recognized in the post-K/T geological record, they are -- like other slowly acting
environmental changes -- of little consequence to mass extinctions, no matter
how much they may inhibit recovery and radiation of new species. For impacts over certain thresholds (that
vary depending on the specific consequence, see Table 2), the effects are
global in extent, notwithstanding possibilities that small refugia may be less
affected. If all individuals starve,
freeze, and die within a year of an impact holocaust, how will the species
reproduce and survive? Adaptation to
such radical environmental shocks is practically impossible.
The
onset of an ice age is something that can be adapted to. (Walls of ice never arrive suddenly in
suburban New Jersey, as fictionally depicted by Thornton Wilder in The Skin
of Our Teeth.) Seas don't suddenly
regress, dramatically decreasing certain ecological niches worldwide, within
the lifetimes of aquatic species.
Species can migrate, evolve new behaviors. Even if competition results in stresses and lowered population
numbers, the survival of small breeding populations within such evolving
ecosystems seems much more likely than in the instant-scorched-but-frozen-Earth
aftermath of an impact. One of the most
popular "causes" for mass-extinctions discussed at the Vienna
conference are episodes millions of years long of enhanced hot-spot
volcanism in certain localities on Earth.
I cannot understand why anyone would regard such localized formation of
a volcanic province like the Deccan traps as possibly resulting in a
mass extinction. What are the killing
mechanisms from such a slowly evolving process on the opposite side of the
planet? Localized volcanism enhanced by
factors of many compared with the modern rate may show up prominently in the
geologic record, but the modest global ecological ramifications would be
readily adapted to by migrations, evolutionary change, and other non-emergency
responses.
Understanding
that slow-acting climatological changes are impotent as causes of
mass-extinctions, some researchers have hypothesized that there are possibilities
for natural, rapid instabilities on Earth -- ranging from sudden melting and
destruction of polar caps, great landslides on continental shelfs, runaway
changes in the carbon dioxide budget, etc.
Such events could possibly stress populations in ways not readily
responded to, but even they are much more slow-acting and less dramatic in
their consequences than are impacts.
They rely on such factors as rising sea-levels (which fail to affect
habitats far from shorelines) and changing climates. Yet none of them transmit their devastating effects at the speed
of many kilometers per second, spreading around the globe in a couple of
hours. And none of them can be as
globally and suddenly effective in changing the climate as the instantaneous
and efficient injection of dust and aerosols into the stratosphere that greatly
dims or blocks out the Sun around the entire globe within a matter of
weeks...and lasting for many months to many years.
Even
despite recent advances (S.A. Bowring, this conference), resolvable timescales
concerning ancient events in the geological record are long compared with human
timescales. It is understandable,
therefore, that geologists try to measure and think about environmental changes
over such resolvable times. However, by
imagining a multi-kilometer asteroid impact occurring today, in our modern
built-up and natural world, we become much more aware of the amazingly sudden
and profound changes that would present dramatic obstacles to survivability.
Huge
impacts, which were nearly instantaneous in their globally devastating effects,
have certainly occurred several times since the pre-Cambrian. Their potency in causing the nearly
instantaneous collapse of ecosystems (within minutes to months) dramatically
exceeds any other suggested mechanism for mass extinction. The smoking guns (like extant, non-subducted
craters) become less likely to remain in the geological record as we search
back in time. But they should hardly be
required as evidence for impact, given the inevitability that the monster
impacts have actually occurred. Other
evidence of the K/T boundary impact, including the famous iridium excess, are
not necessary outcomes of all major impacts (for instance, iridium content varies
among impactors, and survival of projectile material is problematic, depending
on the velocity and angle-of-attack of the impactor). But the impacts have occurred and have the unique attribute of
sudden, global simultaneity. I think it
is no coincidence that, as the techniques for making temporal measurements
improve, the time scales associated with the largest mass extinctions (like the
Permo-Triassic extinction; D.H. Erwin, this conference) continually shrink.
The
huge impacts were so instantly awful, they must have left a paleontological
record. Indeed, they must have caused
mass extinctions of some scale. It is a
testimony to the resilience of life that, through localized, exceptional
circumstances, breeding populations survived so that enough species managed to
make it through the year-long frozen night of terror and death. It then becomes problematic that any other
gradualistic geological or environmental process could have played such a
significant role, if any at all, in mass extinctions. If total lack of evidence (for example, of a layer of shocked
dust) requires searching for another cause in the case of a particular mass
extinction, only then are we compelled to turn to other improbable but still
instantaneously-acting causes, like an immense volcanic explosion, supernova, etc.
Raup's
idea that the record of extinction reflects the cosmic impactor size
distribution, and that impacts may be the cause of essentially all mass
extinctions, was actually first enunciated in 1980 (Alvarez et al.,
1980b):
It
is reasonable to assume that the Permian-Triassic (P-T) and K-T
extinctions
were caused by large Earth-crossers, while lesser extinctions
may
have been caused by more numerous smaller asteroids. If so, the
severity
vs. frequency should relate to the size vs. number of Earth-
crossing
objects.
From the perspective of modern research
on the impact hazard, it seems even more likely now that impacts have been the
dominant cause of mass extinctions during the Phanerozoic.
ACKNOWLEDGEMENTS
This work was supported by the NASA Near
Earth Objects Programs Office at the Jet Propulsion Laboratory and by a
Presidential Discretionary Internal Research and Development grant from
Southwest Research Institute. I thank reviewers
K. Atkinson and B. Ivanov and I appreciate C. Koerberl's work in organizing an
excellent conference and proceedings.
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Fig. 1 Caption:
This schematic diagram illustrates the approximate average annual
worldwide fatality rate for various kinds of accidents and natural disasters of
various magnitudes. Accidents generally
kill only a few people at a time, although accidents involving large
transportation vehicles (like aircraft) can kill hundreds. Natural disasters, as a class, are far less
deadly than automobile accidents, but the largest among them can be far more
deadly per event. Natural
disasters comparable in lethality to Tunguska-class impacts (which occur every
century or two; see downward pointing arrow) are about two orders of magnitude
more frequent than such impacts. Most
natural disasters have natural upper limits to lethality, because they are
confined to particular geographic localities and/or are limited by physics or
by the strength of the Earth's crustal materials in maximum magnitude. The upper limit in lethality of explosive
volcanism is less well known; two alternative limiting curves are shown (see
doubleheaded arrow), one extending to roughly the consequences of an impact of
a 1 to 2 km diameter asteroid. Only
asteroids and comets have no upper bound in size and could, conceivably,
eradicate the human species. In terms
of fatality rate, asteroids dominate over all other natural disasters combined
for individual events that kill more than 100 million people at once, and are
maximized for the civilization-threatening impacts that can kill a billion
people or more.
TABLE
1. CHANCES OF EVENT HAPPENING IN
SPECIFIED DURATION
__________________________________________________________________________________________
Human-Scale Historical Geological Planetary
1y 100y 10,000y 1 Myr 100 Myr
4 Byr
Object (diameter)
_____________________________________________________
K/T Extinctor (10-15 km) 10-8 10-6 10-4
1% 50% 100%
Civilization Ender (2 km) 10-6 10-4 1%
50% 100% 100%
Huge Tsunami (200 m) 10-4 1% 50% 100% 100% 100%
__________________________________________________________________________________________
TABLE 2.
CHIEF ENVIRONMENTAL CONSEQUENCES OF IMPACTS
________________________________________________________________________________________________
Civilization Ender (2 km) K/T
Extinctor (10-15 km)
________________________________________________________________________________________________
Fires ignited by
fireball Fires ignited only
within Fires ignited globally;
and/or re-entering
ejecta hundreds of km of ground
zero. global firestorm assured
(Wolbach et al., 1988).
Stratospheric dust Sunlight drops to "very
cloudy Global night; vision is
obscures sunlight day" (nearly globally);
global impossible. Severe, multi-
agriculture threatened by year "impact winter."
summertime freezes.
Other atmospheric
effects: Sulfates and smoke
augment Synergy of all factors
yields
sulfate aerosols, water
in- effects of dust; ozone layer decade-long winter. Approaches
jected into
stratosphere, may be destroyed. level that would acidify
oceans
ozone destruction,
nitric
(more likely by sulfuric acid
acid, smoke, etc.
than nitric acid).
Earthquakes Significant damage within Modest to moderate damage
hundreds of km of ground zero. globally.
Tsunamis Shorelines of proximate
ocean Primary and secondary tsunami
flooded inland tens of km. flood most shorelines ~100 km
inland, inundating low-lying
areas worldwide.
________________________________________________________________________________________________
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