Norman MacLeod: The Great Extinctions, what causes them and how they shape life (Firefly Books, Ltd, 2013), 194 pages and glossary (impressive), index (exhaustive), references; extensively illustrated (images, photos, graphs, maps).
abbreviations used in this review:
CC climate change
CO2 carbon dioxide gas
GHG greenhouse gas (carbon dioxide, methane..)
GW global warming
K/T Cretaceous / Tertiary extinction event (65 Mya, killed off the dinos)
Mya millions of years ago
NS natural selection
P/T Permian / Triassic extinction event (250 Mya, the greatest extinction)
This is a good reference book for arguing with climate change (CC) "sceptics" (footnote 1) or for informing oneself of the potential consequences of continuing to treat our world like a garbage bin. The author well presents the emerging - or already emerged - consensus among paleontologists, evolutionary biologists, paleo-ecologists and paleo-climatologists regarding the interaction between climate and biological evolution. We now recognize that earth is a highly variable, unstable, dangerous planet - seen from the perspective of individual species - and that CC is the "major driver" of biological evolution on this planet.
MacLeod focuses on the main extinction events of earth's history. These are episodes of sharply increased species mortality. The K/T extinction, 65 Mya, killed off the dinosaurs and the flying reptiles, opening the door for mammals to take rulership of the land and birds that of the air.
Quetzalcoatlus, ancient flying reptile
Interestingly, although a batch of dumb disaster movies popularized the "asteroid that killed the dinos" hypothesis, the K/T extinction was actually a relatively minor one. Furthermore, the major culprit is now seen to be CC, not the asteroid. In the words of one marine extinction specialist, Tony Hallam in Catastrophes and other calamities (2), the asteroid hit was merely the "coup-de-grâce" that finished off the late Cretaceous ecosystems, already stressed out by long term, global CC. Healthy ecosystems, while regionally damaged and locally obliterated, recuperate quite well from impacts the size of the K/T asteroid (10 km or 6 mile diameter).
While mass extinctions may titillate the public's taste for drama and chaos, at least 95% of species extinctions occur outside these periods. In reality, species extinction is an ongoing, natural process and since "mass" extinctions are rare and of relatively short duration, the "background extinction rate" eliminates the lion's share of species extinctions - 99% or more of all known species are extinct, almost all of these died off between mass extinctions.
Interesting tendencies emerge when one steps back and looks at the Big Picture of evolution as science now understands the process. The graphs on page 46-7 reveal a puzzling trend over the Deep History of life on earth: both the intensity and the variability of background extinctions tend to diminish over time as revealed by the fossil record. Several hypotheses have been suggested but none has yet gained the consensus. Some researchers attribute the decline in background extinction intensity and variability over time to biological adaptations. NS has selected for more resilient species, capable of bouncing back after environmental excursions. Other researchers believe the effect is simply an "artifact", a measurement error induced by the relatively greater frequency of recent fossils compared to ancient ones. It's easy to see why: fossils can be destroyed by geological processes but once deposited in the earth they do not multiply. Their number can therefore only decline over time. Older fossils tend therefore to be rarer than recently formed ones. This observational bias introduces an (unknown) error in our calculations of the intensity and frequency of past background extinction levels. MacLeod suggests that both schools of thought contain some truth. Observational bias is real and natural selection (NS) may also be selecting for more resilient species with greater bounce back capability.
It should also be noted that extinctions have a "positive" side: accelerated biological diversification and evolutionary change follow in their wake. Since evolution demonstrably speeds up over time - ?? a "learning curve" expressed at the level of the DNA molecule ?? - extinction may be seen as an agent of evolution in a double sense:
1- elimination of species and ecological niches, preparing the way for new species and niches and
2- exerting selectional pressure favoring genomes capable of more rapid selection.
Emphasizing the complex interactions involved in extinction events, MacLeod distinguishes proximate from ultimate extinction mechanisms. The former - what actually killed - often mask the latter - the real movers that drove the proximate extinction mechanisms. Among the proximate mechanisms are global cooling and changes in ocean - air circulation patterns. Each of these, in turn, can result from several ultimate extinction mechanisms. Thus global cooling can result from:
1- a decline in solar output or
2- an increase in earth's albedo: increased cloud or snow cover reflects more solar energy back to space, cooling the earth; decreased sea levels expose more light colored land, reflecting more solar radiation. These causes mutually interact, further complicating the picture.
Sea level changes are also highly - if confusingly - correlated with extinction events, especially rapid and / or large scale changes in sea level. Sea level changes may be a proximate mechanism of extinction when, for example, tidewater zones are converted to marshland or terra firma. As their environment disappears, so do tidewater organisms. But sea level changes may also act as more distant, ultimate extinction mechanisms: a rise in sea level causing poisonous deep sea anoxic waters to invade shallow coastal waters. In this case, the proximate - immediate - cause of extinction is lack of oxygen and / or toxic sulfur compounds in the anoxic waters.
Sea levels have varied widely over earth's history; today's level are at near historic lows, suggesting that contemporary rising levels are merely moving us back towards the historic norms. This is not good news for an overpopulated planet undergoing Global Warming (GW) since rising seas mean less land for all those people to live on and warming water naturally expands, taking more space. Sea levels in the past have been as much as 400 meters / 1200 feet (!sic!) above modern levels, drowning vast shallow continental shelves.
Sea level changes may result from
1- glaciation / deglaciation cycles (glaciers grow from sea water deposited by precipitation on land). A global cooling trend causes glaciers to grow which, in turn, causes sea levels to drop, exposing more light colored land which reflects solar radiation back to space. In addition, ice and snow are highly reflective, reflecting a high proportion of the light that falls on them back into outer space. Seawater, however, is dark and absorbs most of the light falling upon it. Under appropriate conditions a positive (self-reinforcing) feed back loop can be set up, amplifying the initial cooling trend:
cooling --> more snow and ice cover --> increased reflectivity --> further cooling
2- changes in the geometry of ocean basins due to plate tectonic movements and ocean floor volcanism. A rising sea bed floods shallow continental shelves, a falling sea bed drains them.
Again, since the earth system is an interconnected "holon" - or whole, everything affects everything else. Sea level changes produce multiple, often contradicting, effects in the earth system. They cool the earth by increasing the solar energy reflected back to space as described above. They have a role in producing anoxic oceans zones with reduced oxygen levels and potential for life. They may also expose buried sediments containing organic matter which, when exposed to air, break down, releasing GHGs, thus warming the earth. Which effect - warming or cooling - actually dominates depends upon a host of factors. In addition, sea level drops affect the interior climates of large land masses, exaggerating summer - winter seasonal variation and increasing overall aridity. These continental interior temperature modifications may lead to modified jet stream tracks in the upper atmosphere, radically modifying precipitation regimes. Everything is interconnected!
Marine anoxia or oxygen reduced "Dead Zones", a proximate extinction mechanism which has eliminated many forms of marine life in the past. These kill off marine life in the lower water column, reducing biodiversity and biomass production.
Marine anoxia has, as expected, several ultimate causes. One cause, particularly important today because of agricultural and urban waste run off, is increased delivery of mineral nutrients to the seas from the land. Increased land erosion, for example, will deliver mineral resources to the near offshore water column causing life to blossom at the top of the water column. Increased erosion itself may be due to several causes, often interacting: increased precipitation, changes in chemical composition of the atmosphere or regional mountain building activity (which increases weathering of rock due to increased runoff velocity and / or increased precipitation on slope faces). As a result of increased fertilization of the upper water column, organisms in the lower water column may die from lack of oxygen (the guys in the penthouse take all the oxygen for themselves). Toxic compounds - containing sulfur, for example - may accumulate over the seabed. A movement of these toxic waters into shallow continental shelves - caused by sea level rise - may devastate coastal ecosystems, including terrestrial components (sea birds, aquatic mammals..)
Finally, weather patterns, such as monsoons upon which much of the agriculture of the world depends, and global heat circulation patterns, such as the jet stream or the Gulf Stream of the Atlantic ocean upon which the present climates of Western Europe depend, establish stable patterns of energy and material flows. This is a point climate "sceptics" seem to fail to realize. A small change in, say global temperature, can have a profound geopolitical impact by modifying local and regional precipitation regimes so they no longer coincide with traditional agricultural practices. Civilizations have fallen because of this!
A relatively minor shift in jet stream patterns can turn a viable semi-arid region into a desert (or the contrary). For earlier human cultures, these minor tweakings of the climate machine often meant the difference between life or death: migrate or starve / migrate or freeze.
What do we see when we look for the most basic, primary, ultimate extinction mechanisms? Where are they? What are they?
Variations in solar radiation are often invoked by CC "sceptics" to explain (away) GW. These variations are (quasi-)cyclic and real. Their affect on human culture can be discovered in ancient historical records and archaeological digs. Unfortunately for the "sceptics", short term variations in solar output are too rapid and too weak to account for contemporary GW, (much less for mass extinction events). Longer term variations in solar output, are perhaps of the right order of magnitude but too slow. The time scales don't match..
Plate tectonics have powerful climate modifying effects and since CC is the major driver of biological evolution, their role in evolution is primordial. Their influence however is long term and so cannot explain modern GW. It is believed, for example, that the current rate of CO2 increase is much greater than the rate during the Siberian Traps erruptions which, in part, provoked the great late Permian extinction.
link to Siberian Traps: vulcanism and Permian extinction
A textbook example of the impact of tectonics on evolution is the P/T event, 250 Mya. Tectonic plates assembled the earth's land masses into a single megacontinent, Pangaea ("All Earth").
This produced an intensive "continental" interior climate marked by great seasonal variations. Worse, the imbalances in tectonic forces created by such a logjam of landmasses produced created great interior heating of the earth's mantle. Immense lava flows, the Siberian Traps, expelled GHG into atmosphere, heating the earth.
http://siberia.mit.edu/ a short video describing GHG effects on CC. Shows what Trap country looks like today, a quarter of a billion years later..
One likely scenario is that GW resulting from the Siberian Traps eruptions triggered a massive release of sequestered methane from thawing circumpolar tundra and cold northern seabeds. Since methane is even more potent than CO2 as a GHG, these releases drove planetary temperatures to the point of generalized ecosystemic breakdown. The central core of Pangaea became a hellish desert. At the worst, only a thin line of green ran round the earth, a boreal forest ring at 6o deg North - now the latitude the Arctic Circle begins! I personally believe that the resulting extinction generated, via decomposing biomass, a third wave of GHG warming. The resulting extinction was so severe that it marked the end of a biogeological epoch: the Paleozoic. The Mesozoic - the Age of Dinosaurs - followed the great Permian die off: one age of life on earth ended, another began. Once again, CC was the dominant driver of biological evolution on earth.
The P/T may even provide a possible analog to current manmade - "anthropogenic" - GW. There is growing evidence that sequestered methane in northern tundra and shallow seabeds is destabilizing (?? possibly reaching a tipping point of massive, rapid and irreversible release ??)
video: thawing northern lakes catch fire!
During the late Permian extinction, anoxic - dead, oxygen free - zones in the ocean developed and spread. We see the initiation of similar phenomena today. The primary cause usually cited: runoff from agriculture and city sewage fertilizes the ocean, stimulating overproduction of biomass in surface waters. The lower water column shows a drop in oxygen content due to the increased biological activity at the surface. In the fossil record, anoxic periods are inferred from carbon, sulfur and iron rich ocean sediments: if oxygen were present these elements would oxidize. Anoxic zones - if not totally depleted of oxygen to the point of lifelessness - contain fossils of organisms which tolerate low oxygen levels. Such organisms have low metabolic rates and do not disturb sediment layers by extensive borrowing. Undisturbed sediment layers therefore indicate low oxygen levels.
It is interesting to note, that our present rate of GHG emissions is predicted to bring an approximately 5 C temperature rise by 2100. Yet the truly massive P/T extinction - the biggest of all - is associated with only a SIX C rise in temperature :-0 Such evidence comes from many sources: isotopic analysis, soil type and floral analyses (page 100).
Another reason the P/T extinction might be a fit subject of study for the future evolution of CC: it provides us with an extreme, worst case scenario, thereby lying bare the major causal mechanisms and consequences of CC / GW. "It must be remembered that, in all likelihood, Siberia Trap volcanism was merely one trigger - albeit a major trigger - that combined with the state of the planet at that time - a profound sea-level lowstand, extreme continentality, unprecedented globally high temperatures, marine deep-water stagnation - to cause the late Permian environment to be in a profoundly unstable state. None of these factors was foreordained to have been present and none was more important that the others in terms of the ensuing biodiversity debacle. Indeed, the absence of any one would in all likelihood have resulted in a less intense episode of environmental challenge. But in the late Permian, a "perfect storm" of environmental contingencies came together.. The result was the largest single extinction event the earth has witnessed to date." (page 103)
CLIMATE CHANGE IS THE MAJOR DRIVER OF BIOLOGICAL EVOLUTION ON EARTH
Really? And what, exactly, would it take to kill these off.. (Inostrancevia Alexandri, a synapsid reptile or proto-mammal of the late Permian. Mammals are modern synapsids - lucky for us that some of these dudes did manage to survive..)
The most popular extinction is, of course, the K/T which "killed the dinos". Nuance: by the middle of the Mesozoic, birds had evolved from theropod dinosaurs. So if dinos didn't survive, at least theropods - modern birds - did. Once again, multicausality is stressed: extinction events are not so much unique lethal events as co-incidental lethal combinations of multiple environmental stressors which reach tipping points beyond which local / regional species and ecosystems can no longer adapt. The result: elevated rates of species extinctions which are subsequently labelled "mass extinctions".
The Cretaceous - end phase of the Age of Dinosaurs - experienced massive sea level drops. These produced marine extinctions as ecosystems were eliminated, reduced in size or fragmented (reducing vital biodiversity and resistance to environmental stress).
Increased extinction rates on land resulted from attendant CC: continental surfaces became larger as sea levels dropped. Interior "continental" style climates became more extensive and intense with increased seasonal variation and aridity. Isotopic studies indicate severe global cooling, further stressing marine and terrestrial organisms. A major volcanic episode, the Deccan Traps, India, coincides with the K/T geological and biological boundary (suggesting a link between Traps vulcanism and the K/T boundary extinction event).
The Deccan Traps, a massive lava outpouring in India
This event spewed massive amounts of CO2, acidifying the oceans to the point of shutting down primary phytoplankton production. This obviously provoked a chain of extinctions down the food web, including coastal terrestrial species feeding on ocean biomass. Global temperatures would have soared as much as 10 C (18 deg F) by some estimates, provoking more extinctions as the earth snapped from a stressed cooling period to an even more stressing period of precipitously rising temperatures.
Analogy with today: we are acidifying the oceans with CO2 to the point we, too, will provoke a shut down of primary phytoplankton biomass production.
Finally, then, the coup-de-grâce everyone is waiting for: the asteroid struck, 65 Mya and "killed the dinos". But it was already a world down on its knees from Climate Change..
How much does past climate tell us about today's world?
PETM: the Paleogene-Eocene Thermal Maximum. A very curious event, this one. The modern biological world began with the K/T extinction, 65 Mya. The first half of the modern period was decidedly warmer than the second, during most of which one or both poles were ice covered. One of the curiosities of the warmer first half is a short, extremely high spike in temperature. Over a span of 2 -3 thousand years in the early Eocene, about 55 Mya, global seawater temperatures rose by 6 C (11 deg F) as measured by carbon 13 isotopic measures in ocean sediments. The equatorial tropical zone expanded, pushing subtropical arid zones northward. As expected, extinction rates climbed in response to climate stress on ecosystem integrity.
Many researchers link the PETM to a sudden release of methane from thawing permafrost and "methane-ice" sequestered in circumpolar coastal seafloors. Such a massive release of methane - a much more potent GHG than CO2 - would induce a positive feedback loop in which warming provokes methane release which, in turn, causes further GHG warming. Contrary to some doom-sayers, this process need not result in a "runaway greenhouse effect" leading to earth becoming a hellish desert, devoid of life, like planet Venus. Given the position of the earth from the sun, methane releases should not lead to a Venus-like runaway greenhouse heating effect. The proof lies the pudding! Despite all the excursions of earth's history - chemical, biological, tectonic, astrophysical.. - if the earth's climate were so sensitive to GHG runaway, we would certainly have seen it by now.
The real problem today is that we are pushing atmospheric CO2 levels up to 400 parts per million, the highest level in a few zillion years. Despite CC deniers and "sceptics", the earth IS warming:
This semi-technical article - written by real earth scientists, not untrained "sceptics" and self-pronounced pseudo-experts - explains that the recent "pause" in GW, much publicized by GW "sceptics", is insignificant / spurious. In large part it is "artefactual" - measurement bias - introduced by recalibration of data bases and by lack of measurement stations in Arctic regions. In reality, the whole "GW pause" flap is a red herring: the measurement period is statistically too short to say much of anything with confidence. If somebody repeatedly tossed a coin and came up with heads 50 times in a row, I would have reason to suspect something was fishy. Five times, not so much: there is about 1 chance in 30 of getting 5 heads in a row. You simply can't say too much about the real significance of such statistically short sequences.
From a scientific - not ideological - perspective there is NO solid evidence that GW (which is occurring) is NOT due to human economic and agricultural activity. Therefore, it appears logical, rational, moral that we should be held morally responsible for the consequences of GW. Since the probable consequences are quite serious, the moral responsibility is also quite grave. For example, GW threatens to reduce agricultural output on an already overpopulated world the population of which is still growing. Why is the world population still growing? Who is responsible?
On a purely intellectual level, current GW is a BIG problem because everything we really know about CC tells us that the earth ought to be cooling off, slowly drifting back into an ice age. But this just ain't happenin' folks: the earth she warms. Something ain't right..
book review on ice age science (Imbries)
The following technical article deals honestly with some of the real difficulties determining long term climate cycles like the ice ages.
The biggest problem of all: there is growing evidence that currently rising temperatures in the northern circumpolar regions are beginning to destabilize methane sequestered on seafloors and thawing tundra. Of all climate change mechanisms this is probably the one we should fear most. It is potentially a real game changer. Even if the earth did not suffer the heating of the late Permian extinction, the effects on global agriculture would be geopolitically destabilizing on an unprecedented scale.
I like Prof MacLeod's concluding note, written as a true democrat, page 193:
"Whether the effects of human population growth and economic development are allowed to develop unchecked to the point at which the extinction of living species eventually does reach a magnitude consistent with the great extinctions in earth history will depend on the decisions each individual makes with respect to how they live, what they buy, how they vote and what sort of planet they wish to pass on to future generations" Can't be clearer than that! If we are victims, it is because we, as citizens - not "clients" as neoconservative technocrats would have it, we citizens have let ourselves be duped, let outselves be manipulated to overconsume lots of (admit it!) junk we didn't really need. We let ourselves be "victimized"..
This is a good book, especially as a reference work. It's weak points: a certain stylistic heaviness / clumsiness at times; excessive use of jargon (offset somewhat by the impressive glossary). I found the synoptic images of past ecosystems a bit too small, crowded and lacking sufficient explanatory captioning. Fewer images, better described and explained would have pleased me. Let's be generous, give Prof MacLeod a break, a solid 8 on 10.
1- Why we object to the use of the word by Climate Change "sceptics", Global Warming "sceptics" and Peak Oil "sceptics". We put the word in quotes to indicate our scepticism toward their use. Originally, philosophical skepticism meant a general approach to inquiry that requires all information to be well supported by evidence. It is rather evident to any (relatively) unbiased observer that this is not what our current crew of anti-science "sceptics" are up to. They are ideologues who "know" that right is on their side. "Debate" degenerates to mudslinging matches with self-righteous character assassination the weapon of preference. Modern "sceptics" therefore embody the antithesis of the attitude they claim to hold toward inquiry.
2- Tony Hallam: Catastrophes and other calamities, Oxford University Press, 2004
I love the concluding lines of the above book review: "the subject is open to some discussion and Tony Hallam's view is
certainly no less worthy and clearly the End-Permian is something close
to his academic heart. Consequently students recommended this text need
to be encouraged to read it and then think for themselves. This
last is something that sometimes we forget in today's occasionally
mass-fact regurgitation dominated education regimen". Thinking, the lost art - Amen! brother Amen!