Snow time

744 – 705 million years ago

The United States has been hit by a major winter storm. I’ve been out shoveling snow from the driveway several times today. In-person classes at my university were cancelled, and I’ll probably take the light rail into school tomorrow rather than braving the streets with my car.

But things have been worse. Before 720 million years ago, we find thick limestone deposits left by decaying algae. These were sequestering carbon, taking carbon dioxide out of the atmosphere, and cooling the Earth. At some point a positive feedback cycle kicked in, as polar seas froze and reflected more sunlight, cooling the planet further. The result was a succession of extreme Ice Ages. The Ice Age of the last two million years, which merely covered high latitudes with glaciers, off and on, were nothing compared to the Snowball Earth of the Cryogenian: at a minimum, polar seas were frozen, and tropical seas were slushy with icebergs. It’s possible that things were even more extreme: the entire sea may have been covered by a thick layer of ice, with a few photosynthetic algae surviving in the ice, and other organisms hanging on around deep sea hot water vents. A limited amount of oxygenated meltwater seeping into the ocean from under the glaciers may have kept early oxygen-breathers alive. For a hundred million years, climate oscillated abruptly between two steady states, frozen and warm.

It’s only in the last two decades we’ve begun to figure out this amazing story. If there’s a lesson here, it’s that Earth over the long run is far from a stable system. We will see again and again that the history of life, like human history, has been punctuated by catastrophes.

Above, a rock dropped from an iceberg or glacier into the middle of a tropical ocean

Power laws, earthquakes, and war

On Boxing Day (December 26) 2004, a tsunami resulting from a 9.0+ magnitude earthquake killed about 250,000 people around the Indian Ocean. This was one of the deadliest natural disasters in recorded history. The Indian Ocean tsunami illustrated a major theme on this blog: the importance of catastrophes in human history, and in the history of life and the universe.

Earthquakes are one example of a phenomenon following a power law statistical distribution. The frequency of earthquakes drops off as an exponential function of their magnitude, so that on a logarithmic scale, the magnitude-frequency relationship looks linear. This is known as the Gutenberg-Ritter relation. (The deviation from linearity in the upper left part of the chart below may reflect measurement error, with a lot of tiny earthquakes not being detected.)

Power law distributions are found in many other contexts, for example, in the frequency of wars versus their magnitude (as measured by the number of war deaths). A power law distribution is very different from the more familiar bell-curve Gaussian normal distribution: extreme “black swan” events that are astronomically unlikely under a normal distribution may happen at appreciable frequency under a power law distribution. Depending on the exponent, a power law distribution may not have a well-defined variance, or even a well-defined mean.

For a technical discussion of why small scale processes sometimes aggregate to generate normally distributed outcomes, and other times aggregate to produce power law distributions, here’s an article on The common patterns of nature. A take home lesson – not always covered in introductory treatments of statistics and probability theory – is that catastrophes and extreme outcomes can be an expectable part of the natural order.

Steven Pinker and Nichlas Nassim Taleb have squabbled about the implications of all this for the probability of a peaceful future. Here’s a level-headed review. Several recent books carry this argument further. In Only the Dead: The Persistence of War in the Modern Age, Bear Braumoeller makes the case there is no good reason to think that war is in decline. Rather “international orders” – whether the nineteenth century Concert of Europe, or the post World War II liberal order – have often made for peace among participants and conflicts with non-participants. On the other side, in The Stupidity of War: American Foreign Policy and the Case for Complacency, John Mueller makes the case that the likelihood of war was and is greatly exaggerated, both during and after the Cold War, and argues for the virtues of complacency and appeasement. Of course this year’s Russian invasion of Ukraine forces us to revise our estimate of the probability of major war upward, but it doesn’t prove Pinker wrong: a snow storm in April doesn’t prove that March isn’t a wintrier month.

And here are a couple of blog posts from me about why the bloody early twentieth century was maybe more than just a run of bad luck.

Gradualism

1823 – 1835

Charles Lyell’s great work, Principles of Geology, came out between 1831 and 1833. Lyell advocated an uncompromising uniformitarianism: the same geological forces at work today, causing small changes over the course of lifetimes, were at work in the past, causing massive changes over the course of geological ages. We’ve seen over the course of this blog that uniformitarianism is not a completely reliable guide either to geology or to human history, which have been punctuated often enough by catastrophes – asteroid strikes, continent-scale floods, volcanic eruptions, and devastating wars and plagues. But the theory is nonetheless at least part of the story of history, and Lyell’s work was deservedly influential.

In 1837 Charles Darwin, a careful reader of Lyell, published a short article entitled On the Formation of Mould. This would eventually led to his last book, The Formation of Vegetable Mould through the Action of Worms. Darwin’s work on soil formation was Lyellianism in miniature. He demonstrated, through a combination of careful reasoning and experiment, that the surface layer of pasture soil is formed by earthworms. “Although the conclusion may appear at first startling, it will be difficult to deny the probability that every particle of earth forming the bed from which the turf in old pasturelands springs, has passed through the intestines of worms.” Reading Darwin on worms you get the feeling he identifies with his humble subjects, gradually remaking the world through their patient industry.

The doctrine of progress through gradual change was appealing for more than just scientific reasons. In the 1830s, English liberals (of whom Darwin was one) were attempting to reform their society gradually, without the violence of the French Revolution, and without turning over politics to a Great Man in the style of Napoleon. (Darwin was also a gradualist with regard to his own work: he came up with the theory of natural selection in 1838, but England at the time wasn’t ready for anything so radical, and he didn’t publish On The Origin of Species for another twenty years.)

George Eliot (Mary Ann Evans), a friend of Darwin’s, set her greatest novel, Middlemarch, around the time of the Reform Act of 1832, which moved England one big step closer to a genuinely representative government. The novel’s heroine, Dorothea Brooke, might in another age have been a famous saint, another Theresa of Avila. In the England of her time she has another fate. Here is the famous conclusion of the novel, a paean to gradualism and the cumulative force of small deeds:

Her full nature … spent itself in channels which had no great name on the earth. But the effect of her being on those around her was incalculably diffusive: for the growing good of the world is partly dependent on unhistoric acts; and that things are not so ill with you and me as they might have been is half owing to the number who lived faithfully a hidden life, and rest in unvisited tombs.

Black swan demography

7.03 – 6.65 thousand years ago

The economist Robin Hanson suggests that human population history can be seen as a succession of growth modes, where each mode has a characteristic doubling time. Populations in the hunting mode double roughly every 230,000 years. But by today’s date, enough of the world’s population is practicing agriculture that the farming mode begins to dominate, with a characteristic doubling time of about 860 years.

There’s a puzzle here. Many foraging and farming populations have been observed to grow a lot more quickly than these doubling times would suggest. A recent survey of data on the demography of small-scale societies consistently finds positive growth rates; these would fill the earth many time over in short order. The authors consider various scenarios, with different assumptions about age-specific birth and death rates, to try and reconcile these data with the long-term record of relatively slow growth. Their conclusion is announced in their title: “Periodic catastrophes over human evolutionary history are necessary to explain the forager population paradox.” The expected result is a sawtooth pattern, with populations increasing most of the time, but growth checked by occasional dramatic die-offs.

If this is how human demography has operated in the past, we might expect some consequences for psychology and culture. People should be sensitive to the possibility of catastrophe, and inclined to remember and pass on vivid tales of past catastrophes, even if there is nothing very dangerous going on at the moment.

Also, one response to catastrophe is a military one, ensuring one’s own survival in apocalyptic times at the expense of one’s neighbors. It is notable that the fear of natural disaster is a strong and consistent correlate of warfare.

History became legend, legend became myth, part 2 (Noah’s flood)

7.43 – 7.04 thousand years ago

We’ve already seen that the whole Mediterranean basin once dried out for hundreds of thousands of years, only to be flooded in the course of just a few years once its connection with the Atlantic was restored. This happened 5.3 million years ago. Strikingly, recent findings suggest that there may have been human ancestors in the area to watch it happen. There’s been a lot of interest (and some skepticism) about a report of biped footprints from Crete from this time interval. This would fit with some recent claims that very early human ancestors (just after the chimp/human split) might have lived in Europe. But all this is still very much up in the air, and in any case, if any human ancestors were around in the neighborhood, and survived the flood, they hadn’t reached the stage of passing on the story to the kids.

But a similar story, on a smaller scale, may have happened within the possible limits of human remembrance. At the end of the last Ice Age the Black Sea was a freshwater lake, cut off from the Mediterranean. The water level was lower in Black Sea than in Mediterranean, so this was a potentially unstable situation. According to some evidence, around 7,500 years ago the Mediterranean breached the Bosporus, and water poured in, raising sea levels at the rate of up to six inches a day, until the area of the Black Sea expanded by more than 50%. (However some researchers think the flooding was less dramatic.)

Of course just about any reader knows the famous story of Noah and the Flood. Many readers will also know that Noah’s story seems to be connected with an earlier Babylonian flood story. This is recounted, for example, in the Epic of Gilgamesh, where Gilgamesh travels north from his hometown of Uruk seeking Utnapishtim, who survived the flood that drowned most of his neighbors. (Utnapishtim also tells Gilgamesh about a plant that will grant immortality. Gilgamesh secures the plant. Then a snake eats it. D’oh!)

It’s natural to speculate that the Black Sea flood inspired the Gilgamesh story. But flood stories are found through much of the world, so the story may be an amalgam. Another ingredient may be the story of Ziusudra, maybe a real early Sumerian king from about 2900 BCE who is recorded as surviving a major flood and getting washed into the Persian Gulf.

The bottomlands

5.31 – 5.03 million years ago

There’s a book from back in 1954, now out of print, called Engineer’s Dreams by Willy Ley (who was most notable as a spaceflight advocate). The book lays out various grandiose engineering projects that people have proposed over the years. Some of these dreams have actually been realized: after centuries of people talking about it, there is now a tunnel under the English Channel.

Others … well …

One project the book discusses is damming the Congo River, creating a huge lake in the Congo basin, then sending the water north to create another huge lake in Chad. (There’s a small lake there now, almost dried out, which was a lot bigger 10,000 years ago when the Sahara was wetter.) From Lake Chad, the water would be sent further north to create a great river – a second Nile — running through Libya into the Mediterranean. All that fresh water is just running uselessly into the Atlantic now. Why not send it someplace where it’s needed?

Another engineer’s dream is to refurbish the Mediterranean Sea by building a dam across the Strait of Gibraltar. This actually isn’t an impossible project. The strait is less than nine miles across at its narrowest, and about 3000 feet deep (about 900 meters) at its deepest. A dam across the strait would have some dramatic consequences. The Mediterranean loses more water from evaporation than it gains from the rivers running into it. The difference is made up by a flow of water from the Atlantic. Cut this off, and the sea will start shrinking. You could let the Mediterranean drop 330 feet (about 100 meters) before stabilizing it, run a huge hydroelectric plant at Gibraltar, and open up a whole lot of prime Mediterranean real estate.

Sadly, whenever people have dreamed great dreams, there have always been small-minded carpers and critics to raise objections. Okay, so maybe the mayors of every port on the Mediterranean would complain about their cities becoming landlocked. And maybe massively lowering the sea level in an earthquake-prone region would lead to a certain amount of tectonic readjustment before things settled down.

So probably the Gibraltar dam will never be built (although Spain and Morocco are considering a tunnel). But we’ve seen already that Mother Nature sometimes plays rough with her children, and it turns out (although Ley couldn’t have known this back in the 50s) that damming the Mediterranean has already been done. The story begins back in the Mesozoic (late March), when the Tethys Sea ran between the northern continent of Laurasia and the southern continent of Gondwanaland. The sea was still around 50 million years ago (April 11) when whales were learning to swim. But it has been gradually disappearing over time. When India crashed into Asia and raised the Himalayas, the eastern part of the sea closed off. And as Africa-Arabia moved north toward Eurasia, a whole chain of mountains was raised up, running from the Caucasus to the Balkans to the Alps. The Tethys Sea was scrunched between these: what’s left of it forms the Caspian, Black, and Mediterranean seas.

Starting about 6 million years ago, the story takes a really dramatic turn. The continents were in roughly there present positions, but the northern movement of the African tectonic plate, plus a decline in sea levels due to growing ice caps, shut off the Strait of Gibraltar, sporadically at first. With water from the Atlantic cut off, the Mediterranean began drying out. By 5.6 million years ago, it had dried out almost completely – the Messinian Salinity Crisis. (The Messinian Age is the last part of the Miocene Period). There were just some hyper-saline lakes, similar to the Great Salt Lake in Utah or the Dead Sea in the Near East, at the bottom of an immense desert more than a mile below today’s sea level. The Nile and the Rhone cut deep channels, far below their current levels, to reach these lakes. This lasted until 5.3 million years ago, when the strait reopened and a dramatic flood from the Atlantic restored the Mediterranean.

All this was happening just around that time that hominins were committing to bipedalism. Did the cataclysmic events in the Mediterranean basin have some influence on hominin evolution in Africa? At this point we can’t say.

Harry Turtledove, prolific writer of alternative history, has a novella, Down in the Bottomlands, set on an alternative Earth in which the Mediterranean closed off, dried out, and never reflooded. In the novella, terrorists are plotting to use a nuclear weapon to reopen the Mediterranean desert to the Atlantic – sort of Engineer’s Dreams in reverse.

And here’s a completely unrelated post about bottoms.

The worst day in the history of the Earth

66.5 million years ago

This April 6 on Logarithmic History marks the most famous mass extinction ever, the one that did in the dinosaurs (okay, okay, the non-avian dinosaurs).

Just a few years back, we had news of one of the most extraordinary fossil discoveries ever, in North Dakota: a graveyard of fish piled on one another by a tsunami-like wave, and mixed with burned trees, and the remains of mammals, mososaurs, ammonites, and insects, and a partial triceratops, formed within hours of the asteroid impact that wiped out most life on Earth. Here is a news release, 66 million-year-old deathbed linked to dinosaur-killing meteor, and here is an article from the New Yorker, The Day the Dinosaurs Died.

The end-Cretaceous extinction isn’t the biggest ever, but it’s the one everybody knows about. The Disney movie Fantasia (1940) did a version of the event, set to Stravinsky (and mixing up Jurassic and Cretaceous dinosaurs). In Terence Malik’s movie The Tree of Life, a predatory dinosaur discovers compassion in an encounter with a hadrosaur just before all their kind are wiped out by an asteroid: mass extinction meets the Book of Job.

The discovery that dinosaurs (and about 70% of all species in total) probably went extinct as a result of an extraterrestrial impact did more than anything else to bolster catastrophism. For most of the history of modern geology, geologists have mostly argued instead for uniformitarianism: the same slow processes we see today caused past geological and evolutionary changes. When evidence for an impact was first discovered – a thin layer of iridium, presumably extraterrestrial — paleontologists were pretty uniformly hostile: no physicist was going to tell them how to do science. But by now the evidence is overwhelming that the asteroid impact that left the Chixculub crater, in what is now the Yucatan, was largely responsible for the end-Cretaceous extinctions (although the volcanic eruptions that created the Deccan traps in India may also have played a role).

But at the same time that evidence has increasingly vindicated the catastrophist position, new discoveries in paleontology have increasingly brought home that one group of dinosaurs survived the extinction. Most people think of birds and dinosaurs as two quite distinct kinds of animal. But birds are just as much dinosaurs as bats are mammals. Many dinosaurs had many of the distinctive features of birds – warm-bloodedness and high metabolic rates (probably), wishbones, an advanced respiratory system, feathers (sometimes brightly colored, sometimes used for courtship), and parental care for nests of eggs and juveniles. It’s even possible that some flightless dinosaurs, like the turkey-sized Caudipteryx, were secondarily flightless, descended from flying ancestors like Archeopteryx. We don’t have to hope for The Lost World or Jurassic Parkto come true to see living dinosaurs; a trip to the park, with The Sibley Guide to Birds (or appropriate guidebook for your region) in hand, will do it.

Amborella Day

205-194 million years ago

1 galactic revolution ago

The Triassic ends 201 million years ago with another major mass extinction (the fourth, by the usual count, after the end-Ordovician, end-Devonian, and end-Permian). Not quite as bad as the end-Permian (“only” 75% of species go extinct). This coincides with the formation of the Central Atlantic LIP (Large Igneous Province), which now includes a lot of eastern North America, northeast Amazonia, and western North Africa. So the end Triassic mass extinction may be the result of volcanoes spewing lava and carbon dioxide as Pangaea splits into Laurasia (North America, most of Eurasia) and Gondwanaland (South America, Africa, Antarctica, India, Australia).

The succeeding Jurassic Period will be when dinosaurs become the dominant vertebrates on land. The mammals around are mostly shrew-sized and nocturnal.

Not as conspicuous is another evolutionary innovation: the ancestors of Amborella, a rare shrub found in the wild only on New Caledonia, split off from the other angiosperms, ancestors of all other flowering plants, 200 million years ago. (This was suspected for a while, and confirmed in 2012 with the sequencing of the Amborella genome.) We can call this the origin of flowers. Amborella has clusters of small white flowers, with male and female separate.

Spring is gearing up in the Northern hemisphere; some dwarf irises are in flower in front yard, and the daffodils, celebrating global warming, are about to let loose. If you’re lucky you’ll be able to smell the flowers on Amborella Day, and take your mind off catastrophes past and present.

In memoriam, Paleozoic

256 – 243 million years ago

Alfred, Lord Tennyson, wrote his poem “In Memoriam AHH,” in response to the death of his friend Arthur Henry Hallam. Several cantos consider the bleak lessons of paleontology – not just the myriads of deaths, but the specter of species extinction. Tennyson finished the poem in 1849, a decade before “The Origin of Species,” when the possibility of non-divinely-directed evolution and the reality of mass extinctions like the end-Permian were becoming part of general awareness.

LV

Are God and Nature then at strife,
That Nature lends such evil dreams?
So careful of the type she seems,
So careless of the single life;

That I, considering everywhere
Her secret meaning in her deeds,
And finding that of fifty seeds
She often brings but one to bear,

I falter where I firmly trod,
And falling with my weight of cares
Upon the great world’s altar-stairs
That slope thro’ darkness up to God,

I stretch lame hands of faith, and grope,
And gather dust and chaff, and call
To what I feel is Lord of all,
And faintly trust the larger hope.

LVI

‘So careful of the type?’ but no.
From scarped cliff and quarried stone
She cries, ‘A thousand types are gone:
I care for nothing, all shall go.

‘Thou makest thine appeal to me:
I bring to life, I bring to death:
The spirit does but mean the breath:
I know no more.’ And he, shall he,

Man, her last work, who seem’d so fair,
Such splendid purpose in his eyes,
Who roll’d the psalm to wintry skies,
Who built him fanes of fruitless prayer,

Who trusted God was love indeed
And love Creation’s final law—
Tho’ Nature, red in tooth and claw
With ravine, shriek’d against his creed—

Who loved, who suffer’d countless ills,
Who battled for the True, the Just,
Be blown about the desert dust,
Or seal’d within the iron hills?

For one answer to Tennyson’s anguished question about human extinction, there’s an argument that says we can estimate how much longer humanity has got from just basic probability theory. It comes from astrophysicist Richard Gott, and goes like this: Homo sapiens has been around about 200,000 years. It’s not very likely that we’re living at the very beginning or very end of our species’ history, just like it’s not very likely that a name chosen at random from the phone book will come at the very beginning or the very end. Specifically, there’s only a 2.5% chance that we’re living in the first 2.5% of our species’ life span, and only a 2.5% chance we’re living in the last 2.5% of our species’ life span. So do the math, and there’s a 95% probability that our species will last somewhere between .2 million and 8 million years.

For more on Bayes’ Rule, and the future of humanity, here’s a recent book, The Doomsday Calculation.

Snow time

744 – 705 million years ago

For the past few weeks, tweeting has been sparse, because for a billion years Earth was fairly stable. Any biological evolution towards greater complexity that was going on left little fossil evidence.

Then things changed dramatically. Before 720 million years ago, we find thick limestone deposits left by decaying algae. These were sequestering carbon, taking carbon dioxide out of the atmosphere, and cooling the Earth. At some point a positive feedback cycle kicked in, as polar seas froze and reflected more sunlight, cooling the planet further. The result was a succession of extreme Ice Ages. The Ice Age of the last two million years, which merely covered high latitudes with glaciers, off and on, were nothing compared to the Snowball Earth of the Cryogenian: at a minimum, polar seas were frozen, and tropical seas were slushy with icebergs. It’s possible that things were even more extreme: the entire sea may have been covered by a thick layer of ice, with a few photosynthetic algae surviving in the ice, and other organisms hanging on around deep sea hot water vents. A limited amount of oxygenated meltwater seeping into the ocean from under the glaciers may have kept early oxygen-breathers alive. For a hundred million years, climate oscillated abruptly between two steady states, frozen and warm.

It’s only in the last two decades we’ve begun to figure out this amazing story. If there’s a lesson here, it’s that Earth over the long run is far from a stable system. We will see again and again that the history of life, like human history, has been punctuated by catastrophes.

Above, a rock dropped from an iceberg or glacier into the middle of a tropical ocean