Tag Archives: carbon dioxide

Ice Age gear shift

833-789 thousand years ago

Around today’s date, there was a shift in the nature of glacial cycles.

But let’s back up a bit. Earth’s climate took a turn toward cool in the transition from Eocene to Oligocene, 35 million years ago (although with some warming in the Miocene). It was probably back then that much of Antarctica started being covered by ice. The establishment of open water all the way around Antarctica may have helped isolate and freeze the continent. And declining carbon dioxide levels, partly a result of weathering of rocks in the Himalayas, probably also made a difference. But it was back at the beginning of the Pleistocene, now dated to 2.5 million years ago, that the current Ice Age truly began, with glaciers covering large parts of northern North America and northern Europe.

Current Ice Age? Glaciers covering large parts of northern North America and northern Europe? This isn’t what the climate has been like for the past 10,0000 years. Within the current long Ice Age there have been long glacial periods and shorter interglacials, and we’re currently in an interglacial. Our own activities may have done something to prolong the interglacial, and stave off the return of the ice; more on this another day.

Three astronomical cycles govern the rhythm of glacial and interglacial. There’s a 100,000 year cycle as Earth’s orbit changes from somewhat more elliptical to somewhat more circular. There’s a 40,000 year cycle as Earth’s axis shifts from slightly more tilted (24.5 degrees off vertical) to slightly less (22.1 degrees); it’s currently tilted at 23.5 degrees. And there’s a 21,000 year cycle generated as the Earth precesses – wobbles like a top. Right now the North Pole is pointed at Polaris, and the Sun very recently started rising in the constellation Aquarius at the Spring equinox: hence the Age of Aquarius.

Between 2.5 million and 800,000 years ago, the glacial/interglacial alternation was dominated by the 40,000 year cycle. But beginning about 800,000 years, there has been a gear shift: the 100,000 year cycle has been dominant and swings in climate have been more extreme. (In Africa however the 21,000 year cycle is more important for alternations between rainy and dry. Africa is in a dry state now.)

One of the startling findings to come out of the last few decades of work on ice cores from Greenland and Antarctica is that not only have there have been huge long-term changes in climate, but there have also been extreme short term shifts, probably connected with changes in ocean currents. There have been a number of occasions over the last hundreds of thousands of years during which average temperatures shifted by 10-20 degrees Fahrenheit (5-10 degrees Celsius) for a millennium, or even for a century or less! (During the last 10,000 years, however, the climate has been unusually stable.)

This is bound to have had strong effects on human beings. Two anthropologists, Robert Boyd and Peter Richerson, who work on mathematical models of cultural evolution, have a general theory of how this pattern of oscillations might have affected human evolution. They argue that human adaptation takes place on multiple time scales. On very long time scales, human beings adapt to changes in the environment genetically. On very short time scale, human beings adapt to change through individual learning. But when change happens on intermediate time scales, adaptation takes place through social learning. With changes on intermediate time scales, your ancestors may not have enough time to adapt genetically to the current climate, but things may be stable for long enough that your culture and the wisdom of the elders have a lot to teach you about how to cope. So one of the really distinctive features of human beings – we are, more than any other creature, a cultural animal – may have been shaped by the nature of climate change especially over the last 800,000 years.


Grasses and gases for a cooling world

The standard sort of photosynthesis uses a so-called C-3 chemical pathway. But  maybe from 8-7 million years ago there’s an increasing proliferation of so-called C-4 plants. They use an alternative, more-efficient pathway to incorporate carbon from C02. C-4 plants evolved independently 45-60 times.

Tropical grasslands are mostly C-4. The profusion of grasses, herbivores, and carnivores on tropical savannahs will owe a lot to C-4 plants. This evolutionary transition is probably a sign that C02 levels are declining, and have reached a threshold where C-4 plants are favored.

Going back about 7 million years, C02 levels stood at maybe 1500 parts per million (ppm). (They were higher earlier in the Miocene.) Levels decline pretty steadily, leading to global cooling and eventual Ice Ages. But things never again reach the extremes of Snowball Earth 750 million years ago.

At the beginning of the Industrial Age C02 levels stood below 300 ppm. They went above 400 ppm a few years back.

Ginormous, or The Canseco Conjecture

35.9-34.0 million years ago

The Eocene epoch, which we leave behind, saw super-greenhouse conditions, and tropical forests extending to high latitudes. The Oligocene, starting 34 million years ago, sees a drop in atmospheric COlevels. Glaciers begin forming in Antarctica, and the world cools sharply. There are extinctions in a number of groups (although not on the scale of the Big Five mass extinctions), after which the fauna, at least in Eurasia/North America, starts looking like what we’re used to: versions of horses, deer, camels, elephants, cats, dogs, and many rodent families begin to dominate.

The Oligocene also boasts also the largest land mammal of all time, Indricotherium (or Baluchitherium, discovered 1922), related to living rhinoceroses, but 15 feet high at the shoulders, and weighing as much as three or four African elephants. (The picture below compares them.) Indricotherium was big enough to browse high up on trees. By contrast, living big browsers (giraffes, elephants) use special bits of anatomy (long necks, trunks) to reach that high, and don’t get quite as big.indricotherium

This is still a lot smaller than the biggest dinosaurs, the sauropods. Ginormousness is one of the things dinosaurs are famous for, even though there were plenty of small dinosaurs too. Two things that keep mammals from getting truly huge are probably (1) a different respiratory system, without the extensive airsacs and aerated bones of dinosaurs, and (2) live birth. Gigantic sauropods could lay eggs and produce (relatively) small offspring which grew up quickly, so they didn’t pay as high a reproductive penalty for being big.

There are other possibilities. Jose Canseco, former Major League Baseball player, and authority on being large (he is the author of Juiced: Wild Times, Rampant ‘Roids, Smash Hits, and How Baseball Got Big), published his theory on Twitter in 2013 (February 17-18). “My theory is the core of the planet shifted when [a] single continent formed to keep us in a balanced spin. The land was farther away from the core and had much less gravity so bigness could develop and dominate.” Anticipating possible criticism, he tweeted, “I may not be 100% right but think about it. How else could 30 foot leather birds fly?”

Canseco may not have been the first to come up with this idea. A guy named John Stojanowski wrote to me two years back claiming to have originated the Gravity Theory of Mass Extinction. His main area of expertise is in the design of residential geothermal systems.

Strange relations and island continents

56.2-53.2 Mya

We’re now in the Cenozoic era – our era. The transition from Paleocene to Eocene epochs in the early Cenozoic (55.9 million years ago) saw a spike in CO2 levels and a sharp rise in temperatures that lasted for several hundred thousand years – perhaps an analog for even more rapid human-caused global warming in our own time. (A recent review is here.)

We’ve seen a great many catastrophes in the history of life, and been reminded of the role of sheer chance in evolution. But the Cenozoic also sees a dramatic adaptive radiation and the steady progress of arms races among survivors of the great dinosaur die-off. Four large scale groupings of placental mammals have already appeared: Afrotheres (aardvarks, hyraxes, elephants, and sea cows), Xenarthrans (anteaters, armadillos, and sloths), Laurasiatheres (shrews, hedgehogs, pangolins, bats, whales, hoofed animals, and carnivores), and Supraprimates (aka Euarchontoglires, including rodents, tree shrews, and primates). This grouping of mammals is anything but obvious – it’s only with DNA sequencing that it has emerged. What’s noticeable is the association with different continents: Afrotheres with Africa, Xenarthrans with South America, and the others with the monster content of Laurasia (Eurasia and North America). Looking beyond placental mammals we see other continental associations: marsupials flourish in South America and Australia, and giant flightless “terror birds” carry on rather like predatory dinosaurs in South America.

There is a pattern here. Evolutionary arms races are most intense in the supercontinent of Laurasia (eventually joined by India and Africa). The island continents of South America and Australia stand apart, and they fare poorly when they start exchanging fauna with the rest of the world. We’ll see a similar pattern – large areas stimulate more competition, and more intense evolution, isolated areas are at a disadvantage – when we look at modern history, with ocean voyages effectively reuniting Pangaea. (This is a major theme of Alfred Crosby’s Ecological Imperialism and Jared Diamond’s Guns, Germs, and Steel.)

Coals to Newcastle

339-321 Mya

It seems like Gaia really went on a bender in the late Carboniferous, getting drunk on oxygen. By some estimates, the atmosphere was over 30% oxygen back then, compared to 21% today. Living things took advantage of the opportunity. Insects apparently face an upper limit in size because they rely on diffusion through tracheas instead of forced respiration through lungs to get oxygen into their bodies. With more oxygen in the air, this limit was raised. The Carboniferous saw dragonflies with a wingspan up to 70 centimeters, and body lengths up to 30 centimeters, comparable to a seagull.


This happened because plants were turning carbon dioxide into organic matter and free oxygen, and the organic matter was accumulating. With carbon dioxide being removed from the atmosphere, the late Carboniferous and subsequent early Permian saw a reduced greenhouse effect, and global cooling. This was another Ice Age, with ice caps around the southern pole.

A lot of organic carbon ended up being buried. Much of the world’s coal, especially high quality anthracite, has its origin in Carboniferous tropical forests. Western Europe and eastern North America lay in the tropics at the time, and got a particularly generous allotment of coal. Three hundred million years later this bounty would fuel the early Industrial Revolution. (Thanks partly to some of my Welsh ancestors, who helped dig it up back in the day.)

coal age

Snow time

744-704 million years ago

Tweeting was sparse for the past week, 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 few 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. 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.



Evidence for Snowball Earth, from Namibia. A dropstone, dropped from a melting rock laden iceberg into tropical marine sediments.


2,16-2.04 billion years ago

I turned over my compost heap today. It’s looking good now, but it took me years to get the formula right. For a long time, I just let chopped up leaves sit in a pile in a corner of the yard. Nothing much happened. Eventually I wised up and did some research: proper composting requires the right balance of carbon and nitrogen. The nitrogen and some of the carbon go into building organic molecules. Most of the carbon, though, combines with oxygen to make carbon dioxide, releasing energy in the process for the bacteria that make the compost. The amount of energy is appreciable: on a mild winter day, with air temperature just above freezing, the inside of my compost heap was the temperature of a lukewarm bath.  (My secret, if you want to know, is mixing in coffee grounds from the café down the street. The café workers are happy to fill a five gallon bucket I leave there, and to let me carry it off, every week or so. The grounds supply nitrogen that the leaf mulch is short of.)

But composting goes back way longer than my compost heap, and way longer than Homo sapiens has been around. In fact, composting may have gotten its start more than 2 billion years ago. Here’s the story:

Back when I was in college, a guy I knew was taking a course in organic chemistry. He despaired of mastering the material in time for the next exam. Another student who’d already taken the course advised him, “Just remember, electrons go where they ain’t. Everything else is details” This is a pretty good starting point for thinking about how life and Earth have coevolved. Carbon, sitting in the middle of the periodic table, with four electrons to share, and four empty slots available for other atoms’ electrons, is exceptionally versatile. It can form super-oxidized carbon dioxide, CO2, where carbon’s extra electrons fill in oxygen’s empty slots. Or it can form super-reduced methane, CH4, where carbon’s empty slots are filled by hydrogen’s extra electrons. Or it can form carbohydrates, basic formula CH2O, neither super-oxidized nor super-reduced, where the oxygen atom gives the carbon atom some extra empty slots and the hydrogen atoms give the carbon atom some extra electrons.

Chemically, life is pretty much about reducing and oxidizing carbon compounds. Nowadays, this means, on the one hand, photosynthesis – using the sun’s energy to go from carbon dioxide to carbohydrates – and, on the other hand, aerobic respiration – running the same reaction in reverse to release energy. But this modern arrangement took eons to develop. It depended on geochemistry, specifically on the evolution* of what has been called a planetary fuel cell, with an oxidizing atmosphere and ocean physically separated from buried reduced minerals. Given a breach in this separation, the oxygen and the reduced minerals will react and release energy. In future posts we’ll see how such short-circuits in the planetary fuel cell are (probably) the cause of most major mass extinctions.

The development of the planetary fuel cell was erratic. One puzzling episode is the Logamundi Event, where oxygen levels went up 2.2 billion years ago and then back down 2.08 billion years ago. One theory is that the Event began with photosynthetic cyanobacteria pumping oxygen into the atmosphere. When these bacteria died, they just piled up, leaving a growing accumulation of organic matter in the world’s oceans. According to this theory, the end of the Event happened when other bacteria discovered they could oxidize the accumulated organic matter, and used up a good part of the atmosphere’s oxygen. In other words, the end of the Logamundi Event marks the invention of composting.

The return to higher oxygen levels in the atmosphere would take some time, and deposition of reduced sediments.

*Are we allowed to call this “evolution”? Maybe.