Tag Archives: climate

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 last year.

Ice Age Gear Shift

Today’s date, about 800,000 years ago, sees 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. (And in Africa 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 last year.

Ginormous, or The Canseco Conjecture

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 CO2 levels. 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?”

http://phenomena.nationalgeographic.com/2013/02/25/dinosaur-reproduction-not-ancient-gravity-made-sauropods-super-sized/