Sunday, August 17, 2008

Green means everything

kw: book reviews, nonfiction, evolutionary theory, plants

In 1980 Plate Tectonic theory was sufficiently established to be accepted dogma (as it still is) but new enough that great uncertainties surrounded nearly every major hypothesis. One entire course was devoted to studying when plate motions began and how plate tectonics might have evolved through time. As I recall, the fact that radioactive heating of the earth was six times greater than now, some four billion years ago, was never mentioned.

However, it is a smaller incident that comes to mind today. In a follow-on course I happened to remark that the existence of life, that is the actions of the biosphere, must have had a great effect on the rates of weathering and sedimentation, and may have actually multiplied the rate at which the continents accumulated. The professor objected to this point strongly, and the debate led, over time, to my receiving a B in the course rather than my customary A. This was but one of two Geology courses for which I did not receive the top mark while in graduate school.

As it happens, for more than half the history of life on earth, the biosphere has consisted primarily of bacteria. But by two billion years ago (and perhaps still at one billion), though the oceans were bluish-green with cyanobacteria, the visible color of Earth's continents was still the grayish brown of completely inorganic soils. How things have changed!

Algae of many colors must have brightened up coastal margins, but it was only with the evolution of true plants that living beings came ashore and began to colonize the land. The green-on-blue look of Earth today, shown here in a GOES-8 image from NASA, illustrates that more than 95% of the biomass is now plant life. In The Emerald Planet: How Plants Changed Earth's History author David Beerling explains the part plant life has played, primarily over the past half billion years, and argues for a greater understanding of plants as one of the great driving forces of planetary evolution.

Plants harvest sunlight and so form the basis of all non-bacterial life on earth. So imagine a planet without them, and perforce, without any animals either. Oh, there are perhaps stromatolites in the ocean shallows, and similar bacterial mats in pools and streams of fresh water. But the land surface is no different from that of Mars: brown or reddish-brown, sandy and dusty, or gritty-muddy on a rainy day (as Mars's land would be if it would ever rain there). The cloudiness of the skies might be quite different...I can't say just how. But there would be probably ten to twenty times as much carbon dioxide in the atmosphere, and at most one-third of the oxygen seen today. This bacterial world would be much, much different.

However, I would make one change to the book's title: ...How Plants Determined Earth's History. This is not too strong an expression. For most of the past half billion years, particularly since the evolution of leaves about 450 million years ago, plants have covered much of the land, greatly increasing weathering rates with organic acids, yet holding soils so that bulk erosion rates decreased even as chemical erosion increased.

Carbon dioxide has been successively drawn down, though not at a steady rate. Rates of volcanism are a primary driver of CO2 production, and particularly during the Carboniferous and Permian, this fertilizing gas was produced at much greater rates than at present. In addition, plant remains were being incorporated into sediments at unprecedented rates, which led on the one hand to the Coal Measures, and on the other to a much greater level of Oxygen in the atmosphere, and thus to a higher atmospheric pressure.

Various studies have confirmed that oxygen, which today forms one-fifth of the air, was greater than one-third some 300 million years ago. The nitrogen amount was the same, so this thicker, energy-rich air supported an age of gigantic insects, such as the three-foot wing span dragonfly Meganeura. A gigantic mass extinction episode 250 million years ago nearly eliminated the biosphere, and put an end to the era of big bugs.

200 million years later, the continents were differently configured, and various factors combined to add "extra" greenhouse gases such as nitrous oxide and ozone to the lower atmosphere, multiplying the effect of a rising amount of carbon dioxide. This "Eocene optimum" (optimum for who? I wonder) produced alligators in arctic and antarctic areas, and tropical forests that, paradoxically, had to survive months-long winter darkness, but not winter cold. The average temperature of the planet was 18°C greater than today, primarily because of greater polar warmth.

Finally, the evolution of C4 grasses, which use a more effective type of photosynthesis, led eight million years ago to the spread of savannas in place of dry land forests, and into some semi-arid areas that could not be forested. The author does not mention this, but I wonder if the spread of savannas led to certain chimp-like apes finding it beneficial to stand upright, so they could see over the grasses. Chimps today live in the forest, where there aren't tall grasses to overtop; they would gain little advantage by standing up. Is Grass a parent of Humanity?

And a final question: Is the current series of Ice Ages fundamentally a result of the great draw-down of carbon dioxide that began eight million years ago, and if so, will it continue until the Sun heats up enough to overcome it? Our "global warming" fears ought to center not on the gases we emit, but on the Sun on which we depend.

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