Thursday, June 28, 2007

Meet the mother

... or speak with the Earth, and she will teach you. - Job 12

By now, it must be clear that something special is happening on the Earth that keeps its climate within certain bounds. This "something special" clearly involves more than weather per se; it also involves plant life as well as oceans, soil, and other geophysical processes, plus certain basic gross features of the Earth. Certainly compared with its terrestrial planet siblings in the inner Solar System - we looked at Venus and Mars, but didn't bother with Mercury or the Moon, because neither has an atmosphere - the Earth's configuration of geo- and biophysics seems unusual, if not unique.

In the 1980s, when scientists first began to think about other planets and life in a systematic way based on comparisons with other planets in our system (later, with planets orbiting other stars as well), this striking fact became so obvious that it cried out for a name. So biologists James Lovelock and Lynn Margulis coined one: they called it Gaia, an archaic form of the Greek goddess of the Earth, spelled that way, I suppose, to get people to pronounce it correctly - not Gea or even the Latinized Gaea. Perhaps they were being humorous, although they must have known that using such terminology would bring the New Age weirdos out the woodwork. Maybe they were looking to cash in, living as we do in a media- and celebrity-saturated society with many spiritual seekers dissatisfied by traditional religions.

Gaia is sometimes and misleadingly called a hypothesis, which it certainly is not: everything you know about the biogeosphere tells you it's real now. But it's when you consider the Earth's history that Gaia really hits you over the head. Consider the striking fact that the Earth is a little over 4 billion years (4 Gyr) old, and life appeared on it at least as far back as 3.5 Gyr and liquid water at least as far back as 3.8 Gyr. Now consider that the Sun was much fainter earlier in its 5-Gyr history back then, growing steadily brighter with time. Life on Earth required liquid water, which must have kept from freezing by a temperature-enhancing mechanism like those we know today - or perhaps a different one no longer operative, such as large amounts of other infrared-active gases like methane (CH4) and ammonia (NH3).

We've exhaustively examined the "Goldilocks" climate regime (which is really a range of possible climates), which runs from Ice Ages to mildly tropical. (And don't be deluded by the "global warming" hysteria into thinking that the Ice Ages have ended - they haven't.) Two other, quite different possible Earth climate regimes (ranges) are known. One was touched on briefly months ago: the humid but almost cloud-free supertropical Earth of about 304 oK = 31 oC = 89 oF. The other, so far unmentioned, is the "snowball Earth": mostly covered in ice, which reflects incoming solar radiation as effectively as do clouds, an Earth of less than about 245 oK = -28 oC = -18 oF. Earth has avoided both fates.*

A lot of what goes under the name of Gaia has already been covered - in some sense, we're just slapping a label on a collection of mechanisms and features of the Earth. Where the real Gaian controversy comes is when the role of life and consciousness comes in. I should make it emphatically clear right away that the "life" that mainly matters for Gaia is simple but ubiquitous plant life - nothing so sophisticated as an insect, not to speak of mammals or humans. "Ubiquitous" beats "sophisticated" on most days. So don't get any ideas :-)

Clearly, a complex web of feedbacks are operating on the Earth. We've seen some of them, but by no means all. When you hear "feedback," you sometimes think "alive" or even "conscious" - but Gaia doesn't mean that the Earth as a whole is conscious or even living. That's really getting off into the mystical and well beyond what the science justifies. Some people have tried to do that, including scientists who should know better. The feedbacks here are not all-powerful or mystical and don't by themselves imply consciousness or life. "Feedback" is a more primitive and general concept. But that doesn't stop some people from trying to turn the Gaia thing into a religion - which it isn't.

There are also some gross physical factors involved in Earth's unique biogeosphere that don't involve feedbacks. Its orbital position (close to the Sun, but not too close), its surface gravity, and the continuation of plate tectonics through its whole history are also essential. Additional factors that enhance stable long-term evolution of life include the presence of a large moon and a nearby giant (Jupiter) that sweeps up dangerous comets and asteroids.**

Gaia also has a long but finite life. In a few billion years, the Sun, continuing to brighten, will become so strong that the liquid surface oceans unique to Earth will boil away. That's probably what happened to any liquid water on Venus. We can only hope that our descendants will have terraformed Mars - which will be quite balmy by that point - or left the Solar System altogether for a better spot.

Is the Earth unusual? Almost certainly. Is it unique in the Galaxy or in the Universe? No one has the faintest idea for sure, but probably not. Is such a collection of stabilizing feedbacks a sign of purposive causality, or teleology? Maybe. But if it is, it's like Aristotle's teleology: completely naturalistic - there's nothing supernatural about it, in the literal meaning of that word: something intruding from another reality that suspends natural laws. It's about how those natural laws and the peculiar conditions on Earth fit together to produce a wonderful and unexpected result.
Crescent Earth from unmanned Apollo 4 (1967)* Or maybe not. There's strong evidence that about 600 Myr ago, the Earth did go through a "snowball" phase. But even then, the magical properties of water saved the day (or saved the epoch). Solid water (ice) is less dense than liquid, an unusual material property. So ice floats on top of liquid water, and the surface of an ocean or lake can freeze while life continues in the liquid below - amazing. That's enough for me to believe in a just and benevolent G-d on alternate Tuesdays and Thursdays.

** Our moon is the largest relative to its primary parent planet in the Solar System. There other moons that are larger in absolute size, like Titan - but their parent primaries are much, much larger (Saturn in Titan's case).

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