They may have puzzled somewhat over my insistence that the radially symmetrical among us pose a significant threat to the world hegemony of the bilaterally symmetrical majority, but on the whole, I recall that posts on marine biology, archaeology, and climatology were well received.
So, to reintroduce an old stream of thought, I present to you,
describes the coldest global climate imaginable - a planet covered by glacial ice from pole to pole. The global mean temperature would be about -50°C (-74°F) because most of the Sun's (Solar) radiation would be reflected back to space by the icy surface...The average equatorial temperature would be about -20°C (-10°F), roughly similar to present Antarctica. Without the moderating effect of the oceans, temperature fluctuations associated with the day-night and seasonal cycles would be greatly enhanced. Because of its solid surface, the climate on a snowball earth would have much in common with present Mars.
...oceans smothered by a freezing white blanket nearly a mile thick. Vast glaciers creep across the continents. Nothing else moves. There are no clouds, save perhaps a handful of high wispy streaks made from frozen crystals of carbon dioxide. With the temperature a chilling 40 degrees below zero, only a few living things survive. Algae cling to the meager warmth of volcanic springs, and bacteria eke out a living around hot-water vents deep in the ocean. For millions of years, nothing changes.
This is no far-off planet or alien moon. It's a view of Earth just a few hundred million years ago. A growing number of scientists believe that, after billions of years of comfortable existence, Earth suddenly plunged into a winter so extreme that it makes recent ice ages look warm. Then just as suddenly the ice melted again, and the planet sweltered in a climatic backlash of intense heat. Between 750 million and 590 million years ago, supporters of the "Snowball Earth" theory say, the climate may have swung back and forth between deep freeze and hothouse as many as four times. What's more, they say, the ice might be the reason complex animals like us are around today.
The theory goes, so Wiki tells us, that the Earth was entirely covered by ice in part of the Cryogenian period (850 to 630 million years ago) of the Proterozoic eon, the period before the first abundant complex life on earth. According to the exponents of the hypothesis multicellular evolution began to accelerate after the last big freeze ended.
This will give an idea of how long ago that was:
we're in the teeny tiny coloured bit at the very end.
It was after the thaw, that we got the Cambrian explosion, around 530 million years ago, in which the apparently abrupt appearance and rapid expansion of multicellular life forms started. cf. the Burgess shale.
The Cambrian explosion is interesting since it was only after it that "evolution" or the rapid complexification of species really got going. Before about 580 million years ago, most organisms were simple, composed of individual cells occasionally organised into colonies. "In the following 70 million to 80 million years, the rate of evolution accelerated by an order of magnitude, and the diversity of life began to resemble today’s." (Maybe somebody Said something.)
Charles Darwin said it was one of the main objections that could be made against his theory of evolution by natural selection.
The melting of the Snowball, about 590 million years ago, falls close to the time when most scientists think complex life evolved on Earth. For 3.5 billion years before then, life had been nothing but simple, single-celled bacteria and algae. Then, suddenly, rocks start showing traces of the first multicelled animals. Hoffman and Schrag believe the change was triggered by ice. Perhaps by killing off most of the one-celled creatures in the oceans, the worldwide freeze made room for new forms of life to thrive after things warmed up. Perhaps complex creatures were better at surviving the harsh conditions.
I note with interest that the proponents of the snowball earth theory mention, with parenthetical nonchalance that, as everyone knows, "the global climate has cooled dramatically over the last 50 million years..."
I have to admit, though, that the journalist in me wishes these sciencey people would learn to stick a few "executive summaries" into their explanations.
I got as far as
"The sample, collected by UCLA paleontologist Bruce Runnegar, was a reddish, uncompacted, rhythmically laminated siltstone from the Elatina Formation, a late Neoproterozoic, glacial and periglacial unit widely exposed in the Flinders Ranges and elsewhere in South Australia (Preiss, 1987; Lemon and Gostin, 1990). The rhythmic laminations are interpreted to be lunar tidal bundles (Williams, 2000), implying a shallow marine depositional environment. Glacigenic deposits (diamictites and ice-rafted dropstones) occur in most sections of the Elatina Formation..."
and started kind of glaciating over myself.
Most of it is over my head, but the bits I understand (or more likely think I understand) are pretty cool.
(With thanks to Fr. PJM)