A short history of nearly everything - Bill Bryson [215]
Throughout all this the tubeworms and clams and other life forms adhering to deep ocean vents undoubtedly went on as if nothing were amiss, but all other life on Earth probably came as close as it ever has to checking out entirely. It was all a long time ago and at this stage we just don't know.
Compared with a Cryogenian outburst, the ice ages of more recent times seem pretty small scale, but of course they were immensely grand by the standards of anything to be found on Earth today. The Wisconsian ice sheet, which covered much of Europe and North America, was two miles thick in places and marched forward at a rate of about four hundred feet a year. What a thing it must have been to behold. Even at their leading edge, the ice sheets could be nearly half a mile thick. Imagine standing at the base of a wall of ice two thousand feet high. Behind this edge, over an area measuring in the millions of square miles, would be nothing but more ice, with only a few of the tallest mountain summits poking through. Whole continents sagged under the weight of so much ice and even now, twelve thousand years after the glaciers' withdrawal, are still rising back into place. The ice sheets didn't just dribble out boulders and long lines of gravelly moraines, but dumped entire landmasses—Long Island and Cape Cod and Nantucket, among others—as they slowly swept along. It's little wonder that geologists before Agassiz had trouble grasping their monumental capacity to rework landscapes.
If ice sheets advanced again, we have nothing in our armory that could deflect them. In 1964, at Prince William Sound in Alaska, one of the largest glacial fields in North America was hit by the strongest earthquake ever recorded on the continent. It measured 9.2 on the Richter scale. Along the fault line, the land rose by as much as twenty feet. The quake was so violent, in fact, that it made water slosh out of pools in Texas. And what effect did this unparalleled outburst have on the glaciers of Prince William Sound? None at all. They just soaked it up and kept on moving.
For a long time it was thought that we moved into and out of ice ages gradually, over hundreds of thousands of years, but we now know that that has not been the case. Thanks to ice cores from Greenland we have a detailed record of climate for something over a hundred thousand years, and what is found there is not comforting. It shows that for most of its recent history Earth has been nothing like the stable and tranquil place that civilization has known, but rather has lurched violently between periods of warmth and brutal chill.
Toward the end of the last big glaciation, some twelve thousand years ago, Earth began to warm, and quite rapidly, but then abruptly plunged back into bitter cold for a thousand years or so in an event known to science as the Younger Dryas. (The name comes from the arctic plant the dryas, which is one of the first to recolonize land after an ice sheet withdraws. There was also an Older Dryas period, but it wasn't so sharp.) At the end of this thousand-year onslaught average temperatures leapt again, by as much as seven degrees in twenty years, which doesn't sound terribly dramatic but is equivalent to exchanging the climate of Scandinavia for that of the Mediterranean in just two decades. Locally, changes have been even more dramatic. Greenland ice cores show the temperatures there changing by as much as fifteen degrees in ten years, drastically altering rainfall