Frozen Earth: The Once and Future Story of Ice Ages Read Online Free PDF

recent ice age comprises a long succession of ice incursions deep into Europe (although not as far as the Mediterranean) and North America, separated by much warmer periods.
    It is often not appreciated that today’s climate is just a geologically short warm spell in this continuing ice age.But in addition to the ice sheets of Greenland and Antarctica, mountainous regions today sustain permanent ice fields even in the tropics.The brilliant white cap on Mt.Kilimanjaro described by Hemingway in The Snows of Kilimanjaro is actually a permanent glacier, in spite of the fact that Kilimanjaro is only 300 km (roughly two hundred miles) from the equator.The Andes too host equatorial glaciers.If you were an astronaut circling the Earth at the end of a northern winter, you would observe that nearly half the land area and more than a quarter of the oceans were white with snow and ice.Only a fraction of this is permanent glaciers, but still, about 75 percent of all the fresh water on our “blue” planet is frozen in glaciers.Even so, in comparison with the average of the past few million years, the present-day interglacial climate is benign.The last time the Earth was as warm as it is today was about 120,000 years ago; for most of the time since then it has been much, much colder.
    All of the evidence we have about past climates suggests that the Earth has been progressively cooling for the past 50 or 60 million years.Before then, most of the world had experienced warm temperatures—the fossil remains of tropical and subtropical plants and animals from those times are found even north of the Arctic Circle.Sometime near 35 million years ago, there was an especially sharp drop in global temperatures—this is when, most researchers believe, glaciers began to form in Antarctica.However, although temperatures continued to fall as the Antarctic icecap grew, it was not until about 3 million years ago that permanent glaciers appeared in abundance in the Northern Hemisphere, again accompanied by an abrupt temperature decrease.This is generally agreed to be the start of the current ice age, and since that time, most climate changes around the globe have been associated with the waxing and waning of ice sheets in the Northern Hemisphere.Fortunately for us, the glaciers have withdrawn to high altitudes and latitudes during the present warm period.But on average, for the past few million years, the Earth has been considerably colder than over most of its four and a half billion years of existence.During much of Earth history, except for short, rare, intervals, glaciers such as the one on Kilimanjaro have been absent.In contrast, within the current ice age, warm periods with moderate climates similar to the present have been short by geological standards, generally lasting only ten to twenty thousand years.We are already about ten thousand years into the current warm episode.If history is any guide, and if human activities don’t warm the Earth too severely, the ice will return, and quite soon on a geological timescale.The sites of cities such as Montreal and Edinburgh and Stockholm, and perhaps even New York and Chicago, will be buried deep in glacial ice, as they were in the past.
    You might reasonably ask: How do we know these things?How do we know that the Earth has been cool for the past few million years, compared to the rest of its history?How do we know that the Earth is still locked in an ice age characterized by a series of advances and retreats of ice over North America and Europe?One of the aims of this book is to answer these questions, and also to delve into some of the history behind the answers.In doing so, I hope also to illustrate thestartling ingenuity of some of the scientists who have investigated such questions, and the deep curiosity about how the Earth works that has pushed them toward their goals.And also to show why such work is important for understanding—and perhaps even shaping—man’s impact on our small