Over most of the unimaginably long history from our distant prehuman precursors to our more immediate and fully human ancestors, we have been passive (or at best reactive) participants in whatever kinds of changes in climate that nature has thrown at us. Our remote ancestors were simply too few in number, too unsophisticated in technology, and too transiently present in any region to leave significant footprints on the landscape or alter the climate system. Climatic changes over these immense spans of time were entirely under nature's control.
These spans of time are beyond imagining. I suspect that even those of us who spend our entire careers studying aspects of Earth's history have no real grasp of their immensity. The last 2.5 million years since the time our immediate ancestors (hominids) first appeared on Earth is a tiny portion (less than 0.1%) of this planet's entire history, yet even it is far beyond the spans of not just our individual lives but also all written human history (covering little more than 2,000 years). As students entering this field, we learn early on to store this information in a mental file cabinet marked "time," with drawers labeled as billions, millions, and thousands of years ago.
In this way we file away the basic outline of Earth's story: Earth formed 4.6 billion years ago. . . . The atmosphere became oxygen-rich nearly 2.2 billion years ago. . . . Marine organisms first left hard fossil shells after 600 million years ago. . . . Complex life on land arrived by 400 million years ago. . . . All the continents were mashed together into the giant supercontinent Pangaea ("all Earth") between 325 and 250 million years ago, and ice sheets formed on the parts of it near the South Pole. . . . Pangaea broke up after 175 million years ago, and the Atlantic Ocean came into being, while a once-larger Pacific Ocean shrank . . . No ice sheets existed on land at the South Pole 100 million years ago in a climate that was warmer than today. . . . India broke loose from Antarctica about 70 million years ago and began slowly crunching into Asia 20 million years later, heaving up the Tibetan Plateau and the Himalaya. . . . Near that time, ice appeared on Antarctica. . . . The closing of the Isthmus of Panama joined North and South America by 4 million years ago, and, soon after, ice-age cycles began in the Northern Hemisphere.
Later, we choose particular intervals to study, and we learn the detailed sequences of geological and climatic changes within those intervals. By degrees, and by repetition, we become thoroughly familiar with the interval we investigate, and eventually we become "experts." But do we really comprehend the immensity of all that time? I doubt it.
Still, time provides us with information vital to the detective story told here. The greatest success story to emerge to date from the fourth great earth science revolution—the study of Earth's climatic history—is an understanding of the causal links between relatively small changes in Earth's orbit and relatively large changes in its climate. This important discovery arose from the convergence of knowledge from two very different disciplines: geology and astronomy. And this new understanding has two major facets: the cause of ice-age cycles that have dominated climate change at north polar latitudes, and the cause of fluctuations in the monsoons that have prevailed in the tropics.
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