It is prudent, both intellectually and practically, to accept that the atmosphere and oceans are indeed warming, as the evidence tells us, and that this trend will accelerate in the decades ahead. While we do not and cannot know just how much warming will occur, nor how fast, we can safely say that the rapidity of warming, now and in all likelihood over the next decades, has few precedents in the history of the Earth and none in the history of civilization. This is true regardless of which of the three versions of the future offered in this book one prefers.
No instrumental records exist for prior episodes of climate change. The proxy evidence used for the reconstruction of climate history— measurements of fossil pollen, foraminifera, tree rings, oxygen isotopes in air bubbles trapped in ancient ice, and other tools—can give a good but not precise idea of past temperature and precipitation patterns.
Earth's climate has never been static. For the past 2.7 million years, it has shown a pattern of alternating long ice ages and shorter interglacials, governed by cycles in the Earth's orbit around the sun. The last ice age was at its height around 20,000 years ago. Its end (circa 11,000 to 6,000 years ago) was probably crucial for human history as it coincided with the emergence of agriculture in multiple locations. After that bout of warming—generally much slower than what we have witnessed in the last hundred years but not without sudden lurches now and again—global climate changed only modestly and slowly until the industrial age.1 Although our Paleolithic ancestors did have to cope with rapid climate change from time to time, when they did so the Earth had fewer people (or hominids) than Chicago has today, and they were accustomed to migrating with their scant possessions as a matter of course. Their response to adverse climate change (as to much else) was to walk elsewhere. Since the emergence of agriculture, seden-tarism, civilization, and the settlement of all habitable parts of the globe, the Paleolithic response has become more and more impractical. Thus, although there are analogues in Earth's history for the climate change now under way, there are none in human history. We have entered uncharted terrain.
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