What Has Controlled Earths Climate

Imagine Earth viewed from a satellite. Blue oceans cover more than two-thirds of the planet and brown or green land the rest. White ice sheets over a mile thick bury a small fraction of the land (Antarctica and Greenland). Whitish sea ice forms a cap a few feet thick over the polar oceans, and its seasonal fluctuations in the two hemispheres occur at exactly opposite tempos (one large when the other is small). Surrounding everything is a thin blue envelope of atmosphere with swirls of clouds.

In comparison to these fundamental and massive parts of the natural climate system, the largest structures built by humans are insignificant to or even undetectable by the unaided eye. Pyramids, dams, and roads are invisible from space without high-powered telescopes. On the side of Earth lying in the dark of night, even the brightly lit cities are just tiny islands of light.

From this perspective, the possibility that humans could have any major impact on the workings of these vast parts of the climate system sounds ridiculous. How could we possibly cause changes in the size of these immense regions of blue and green and white? Yet we are. No credible climate scientist now doubts that humans have had an effect on Earth's climate during the last two centuries, primarily by causing increases in the concentrations of greenhouse gases like carbon dioxide and methane in the atmosphere. These gases trap radiation emitted from Earth's surface after it has been heated by the Sun, and the added heat retained in Earth's atmospheric envelope makes its climate warmer.

Because increases in both greenhouse gases and Earth's temperature during the last century have been measured, the so-called global warming debate is not about whether humans are warming climate or whether we will warm climate in future decades—we are warming it, and we will warm it more in future decades as greenhouse-gas concentrations rise.

The only issue under serious debate is: By how much? Will we make Earth's climate only slightly warmer, a change that might be hardly noticeable? Or will we alter the climate system in much more extensive ways, for example by melting most of that white sea-ice cover near the North Pole and turning the Arctic to an ocean blue? For now, the answer to this question of "how-much" is not so clear.

Another part of the global-warming debate is whether these changes will be "good" or "bad." This question has many answers, all of which turn on the value system of the person asking it. The world is complicated; no single answer of good or bad is sufficient when the many complexities of such an issue are taken into account. But most of the story this book has to tell is not about highly charged political or media debates under way today and forgotten a few years hence. The focus here is on what we can learn from the past.

For most of the time that human beings and our recognizable ancestors lived on Earth, we did not affect climate. Few in number, and moving constantly in search of food and water, our Stone Age predecessors left no permanent "footprints" on the landscape for several million years. Throughout this immensely long span of time, climate changed for natural reasons, primarily related to small cyclical changes in Earth's orbit around the Sun. Nature was in control of climate.

But the discovery of agriculture nearly 12,000 years ago changed everything. For the first time, humans could live settled lives near their crops, rather than roaming from area to area. And gradually, the improved nutrition available from more dependable crops and livestock began to produce much more rapid increases in population than had been possible in the earlier hunting-and-gathering mode of existence. As a result, the growing human settlements began to leave a permanent footprint of ever-increasing size on the land.

If you could watch a time-lapse film showing Earth's surface since agriculture began, you would see a subtle but important change spread across southern Eurasia during the last several thousand years. In China, India, southern Europe, and northernmost Africa, you would see darker shades of green slowly turning a lighter green or a greenish brown. In these areas, the first villages, towns, and cities were being built, and vast areas of dark-green forest were slowly being cut for agriculture, cooking, and heating, leaving behind the lighter-green hues of pastures or the green-brown of croplands.

Until very recently, scientists thought that humans first began altering climate some 100 to 200 years ago, as a direct result of changes brought about by the gassy effusions of the Industrial Revolution. But here I propose a very different view: the start of the switch-over from control of climate by nature to control by humans occurred several thousand years ago, and it happened as a result of seemingly "pastoral" innovations linked to farming. Before we built cities, before we invented writing, and before we founded the major religions, we were already altering climate. We were farming.

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