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S itting at a sidewalk café in Yellow Springs, Ohio, I was surprised to feel a drop of perspiration sliding down my temple. Moving that pen around the page hardly qualiWed as sweat-producing activity, although I suppose drinking hot coffee on a muggy July day did. Over the next few minutes, I felt tension building—in the air and in me. As the humidity grew more stiXing, my body responded by producing bolts of impatience along with beads of sweat. Why couldn’t that person keep her dog quiet? And what was wrong with this stupid pen? Suddenly, raindrops began to materialize in front of my eyes, as if arriving from another dimension. I had never before seen air so sodden that raindrops took shape at eye level. I laughed in relief as the oppression of one hundred percent humidity lifted. Every ounce of liquid rain meant that much less water weighing down the air as vapor. As the spattering drops multiplied into a downpour, I grabbed my notebook and retreated inside. That experience comes to mind now as an apt comparison between two different bodies’ natural defenses against overheating—our own and our planet’s. As temperatures rise with global warming, we’ll sweat more. Similarly, Earth will produce more rain. Both responses have cooling effects. The Earth has other ways of regulating temperature, too. Excess heat spirals into stronger hurricanes and bigger Xoods. Forests invade grasslands and tundra. Melting glaciers spur the sea to rise up and claim more ground for wetlands. In fact, all these things are happening now. Much as humans produce sweat to cool off on a hot day, Earth produces hurricanes and Xoods, wetlands and forests to cool off during a hot century. And this promises to be an unusually hot century. Because of all the greenhouse gases we’re producing around the world, the question is not whether the planet’s temperature will increase overall—it’s how high the mercury will rise, and how quickly. By the end of this century, we face a global temperature change akin to the difference Y Introduction The Sweat of the Earth between an ice age and an interglacial warm period. This time around, though, we’re already starting from an interglacial warm period. Also, this time around, our own actions will affect how much the Earth warms. At this point, we’re not necessarily holding a one-way ticket to hellish heat. Steps we take could slow down the temperature trajectory. They can also moderate local impacts of global warming. Our actions do matter—especially if we follow the planet’s lead. The Earth’s regulatory system contains some features that can dampen the temperature rise and make it less dire for life, both locally and globally. The concept that the planet has some means of controlling its temperature comes from Gaia theory, proposed by James Lovelock in the mid-1960s with some later reWnement by collaborator Lynn Margulis. With help from the catchy name that refers to the Greeks’ Earth goddess , “Gaia” has caught on as an idea that the planet is a living system. For this book, the most relevant aspect of the many-faceted theory involves Earth’s ability to keep the global climate within a livable range. In other words, the biosphere—the interconnected web of life inhabiting Earth’s land, air, and sea—doesn’t just sit around passively, making do with the existing environment. It takes an active role in moderating the environment, including the climate, in ways that promote life’s continued survival. Rainfall and other physical mechanisms also contribute to the balancing act. For the past roughly half a billion years, since life branched out to include stemmed plants and insects, Earth’s temperature has remained within a livable range. It has shifted between hot and cold without reaching extremes that could turn the world into a snowball or a steambath. Life has clearly played a role in some of these temperature adjustments. Keep this in mind, though: Gaia keeps conditions livable—not necessarily comfortable. Clearly Earth lacks the precise type of climate-control system found in some modern buildings, with ambient temperature continually adjusted to around 70 degrees Fahrenheit. Temperature can lurch from one extreme to the other, in space and in time. Gaia’s climate-control system allows the tropics to persist in never-ending summer and the poles to face relentless winter while the rest of the globe marches through changing seasons. Gaia...

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