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all acting on different timescales. The need for an understanding of Earth System functioning that is firmly rooted in knowledge of the past is essential.⁴

      To achieve that understanding, researchers needed information about not just a few decades or centuries, but about tens and hundreds of thousands of years for which there are no written or instrumental records. When the IGBP started work, scientists knew some of the deep history of climate in broad outline—when ice ages had occurred, for example—but detail was lacking. During the 1990s, scientists associated with PAGES conducted unprecedented studies into the physical records that global change leaves behind, including tree rings, coral reefs, ocean and lake sediments, and especially glaciers in which ice has been accumulating in layers for millennia. New methods of extracting and analyzing deep cores from glaciers provided a wealth of new data on the history of temperature, atmospheric composition, ocean levels, and more.

      Two cores, each over 3,000 meters deep, were drilled in Greenland early in the 1990s—they provided a record of conditions going back 100,000 years. Later in the decade, a French-Russian team working in the Vostok region of Antarctica extracted and analyzed a core that was 420,000 years old at its deepest point. Data from the Vostok study, published in 1999, has been described as “arguably among the most important produced by the global change scientific community in the twentieth century.”⁵ Subsequent drilling has extended the record to 800,000 years. It is no exaggeration to say that this research has revolutionized our understanding of Earth’s past—and consequently, that it has revolutionized our understanding of Earth’s present and future.

      It has been known since the 1850s that small amounts of carbon dioxide in the atmosphere help to control Earth’s temperature—CO₂ lets sunlight in, but won’t let heat out. If the greenhouse effect did not exist, Earth’s average temperature would likely be about 35°C colder than it is now, far colder than in the most extreme ice ages. We now also know that carbon dioxide constantly cycles between atmosphere and oceans, keeping the overall levels roughly stable.

      It has also been long known that the angle at which sunlight hits Earth changes slightly over periods of approximately 100,000, 40,000, and 20,000 years—cycles produced by complex combinations of very slow changes in the shape of Earth’s orbit and the tilt and orientation of the planet’s axis. Climatologists have long believed that these Milankovitch cycles (named after the Serbian engineer who painstakingly calculated them in the 1920s) must play a role in the coming and going of ice ages, but the solar energy changes involved are simply too small to have had so much effect by themselves.

      Detailed analysis of the composition of 800,000 years of Antarctic ice has now shown that the two apparently separate processes—wobbles in space and the terrestrial carbon cycle—are in fact closely linked as fundamental components of the Earth System. To oversimplify: the small amounts of cooling or warming caused by Milankovitch cycles act as triggers that cause CO₂ to be absorbed or released by the oceans, producing “changes that are abrupt and out of all proportion to the changes in incoming solar radiation.”⁶

      At the Amsterdam Global Change conference in 2001, the chair of the IGBP’s Scientific Committee, Berrien Moore, pointed out that the cycles found in the Vostok ice core show a remarkably consistent pattern over hundreds of thousands of years:

      The repeated pattern of a 100 ppmv [parts per million by volume] decline in atmospheric CO₂ from an interglacial value of 280 to 300 ppmv to a 180 ppmv floor and then the rapid recovery as the planet exits glaciation suggests a tightly governed control system with firm stops at 280–300 and 180 ppmv. There is a similar CH₄ [methane] cycle between 320–350 ppbv [parts per billion by volume] and 650–770 ppbv in step with temperature.⁷

      IGBP executive director Will Steffen wrote that “no record is more intriguing than the rhythmic ‘breathing’ of the planet as revealed in the Vostok ice core records.” The “remarkably regular planetary metabolic pattern embodied in the Vostok ice core” provided “a fascinating window on the metabolism of Earth over hundreds of thousands of years.”⁸

      The exact mechanisms of this “tightly governed control system” are still not fully understood, but there is no doubt that atmospheric CO₂ is the control knob on Earth’s thermostat.

      External factors can disrupt these cycles. About 56 million years ago, for example, a massive release of buried carbon dioxide, probably triggered by super-volcanoes or a comet collision, overwhelmed the normal process, increasing global temperatures by 5 to 9°C in a geological instant. It then took about 200,000 years for the excess CO₂ to be reabsorbed, and for temperatures to return to normal.⁹

      The amount of CO₂ released in that episode was about equal to what will be produced if we burn all remaining reserves of coal, oil, and natural gas. Today’s situation is different in many ways, so we should not expect a replay, but one important similarity should be noted. As Berrien Moore went on to say, atmospheric CO₂ levels are now far out of their normal range:

      Today’s atmosphere, imprinted with the fossil fuel CO₂ signal, stands at nearly 100 ppmv above the previous “hard stop” of 280–300 ppmv. The current CH₄ value is even further (percentage-wise) from its previous interglacial high values. In essence, carbon has been moved from a relatively immobile pool (in fossil fuel reserves) in the slow carbon cycle to the relatively mobile pool (the atmosphere) in the fast carbon cycle.¹⁰

      Figure 4.1 (page 65) illustrates the point. As the IGBP says in Global Change and the Earth System, “Human-driven changes are pushing the Earth System well outside of its normal operating range.” And as climate change historian Spencer Weart says, learning the causes of ice ages showed that “the system is delicately poised, so that a little stimulus might drive a great change.”¹¹ Burning fossil fuels has disrupted the carbon cycle, and global warming is an inevitable result—the questions are: how much, and how fast?

       Tipping Points

      The transformation of quantity into quality has been a fundamental postulate of dialectics for two centuries. Hegel stated and explained it as a law of thought; Marx and Engels applied it to the material world. Small changes accumulate, creating ever greater complexity, until the object or being or system suddenly shifts from one state to a radically different one, in what is often called a phase change. Water is a liquid until its temperature reaches 100°C when it becomes a gas. Overfishing produces large catches, right up until the fish population abruptly collapses. Social and economic stresses accumulate gradually until a revolutionary upsurge imposes a new social order, qualitatively different from the old society.

      Few scientists today are familiar with dialectics, and even fewer use it consciously, but the fundamental dialectical concept of the transformation of quantity into quality has been absorbed into scientific thought under labels such as emergence, quantum leaps, and punctuated equilibrium.

      Colloquially, those transitions are called “tipping points,” a term originally used by physicists for the point at which adding weight or pressure to a balanced object suddenly causes it to topple into a new position. In the Earth System, tipping points are not unusual—they are the norm.

      Until a few decades ago it was generally thought that large-scale global and regional climate changes occurred gradually over a timescale of many centuries or millennia, scarcely perceptible during a human lifetime. The tendency of climate to change relatively suddenly has been one of the most surprising outcomes of the study of earth history.¹²

      Despite this change in the scientific understanding of the climate, in most accounts, including the reports of the Intergovernmental Panel on Climate Change, there is an unspoken assumption that climate change will be gradual. The twenty-first century will be a warmer, stormier, and less biodiverse version of the twentieth—less pleasant, but not fundamentally different. As research commissioned by the U.S. National Research Council points out, that assumption leads to particular conclusions about society’s ability to respond to change:

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