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Tipping Points, Climate Chaos, and Planetary Boundaries

The Anthropocene raises a new question: What are the non-negotiable planetary preconditions that humanity needs to respect in order to avoid the risk of deleterious or even catastrophic environmental change at continental to global scales?

—JOHAN ROCKSTRÖM¹

After listing recent critical changes to the Earth System, the authors of Global Change and the Earth System insisted that such lists do not give the whole picture: “Listing the broad suite of biophysical and socioeconomic changes that is taking place fails to capture the complexity and connectivity of global change since the many linkages and interactions among the individual changes are not included.” The listed crises, and others, reinforce and transform one another, producing complex “syndromes of change,” and “many changes do not occur in a linear fashion, but rather, thresholds are passed and rapid, non-linear changes ensue.²

That understanding of ecological volatility, a recent development in Earth System science, is a direct result of IGBP projects conducted around the world in the 1990s.

The Past as Guide to the Future

From the early 1990s, the International Geosphere-Biosphere Program organized its work into nine projects that focused on broad aspects of the Earth System, including terrestrial ecosystems, atmospheric chemistry, and ocean ecosystems. Each project included a multitude of specific studies conducted by scientists around the world.

All the projects contributed to IGBP’s goal of producing an integrated picture of the nature and direction of global change, but arguably the most important, in both objectives and results, was the Past Global Changes (PAGES) project, charged with “providing a quantitative understanding of the Earth’s past climate and environment.”³ The importance of this work can be stated simply: we cannot understand the dynamics and direction of today’s changing Earth unless we know how current conditions differ from those of the past:

Understanding the expression, causes, and consequences of past natural variability is of vital concern for developing realistic scenarios of the future. Moreover, the complex interactions between external forcings and internal system dynamics on all timescales implies that at any point in time, the state of the Earth System reflects not only characteristics that are an indication of contemporary processes, but others that are inherited from past influences, 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:

FIGURE 4.1: Global Climate Change

450,000 years of atmospheric CO₂. The dotted line indicates the upper bound of natural CO₂ variation as found in the Vostok Ice Core. By 1945, CO₂ was 25 parts per million above the preindustrial level; in 2015 it was 120 ppm above. Source: NASA, http://climate.nasa.gov/climate_resources/24/.

Many projections of future climatic conditions have predicted steadily changing conditions, giving the impression that communities have time to gradually adapt, for example, by adopting new agricultural practices to maintain productivity in hotter and drier conditions, or by organizing the relocation of coastal communities as sea level rises.

But the authors emphasize that the actual experience could be very different:

The scientific community has been paying increasing attention to the possibility that at least some changes will be abrupt, perhaps crossing a threshold or “tipping point” to change so quickly that there will be little time to react. This concern is reasonable because such abrupt changes—which can occur over periods as short as decades, or even years—have been a natural part of the climate system throughout Earth’s history. The paleoclimate record—information on past climate gathered from sources such as fossils, sediment cores, and ice cores—contains ample evidence of abrupt changes in Earth’s ancient past, including sudden changes in ocean and air circulation, or abrupt extreme extinction events.¹³

Many Earth System scientists argue that abrupt environmental change is not only possible, but virtually certain:

In reality, Earth’s environment shows significant variability on virtually all time and space scales…. Nonlinear, abrupt changes in key environmental parameters appear to be the norm, not the exception, in the functioning of the Earth System. Thus, global change is not likely to be played out as a steady or pseudo-linear process under any conceivable scenario but will almost surely be characterized by abrupt changes for which prediction and adaptation are very difficult.¹⁴

The Imbalance of Nature

The idea that the natural world is fundamentally stable and unchanging has a long history. In its oldest version, it is religious: God created a perfect world, and if humans disturbed that perfection, God would in time restore it. A secular equivalent was expressed in 1864 by the pioneering U.S. naturalist George Perkins Marsh:

In countries untrodden by man the proportions and relative positions of land and water, the atmospheric precipitation and evaporation, the thermometric mean, and the distribution of vegetable and animal life are subject to change only from geological influences so slow in their operation that the geographical conditions may be regarded as constant and immutable.¹⁵

That view remains influential: one of the most quoted passages in all naturalist literature is Aldo Leopold’s 1949 call for a “land ethic” based on preserving the “integrity, stability, and beauty of the biotic community.”¹⁶ It never occurred to Leopold, nor has it occurred to most of his contemporary admirers, that the natural world might be inherently unstable, subject to rapid change even in the absence of human activity.

Of course, naturalists had been aware since the mid-nineteenth century that glaciers on Earth had at least once advanced to cover much of the world with ice, and that animals now unknown had once walked the Earth, but changes of that magnitude were believed to occur extremely slowly, and to be of no relevance to human history and activity. Like the painted backdrop in a stage play, the natural world was the unchanging context, not an active player in any human drama.

That view is no longer tenable. Scientific research now shows that even in times of relative stability, like the Holocene, the Earth System is constantly changing on every scale of space and time, and that the most drastic changes often occur with remarkable speed.

Climate Chaos

In Global Change and the Earth System, Will Steffen and his colleagues wrote:

The behavior of the Earth System is typified not by stable equilibria but by strong non-linearities, whereby relatively small changes in a forcing function can push the System across a threshold and lead to abrupt changes in key functions. Some of the modes of variability noted above contain the potential for very sharp, sudden changes that are unexpected given the relatively small forcing that triggers such changes The potential for abrupt change is a characteristic that is extremely important for understanding the nature of the Earth System. The existence of such changes has been convincingly demonstrated by paleo-evidence accumulated during the past decade.¹⁷

Figure 4.2, adapted from a study of ice-core data by scientists at the Potsdam Institute for Climate Impact Research,¹⁸ shows the average annual temperature in Greenland over the past 100,000 years. Our current epoch, the Holocene, is the nearly flat segment at the top right.

Ninety percent of the time shown in that graph was the end of the Pleistocene, a 2.6 million-year-long epoch characterized by repeated glaciations and interglacial retreats: the global climate was not only cold, it was extremely variable. Modern humans walked the Earth for the entire time shown in this graph, but until the Holocene they all lived in small nomadic groups of hunter-gatherers. Climate historian William J. Burroughs, who calls that time the “reign of chaos,” argues compellingly that so long as rapid and chaotic climate change was the norm, agriculture and settled life were impossible, even in parts of the world that the glaciers never reached. To succeed, agriculture needs not just warm seasons, but a stable and predictable climate—and indeed, in just a few thousand years after the Holocene began, humans on five continents independently took up farming as their permanent way of life. “Once the climate had settled down into a form that is in many ways recognizable today, all the trappings of our subsequent development (agriculture, cities, trade, etc.) were able to flourish.”¹⁹

For 11,700 years, the average annual global temperature has not varied up or down by more than one degree Celsius. But averages can conceal large variations: despite being warm and stable on average, the Holocene has not been an unmitigated climate paradise. That one degree average variation included uncounted droughts, famines, heat waves, cold snaps and intense storms—extreme weather events in which millions of people died.

The Pleistocene was far worse: temperature variations were five to ten times greater than anything humanity has experienced since. Furthermore, as geologists Jan Zalasiewicz and Mark Williams describe, the transition from Pleistocene cold to Holocene warmth was itself an abrupt and chaotic process;

FIGURE 4.2: Average Annual Temperature in Greenland over the Past 100,000Years.

Coming from the Glacial Maximum, temperatures suddenly rose, 14,700 years ago, with the average temperature of the North Atlantic and surrounding areas increasing rapidly by some 5°C (over Greenland, the temperature hike approached 20°C). Temperatures remained around these levels for nearly two millennia—and then fell suddenly by a similar amount, as the whole region went into a deep freeze once more.

The new cold period, known to geologists as the Younger Dryas, lasted a thousand years. “Then, 11,700 years ago, temperatures suddenly soared again in another climate transformation—only this time the warm temperatures stayed, and this transition marks the beginning of the Holocene.”²⁰

How sudden is sudden? Each of the temperature jumps took a few decades, almost no time in geological terms, and not much time in human terms. More astonishing, in both cases the change in atmospheric circulation that drove the warming “seems to have been accomplished in something between one and three years.”²¹

Holocene to Anthropocene

In 1999, the first scientists to study the Vostok ice core reported with surprise that “the Holocene, which has already lasted 11,000 years, is, by far, the longest stable warm period recorded in Antarctica during the past 420,000 years.”²² Will the transition to Anthropocene conditions be gradual, or can we expect sudden shifts comparable to those that initiated the Holocene?

The specific causes of past climate chaos are unlikely to repeat, so the pattern of change will certainly be different. But the most significant difference between then and now is the unprecedented impact of human activity in the past sixty years—and that makes it very likely, as a team headed by the noted U.S. biologist Anthony D. Barnosky concluded, that Earth is “approaching a state shift.”

Comparison of the present extent of planetary change with that characterizing past global-scale state shifts, and the enormous global forcings we continue to exert, suggests that another global-scale state shift is highly plausible within decades to centuries, if it has not already been initiated.²³

If that occurs, the relative stability of the Holocene could be replaced by a new and unprecedented climate state, unlike anything any human society has experienced. And, as geoscientist Richard B. Alley points out, the transition is likely to be fast:

Large, abrupt climate changes have repeatedly affected much or all of the earth, locally reaching as much as 10°C change in 10 years. Available evidence suggests that abrupt climate changes are not only possible but likely in the future, potentially with large impacts on ecosystems and societies….