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
