Cause and Direction in Global Environmental History
Written by: John L. Brooke
Exogenous Forces and Endogenous Pressures
John L. Brooke, the author of Climate Change and the Course of Global History, explores the currents that shape our civilizations and how we have become a part of our own global environmental history.
Since the end of the Second World War modern humanity has been haunted by the specter of crisis and civilizational collapse. First it was the atomic bomb, then the population bomb; now we are beginning to understand the accelerating impacts driven by one hundred and fifty years of advancing industrial greenhouse emissions. Modern economic growth is altering in the earth system, and we worry about the costs of mitigation, even a sudden systemic breakdown, in the face of heat-waves, droughts, rising seas, super-storms and storm clusters: some of the effects of global warming.
What does history have to tell us about the stress and collapse in past human societies? This is the central and pressing problem for the field of environmental history. Environmental history emerged with the post-war rise of the environmental movement, as the impacts of radioactive fallout, chemical pollution, and rapid population growth began to seep into the public consciousness. In the United States environmental consciousness gained a legitimate place in the public mind with Earth Day 1969, the year after Paul and Ann Ehrlich put out the Population Bomb, describing a world imminently threatened by the vast expansion of human numbers.
The Ehrlich’s powerful invocation of the Malthusian equation of rising population, limited resources, and ecological degradation was quickly assimilated as a core paradigm by founding environmental historians as an explanation for the entire sweep of the human condition. A generation of influential scholars – among them Clive Ponting, Donald Hughes, Mark Nathan Cohen, Marvin Harris, Charles Redman, David Christian, Joachim Radkau, and Jared Diamond – applied this Malthusian framework of human sustainability crises to paleo, ancient, and medieval circumstances. The explicit lesson was that fate of these societies should stand as object lessons to the present.
Thus was forged what can be called the “endogenous paradigm” in environmental history. Humanity was the sole actor in this story, and humanity’s trials and tribulations over centuries and millennia were caused simply by the pressures of growing populations and expanding economies, degrading essentially inert natural systems. The long sweep of human history was thus defined as a series of endogenously driven crises of sustainability. This was a model that I accepted and elaborated for classes in global environmental history for over a decade, at Tufts and Ohio State, assigning Marvin Harris’s Cannibals and Kings as a wonderfully accessible grand synthesis.
But new evidence has called into question this long-established unitary “endogenous” model of deep historical time. Quite simply, our world is very different that of our premodern ancestors. Ironically, the evidence developed as the United States government worked to avoid action when the problem of climate change and global warming was put before the public in the late 1980s. Delay action while we “study the problem,” we were told. Climate scientists were put to the task and over the past twenty-five years their efforts have been amazingly fruitful. Their core public mandate has been to assess the degree to which modern global climates are deviating from “historical norms.” In doing this arduous work they have generated a vast body of carefully calibrated data describing the shape and history over decades, centuries, millennia, and even millions of years into the past, from far before humanity first began to evolve, several million years ago in the scrub forests of East Africa. We now have at hand an amazing body of knowledge about climatic and environmental change in times past, for the entire sweep of earth history and the human experience of society, agriculture, and civilization.
This data calls into question the “endogenous paradigm.” As historians we can no longer assume that climate history is unknowable, and thus irrelevant. We now know its patterns, and we need integrate natural forces “exogenous” to the human system into our historical accounts. In short, we as historians need to move to a new model of the field of historical causation. Rather than simply assume that human agency is and was the only legitimate active force in shaping our past, we need to take much more seriously the active forces of nature.
It is abundantly clear that vast environmental shifts shaped both biological and human evolution in deep paleo-time. But let us consider briefly the evidence for the reasonably warm Holocene, the last ten thousand years, which points to two very different histories. In the first history, running from the paleo-past up to the rise of modernity between 1400 and 1800, human circumstances were shaped by global climate forces, in rather eerie syncrecity, what Victor Lieberman has termed “strange parallels.” The material fate of human societies around the world was shaped by the force of large-scale shifts in Holocene climate, in recurring “optimums and “Dark Ages.” In short, the millennial-scale pattern of solar maxima and minima known as the Hallstatt cycle has been the foundational driver of the material condition of humanity for the last six thousand years. Since patterns of temperature and particularly precipitation had different regional manifestations during the same climatic regimes, regional experience of immiseration and crisis might vary. But the general pattern is clear: the global climate regime, shifting on a periodicity shaped by millennial-scale solar variation, has had a profoundly shaping force in our collective history. Rather than population pressure, pure and simple, crisis came from outside the human system, in virtually every case the “exogenous” forces of nature played a role in civilizational crisis and collapse in the pre-modern past.
Thus in the first history, premodern time, “exogenous” natural forces were the ultimate actors on the global stage. In the second history, modern time, the “endogenous” pressure generated by humanity has become a primary earth system force of its own. Modern economic growth, shaped equally by explosive developments in scientific knowledge and industrial application, in politics and governance, make our world utterly different that of just a few generations in the past.
The central differences in the material condition of premodern and modern societies involve numbers and chronology, and earth system scale. On numbers: the background reality is that ancient and medieval societies never really had capacity to achieve populations that would threaten endogenously their ecological sustainability. And as their populations grew, slow shifts in technology could and did rise to meet the challenge. But the essential reality is that pre-scientific societies were constantly struggling to maintain their numbers against the exogenous pressures of disease and climatic shifts; massive infant mortality kept life expectancy low. Conversely, the societies in which these people lived were remarkably long-lived: they may not have been pleasant places to live, but they could last in coherent form for centuries on end. Ironically, we have reversed this equation in modernity: our long life expectancies of eighty or more years loom very large against the shallow chronology of modern industrial society.
Finally, there is the issue of earth system scale. Ancient and medieval societies did not collapse at the slightest push of climate change: the great collapses were shaped by great global forces, ultimately determined by the multi-millennial Hallstatt solar cycle. But here lies the problem. Measured against the know variations in the atmospheric system, nothing in the Holocene remotely resembles the speed and scale of modern anthropogenic climate change and its driver, industrial greenhouse emissions.
The history of atmospheric CO2directly illustrates the problem. Through the operation of the earth system’s greenhouse bubble, atmospheric CO2 powerful determines global temperature: the more CO2 to hotter the world. During the last great global climate crisis, the Little Ice Age of the sixteenth and seventeenth centuries, atmospheric CO2 dropped from about 282 part per million to roughly 276 parts per billion. Geoffrey Parker’s new book The Global Crisis charts the devastating human impacts correlated with this droop in CO2, and associated cooling. Recovering from Little Ice Age lows by 1800, atmospheric CO2 climbed thirteen points over the next century, to about 296ppm in 1900. In 1958, when Charles Keeling first began measuring CO2 on Mona Loa in Hawaii, when I was five years old, the CO2 level stood at 315ppm. Last May, 56 years later, they hit a monthly average of 400PPM. Our best estimates are that the last time that global atmosphere held this volume of CO2 was in the Oligocene, 35-40 million years, just before a balmy, forested Antarctica began to freeze over.
This spring it became clear that Antarctica is beginning to melt. Clearly, modernity “endogenous” human pressures have met and exceeded “exogenous” natural forces, indeed fused with them in a perfect storm. Population, economic output, and atmospheric CO2 have risen in perfect tandem since 1800, and accelerated with the super-cycles of modern economic growth.
Humanity in the paleo, ancient and medieval past lived in a world fundamentally different from ours. We now live in the Anthropocene: we have become a fundamental cause in the earth system, in global environmental history. We have created this new world; we need to face this fact and take responsible action.
 Henry M. Paulson, Jr., “The Coming Climate Crash,” New York Times, June 21, 2014; Risky Business: The Economic Risks of Climate Change in the United States, June, 2014 [http://riskybusiness.org/uploads/files/RiskyBusiness_PrintedReport_FINAL_WEB_OPTIMIZED.pdf]
 J. Mouginot, E. Rignot, and B. Scheuchl, “Sustained increase in ice discharge from the Amundsen Sea Embayment, West Antarctica, from 1973 to 2013,” Geophysical Research Letters 41 (Mar., 2014), 1576-1584; Ian Joughin, Benjamin E.Smith, Brooke Medley, “Marine Ice Sheet Collapse Potentially Under Way for the Thwaites Glacier Basin, West Antarctica,” Science 344 (May, 2014), 735-738.