Underlying the environmental changes of the twentieth century were three factors that vastly magnified the human impact on earth’s ecological systems far beyond anything previously known.25 One was the explosion of human numbers, an unprecedented quadrupling of the world’s population in a single century, leaving the world of 2014 with about 7.2 billion people compared to about 1.6 billion in 1900. It was a demographic revolution born of medical and sanitation advances that dramatically lowered death rates and Green Revolution technologies such as genetically modified seeds and fertilizers that substantially increased world food supplies.

This vast enlargement of the human population meant more consumption and thus more demands on the earth’s resources. It also fostered massive urbanization and global migration, even as it contributed to many political and social upheavals, especially in the second half of the century. By the end of the century, the rate of global population growth had begun to slow. From a peak of over 2 percent per year in the 1960s, it had dropped to 1.14 percent by 2014. This transition had occurred first in the more developed countries, where birth control measures were widely available, women were educated and pursuing careers, and large families were economically burdensome. This pattern began to take hold in developing countries as well, assisted by vigorous family-planning programs in many places, the most dramatic of which was China’s famous “one-child family” policy. Experts predict that the modern population explosion will level off by the mid-twenty-first century at some 9 to 12 billion people, although whether the world economy and its resource base can support these enormously enhanced numbers remains an open question.

A second cause of environmental stress lay in the amazing new ability of humankind to tap the energy potential of fossil fuels—coal in the nineteenth century and oil in the twentieth. These fuels drove the industrialization process everywhere, with coal providing the major source for electricity generation and oil giving rise to the immense automobile industry. Hydroelectricity, natural gas, solar power, and nuclear power added to the energy resources available to our species.

These new sources of energy made possible a third contribution to environmental transformation—phenomenal economic growth—as modern science and technology immensely increased the production of goods and services. Between the 1890s and the 1990s, global industrial output grew by a factor of forty, although very unevenly across the planet. But almost everywhere—in capitalist, communist, and developing countries alike—the idea of economic growth or “development” as something possible and desirable took hold as a novel element of global culture.

These three factors were the foundations for the immense environmental transformations of this most recent century. Human activity had always altered the natural order, usually on a local basis, but now the scale of that impact assumed global and perhaps even geological proportions. The growing numbers of the poor and the growing consumption of the rich led to the doubling of cropland; a corresponding contraction of the world’s forests, wetlands, and grasslands; and dramatic increases in the rate of erosion. Huge urban complexes have transformed the landscape in many places. With diminished habitats, numerous species of plants and animals either disappeared or were threatened with extinction at a rate many times greater than the background level. Certainly, massive species extinctions have occurred much earlier in the history of the planet (the dinosaurs, for example), but this wave of extinctions is happening at the hands of humankind. The human remaking of the ecosystem has also greatly increased the presence of plants and animals that have benefited from human activity—cattle, pigs, chickens, rats, wheat, corn, and dandelions. By some estimates, 90 percent of all plant activity now occurs in environments shaped by human action.

The global spread of modern industry, heavily dependent on fossil fuels, generated dramatic changes in the air, water, soil, and atmosphere with profound impacts on human life. China’s spectacular economic growth since the 1980s, fueled largely by coal, has resulted in the equally spectacular pall of air pollution in its major cities. In 2004, the World Bank reported that twelve of the world’s twenty most polluted cities were in China. Degradation of the world’s rivers, seas, and oceans has also mounted as pesticides, herbicides, chemical fertilizers, detergents, oil, sewage, industrial waste, and plastics have made their way from land to water. By the 1960s, Lake Erie in the United States was widely reported as “dead.” The Great Pacific Garbage Patch, an area of about 7 million square miles in the North Pacific, has trapped an enormous quantity of marine debris, mostly plastics, endangering oceanic food webs and proving deadly to creatures of the sea, which ingest or become entangled in this human garbage. Industrial pollution in the Soviet Union rendered about half of the country’s rivers severely polluted by the late 1980s, while fully 20 percent of its population lived in regions defined as “ecological disasters.” In addition, the release of chemicals known as chlorofluorocarbons thinned the ozone layer, which protects the earth from excessive ultraviolet radiation.

The most critical and intractable environmental challenge of recent decades has been global warming. Scientists became concerned about this phenomenon in the 1970s, although their research drew on earlier studies dating to the nineteenth century. By the end of the twentieth century, a worldwide scientific consensus had emerged that a dangerously warming climate was well under way, driven by human actions. Particularly responsible for this global warming has been the vastly increased burning of fossil fuels, which release heat-trapping greenhouse gases such as carbon dioxide into the atmosphere, as well as by the loss of trees that would otherwise remove the carbon dioxide from the air. By 2014, carbon dioxide concentration in the atmosphere, which had been roughly 275 ppm (parts per million) before the Industrial Revolution, had risen to 400 ppm, well above the level of 350 ppm that is generally considered “safe.” Scientists have associated this global warming with all manner of environmental changes, both current and projected: the melting of glaciers and the rising of sea levels; extreme weather events such as floods, droughts, hurricanes, and typhoons; increased acidification of the oceans; decreased crop yields; disruptions of ecosystems and the extinction of many species. All of this has varied and will continue to vary substantially from region to region, but serious observers have begun to speak about the possibility of “a crisis that threatens our survival as a species.”26

Beyond these weather- and climate-related changes, global warming has interacted with a variety of social conditions—poverty, inequality, oppression—to generate or exacerbate conflict and upheaval. Syria’s bitter civil war, which has killed over 200,000 people and displaced or made refugees of many millions more since 2011, followed on the heels of a severe and prolonged drought and related crop failures. More broadly, in 2010 and 2011 extreme weather conditions characteristic of global warming—droughts, dust storms, fires, heavy rainfall—afflicted many grain-producing regions of the world, including Canada, Russia, China, Argentina, and Australia, causing a sharp spike in grain prices on the world market. The Middle East and North Africa, heavily dependent on grain imports, experienced sharply rising food prices, arguably aggravating social unrest and contributing to the political protests of the Arab Spring. Even the American military has taken climate change seriously. A 2007 report by a number of senior retired officers concluded that “climate change poses a serious threat to America’s national security … [and] acts as a threat multiplier for instability in some of the most volatile regions of the world.”27