The worst epidemics have killed 30 million to 100 million people, even if one considers the bubonic plague pandemic in fourteenth-century Eurasia (and possibly Africa) as a single event (a pandemic is an especially widespread epidemic, often global in scope). The most recent epidemic on such a scale, the influenza that raged from 1918 to 1919, killed perhaps 40 million (about 2 percent of the global population). The ongoing AIDS pandemic has so far killed 25 million to 30 million, about 0.5 percent of the current world population.4 Such pandemics have been mercifully rare, but past epidemics that affected regions or single cities were not, and they routinely killed 5 to 10 percent or even more of the affected population.
Droughts, at their worst, have resulted in a few million deaths. The long history of drought is notably fuzzy, and whether or not deaths ought to be laid at drought's door is often unclear, especially for the deeper past. In the twentieth century, where the uncertainties are reduced, the deadliest droughts occurred in China from 1928 to 1931, in 1936, and in 1941, with 2 million to 5 million deaths on each occasion, generally because of starvation. The famous droughts in West Africa's Sahel region of 1967 to 1973 and again in the early 1980s each killed about 1 million people. In all probability some of the drought-induced Indian famines of the nineteenth century killed greater numbers, but the figures are in dispute.5
Floods, too, could kill thousands, even millions, although flood control and evacuation procedures have made a big difference in flood mortality. Since 1953 the annual average number of flood-caused deaths in India, the country most afflicted by floods, has been about 1,500. The worst flood in recent Chinese history, when the Yangtze surged in 1954, killed 30,000 people. Yangtze floods in 1931, perhaps the most costly ever, killed 1 million to 4 million, and those on the Hwang He (Yellow River) in 1887 resulted in perhaps 1 million to 2 million deaths. The great North Sea floods of December 1953 killed some 2,400 in the Netherlands, whereas earlier floods, in 1212, had killed 60,000. A 1342 megaflood in central Europe affected dozens of rivers,
caused half of all the soil erosion over German lands in the past millennium, and probably drowned hundreds of thousands of people.6 In 1927 the worst flood in U.S. history—until those caused by Hurricane Katrina in 2005 killed 243 people along the lower Mississippi River.7
Of the many thousands of deadly earthquakes, only ten have killed more than 100,000 people. The worst occurred in China in 1566; perhaps 800,000 died. The recent tsunami of December 2004, created by an undersea earthquake, killed 284,000, and the 2005 earthquake in Pakistan killed about 79,000. The San Francisco earthquake of 1906, the worst in U.S. history, killed about 3,000.8
Of the countless volcanic eruptions, only six are likely to have killed more than 10,000 people. The worst case, the explosion of Mount Tambora, on the northern coast of Sumbawa island, Indonesia, in 1815, took perhaps 92,000 lives; Krakatoa, in 1882, cost 36,000. The famous eruption of Mount Vesuvius in 79 C.E. killed about 3,600, and the worst in U.S. history, that of Mount St. Helens, Washington, in 1980, killed 57.
With the exception of the richer parts of the world since 1919, every generation everywhere lived under the threat of devastatingly lethal epidemics, floods, droughts, and other kinds of natural risks.9
As a result, all societies had to build resilience to nature's shocks. By and large, they did not intentionally build resilience or resistance to nature's slow-acting stresses, such as desiccation (the gradual drying of climate) or soil salinization, because these progressed too slowly to cause alarm, and often too slowly even to be noticed from one generation to the next. But resistance and resilience to the easily observable short, sharp shocks were, always and everywhere, an important priority.
Resistance and resilience are not the same thing. Resistance to flood, for example, can take the form of the construction of seawalls and dikes, as the Dutch have done for seven hundred years to keep the North Sea at bay. Resilience to flood means the capacity to recover as quickly and easily as possible, which might take the form of leaving a river floodplain uninhabited and using it only for seasonal pasture, as was done along the Rhine until engineers straightened and narrowed its channel beginning in 1817.
Societies built resistance to nature's shocks as a conscious enterprise. In regions of the world prone to drought, they developed water-storage infrastructure such as cisterns. In flood-prone regions, they built levees. Cities developed quarantine routines to try to prevent epidemics. By the eighteenth century, China's Qing dynasty had constructed an elaborate system of state granaries intended to prevent famine from whatever cause. (The
Aztecs had done this on a smaller scale in the fifteenth century.) By the nineteenth century, richer societies undertook to control river floods with dikes, dams, and canalization, as on the Rhine, for example.10 Since the 1880s public health services have made major efforts—by and large crowned by suc-cess—to prevent epidemics, by means of sanitation reforms and vaccination regimes. Otherwise there would not be 6.4 billion people today.
There have always been limits to the degree to which resistance can be built. Preventing volcanic eruptions remains impossible and stopping lava flows implausibly expensive. Flood control is feasible but only within limits; levees and dikes occasionally are overwhelmed, as occurred in the Mississippi basin in 1927 and 1993 and most recently in New Orleans in 2005. Moreover, as the Mississippi and New Orleans floods show, societal faith in the infrastructure of resistance can undermine resilience: the opportunity cost of leaving a floodplain unoccupied seems excessive if one trusts the levees and dikes.
Resilience, on the other hand, has to date proved to be in abundant supply: our species has survived countless shocks and now covers the globe as never before. In our earliest years, resilience consisted mainly of mobility—the ability to escape the worst of a natural shock through migration and to start afresh in a new landscape. Until recent decades, this remained an option for millions of pastoralists and the few remaining hunting and foraging populations. As recently as 1912 to 1915, when severe droughts affected the West African Sahel, millions of people adapted by migrating southward—a feasible response because in those days West Africa had about one-eighth the population it carries today, and there were no effective border control regimes to inhibit migration. For the great majority of our historical experience, mobility was the survival response to nature's shocks. Today it is severely restricted.
A second source of resilience in times past was simplicity combined with fertility. Societies with minimal infrastructure lost little except people when a natural disaster struck, and new people were easily created. Rebuilding a complex city in the aftermath of a flood or earthquake requires much more knowledge, investment, coordination, and cooperation than does rebuilding a patchwork of fields and villages. Most peasant societies prior to the twentieth century had a large number of unmarried young people who, in the wake of deadly catastrophe, would stampede into marriage and within a year sharply raise birth rates. This was not a conscious strategy, but a result of custom and economic preferences. Nonetheless, it provided resilience in the form of the ability to ramp up fertility quickly and jump-start demographic recovery.11
For many centuries societies have also consciously created more mechanisms to improve resilience. Storing food in state warehouses to cope with famine is a strategy intermittently practiced since ancient times and brought to a high level of reliability by the Qing dynasty in eighteenth-century China.12 Transportation infrastructure, although built for other reasons, also provided resilience in that it allowed both faster evacuations from affected zones and quicker rescue and relief. Thus societies with extensive and dense road or canal networks, or both, eliminated famine by the end of the eighteenth century, while those lacking transportation infrastructure remained vulnerable.
Organized relief efforts also improved resilience in modern history. The practice of maintaining contingency funds against disasters is probably nearly as old as money and treasuries. The practice by governments of providing funds for disaster victims in other countries dates back at least to a great Jamaican hurricane of 1783 and a Venezuelan earthquake of 1812. The first standing international body devoted to disaster relief probably was the Red Cross, founded in 1863, though until the late 1940s it concerned itself almost entirely with victims of war, rather than nature's shocks.13 The total effect of such efforts and organizations upon societal resilience has to date been modest, but they have eased the suffering of millions.
In the last two or three centuries, as societies have grown more complex and as mobility has become less feasible as a societal response, resistance and resilience have come to take more technological and bureaucratic forms, such as granaries, seawalls, and international relief organizations. Since 1950 or so, the ability to evacuate millions and to bring large quantities of food and other supplies, quickly and over great distances, has improved immensely. As a result, the causes of modern famines have typically been war and totalitarian politics, rather than environmental factors.14 Ironically, the logistical capacity to do such things was in large part developed to meet the military requirements of global war, especially in World War II.
As a consequence of this technological and organizational progress, disease, droughts, floods, and earthquakes that a century or more ago might have killed millions more recently would only kill thousands. This extraordinary ability to mitigate disaster has hinged on the relative stability of international politics since 1945, which has provided an opportunity for what we might call "regimes of resilience" to develop. However, the rapid population growth in these same decades imperils resilience by making it harder to maintain uninhabited or sparsely populated buffer zones, wetlands, mangrove forests, floodplains, and so forth. Resilience in the face of drought or similar shocks can be harder to maintain in more crowded circumstances, as can resistance to infectious disease.
In the past, vulnerability to shock had several components. First and most obvious, the intensity and duration of natural shocks often made all the difference between survival and catastrophe. Societies that could withstand one drought a year with only hunger could not withstand two without starvation. Second, some societies had, by design or accident, less in the way of buffers or resilience than others. For example, a society that had few or no domestic animals could not survive a harvest failure as reliably as could a society that could eat its animals one by one if circumstances required it. Societies that had poor transportation infrastructure could not import food as readily or cheaply as could those with good roads, canals, or, eventually, railroads. Nor could the isolated receive any available government or charitable assistance as easily. Societies that used nearly every available acre as farmland and preserved very little in the way of woodlands or wetlands, such as that of early-twentieth-century rural China, proved more vulnerable to floods than did those that (by accident or design) kept land in reserve, because floodwaters sitting on unpopulated or uncultivated land were merely an inconvenience, not a catastrophe. Societies without active and able public health systems suffered more from epidemics than did those that had such systems.
Less obvious, perhaps, were differences in levels of ecological ignorance. Populations that have lived in one environment for several generations gradually acquire, and usually take pains to transmit, knowledge of how to survive and prosper within the limits of their environment. They also gradually form a sense of the boundary conditions to be expected and know from oral tradition that they must be prepared for adversities—locust invasions, prolonged drought, and so forth—beyond their own personal experience. Populations present for dozens of generations normally had exquisitely fine-tuned ecological knowledge and knew where to find edible plants to see them through famine, where to find underground water when there was none on the land's surface, and so forth. Such knowledge contributed materially to resilience.
Conversely, in many instances, especially in the last two centuries, the prevalence of cheap transportation and more long-distance migration has meant that many populations found themselves operating experimentally in new environments. This was true of the British and Irish settlers in Australia after 1788, who inevitably misunderstood antipodean ecology and often paid a price for it.15 It was true of the American farmers on the southern plains, almost all of whom came from more humid climes, who during the 1930s drought naturally presumed that the wetter years of 1915 to 1930 were normal. They were ignorant of the cyclic drought patterns of the plains; through their farming practices they inadvertently turned the southern plains into the Dust Bowl in a routine drought. Ecological ignorance also lay behind the failures of the Soviet Virgin Lands scheme of the 1950s, in which Premier Nikita Khrushchev ordered an area of dry Siberian and Kazakh steppe land the size of California to be planted in wheat, only to see the region experience, within a few years, disastrous drought, dust storms, and harvest failure.
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