Reconstructions of Mangaia's human population mirror those of Easter Island, albeit on a smaller scale. Starting with perhaps a few dozen colonizers around 500 BC, the island's population grew steadily to about five thousand people by AD 15oo. The population fell dramatically over the next two centuries, hitting a low soon after European contact and then rebounding to a modern population of several thousand.

The environmental and cultural history of Tikopia, a British protectorate in the Solomon Islands, provides a striking contrast to Mangaia despite very similar backgrounds. With a total area of less than two square miles, Tikopia is smaller than Mangaia. Even so, the two islands supported comparable populations at the time of European contact. With a population density five times greater, Tikopia sustained a relatively stable and peaceful society for well over a thousand years. This tiny island offers a model for sustainable agriculture and an encouraging example of cultural adaptation to limited resources.

Land use on Tikopia began much as that on Mangaia did. After people arrived about 900 BC, a shifting pattern of forest clearing, burning, and cultivation increased erosion rates and began to deplete the island's native fauna. After seven centuries on the island, the islanders intensified pig production, apparently to compensate for loss of birds, mollusks, and fish. Then instead of following the path taken by the Mangaians and Easter Islanders, Tikopians adopted a very different approach.

In their second millennium on their island, Tikopians began adapting their agricultural strategy. Plant remains found in the island's sediments record the introduction of tree crops. A decline in the abundance of microscopic charcoal records the end of agricultural burning. Over many generations, Tikopians turned their world into a giant garden with an overstory of coconut and breadfruit trees and an understory of yams and giant swamp taro. Around the end of the sixteenth century, the island's chiefs banished pigs from their world because they damaged the all-important gardens.

In addition to their islandwide system of multistory orchards and fields, social adaptations sustained the Tikopian economy. Most important, the islanders' religious ideology preached zero population growth. Under a council of chiefs who monitored the balance between the human population and natural resources, Tikopians practiced draconian population control based on celibacy, contraception, abortion, and infanticide, as well as forced (and almost certainly suicidal) emigration.

Arrival of Western missionaries upset the balance between Tikopia's human population and its food supply. In just two decades the island's population shot up by 40 percent after missionaries outlawed traditional population controls. When cyclones wiped out half the island's crops in two successive years, only a ma.s.sive relief effort prevented famine. Afterward, the islanders restored the policy of zero population growth, this time based on the more Western practice of sending settlers off to colonize other islands.

Why did Tikopians follow such a radically different path than their counterparts on Mangaia and Easter Island? Despite similar settings and natural resources, the societies that colonized these islands met radically different fates. Tikopia developed into an idyllic island paradise, while Mangaia and Easter Island descended into perpetual warfare. Recalling that Tikopia's utopian system was maintained at the cost of lives prevented or eclipsed in the name of population control, we can justifiably ask which was the higher price. Nonetheless, Tikopian society prospered for thousands of years on a tiny isolated outpost.

An essential difference between the stories of these islands lies in their soils. Deeply weathered soils in Mangaia's sloping volcanic core are nutrient poor. The sharp coral slopes of the uplifted reef hold no soil at all. In contrast, Tikopia hosts young phosphorous-rich volcanic soils. The greater natural resilience of Tikopia's soils-because of rapid weathering of rocks with high nutrient content-enabled Tikopians to sustain key soil nutrients, using them at about the rate that they were replaced from the underlying rocks through intensive, multistory gardening that protected topsoil.

After deciphering the environmental history of both Tikopia and Mangaia, Patrick Kirch suspects that geographic scale also influenced the social choices that shaped these island societies. Tikopia was small enough that everyone knew everyone else. Kirch suggests that the fact that there were no strangers on the island encouraged collective decision making. By contrast, he suggests, Mangaia was just large enough to foster an us versus them dynamic that fueled compet.i.tion and warfare between people living in neighboring valleys. Easter Island supported a larger and less cohesive society, leading to even more disastrous results. If Kirch is right and larger social systems encourage violent compet.i.tion over collective compromise, we need to take a sober look at our global prospects for managing our island in s.p.a.ce.

The story of dramatic soil loss following human colonization of islands is not restricted to the South Pacific. Viking colonization of Iceland in AD 874 catalyzed an episode of catastrophic soil erosion that continues to consume the country. At first the new colony prospered raising cattle and growing wheat. The population rose to almost eighty thousand people by AD 1100. Yet by the late eighteenth century the island's population had dwindled to half the medieval population. Cooling during the Little Ice Age from about AD 1500 to 1900 certainly influenced the fortunes of the Iceland colony. So did soil erosion.

Iceland had an extensive forest cover when first colonized. In compiling the fslendingabok in the late twelfth century, Ari the Wise described the island as "forested from mountain to sea sh.o.r.e." 2 Since human settlement, more than half of Iceland's vegetation cover has been removed. The native birch forest that covered thousands of square miles now occupies less than 3 percent of its original area.

Over time, herds of sheep increasingly disturbed the landscape. By the start of the eighteenth century more than a quarter of a million sheep roamed the Icelandic countryside. Their numbers more than doubled by the nineteenth century. Visitors began describing Iceland as a bare land devoid of trees. The combination of a deteriorating climate and extensive overgrazing led to severe erosion and abandoned farms. Today, threequarters of Iceland's forty thousand square miles of land are adversely affected by soil erosion; seven thousand square miles are so severely eroded as to be useless.

Once Iceland's slopes were deforested, strong winds blowing off its central ice caps helped strip the soil from roughly half the once forested area of the island. Large herds of sheep broke up the soil, allowing wind and rain to dig their way down to bedrock last exposed by melting glaciers. Soils built up over thousands of years disappeared within centuries. The central part of the island where the soil has been completely removed is now a barren desert where nothing grows and no one lives.

Some areas eroded soon after the Vikings arrived. During the relatively warm period in the eleventh and twelfth centuries, before the Little Ice Age, severe soil erosion caused the abandonment of mostly inland and some coastal farm sites. Later erosion in the lowlands primarily involved farms in marginal locations.

Many theories have been advanced to explain Iceland's abandoned farms. Inland areas have been vacated for centuries, some valleys literally deserted. Until recently, the abandonment was primarily attributed to climate deterioration and a.s.sociated epidemics. But recent studies have doc.u.mented the role of severe soil erosion in converting farms and grazing land into barren zones. The history of Icelandic soils can be read through the layers of volcanic ash. Frequent volcanic eruptions imprinted Iceland's dirt with a geologic bar code. Each ash buried the soils onto which they fell. The layers gradually became incorporated into the soil as wind deposited more dirt on top.

In 1638 Bishop Gisli Oddson described layers of volcanic ash in Icelandic soils. The observant bishop noticed that thick layers of ash separated buried soils, some of which contained the rooted stumps of ancient trees. Since Oddson's day, it has been recognized that hundreds of volcanic eruptions after the last glaciation produced fine-grained soils readily eroded if exposed to high winds sweeping across the island. Windblown material acc.u.mulates where vegetation stabilizes the ground surface, combining with layers of volcanic ash to build Icelandic soils. Based on ages of the different layers of ash in soil profiles, Icelandic soils acc.u.mulated at about half a foot every thousand years, roughly half an inch per century. The loss of vegetation not only accelerates erosion, but keeps soil from acc.u.mulating once there is nothing on the surface to trap volcanic ash and windblown silt.

In prehistoric times, relatively loose soil held together by thick native vegetation slowly built up on top of more cohesive lava and glacial till (an unstratified mix of clay, sand, and boulders deposited by glaciers). In areas where the soil sits directly on top of the till, soil acc.u.mulated continuously over ten thousand years. In some areas, exposed layers of soil and ash preserve evidence for erosion before the Vikings arrived, during periods when climatic deterioration stressed Iceland's native vegetation. The combination of overgrazing and climatic deterioration during the Little Ice Age triggered the most extensive episode of soil erosion in Iceland's postglacial history.

During the light-filled Icelandic summer, sheep graze twenty-four hours a day, roaming over both heath and wetlands. Trampling generates bare spots up to several feet in diameter. Shorn of a dense root mat, Iceland's volcanic soils offer little resistance to wind, rain, or snowmelt. Patches of bare earth erode rapidly down to hard rock or glacial till, carving little cliffs ranging in height from one to almost ten feet, depending on the local depth of the soil. Once started, these miniature escarpments sweep across the landscape eating away at the remaining pillars of soil and transforming rich grazing lands into windswept plains of volcanic tephra and rock fragments. Soil erosion since Norse settlement removed the original soil from about half the island. Although many factors contribute, overgrazing by sheep is generally acknowledged as the primary cause. Worms may have shaped Darwin's England (once glaciers got through with it), but sheep shaped Iceland.

Rofabards-the Icelandic name for soil escarpments-erode back half an inch to a foot and a half per year. On average, rofabard advance amounts to an annual loss of 0.2 to 0.5 percent of the soil cover from areas across which rofabards presently occur. At this rate it would take just a few hundred more years to finish stripping the soil from the whole island. Since Viking settlement, rofabard erosion has removed the soil from about five square miles per year. Icelandic scientists fear that many areas of the country have already pa.s.sed a threshold that makes further erosion inevitable. They also know that once stripped of soil the land is pretty much useless.

Figure 25. Professor Ulf h.e.l.lden standing on top of a rofabard, the last remnant of soil that formerly covered the surrounding plain, Iceland (courtesy of Professor h.e.l.lden, Lund University).

Even though Iceland has lost 6o percent of its vegetative cover and 96 percent of its tree cover, after i,ioo years of inhabitation most Icelanders find it difficult to conceive of their modern desert as having once been forested. Most don't comprehend how severely their landscape has been degraded. Just as at Easter Island, people's conception of what is normal evolves along with the land-if the changes occur slowly enough.

The Caribbean islands of Haiti and Cuba provide another dramatic contrast in how island nations treat their soil. Haiti, which means "green island" in the native language, Arawak, is a modern example of how land degradation can bring a country to its knees. Cuba provides an example of a nation that, out of necessity, transformed a conventional agricultural system into a model for feeding a post-petroleum world.

The history of Haiti, the western third of the island of Hispaniola, shows that small hillside farms can lead to devastating soil loss even without a disastrous hurricane. Within twenty-five years after Columbus discovered Hispaniola in 1492, Spanish settlers had annihilated the island's native inhabitants. Two centuries later, in 1697, the Spanish ceded the western third of the island to the French, who imported African slaves to work timber and sugar plantations serving European markets. The colony's half million slaves revolted in the late eighteenth century, and in 1804 Haiti became the world's first republic of freed citizens to declare independence-from France, Europe's first republic.

Figure 26. Map of Iceland showing the extent of areas considerably to severely eroded, glacial ice, and uneroded soils (created from data provided courtesy of Einar Gretarsson).

Subsequent cultivation on steep slopes converted about a third of the country to bare rocky slopes incapable of supporting agriculture. In colonial times, there were reports of extensive erosion on upland coffee and indigo plantations and plantation owners could count on only three years of productive crops from upland fields. Widespread cultivation of steep slopes began again in the mid-twentieth century when subsistence farmers spread back into the uplands. By 199o, 98 percent of Haiti's native tropical forest was gone. Common erosion control measures such as piling up soil into mounds, or piling up soil against stakes placed along contour to create small terraces, were not very effective in controlling erosion on steep slopes.

Soil loss from the uplands in the rainy season is so severe that bulldozers function as tropical snowplows to clear the streets of the capital, Portau-Prince. The United Nations estimates that topsoil loss over at least half the country is severe enough to preclude farming. The U.S. Agency for International Development reported in 1986 that about a third of Haiti was extremely eroded and practically sterile from soil loss. Farmers worked an area six times larger than the area well suited for cultivation. The UN Food and Agriculture Organization estimated that soil erosion destroyed 6,ooo hectares of arable land a year in the 198os. For the past few decades estimates of the remaining area of "good" farmland showed a long-term decline of several percent a year. With little more than 50 percent of the island's potential farmland still arable, the island's growing population no longer can feed itself.

Prosperity disappeared along with Haiti's topsoil. As subsistence farms literally disappeared many rural families resorted to felling the last remaining trees to sell as charcoal to buy food. Desperate peasants flocking to cities created huge slums that fostered the insurgency that toppled the government in 2004.

Haiti's crippling soil loss is not simply a colonial legacy. Land distribution in Haiti is far more egalitarian than elsewhere in Latin America. After independence the Haitian government confiscated colonial estates and freed slaves began farming unclaimed lands. Early in the nineteenth century, Haiti's president distributed a little more than 15 hectares of land to each of some ten thousand beneficiaries. Since then, land holdings generally were divided upon inheritance and several centuries of population growth gradually reduced the size of the average peasant farm to the point where by 1971, the average farm size was less than 1.5 hectares. With an average of between 5 and 6 people per household, this comes to about 0.25 to 0.3 hectares per person. More than three-fourths of rural households fall below the poverty line and two-thirds of Haitian households fall below the UN Food and Agriculture Organization's minimum nutritional standard. This is Ireland all over again, this time without the landlords.

As the population grew, the land inherited by each successive generation was subdivided into smaller plots that eventually became too small to allow fallow periods. Declining farm income reduced the ability to invest in soil conservation measures. Unable to support themselves, the poorest farmers move on to clear steeper hillsides-the only remaining land not already cultivated-and start the cycle all over on land that can last only a few years. Eventually the shortage of arable land and rising rural poverty pushes peasants from hillside subsistence farms to search for work in Portau-Prince, where the concentration of desperate people in slums contributes to the country's tragic history of civil strife.

In Haiti, the majority of peasants own their own small farms. So small farms per se are not the answer to stopping erosion. When farms become so small that it is hard to make a living from them, it becomes hard to practice soil conservation. In Cuba, fifty miles from Haiti across the Windward Pa.s.sage, the collapse of the Soviet Union set up a unique agricultural experiment. Before the 1959 Cuban revolution, the handful of people who controlled four-fifths of the land operated large export-oriented plantations, mostly growing sugar. Although small subsistence farms were still common on the remaining fifth of the land, Cuba produced less than half its own food.

After the revolution, in line with its vision of socialist progress, the new government continued sponsoring large-scale, industrial monoculture focused on export crops-primarily sugar, which accounted for threequarters of Cuba's export income. Cuba's sugar plantations were the most mechanized agricultural operations in Latin America, more closely resembling those in California's Central Valley than on Haiti's hillsides. Farm equipment, the oil to run them, fertilizers, pesticides, and more than half of Cuba's food were imported from the island's socialist trading partners. The end of Soviet support and an ongoing U.S. trade embargo plunged Cuba into a food crisis. Unable to import food or fertilizer, Cuba saw the calories and protein in the average diet drop by almost a third, from 3,000 calories a day to i,9oo calories between 1989 and 1994.

The Soviet collapse resulted in an almost 9o percent drop in Cuba's external trade. Fertilizer and pesticide imports fell by 8o percent and oil imports fell by 50 percent. Parts to repair farm machinery were un.o.btainable. The New York Times editorial page predicted the imminent collapse of Castro's regime. Formerly one of the best-fed nations in Latin America, Cuba was not quite at the level of Haiti-but not much above it. Isolated and facing the loss of a meal a day for everyone on the island, Cuban agriculture needed to double food production using half the inputs required by conventional agriculture.

Faced with this dilemma, Cuba began a remarkable agricultural experiment, the first nation-scale test of alternative agriculture. In the mid-198os, the Cuban government directed state-run research inst.i.tutions to begin investigating alternative methods to reduce environmental impacts, improve soil fertility, and increase harvests. Within six months of the Soviet collapse, Cuba began privatizing industrialized state farms; staterun farms were divided among former employees, creating a network of small farms. Government-sponsored farmers' markets brought peasant farmers higher profits by cutting out intermediaries. Major government programs encouraged organic agriculture and small-scale farming on vacant city lots. Lacking access to fertilizers and pesticides, the food grown in the new small private farms and thousands of tiny urban market gardens became organic not through choice but through necessity.

Charged with subst.i.tuting knowledge-intensive agriculture for the embargoed inputs needed for conventional agriculture, the country's research infrastructure built on experiments in alternative agriculture that had languished under the Soviet system but were available for widespread, and immediate, implementation under the new reality.

Cuba adopted more labor-intensive methods to replace heavy machinery and chemical inputs, but Cuba's agricultural revolution was not simply a return to traditional farming. Organic farming is not that simple. You cannot just hand someone a hoe and order them to feed the proletariat. Cuba's agricultural transformation was based as much on science as was the Soviet era's high-input mechanized farming. The difference was that the conventional approach was based on applied chemistry, whereas the new approach was based on applied biology-on agroecology.

In a move pretty much the opposite of the green revolution that transformed global agriculture based on increased use of irrigation, oil, chemical fertilizers and pesticides, the Cuban government adapted agriculture to local conditions and developed biological methods of fertilization and pest control. It created a network of more than two hundred local agricultural extension offices around the country to advise farmers on low-input and no-till farming methods, as well as biological pest control.

Cuba stopped exporting sugar and began to grow its own food again. Within a decade, the Cuban diet rebounded to its former level without food imports or the use of agrochemicals. The Cuban experience shows that agroecology can form a viable basis for agriculture without industrial methods or biotechnology. Unintentionally, the U.S. trade embargo turned Cuba into a nation-scale experiment in alternative agriculture.

Some look to the Cuban example as a model for employing locally adapted ecological insight and knowledge instead of standardized mechanization and agrochemistry to feed the world. They see the solution not simply as producing cheap food, but keeping small farms-and therefore farmers-on the land, and even in cities. Thousands of commercial urban gardens grew up throughout the island, hundreds in Havana alone. Land slated for development was converted to acres of vegetable gardens that supplied markets where local people bought tomatoes, lettuce, potatoes and other crops. By 2004 Havana's formerly vacant lots produced nearly the city's entire vegetable supply.

Cuba's conversion from conventional agriculture to large-scale semiorganic farming demonstrates that such a transformation is possible-in a dictatorship isolated from global market forces. But the results are not entirely enviable; after almost two decades of this inadvertent experiment, meat and milk remain in short supply.

Cuba's labor-intensive agriculture may not produce basic crops as cheaply as American industrial farming, but the average Cuban diet did recover that lost third meal. Still, it is ironic that in retreating from the socialist agenda, this isolated island became the first modern society to adopt widespread organic and biologically intensive farming. Cuba's necessity-driven move toward agricultural self-sufficiency provides a preview of what may come on a larger scale once we burn through the supply of cheap oil that presently drives modern agriculture. And it is somewhat comforting to know that on at least one island the experiment has already been run without social collapse. Less comforting is the question of whether something similar could be pulled off in a society other than a one-party police state.

After Darwin's famous sojourn in the Galapagos, the isolated nature of islands strongly influenced biological theory. But it is only in the last several decades that such thinking reached the realm of anthropology. While people may someday migrate into s.p.a.ce to colonize other planets, the vast majority of us remain trapped on our planet for the foreseeable future. Although a global rerun of Haiti, Mangaia, or Easter Island is by no means inevitable, the experiences of societies on islands around the world remind us that Earth is the ultimate island, an oasis in s.p.a.ce rendered hospitable by a thin skin of soil that, once lost, rebuilds only over geologic time.

LIFE SPAN OF CIVILIZATIONS

Speak to the earth, and it shall teach thee.

JOB 12:8.

AFTER TWO HUNDRED YEARS, THE CONTRASTING VISIONS of Malthusian pessimism and G.o.dwinian optimism still frame debate over whether technological innovation will keep meeting society's growing agricultural needs. Preventing a substantial decline in food production once we exhaust fossil fuels will require radically restructuring agriculture to sustain soil fertility, or developing ma.s.sive new sources of cheap energy if we continue to rely on chemical fertilizers. But the future is clear if we continue to erode the soil itself.

Estimating how many people Earth can support involves a.s.sumptions about trade-offs between population size, quality of life, and environmental qualities such as biodiversity. Most demographic estimates antic.i.p.ate more than ten billion people on the planet by the end of this century. Whether we endorse the National Conference of Catholic Bishops' apparent belief that the world could comfortably support forty billion people, or Ted Turner's view that four hundred million would be plenty, feeding even the middle range of such estimates presents an impossible challenge. For even if we were we to somehow harness Earth's full photosynthetic production with the same efficiency as the 40 percent now devoted to supporting humanity, we could support fifteen billion people-and share the planet with nothing else.

Credible scientists also disagree on Earth's carrying capacity. Norman Borlaug, the n.o.bel Prize-winning green revolution pioneer, claims that Earth can support ten billion folks, although he acknowledges that it will require major advances in agricultural technology. This is the same guy who warned at his n.o.bel acceptance speech that the green revolution had bought us only a few decades to deal with overpopulation. Now, more than three decades later, he trusts scientists will pull more rabbits out of the hat. At the other end of the spectrum are Stanford University biologists Paul and Anne Ehrlich who maintain that we have already pa.s.sed the carrying capacity of the planet, which they put at about three billion people. In their view, we've already ensured disaster.

Regardless of who is right, a key issue for any long-term scenario is reforming agriculture in both industrialized and developing countries. Conventional industrial farmers sacrifice soil to maximize short-term returns to pay rent, service debt for machinery, and buy pesticides and fertilizers. Peasant farmers mine the soil because they are trapped farming plots too small to feed their families. While the underlying economic and social issues are complex, sustaining agricultural productivity in both the developed and developing world depends on retaining fertile soil.

Irreplaceable over human timescales, soil is an awkward hybrid-an essential resource renewable only at a glacial pace. Like many environmental problems that become harder to address the longer they are neglected, soil erosion threatens the foundation of civilization over timescales longer than social inst.i.tutions last. Yet as long as soil erosion continues to exceed soil production, it is only a matter of time before agriculture fails to support a growing population.

At its peak, the Roman Empire relied on slave labor to work the plantations that replaced the conservative husbandry of farmer-citizens in the early republic. Before the Civil War, the American South became addicted to similar methods that destroyed soil fertility. In both cases, soil-destroying practices became entrenched as lucrative cash crops seduced large landowners and landlords. Soil loss occurred too slowly to warrant societal attention.

There are plenty of reasons to argue for smaller, more efficient government; market efficiencies can be effective drivers for most social inst.i.tutions. Agriculture is not one of them. Sustaining our collective well-being requires prioritizing society's long-term interest in soil stewardship; it is an issue of fundamental importance to our civilization. We simply cannot afford to view agriculture as just another business because the economic benefits of soil conservation can be harvested only after decades of stewardship, and the cost of soil abuse is borne by all.

The idea of free markets for labor, land, and capital developed alongside Malthus's controversial theory. Adam Smith, the father of modern economic theory, wrote his Inquiry into the Nature and Causes of the Wealth of Nations in 1776. In it he argued that compet.i.tion between individuals acting in their own interest, whether as buyers or sellers, would produce the greatest societal benefit. Clearly, the past few centuries proved that selfregulating, free markets can effectively set prices and match production to demand. Yet even Smith acknowledged that governmental regulation is needed to steer markets toward desirable outcomes.

Almost unquestioningly accepted in Western societies, cla.s.sical economics distilled from Smith's views, as well as variants like Keynesian economics, neglect the fundamental problem of resource depletion. They share the false a.s.sumption that the value of finite resources is equal to the cost of using them, extracting them, or replacing them with other resources. This problem is central to soil exhaustion and erosion, given the long time required to rebuild soil and the lack of any viable subst.i.tute for healthy soil.

Marxist economics shares this critical blind spot. Marx and Engels viewed the value of products as derived from the labor that went into their production. To them, the level of effort needed to find, extract, and use a resource accounted for issues deriving from resource scarcity. Focused on harnessing nature to advance the proletariat, they never put the idea that society could run out of key resources in their lexicon. Instead, Engels tersely dismissed the problem of soil degradation. "The productivity of the land can be infinitely increased by the application of capital, labour and science."' Contrary to his dour image, Engels was apparently an optimist.

In effect, economic theory-whether capitalist or Marxist-implicitly a.s.sumes that resources are inexhaustible or infinitely subst.i.tutable. Given either scenario, the most rational course of action for individuals pursing their own self-interest is to simply ignore the interests of posterity. Economic systems of all stripes are biased toward using up finite resources and pa.s.sing the bill on to future generations.

Concern over long-term productivity of the soil is almost universal among those who have examined the issue. Predictably-and understandably-more pressing problems than saving dirt usually carry the day. Long-term issues seldom get addressed when more immediate crises compete for policymakers' attention. When there is lots of land, there is little incentive to preserve the soil. It is only when scarcity arrives that people notice the problem. Like a disease that remains undetected until its last stages, by then it has already become a crisis.

Just as lifestyle influences a person's life expectancy within the constraints of the human life span, the way societies treat their soil influences their longevity. Whether, and the degree to which, soil erosion exceeds soil production depends on technology, farming methods, climate, and population density. In the broadest sense, the life span of a civilization is limited by the time needed for agricultural production to occupy the available arable land and then erode through the topsoil. How long it takes to regenerate the soil in a particular climate and geologic setting defines the time required to reestablish an agricultural civilization-providing of course that the soil is allowed to rebuild.

This view implies that the life expectancy for a civilization depends on the ratio of the initial soil thickness to the net rate at which it loses soil. Studies that compare recent erosion rates to long-term geologic rates find increases of at least twofold and as much as a hundred times or more. Human activities have increased erosion rates severalfold even in areas with little apparent acceleration of erosion, while areas with acknowledged problems erode a hundred to even a thousand times faster than what is geologically normal. On average, people appear to have increased soil erosion at least tenfold across the planet.

Several years ago, University of Michigan geologist Bruce Wilkinson used the distribution and volume of sedimentary rocks to estimate rates of erosion over geologic time. He estimated that the average erosion rate over the last 500 million years was about an inch every i,ooo years, but that today it takes erosion less than 40 years, on average, to strip an inch of soil off agricultural fields-more than twenty times the geologic rate. Such dramatic acceleration of erosion rates makes soil erosion a global ecological crisis that, although less dramatic than an Ice Age or a comet impact, can prove equally catastrophic-in time.

With soil production rates of inches per millennia and soil erosion rates under conventional, plow-based agriculture of inches per decade to inches per century, it would take several hundred to a couple thousand years to erode through the one- to three-foot-thick soil profile typical of undisturbed areas of temperate and tropical lat.i.tudes. This simple estimate of the life span of civilizations predicts remarkably well the historical pattern for major civilizations around the world.

Except for the fertile river valleys along which agriculture began, civilizations generally lasted eight hundred to two thousand years, roughly thirty to seventy generations. Throughout history, societies grew and prospered as long as there was new land to plow or the soil remained productive. Things eventually fell apart when neither remained possible. Societies that prospered for longer either figured out how to conserve soil, or were blessed with an environment that naturally refreshed their dirt.

Even a casual reading of history shows that under the right circ.u.mstances any one, or any combination of political turmoil, climatic extremes, or resource abuse can bring down a society. Alarmingly, we face the potential convergence of all three in the upcoming century as shifting climate patterns and depleted oil supplies collide with accelerated soil erosion and loss of farmland. Should world fertilizer or food production falter, political stability could hardly endure.

The only ways around the boom-and-bust cycle that has characterized agricultural societies are to continuously reduce the amount of land needed to support a person, or limit population and structure agriculture so as to maintain a balance between soil production and erosion. This presents several near-term alternatives: we can fight over farmland as the human population keeps growing and soil fertility declines, maintain blind faith in our ability to keep increasing crop yields, or find a balance between soil production and erosion.

Whatever we do, our descendants will be compelled to adhere to something close to a balance-whether they want to or not. In so doing they will face the reality that agricultural reliance on fossil fuels and fertilizers parallels ancient practices that led to salinization in semiarid regions and soil loss with agricultural expansion from floodplains up into sloping terrain. Technology, whether in the form of new plows or genetically engineered crops, may keep the system growing for a while, but the longer this works the more difficult it becomes to sustain-especially if soil erosion continues to exceed soil production.

Part of the problem lies in the discrepancy between rates at which civilizations and individuals respond to stimuli. Actions that are optimal for farmers are not necessarily consistent with their societies' interests. Evolving gradually and almost imperceptibly to individual observers, the ecology of economies helps define the life span of civilizations. Societies that deplete natural stocks of critical renewable resources-like soil-sow the seeds of their own destruction by divorcing economics from a foundation in the supply of natural resources.

Small societies are particularly vulnerable to disruption of key lifelines, such as trading relations, or to large perturbations like wars or natural dis asters. Larger societies, with more diverse and extensive resources, can rush aid to disaster victims. But the complexity that brings resilience may also impede adaptation and change, producing social inertia that maintains collectively destructive behavior. Consequently, large societies have difficulty adapting to slow change and remain vulnerable to problems that eat away their foundation, such as soil erosion. In contrast, small systems are adaptable to shifting baselines but are acutely vulnerable to large perturbations. But unlike the first farmer-hunter-gatherers who could move around when their soil was used up, a global civilization cannot.

In considering possible scenarios for our future, the first issue we need to consider is how much cultivatable land is available, and when we will run out of unused land. Globally about one and a half billion hectares are now in agricultural production. Feeding a doubled human population without further increasing crop yields would require doubling the area presently under cultivation. But we are already out of virgin land that could be brought into long-term production. Such vast tracts of land could be found only in tropical forests and subtropical gra.s.slands-like the Amazon and the Sahel. Experience shows that farming such marginal lands will produce an initial return until the land quickly becomes degraded, and then abandoned-if the population has somewhere to go. Look out the plane window on a flight from New Orleans to Chicago, or Denver to Cincinnati. Everything you see is already in agricultural production. This huge expanse of naturally fertile ground literally feeds the world. The suburbs growing around any city show that we are losing agricultural land even as the human population continues to grow. With the land best suited for agriculture already under cultivation, agricultural expansion into marginal areas is more of a delaying tactic than a viable long-term strategy.

Second, we need to know how much soil it takes to support a person, and how far we can reduce that amount. In contrast to the amount of arable land, which has varied widely through time and across civilizations, the amount of land needed to feed a person has gradually decreased over recorded history. Hunting and gathering societies needed 20 to ioo hectares of land to support a person. The shifting pattern of cultivation that characterized slash-and-burn agriculture took 2 to io hectares of land to support a person. Later sedentary agricultural societies used about a tenth as much land to support a person. An estimated 0.5 to 1.5 hectares of floodplain fed a Mesopotamian.

Over time, human ingenuity increased food production on the most intensively farmed and productive land so that today, with roughly 6 bil lion people and 1.5 billion hectares of cultivated land, it takes about 0.25 hectares to feed each person. The world's most intensively farmed regions use about 0.2 hectares to support a person. Increasing the average global agricultural productivity to this level would support 7.5 billion people. Yet by 2050 the amount of available cropland is projected to drop to less than o.i hectare per person. Simply staying even in terms of food production will require major increases in per hectare crop yields-increases that simply may not be achievable despite human ingenuity.

Before 1950 most of the increase in global food production came from increased acreage under cultivation and improved husbandry. Since 1950 most of the increase has come from mechanization and intensified use of chemical fertilizers. Dramatic intensification of agricultural methods during the green revolution is credited with averting a food crisis over the past three decades. Increased harvests stemmed from development of highyield "miracle" varieties of wheat and rice capable of producing two or three harvests a year, increased use of chemical fertilizers, and ma.s.sive investments in irrigation infrastructure in developing nations. The introduction of fertilizer-responsive rice and wheat increased crop yields between the 195os and 197os by more than 2 percent a year.

Since then, however, growth in crop yields has slowed to a virtual standstill. The great postwar increase in crop yields appears to be over. Wheat yields in the United States and Mexico are no longer increasing. Asian rice yields are starting to fall. Crop yields appear to have reached a technological plateau. Thirty-year experiments on response to nitrogen fertilization at the International Rice Research Inst.i.tute in the Philippines found that increasing nitrogen inputs were needed just to maintain crop yields. "At best, we have been able to keep rice yields from decreasing despite considerable investment in breeding efforts and agronomic research to improve crop management." 2 We're still waiting for the next innovation to crank up food production despite the reality that over the coming decades further annual increases of more than i percent are needed to meet projected demand for wheat, rice, and maize. Achieving and sustaining such increases through conventional means will require major breakthroughs as agricultural productivity approaches biological limits. It is getting harder just to stay even, let alone increase crop yields.

In the second half of the twentieth century, food production doubled thanks in great part to a sevenfold increase in nitrogen fertilization and a three-and-a-half-fold increase in phosphorus fertilization. Repeating this story simply is not possible because you can apply only so much fertilizer before plants have all they can possibly use. Even tripling fertilizer applications won't help much if soils are already saturated with biologically useful nitrogen and phosphorus. Since crops don't take up half the nitrogen in the fertilizers farmers apply today, it may not do much good to add more-even if we could.

Growing food hydroponically-by pumping water and nutrients through dirt in a laboratory-can produce far more per unit area than growing food in natural soil, but the process requires using large external inputs of nutrients and energy. This might work on small-scale, laborintensive farms, but it cannot feed the world from large operations without huge continuous inputs of fossil fuels and nutrients mined from somewhere else.

Finally, in all likelihood the easiest-and greatest-increases in crop yields from plant breeding have already been achieved. Given a fixed gene pool already subjected to intensive natural selection over millions of years, further major gains in crop yields would require working around morphological and physiological constraints imposed by evolution. Growth in crop yields has already slowed while the cost of research to bring even incremental increases in crop production has skyrocketed. Perhaps genetic engineering might yet substantially increase crop yields-but at the risk of releasing supercompet.i.tive species into agricultural and natural environments with unknowable consequences.

Meanwhile, global grain reserves-the amount of grain stored on hand at a given time-fell from a little more than a year's worth in 2000 to less than a quarter of annual consumption in 2002. Today the world is living harvest-to-harvest just like Chinese peasants in the 1920s. Now that's progress.

Clearly, more of the same won't work. Projecting past practices into the future offers a recipe for failure. We need a new agricultural model, a new farming philosophy. We need another agricultural revolution.

Agricultural philosopher Wendell Berry argues that economies can be based on either industrial or agrarian ideals, and that an agrarian society need not be a subsistence society lacking technological sophistication and material well-being. He sees industrial societies as based on the production and use of products, whether fundamental to survival (food) or manufactured along with the desire for it (pop tarts). In contrast, an agrarian economy is based on local adaptation of economic activity to the capacity of the land to sustain such activity. Not surprisingly, Berry likes to talk about the difference between good farming and the most profitable farming. Still, he points out that everybody need not be a farmer in an agrarian society, nor need industrial production be limited to the bare necessities. The distinction in Berry's view is that agriculture and manufacturing in an agrarian society would be tailored to the local landscape. While it is difficult to reconcile current trends with this vision for an agrarian economy, a reoriented capitalism is not unimaginable. After all, today's quasi-sovereign global corporations were inconceivable just a few centuries ago.

Agriculture has experienced several revolutions in historical times: the yeoman's revolution based on relearning Roman soil husbandry and the agrochemical and green revolutions based on fertilizer and agrotechnology. Today, the growing adoption of no-till and organic methods is fostering a modern agrarian revolution based on soil conservation. Whereas past agricultural revolutions focused on increasing crop yields, the ongoing one needs to sustain them to ensure the continuity of our modern global civilization.

The philosophical basis of the new agriculture lies in treating soil as a locally adapted biological system rather than a chemical system. Yet agroecology is not simply a return to old labor-intensive ways of farming. It is just as scientific as the latest genetically modified technologies-but based on biology and ecology rather than chemistry and genetics. Rooted in the complex interactions between soil, water, plants, animals, and microbes, agroecology depends more on understanding local conditions and context than on using standardized products or techniques. It requires farming guided by locally adapted knowledge-farming with brains rather than by habit or convenience.

Agroecology doesn't mean simply going organic. Even forgoing pesticides, California's newly industrialized organic factory farms are not necessarily conserving soil. When demand for organic produce began to skyrocket in the 199os, industrial farms began planting monocultural stands of lettuce that retained the flaws of conventional agriculture just without the pesticides.

Agroecology doesn't necessarily mean small farms instead of large farms. Haiti's tiny peasant farms destroyed soil on steep slopes just as effectively as the immense slave-worked plantations of the American South. And the problem isn't just mechanization. Roman oxen slowly stripped soil as effectively as the diesel-powered descendants of John Deere's plows. The underlying problem is confoundingly simple: agricultural methods that lose soil faster than it is replaced destroy societies. Fortunately, there are ways for very productive farms to operate without cashing in the soil. Put simply, we need to adapt what we do to where we do it.

Clues about how to do so may lie in the experiences of labor-intensive and technology-intensive agricultural societies. In labor-intensive systems people tend to adapt to the land. In technology-intensive systems people typically try to adapt the land to their techniques. Labor-intensive cultures that invested in their soil by increasing soil organic matter, terracing hillslopes, and recycling critical nutrients survived for long periods of time in lowland China, Tikopia, the Andes, and the Amazon. Technologyintensive societies that treated the soil as a consumable input developed systems where tenant farmers and absentee landlords extracted as much as they could from the soil as fast as possible by exchanging soil fertility for short-term profits.

This fundamental contrast highlights the problem that dirt is virtually worthless and yet invaluable. The cheapest input to agricultural systems, soil will always be discounted-until it is too late. Consequently, we need to consciously adapt agriculture to reality rather than vice versa. Human practices and traditions shaped to the land can be sustained; the opposite cannot.

Some changes in practices or habits simply require a different mind set, like no-till agriculture, which is effective at r.e.t.a.r.ding soil loss and compatible with both conventional and organic agricultural practices. Nothing really stands in its way, and as experience grows it is being adopted by many U.S. farmers. For other alternative ideas, like organic practices and biological pest control, it is consumers rather than governments who are driving the process of change in today's global economy without a global society.

But governments still have an important role to play. In the developed world, through policies and subsidies they can reshape incentives to promote both small-scale organic farms and no-till practices on large, mechanized farms. In developing countries, they can give farmers new tools to replace their plows and promote no-till and organic methods on small labor-intensive farms. Governments can also support urban agriculture and much needed research on sustainable agriculture and new technologies, especially precision application of nitrogen and phosphorus, and on methods for retaining soil organic matter and soil fertility. What governments need not promote are genetic engineering and more intensive fertilizer- and irrigation-based farming-the very practices pushed by industry as the key to extending reliance on its products.

Emerging interest in supporting an agrarian land ethic is embodied in the slow food and eat-local movements that try to shorten the distance between crop production and consumption. Yet energy efficiency in the delivery of food to the table is not some radical new idea. Romans shipped grain around the Mediterranean because the wind provided the energy needed to transport food long distances. That's why North Africa, Egypt, and Syria fed Rome-it was too inefficient (and difficult) to drag western European produce over the mountains into central Italy.

Similarly, as oil becomes more expensive it will make less sense to ship food halfway around the world: the unglobalization of agriculture will become increasingly attractive and cost effective. The average piece of organic produce sold in American supermarkets travels some 1,500 miles between where it is grown and where it is consumed. Over the long run, when we consider the effect on the soil and on a post-oil world, markets for food may work better (although not necessarily more cheaply) if they are smaller and less integrated into a global economy, with local markets selling local food. As it becomes increasingly expensive to get food produced elsewhere to the people, it will become increasingly attractive to take food production to the people-into the cities.

Despite its seemingly contradictory name, urban agriculture is not an oxymoron. Throughout much of preindustrial history city wastes were primarily organic and were returned to urban and quasi-urban farms to enrich the soil. In the mid-nineteenth century, one sixth of Paris was used to produce more than enough salad greens, fruits, and vegetables to meet the city's demand-fertilized by the million tons of horse manure produced by the city's transportation system. More productive than modern industrial farms, the labor-intensive system became so well known that intensive compost-based horticulture is still called French gardening.

Urban farming has been growing rapidly-worldwide more than 8oo million people are engaged in urban agriculture to some degree. The World Bank and the UN Food and Agriculture Organization encourage urban farming in efforts to feed the urban poor in developing countries. But urban farming is not restricted to developing countries; by the late 199os one out of ten families in some U.S. cities were engaged in urban agriculture, as were two-thirds of Moscow's families. Urban farms not only deliver fresh produce to urban consumers the same day it is harvested, with lower transportation costs and the use of far less water and fertilizer, they can absorb a significant amount of solid and liquid waste, reducing urban waste disposal problems and costs. Eventually it may well be worth recon figuring the downstream end of modern sewage systems to close the loop on nutrient cycling by returning the waste from livestock and people back to the soil. As archaic as it may sound, someday our collective well-being is likely to depend on it.

At the same time, we can't afford to lose any more farmland. Fifty years from now every hectare of agricultural land will be crucial. Every farm that gets paved over today means that the world will support fewer people down the road. In India, where we would expect farmland to be sacred, farmers near cities are selling off topsoil to make bricks for the booming housing market. Developing nations simply cannot afford to sell off their future this way, just as the developed world cannot pave its way to sustainability. Agricultural land should be viewed-and treated-as a trust held by farmers today for farmers tomorrow.

Still, farms should be owned by those who work them-by people who know their land and who have a stake in improving it. Tenant farming is not in society's best interest. Private ownership is essential; absentee landlords give little thought to safeguarding the future.

Viewed globally, humanity need not face a stark choice between eating and saving endangered species. Protecting biodiversity does not necessarily require sacrificing productive agricultural land because soils with high agricultural productivity tend to support low biodiversiry. Conversely, areas with high biodiversity tend to be areas with low agricultural potential. In general, species-rich tropical lat.i.tudes tend to have nutrient-poor soils, and the world's most fertile soils are found in the species-poor loess belts of the temperate lat.i.tudes.

Much of the recent loss of biodiversity has been encouraged by government subsidies and tax incentives that allowed clearing and plowing of lands (like tropical rainforests) that can be profitably farmed for only a short period and are often abandoned once the subsidies lapse (or the soil erodes). Unfortunately, most developing countries are in the tropical lat.i.tudes where soils are both poor in nutrients and vulnerable to erosion. Despite this awkward geopolitical asymmetry, it is myopic to ignore the reality that development built upon mining soil guarantees future food shortages.

There are three great regions that could sustain intensive mechanized agriculture-the wide expanses of the world's loess belts in the American plains, Europe, and northern China, where thick blankets of easily farmed silt can sustain intensive farming even once the original soil disappears. In the thin soils over rock that characterize most of the rest of the planet, the bottom line is that we have to adapt to the capacity of the soil rather than vice versa. We have to work with the soil as an ecological rather than an industrial system-to view the soil not as a factory but as a living system. The future of humanity depends as much on this philosophical realignment as on technical advances in agrotechnology and genetic engineering.

Capital-intensive agricultural methods will never provide the third of humanity that lives on less than two dollars a day a way out of hunger and poverty. Labor-intensive agriculture, however, could-if those people had access to fertile land. Fortunately, such methods are also those that could help rebuild the planet's soil. We should be subsidizing small subsistence farmers in the developing world; teaching people how to use their land more productively invests in humanity's future. Too often, however, modern agricultural subsidies favor large industrial farms and reward farmers for practices that undermine humanity's long-term prospects.

The more than three hundred billion dollars in global agricultural subsidies amounts to more than six times the world's annual development a.s.sistance budget. Oddly, we are paying industrial farmers to practice unsustainable agriculture that undercuts the ability of the poor to feed themselves-the only possible solution to global hunger. Political systems perpetually focused on the crisis du jour rarely address chronic problems like soil erosion; yet, if our society is to survive for the long haul, our political inst.i.tutions need to focus on land stewardship as a mainstream-and critical-issue.

Over the course of history, economics and absentee ownership have encouraged soil degradation-on ancient Rome's estates, nineteenth-century southern plantations, and twentieth-century industrialized farms. In all three cases, politics and economics shaped land-use patterns that favored mining soil fertility and the soil itself. The overexploitation of both renewable and nonrenewable resources is at once well known and almost impossible to address in a system that rewards individuals for maximizing the instantaneous rate of return, even if it depletes resources critical for the long term. The worldwide decimation of forests and fisheries provide obvious examples, but the ongoing loss of the soil that supplies more than 95 percent of our food is far more crucial. Other, nonmarket mechanismswhether cultural, religious, or legal-must rise to the challenge of maintaining an industrial society with postindustrial agriculture. Counterintuitively, for the world beyond the loess belts this challenge requires more people on the land, practicing intensive organic agriculture on smaller farms, using technology but not high capitalization.