The Natural History of the Present, Chapter 6

Chapter 6: The End of the Golden Age: the Effects of Agriculture and “High Civilizations” on the Environment

Human transformation of the landscape, like its transformation by trees, whose leaves and fruits provide the livelihood of animals, and whose fire cycles, evapotranspiration rates and root secretions change the environment for other plants, falls along a continuum. Its effects depend on the size of the human population and on its relative demands. One looks for a discontinuity: when did an edible landscape like that of the Cree, or even the horticultural Huron, become a marketable one, like that of sixteenth-century Europe, Anasazi Arizona, Sumerian Kish? The permanently cultivated fields of agriculturalists, whether irrigated or not, mark an intensification of use in a defined habitat. Regions of early agricultural civilization include the Mesopotamian uplands of modern-day Syria and Jordan; the valleys of the Tigris and Euphrates; the valley of the Indus; the lower Nile Valley; the terraced uplands and riverine lowlands where wet rice cultivation developed in Asia. Somewhat later came the Americas, lost in their post-glacial isolation, with the Valley of Mexico, settled at the time of the Spanish conquest more densely than China; the dry uplands and swampy lowlands of Mayan Central America (whose salty ground waters made the area chemically hostile to dense settlement, requiring paved water catchments and plastered water reservoirs, the plaster underlain with crushed limestone); the grassy altiplano about Lake Titicaca, probably deforested by hunters and herders for llama pasture thousands of years before the farmers, along with the straightened rivers, terraced fields, and the ditched shore of the lake itself. Because of its tropical location and because more moisture is available at higher elevations, much of Peru’s cropland lies at elevations over 9000 feet. The people of the pre-Incan Wari culture constructed terraced fields at high elevations, irrigated by water from mountain glaciers, throughout the Andes. Most, like the raised fields of the Tiahuanaco people beside Lake Titicaca, are now abandoned. (Terraces in Peru’s Cola Valley, in use for 1500 years, have topsoil horizons 1 to 4 inches thicker than nearby uncultivated soils, and greater amounts of soil nutrients.)

All early agricultural regions sustained intensive development of limited areas through the creation of fields, the diking or diversion of rivers, the construction of irrigation canals, the digging of raised fields in wetlands. Such developments were necessary to produce the agricultural surpluses that supported high civilizations, that is, those with grandiose, permanent buildings; calenders derived from astronomical observation; armies; craftspeople (doing weaving, pottery, metalwork, stonework); some sort of record-keeping ability. An Egyptian peasant produced 5 times the food required by his family, a Mayan farmer less than double his family’s needs; which makes the Mayan achievement more surprising and explains why the Mayan states, despite their bellicosity, could not afford standing armies, or support them long in the field. Perhaps their very bellicosity was a result of their tenuous holds on their material lives, their long history of nutritional stress. The floodplain of the lower Amazon supported a population of maybe 3 million people (some writers claim more), but they for the most part lacked permanent buildings or permanent records. The area exposed by the seasonal rise and fall of the river, some 40 to 60 vertical feet for 1000 miles above the estuary, was used for cultivating crops, as was the forest itself, many of whose trees (especially the palms) produce edible fruit in amounts that rival or surpass the yield of grains. This population melted away upon contact with the Portuguese, leaving little trace, except in the cultures of some of the forest tribes (Levi Strauss suspected some had descended from more complex cultures) and in the structure of the floodplain forests, many of whose tree populations seem to be the result of human manipulation, and in the extensive areas of black earth (terra prieta) soils. (Terra Prieta soils are soils suitable for growing crops. It is now thought man-made terra prieta “black earths” may constitute 10% of Amazon soils. At the mouth of the Tapajos River in the Amazon is a zone of black earth soil 3 miles long and 0.5 mile wide. Accessible black earths are now mined and sold to garden centers.)

The effects of large agricultural civilizations extend far beyond their fields. Large, settled agricultural populations, with their ability to accumulate things, their constant growth of population, their rising wealth, their need for building material and cooking fuel, their technologies (pottery-making, brick-firing, metal smelting) that further increase their need for fuel, their domestic animals, construction programs and wars, have a tremendous effect on the surrounding landscape. Primarily this effect is through deforestation: trees cut for building material, for cooking fuel, for fuel for metal-smelting, pottery-making, firing bricks (1 cubic meter of burnt brick required 150 cubic meters of wood in Roman times). Deforestation in ancient Eurasia seems to have been essentially market-related, that is, the timber was cut as needed. To a certain extent in the ancient world, forest exploitation was limited by the technology of transport, but for example, forests near mines, such as in Cyprus or Spain, might be cut and recut several times to smelt the ores, and the (more valuable) metal shipped much greater distances than the wood itself could be. The plain of Cyprus was cut over several times to smelt Cyprus’s copper ores and for material to build ships. Finally the land (whose trees were probably kept from regrowing by browsing goats) was granted free to farmers, in order to enrich the polis. Herds of sheep, cattle, camels, goats, used for meat, milk, wool, traction power and transportation, were pastured on the areas surrounding the agricultural zone, sometimes at great distances, the animals moving from pasture to pasture up the slopes of distant mountains as summer progressed, returning to the lowlands near the villages in the winter. In Mesopotamia, pastoralists grazed their animals near the rivers in winter, moving them out onto the plain in summer, until growing droughts after 2200 BC forced them to stay near the rivers all year. Overgrazing causes erosion. Pasturing cutover forests prevents their regrowth. Greek shepherds girdled trees to encourage the growth of grass. For Greeks, cutting the forest meant the desecration of paradise, for man had sprung from an oak; but also the birth of civilisation. Cutting forests and grazing would change the local climate and hydrology, increase temperature extremes near the ground and run-off from rain or snow, raising flood levels and levels of salts and silt in rivers. Cutover forests would become scrub, or grassland; grassland, if overgrazed or if its native grasses were not adapted to grazing (as in the American Southwest), would turn into more unpalatable vegetation: the thymy uplands of Greece, the sage, creosotebush and mesquite of the Great Basin of North America. In this way over thousands of years whole landscapes were altered. By the time of the fall of Rome, the Mediterranean basin was a largely denuded landscape. The marshes at the mouths of its rivers were human artifacts, created with soil washed off the uplands, its steeper hills were eroded, often to bare rock, deforested, sometimes terraced, or used for pasture. (Terraces accumulate soil by mass wasting, and help store run-off water, passing it slowly down the hill, thus making it more available to crop plants; by slowing water movement downhill, they help recharge ground water levels.) The vine and olive, with wheat grown on the flat patches among the trees, were crops that would grow on the eroded soils.

Humans didn’t do it all: a slowly drying climate, which deforestation reinforced, modified the air circulation patterns over the Mediterranean and further reduced rainfall, especially over northwest Africa. When the rain was hard, the soil slid away: the natural erosional processes of the Mediterranean proceed by catastrophic rain events that rearrange landscapes in a stroke. But people created many of the changes. Anatolian and North African lands on the south side of the Mediterranean basin, those “fat lands” colonized by the Greeks, and used, along with the Nile Valley, as a source of grain by the Romans, were also severely eroded by deforestation and agriculture. (By the time of the Peloponnesian Wars in the early 400s BC, Egypt and Sicily were providing 30-75% of the food for Greek cities; and 900 years later the silt carried by the Nile was feeding Rome.) It was said that in early Roman times one could walk from Egypt to Morocco in the shade. But by 100 AD, the Middle East of the Roman Empire was completely deforested. Most wheat-growing North African soils were degraded by 200 AD. Former Greek port cities in Turkey lie several kilometers inland from the modern coast, and tens of meters of soil and gravel cover valley bottoms in Israel and Lebanon. Deep under such spoil lie Roman bridges. Ancient Antioch, once in Syria, now in Turkey, lies under 28 feet of water-eroded soil. Such bare lands no longer hold much water. There are few springs; houses are built with cisterns. Similarly, the irrigated landscapes of Mesopotamia and the Indus valley turned into deserts; the Thar desert in India was a jungle at the time of Mohenjo-Daro, and Mesopotamian Iraq was a fertile land of swamps and partly wooded uplands, with tigers, date palms, and herds of gazelle. The Thar and Mesopotamia also suffered from long-term post-glacial drying, but again people helped things along. Life was still possible after a collapse, if “high civilization” wasn’t, and the rise and fall of human affairs gave the landscape some respite. The Maya population in the 800s fell from 2.5-5 million to 500,000 with the civilization’s collapse and the tropical jungle recovered. Iron age agriculture in Britain left erosional terraces a meter or more high along rivers like the Severn; then the people disappeared and the land regrew to trees before the next invasion. Climates change and cultures come and go. Settlers of “new land” in times of population expansion in Spain would come upon the stone walls and old olives trees of their predecessors amidst the fragrant scrub. Some writers trace the rise and fall of civilizations to the 30 to 70 generations (800 to 2000 years) it takes a civilization to occupy and erode its soils. During the Bronze Age, Mediterranean uplands were settled, abandoned and resettled over 1000-year periods, as their soils eroded under the plow (fields were plowed 3 times a year), were abandoned to pasture or scrub, the population of the hills declining, until the topsoil recovered and the land was settled again.

All previous “high civilizations” have collapsed, some from conquest, some from internal dissolution, often helped along by environmental problems, such as a cooling climate, soil depletion, drought or new diseases. It has become the fashion to put such collapse down to abuse of the environment; that is, to exploitation of fields or forests at an unsustainable level. But some economies collapsed because demand for their products fell (the southern Arabian trading cities that depended on the collection of frankincense and myrrh are examples), or caravan routes were cut off by wars, or were no longer usable because of a drying climate (both seem to apply to the overland route from China to western Asia and the Mediterranean between Roman and Renaissance times). A hundred year drought brought down the Tiahuanaco civilization (a pre-Inca culture) in the Andes. A decline in rainfall of 10-15% caused a fall of 40 to 45 feet in the level of Lake Titicaca, which dried up the raised fields about its shore, and slowly depleted the ground water levels that fed the rivers and the fields that were irrrigated from them. Ground water reserves are large. They take a long time to dry up and a long time to recharge. In Tiahuanaco, ground water also fed fields that had been excavated down to just above the ground water level.

The climate change that caused the drought in the Andes was world wide. At the same time a shift in rainfall patterns in Central America weakened the Maya civilization and eventually caused the collapse of the southern cities. Rainfall seems to have decreased in the uplands, terraced to grow corn, and increased near the coast, where crops were grown on ditched swampland. (Severe droughts in the 800s and early 900s correspond with the abandonment of many Mayan cities.) The Maya uplands were vulnerable to erosion, the ditched swampland to siltation. Both were labor intensive habitats to maintain. The canals among the swamp fields were useful for transporting crops and also a source of fish. The Yucatan is a karst landscape of eroded limestone with little surface water. Much of it was cleared during Maya times and remained cleared for centuries. (The Maya may have planted woodlots for building material and thatch.) In the southern lowlands, with 100 inches of rain a year, cities got their drinking water from plastered reservoirs fed by paved or plastered catchments, in the north from wells or natural pools. Lake cores show episodes of severe erosion, presumably from the terraced cornfields on the uplands about the lakes. Erosion would have started with forest clearance (for wood for building timbers, cooking fires, to manufacture plaster) but worsened under a high rainfall regime. After the erosional events, rainfall seems to have dropped off. The corn crops would then have suffered from lack of water, as well as from the declining nutrients of continuous cultivation. The swamp fields were kept fertile with silt dug from the canals and with composted water lily plants, but North America had no large domesticated animal like the cow or the water buffalo to bring in nutrients from the surrounding landscape to upland fields in the form of urine and manure. Without animal manure (whether the Maya used human manure isn’t certain) maintaining the fertility of their terraces must have been a problem. Erosion, which carries away soil nutrients with the soil, was a perennial problem: soil traps were constructed so that eroded soil could be carried back uphill to the terraces. The traditional milpas of the Maya were a mixed forest garden, planted in a patch of cleared forest, where a variety of crop plants were grown, annual crops giving way after a few years to perennial and tree crops, as the land slowly returned to forest. Thus the soil was almost always covered by vegetation. (Even so, modern milpas in the Yucutan erode to bedrock in two decades.) New land has to be periodically cleared, and in modern times in the face of population pressure the fallow may be reduced to as little as four years. Higher populations require constant production and permanent fields; thus the terraces, with their exposed erodable soils and successive crops of nutrient-demanding corn. (Central Mexico also shows signs of extensive pre-Columbian soil erosion. Old corn-growing soils there—now still plowed for crops—consist of thin mantles of broken rock while nearby soils with little evidence of cultivation have 18 inches of topsoil.)

Disaster in the Americas was associated with a warming trend (1º C., 1.75º F.) in Europe: the Medieval Climate Optimum. The rise of 1º C. increased growing seasons in northern Europe by a month and extended the range of many crops. The Mediterranean ecotone, with its mild wet winters and hot dry summers, shifted north to include most of Europe. Grapes were grown on south-facing terraces in England, and human settlement and the tree line moved further up in the Alps, and into the uplands of Scandinavia. The warmth corresponded with a burst of population growth and forest clearance. The wheeled plow was invented in the sixth century. It wouldn’t come into common use until the eleventh century, when, pulled by a yoke of eight oxen, it allowed the heavy clay soils of the European oakwoods to be broken, and settlement to increase throughout Europe.

While food production was the purpose of both horticulturists like the Onondaga and of “high civilizations” like the Inca, the “high civilizations” required more per farmer. This meant more land under cultivation and more work per person. Generally speaking, peasants under Inca rule were supposed to produce a third for themselves, a third for the gods (the priesthood and their temples), and a third for the Inca, the god-head of the state. Besides cultivating crops, peasants had to provide labor in construction, in transportation of crops and goods, and in mining; they also had to provide military service. Food was hard to transport without wheeled vehicles, and heavy foods like potatoes were usually stored in stone warehouses near fields. Domesticated llamas brought warm season crops like maize (eaten and used for brewing beer) and cotton, as well as beans, cocoa, squash, and chilis from terraced fields lower down on the slopes of the warm and humid Amazon basin to the east. The llamas also carried forest products such as woods, resins, honey, feathers, animal skins, wild fruits, medicinal and hallucinagenic plants. Such luxury goods were grown, collected, and transported by free peasant labor. Under the Incas, more people created a more powerful state and more wealth, and put more stress per person on the landscape. The Inca state, like many of the pre-contact American states, was not a trading state like those of Europe (or as the European states were becoming), but a theocratic one, whose power was based on the success of its agriculture and thus on its successful connection with the mountain gods. The size and impressiveness of its monuments, and the wealth of its upper classes, celebrated this. (A massive Tiahuanaco fountain building seems to show the mountains yielding their water to the valleys below.) The Inca economy depended on traditional labor-sharing agreements and on the ceremonial exchange of goods among kin groups (of lowland crops, for instance, for wool and potatoes) and not on monetized exchanges calculated in coins.

Inca agricultural systems were capitalist systems in that their cultivation produced a profit. Roughly speaking, it would seem that part-time adult labor produced a surplus double or more that of the subsistence needs of the farmer and his family. The situation is a little cloudy because other labor was required of the peasantry and the participants in the labor brigades were probably fed with food they themselves had grown, re-distributed to them. The profits from agricultural activity accrued to the organizers of the state: the Inca, his nobles, the priesthood. Profits were re-invested in roads and bridges, in monumental public buildings, and in maintaining the lifestyle of the priests and nobles. All this was made possible by an organized, labor-intensive manipulation of the local landscape.

The Tiahuanaco people that preceded the Inca constructed extensive raised fields on the shores of Lake Titicaca. These fields were not reconstructed by the Inca state and are now an undulating upland used for cattle pasture. Originally however, the fields were an example of large-scale, sustainable agricultural production. Lake Titicaca lies 12,500 feet above sea level. It is a large, deep lake and its great mass of water (it never freezes) moderates the local climate. It fluctuates unpredictably in level, by as much as 15 feet over 2 years in historic times and by much more over the last 12,000 years. The lake is home to substantial populations of fish and waterfowl (ducks and flamingos) and has extensive shallows of totora reed, parts of which are eaten, and whose stems and leaves are used for thatch and for making textile boats. Raised planting beds are common in peasant agricultures. Their primary purpose is drainage, but those about Lake Titicaca also functioned as solar collectors. These raised fields were large (15 to 30 feet wide and up to 600 feet long), their planting surface raised about 5 feet above the bottoms of the canals, from where the earth was excavated to build them. The water in the canals, warmed by the strong tropical sun, irrigated the growing plants with warm water from below, drawn upward by capillary action. The heat stored in the water, and transferred to the beds, improved plant growth (yields in raised beds nowadays are double those of dryland crops nearby) and protected the plants from summer frosts, which are not uncommon at this altitude. The depth of the water in the canals was regulated, so as to provide sufficient heat storage and also water for irrigation. Water came from the lake, from groundwater, and from rivers that flowed into the lake. The canals thus constituted a sort of extended delta in the lake. (The rivers were also used for other sorts of irrigation.) This is certainly a modified landscape, though not necessarily a degraded one. The modifications of the lakeshore and river deltas may have improved things for the fish and waterfowl of the Lake Titicaca basin. These raised-bed fields are in some ways similar to rice paddies, except that the plants are cultivated above the water table. The crops included quinoa (a grain), potatoes, and various other Andean roots. The azolla and blue-green algae that colonized the canals fixed nitrogen and provided one base of the food chain that lived in the canals, itself a part of the food web that connected the waterfowl, fish, birds, turtles, amphibians, and higher plants of the lake. The silt and bacterial scum dug from the canals made the fertility of the fields self-maintaining; they needed no fallow and could be planted to crops every year. The Tiahuanaco people, somewhat unusual in the Americas, also used human manure on their fields. (They may also have pastured their large flocks of llamas and alpacas on them after harvest.) The nutrients added to the soil and produced in the canals were recycled in the canal-field ecosystem, or incorporated into the larger food chain; they didn’t leach out significantly as mineral elements into the lake water downstream and cause algal blooms, as modern agricultural nutrients do.

Similar agricultural systems include the Maya’s planting beds in lowland swamps; and the raised beds dug in seasonal wet prairies in the American Middle West. The disadvantage of such systems to a modern farmer is that probably half the area is lost to waterways. This is not a disadvantage in a biologically friendly system (where agriculture is considered part of the whole environment); the water is not only necessary as a heat trap, but for fertilization and irrigation; and the ditches extend the habitat of the lake or swamp. It is a disadvantage in a modern capitalist agricultural system where land in crops is maximized, machines are available to install drainage tile, fertilization comes from artificial fertiliser, irrigation water from pumps, and the polluted runoff is exported downstream. Heat storage (as with oranges grown among the lakes of central Florida), is provided by the lakes themselves. Natural wetlands in Florida, such as those that once occupied the inlet to Lake Apopka (since drained to grow oranges) are about 2º C. warmer on frosty mornings than drained farmland. The cold temperatures also last less long. John Bartram pointed out the sheltering effect of the palm canopy over wild orange groves in Florida on cold mornings in the 1770s. An agro-ecosystem that grew oranges under a canopy of palms (also useful for various fruits), the trees planted on raised beds that allowed the wetland to function, and to maintain a flow of clean water downstream, would provide more frost protection and filter the water entering the Lake. In this case Lake Apopka might still be a clear sandy-bottomed swimmable lake, with a natural bass fishery, and not a murky and eutrophicated one.

Inca landscapes were modified much more than those of the Onondaga. Rivers were straightened, fields levelled, or excavated down to ground water level, irrigation canals dug. Mountain springs were led into stone-lined trenches to irrigate descending terraces, changing (probably not eliminating), their watery contribution to rivers. Much terraced land was inherited from the Wari. The deforestation of the Andean uplands, perhaps partly for fuel, but mostly to create better pasture for deer and the semi-domesticated cameliids (llama, alpaca, vicuna), happened beyond the time of folk memory. The llama and alpaca have been domesticated for 7000 years. Like the Tibetan plateau, which was once wooded with cypress, and which was also cleared several thousand years ago, to grow barley and graze animals, the Andean highlands were once wooded. So the natural vegetation of the Altiplano disappeared a long time ago. A change in vegetation, without a concurrent increase in erosion and in nutrient losses, is not necessarily degradation, especially if the new system turns into an adequately functioning ecosystem. (Fire clearing, which was probably used here, certainly involves an initial loss of nutrients.) The new landscape may be a little more leaky of nutrients and have a different hydrology than the former one. The modern landscape, dryfarmed where possible, and otherwise pastured by sheep, is clearly degraded. A similar upland in Andean Columbia, planted by an utopian community to forest, is reverting to a natural forest. Trees and shrubs, with their associated birds and animals, have appeared that were not planted, undoubtedly planted in the droppings of those animals and birds. Is this better, or only different? It is certainly better for the hydrological environment than modern grazing and farming practices. Whether turning the Andean uplands into pasture for the native camiliids significantly degraded the Altiplano depends on many things, including the rate of stocking and the response of the vegetation to the animals. The buffalo and elk barrens in the forests of the eastern United States were not degraded landscapes. They were human artifacts, maintained by the animals and by man-made fire, but not degraded. Nor were forests burned to maintain their crops of nuts, at the expense of other, later successional trees degraded; such systems were capable of indefinite survival and fitted seamlessly into the surrounding landscape, including the hydrological landscape; though the forests that resulted were not those that would have been there without human intervention.

What is interesting is the difference, so much as one can make it out, between the agricultural (proto-capitalist, marketable: these words probably work better with Eurasian cultures than with American ones) and the edible landscape. One thinks of the edible landscape as subsistence and sustainable, but this is not necessarily so. Subsistence landscapes, even gathering and hunting ones, can be over-exploited and thus become unsustainable in the long run. Modern agricultural landscapes are for the most part unsustainable, but older marketable landscapes were sometimes sustainable and sometimes not. The landscape of Tiahuanaco was proto-capitalist and some of it was sustainable. Fish remained an important part of the diet of people about Lake Titicaca. The Maya terraces were not sustainable because of soil erosion and nutrient depletion from the continuous crops of corn, but their swamplands were. The continued productivity of waterways is a good indication of the health of terrestrial environments.

Some writers argue that the large-scale adoption of agriculture 8000 years ago helped create our present climate. During previous interglacial periods, levels of carbon dioxide and methane (two natural greenhouse gases) fell, as the intensity of summer sunshine decreased due to cyclical changes in the earth’s orbit. These changes reinforced each other and would have set the earth into another cycle of cooling. (The reason for the falls in the levels of the greenhouse gases aren’t completely known, but have also occurred in past glacial cycles; the gases may have fallen in response to falling global productivity, and at the same time the cooling ocean and cooling northern wetlands may have stored more of them.) In the current interglacial (our time), carbon dioxide began to increase about 8000 years ago, when it should have continued to fall. The increase corresponds with the extensive clearing of forests in India, northern China and southern Europe for agriculture, and the corresponding release of carbon dioxide from burning trees and cultivated soils. Estimates of the amount of cropland and modified forest needed to feed a person with Neolithic agriculture are quite high (about 7 acres), so the amount of carbon dioxide released to the atmosphere by a relatively small farming population (a few tens of millions by 7000 to 8000 years ago; perhaps 200 million by 2000 years ago) would be considerable. Methane, another greenhouse gas, began to increase about 5000 years ago, with the cultivation of paddy rice. (Like natural wetlands, rice paddies produce methane.) The growing abundance of livestock, which produce methane in their digestive tracts, also added to the amount in the atmosphere. Cattle were domesticated in Greece or the Balkans about 8000 years ago and spread to the Middle East, sheep and goats at least a thousand years earlier. Looked at this way, agriculture may have contributed 0.8º C. of warming globally before 1700, 2º C. at higher latitudes. The temperature increase at higher latitudes prevented the re-glaciation that should have started 4000 to 5000 years ago. In this case agriculture created a new golden age. (Other writers argue that the current orbital configuration implies a long current interglacial, an unheard of 60,000 years; in either case we’re lucky.)