An Image Reconnaissance: Agricultural Patterns and Related Environmental Impacts Viewed From Space
Summary and Keywords
Satellite reconnaissance of the Earth’s surface provides critical information about the state of human interaction with the natural environment. The strongest impact is agricultural, reflecting land-use approaches to food production extending back to the dawn of civilization. To variable degrees, depending upon location, regional field patterns result from traditional farming practices, surveying methods, regional histories, policies, political agendas, environmental circumstances, and economic welfare. Satellite imaging in photographic true or false color is an important means of evaluating the nature and implications of agricultural practices and their impacts on the surrounding world. Important platforms with publicly accessible links to satellite image sets include those of the European Space Agency, U.S. National Aeronautics and Space Administration, the Centre D’etudes Spatiales, Airbus, and various other governmental programs. Reprocessing of data worldwide in scope by commercial concerns including Digital Globe, Terrametrics, and GoogleEarth in the 21st century enable ready examination of most of the Earth’s surface in great detail and natural colors. The potential for monitoring and improving understanding of agriculture and its role in the Earth system is considerable thanks to these new ways of viewing the planet.
Space reconnaissance starkly reveals the consequences of unique land surveys for the rapid development of agriculture and political control in wilderness areas, including the U.S. Public Land Survey and Tierras Bajas systems. Traditional approaches toward agriculture are clearly shown in ribbon farms, English enclosures and medieval field systems, and terracing in many parts of the world. Irrigation works, some thousands of years old, may be seen in floodplains and dryland areas, notably the Maghreb and the deep Sahara, where center-pivot fields have recently appeared in areas once considered too dry to cultivate. Approaches for controlling erosion, including buffer zones, shelter belts, strip and contour farming, can be easily identified. Also evident are features related to field erosion and soil alteration that have advanced to crisis stage, such as badland development and widespread salinization. Pollution related to farm runoff, and the piecemeal (if not rapid) loss of farmlands due to urbanization can be examined in ways favoring more comprehensive evaluation of human impacts on the planetary surface. Developments in space technologies and observational platforms will continue indefinitely, promising ever-increasing capacity to understand how humans relate to the environment.
“The landscape is a map drawn by history; reading the signposts to the past.”
— Cornwall County Council Historic Environment Service, UK
The physical appearance of planet Earth as visible from apace has undergone significant change since the end of the latest Ice Age, which was 10,600 yr bce. At the height of the Pleistocene glaciations around 18,000 years ago, approximately one third of the Earth’s surface was covered with land-fast ice, three times the expanse in the 21st century. The ocean surface lay as much as 120 meters below the current mean sea level. Over the centuries, sea level rose and fell across much larger ranges than it does in the 21st century, corresponding to a greater degree of global climate instability. The overall planetary albedo (reflectivity) was much greater owing to wider-spread, year-round snow and ice and to reduced extent of forests and seas (Ehlers, Hughes, & Gibbard, 2016). The conditions for planetary civilization as we know it simply did not exist, and nomadic hunting and gathering appears to have been humanity’s only effective survival strategy for virtually all of Pleistocene times (Richardson, Boyd, & Bettinger, 2001; Gupta, 2004).
The natural driver for the visual transformation marking the beginning of the Holocene Epoch was climate change. However, it can be argued that a comparably significant change in the Earth’s surface appearance and planetary environment has occurred with the development of agriculture since then, especially during the past 2,500 years (Diamond, 1999; Ruddiman, 2016). In lockstep with growing human population, the percentage of the Earth’s surface dedicated to farming and livestock grazing has accelerated over time and only abated somewhat with the intensification of crop production and the Green Revolution beginning in the mid-20th century. While in 1700 ce people utilized only about 7% of the Earth’s surface for farms and pastures, the area presently appropriated is roughly 37% (Ramankutty & Foley, 1999; Owen, 2005; World Bank, 2017). This is nearly 20% greater than the entire planet’s forestland (FAO, 2014). The related environmental impacts, including soil erosion, biodiversity loss, ocean and fresh water pollution, carbon redistributions, and destruction of habitats inland to near-offshore in heavily cultivated coastal regions, impart to agriculture the distinction of being humankind’s greatest environmental impact and severest challenge to our fellow species (Tilman, 1999; Foley, 2009). Envisioning a sustainable future must take into consideration a revolution in agricultural practices, perhaps in part already underway, which acknowledges practical environmental limits, even significant cultural restraint and forms of resilience yet to be understood, accepted, and achieved in a fast-growing, globally integrated world (Rowe, 2014; Legnick, 2015).
This article provides a look at some important landscapes of agriculture visible from space today, together with brief explanations of their meanings and environmental significance. It helps provide geographical context for many other articles in Oxford Research Encyclopedia on agriculture and the environment, though we emphasize that the topic of agricultural geography may be usefully explored much more deeply: linking in important ways to the fields of rural sociology, history, economics, climatology, soil science, hydrology, politics, and others. The brief collection of annotated images below can only be taken as an academic teaser, and the reader is referred to the vital discussions of geographers Sauer (1925) and Cosgrove (2004) to appreciate fully their meaning or potential significance, as expressions of both nature and culture. Individual images are satellite or orbiter based and encompass areas ranging from a few hundred meters to tens of kilometers across. Most are true color or camera images, while those which are not are so indicated. “North” is toward the top of each image.
Online searches of agricultural images are generally constrained by the limited range and explanation of materials available to the public. Metadata are challenging if not impossible to obtain. As such, this collection represents a wide scattering of highly generalized themes. Nevertheless, it underscores the utility of using satellite and high-altitude aerial imagery for studying a range of important agricultural modifications and impacts upon Earth’s surface. There is much practical significance of satellite and high-altitude aerial reconnaissance in land management. Uses in many parts of the world have only expanded and improved since launch of the first continuously monitoring satellite in 1972 (LANDSAT 1). Two broad utilities include tracking changes in landscapes and the classification of land uses. An example of the former is the impact of grazing on grasslands and prairies in the American northwest, a study useful both to sustainability minded ranchers and the Nature Conservancy (Rangeland Center, 2017). As latter examples, satellite imagery layered into global information systems provides managers with timely and accurate ways of identifying and assisting distressed farmlands, locating areas with different kinds of crops for the purposes of seed company production, estimating grain production and harvests, evaluating soil moisture conditions, the relative success of farm policies, and many other important applications (e.g., Freeman, 2009; Wyland, 2009).
The overview provided in this article, however, is not simply a matter of terrain analysis—the study of topography, drainage, land-use, and ecology revealed through image reconnaissance (Wilson & Gallant, 2000). It is about interpretation of enduring land-use patterns in the context of history, economics, policy, and environmental conditions for each selected area. Most climates hospitable to cultivation are represented, including arid, temperate, and tropical landscapes.
The following sections begin with consideration of how lands have been apportioned to farmers, including both traditional and more rigidly pursued modern land surveys, motivated for both practical and political reasons. In some regions, irrigation has been an essential part of sustaining agriculture, so the narrative passes next to that topic. Another key aspect of sustainability, presented in the ensuing section, is the recognition and control of soil erosion resulting from farming and grazing (e.g., Montgomery, 2007). Both soil erosion and irrigation represent significant environmental impacts related to agriculture, though much more can be said about the relationship of agriculture to the natural world. The next-to-last section continues exploration of that relationship with brief considerations of water depletions, pollution from runoff, and deforestation. Finally, the text contrasts these destructive processes with a broad economic and demographic threat to agriculture: the one form of human activity that deliberately eliminates farm and grazing lands—encroaching urbanization. Overall, the article moves from the past to future prospects for agriculture.
Agricultural Land Surveys
Along with the development of agriculture in early Holocene times came permanent settlement on or near fertile lands. How particular areas came to be identified or designated by, or for, the earliest farmers remains unclear, but clues to first approaches may be recorded in more recent history. Metes and bounds is an antiquated land demarcation system that originated in England as a way of defining property boundaries. The description of an enclosure border was defined by distances along a straight line between two terminal points (i.e., metes) and explicitly identified margins provided by geographical features such as rivers and foothills (i.e., bounds). The provisions for a land deed under this system started out by identifying a starting point, usually an obvious landmark, and delineating the path walked around the perimeter back to the beginning guided by metes and bounds. Directions of property-line travel from starting points and around corners were eventually aided by the use of compasses. Recorded boundary descriptions, stored in churches, courthouses, or state offices, could be quite long and difficult to decipher.
Because pursuing metes and bounds was a decentralized exercise with no regulating authority, individualized demarcation led to misalignment of borders, idiosyncratic plot shapes, noncontiguous parceling of land, uncoordinated groundwork for infrastructure, and frequent disagreements between neighboring land users (Libecap, Lueck, & O’Grady, 2011). Resolving disputes was complicated by the fact that boundaries referenced in a property description may have changed from the time the parcel was originally defined. There were many reasons for this, including the redrawing of district limits of cities and townships, the disappearance or transformation of natural or man-made landmarks or features, and the deflection of compass directions due to magnetic field flux after compasses were first employed.
Boundaries surveyed by metes and bounds have been inherited and preserved up to modern times in many regions where agricultural practices precede the implementation of modern surveying practices and land-use policies, as explored further later in this article. They may appear to be arbitrary, especially given changes that have taken place since their original designation (Figure 1). But ultimately, the patterns should not be regarded as random. Enclosures sought to include highest-quality soils available, useful forms of vegetation, and favorable topography for individual landowners. These factors were generally met, historically, in previously forested areas, rather than in the flats and prairies where water was scarce and drainage might be poor or nonexistent. But truly enormous properties could be laid out in drier, more open terrain using essentially the same surveying approach, as for example, in the land grant ranchos of Spanish Colonial California (Chávez, 1998).
While metes and bounds is an older approach toward laying out individual farm plots, it is difficult even now to envision a modern uniform grid system developing in landscapes that are hilly and/or dissected intricately by watercourses. For some parts of the world, this age-old approach remains essentially practical even in the 21st century (Lai, 2014). It should be recognized, however, that the traditional patterns of farmland seen in Britain represent more than the manual surveying of individual medieval farmers. Neolithic field systems, marked by rows or piles of stones at the edges and corners of fields, still define modern field boundaries in parts of Cornwall thousands of years later. This includes long, parallel field boundaries, generally stretching downslope, which make up what are locally called “reave” or “coaxial” fields. The Romans termed them “centuriatons” (Taylor, 1975). The feudal open-field system intermixed manorial lord’s holdings with those of tenant peasants and graziers on a largely communal and fluid basis within single fields. This system, too, represented a legacy stretching back to pre-Roman times. And, from the 12th to 15th centuries onward, English law encouraged the consolidation of smaller landholdings used communally to create much larger farms that could be enclosed by fences or walls and become the sole property of one or more owners. This reached its peak with the Parliamentary Enclosures of the 17th and 18th centuries. Viewing the English landscape from above requires recognition of this complex, multilayered history—deeper than in most parts of the cultivated world.
The British introduced a system of metes and bounds into the United States during colonial times, and it persisted until Congress passed the Land Ordinance Act of 1785 to divide up the land west of the Thirteen Colonies for soldiers after the Revolutionary War, as well as a means to raise money for paying off the war debt. Thomas Jefferson, who proposed an earlier land ordinance measure, oversaw the designing committee that made sure the surveyed land was parceled into orderly plots to avoid the inconsistencies inherent in metes and bounds, to simplify land deeds, and “to provide for the buyer to take poor land along with the good” (Cole & Wilson, 2016, p. 153). After Jefferson negotiated the Louisiana Purchase from France in 1803, effectively doubling the size of the United States, tremendous effort was undertaken for systematic survey and sale of land in a vast new frontier to encourage western expansion and development (Smith, 2012). A centrally planned cadastral (tax survey) system enabled greater confidence in transactions between buyers and sellers, who could trust standardized information about parcel boundaries without the need to see land purchases under consideration firsthand (Libecap et al., 2011).
The result is the modern American Public Land Survey (PLS) System, which has generated a symmetrical, quilt-like layout of farmlands according to a range-and-township location framework (see Figure 2 and Figure 3). Also referred to as the rectangular survey system, the PLS was “the first mathematically designed and nationally conducted cadastral system in any modern country” (Cole & Wilson, 2016, p. 153). Boundaries between farms follow cardinal directions parallel to baselines running east to west and meridians running north to south. Alphanumeric coding identified land blocks down to the scale of a square mile (a “section”) each. Sections are further subdivided into smaller parcels according to the needs of the agricultural economy; this is an important allowance. Small private holdings, for example, make up about 88% of American farms in the 21st century. They average a little more than a third of a square mile apiece (USDA, 2007).
The U.S. PLS enabled rapid establishment of political and economic control over broad areas of the American West as the young United States expanded into lightly populated wilderness. A similar agenda motivates other land development schemes implemented along the frontiers of wilderness elsewhere in the world in the 21st century—notably Amazonia, where the world’s largest rainforest is rapidly succumbing to logging and agriculture (Figure 4). This process has significant implications for global natural carbon sequestration and climate change (Fearnside, 2008). It also contributes to an accelerated rate of species extinction. As forests are cleared to make way for other land uses, forest fragmentation inevitably occurs resulting in smaller woodland habitats, smaller forest interiors, and patches that become more spatially separated (Millington, Velez-Liendo, & Bradley, 2003). These effects tend to increase as intensification of land use grows, with drastic, non-linear impacts on biodiversity (Drakare, Lennon, & Hillebrand, 2006).
Originally colonized as a settlement area for migrants of the Andean highlands by the U.S. government through the U.S. Agency for International Aid in the late 1970s (and then by the Bolivian government for Mennonites in the 1980s), the Tierras Bajas region of Bolivia is considered one of the most rapidly deforested areas in the world (Steininger et al., 2001; see Figure 4). A broad plain of deciduous dry forest with fertile alluvial soils located to the east of the city of Santa Cruz de la Sierra, the Tierras Bajas was subject to a land development survey by the World Bank in the 1990s, which was dedicated to building infrastructure and promoting land clearance beyond initial settlements to facilitate cultivation of soybeans for commodity crop export (World Bank, 1997). In a long-term study of the region from 1975 through 1998, Steininger et al. (2001) used satellite images to evaluate deforestation in the region, which increased from an average rate of 87 km2/year in 1975–1984 to 165 km2/year in 1984–1990 and then 890 km2/year between 1990–1998. Of the 9,400 km2 of forest cleared in the study period, accounting for 48% of total regional forest cover, more than 50% occurred after 1992, with 65% of total land use transformed to farmland.
Agricultural expansion has continued in Bolivia despite growing environmental concerns. Boillat et al. (2015) studied deforestation in Bolivia in relation to socioeconomic factors revealed in census data. They underscore that government policy is a primary driving factor in patterned forest conversion to cropland, similar to American actions “opening up the wilderness” in the 19th century. Unlike the American case, however, a strong correlation links unprecedented rates of land conversion to high levels of rural poverty. Rapid deforestation enabled by expensive technologies (funded by foreign investments) have stimulated migration and resettlement of poor Andean farmers throughout the region (Boillat et al., 2015). Whether these migrants can sustain freshly opened farmlands and achieve prosperity appears to be a major economic gamble.
National development schemes alone are not the only incentive for wilderness-to-farm land conversion. Rather than consolidating efforts by investing in aquifer irrigation and regional fertilizer production to increase yields on existing plots, mechanized farmers and slash-and-burn agriculturalists focus on short-term benefits in response to spikes in global demand for commodity crops by further clearing wilderness for cultivation (Killeen et al., 2008). Consumers are often unaware of the farming practices at the heart of these complex global commodity chains (Waroux, Lambin, Garrett, & Heilmayr, 2016). A mandated reduction of deforestation in one area may lead to increased deforestation in another. Such is likely the case, for instance, in the Brazilian Cerrado, the most diverse floral savannah biome in the world: here agriculture has grown significantly in response to concerns about the negative environmental impacts of beef and soy production in adjoining Amazonia (Waroux et al., 2016; see Figure 5). From 39% to 55% of the natural Cerrado has already been transformed for agricultural purposes (Bianchi & Haig, 2013). Areas converted to new pastureland in habitats such as this are especially vulnerable to erosion, compaction, and long-term fertility loss (Steinfeld, Gerber, Wassenaar, Castel, & de Haan, 2006).
Other regions of rapidly diminishing tropical rainforest owing to expanding agriculture are Southeast Asia and western Africa. Accelerated timber cutting and the development of palm oil plantations related to booming international markets are major causal agents. One estimate provided by the United Nations warns that as much as 98% of Indonesia’s remaining primeval forest could disappear due to illegal cutting, forest fires, and palm oil plantation development by 2022 (Nellemann, Miles, Kaltenborn, Virtue, & Ahlenius, 2007; see Figure 6).
Most of the world’s farmlands are watered through rainfall and snowmelt. But 16% of the total is irrigated by long-range water transfers from natural surface supplies and reservoirs, or by groundwater pumping—as illustrated by the example of the Ogallala Aquifer in Kansas (Figure 2). Average yields from irrigated fields greatly exceed those of crops watered by precipitation alone (by 100%–400%) (FAO, 1996; Siebert, 2010). Irrigation is the favored practice in many agricultural marginal lands—territories that could be well cultivated with increased water supply—where economic resources allow for groundwater exploitation (see Figure 7 and Figure 8), and in floodplains and deltas, the birthplace of much of the world’s earliest agriculture. Irrigation also overwhelmingly claims most of the world’s freshwater use—around 92% according to Hoekstra and Mekonnen (2012). About three-quarters of irrigation water applied to a field simply evaporates (Siebert et al., 2010), while runoff both from irrigation and natural precipitation carries large amounts of nutrients and related pollutants into drainages, promoting eutophication and reduction of water use for most other purposes. The relatively small percentage of irrigation water that infiltrates sustains crops, accounting for roughly 35% of total global harvest according to one United Nations study (FAO, 1996).
Ribbon farms accentuate the importance of river floodplains in supporting agricultural production around the world. They are long, narrow strips of cultivated land. Synonyms include long lots, and French farming system, the latter derived from ribbon farm settlement patterns widespread in areas of North America formerly colonized by the French, based on the older manorial system inherited from feudal times (Jordan, 1974; Harris, 2008). From space, they contrast notably from the square platting of PLS (Figure 7 and Figure 8). The dimensions vary from place to place; however, the defining aspect is that the length is much greater than the width of each plot usually by a factor of 10 but in some cases as much as 100 (Harris, 1984).
As a matter of practicality, a typical set of ribbon farms are lined up width-wise perpendicular to a river, irrigation channel, or roadway, in this way providing direct, roughly equal access to transportation routes and water supply for the maximum number of farmers—many (if not most) of whom are low income or tenant based. In the classic French long lots, farmhouses lay along the river banks at the far ends of fields and served as a proto-neighborhood during winter months when rivers froze and people could socialize by crossing them: a natural sidewalk (Davis, 2000).
Ribbon farming, of course, is not uniquely French or Canadian, but rather for practical purposes has been taken up in many parts of the world, notably along Middle Eastern and Southeast Asian waterways and related floodplains. Within a single long lot there may exist a gradient of soil types and moisture levels, allowing for greater flexibility in land management practices where a range of crops is grown, each crop faring best in a particular type of soil or micro-climatic condition. In addition, a multiplicity of plot owners growing different crops under different land-use practices mitigates the negative ecological impacts of crop diseases and pest outbreaks, allowing for greater overall food security (Margosian, Garrett, Hutchinson, & With, 2009). On the downside, the smaller holdings characteristic of ribbon farming significantly restrict the use of modern tractors and harvesting equipment, requiring greater manual labor inputs.
In arid or semi-arid marginal landscapes where rivers are few and far between, ambitious water diversions for irrigation in pre-industrial times sustained agricultural communities in some instances tens of kilometers from water sources. These include the aqueducts of the Roman Empire and the older Middle Eastern qanat system—both products of strong central government or cooperative planning, sophisticated hydraulic engineering, and a tremendous amount of human labor. Qanats are gently sloping subterranean canals that link an artesian spring or well-fed water supply (a “mother well”) in highlands to distant farms and communities (e.g., Figure 9). No machinery or pumps are needed; water flows non-turbulently through passages several tens of meters underground entirely under the influence of gravity. Developed by the Persians during the 1st millennium bce in what is now Iran, qanats are still the most sustainable and efficient means of providing water for agricultural purposes in many desert regions in the Middle East. Efficiency may be measured in terms of low evaporative loss and unforced, nearly continuous supply despite fluctuations in seasonal conditions (Motiee, Mcbean, Semsar, Gharabaghi, & Ghomashchi, 2006; Mostafaeipour, 2010). An added benefit is typically fine water quality. Owing to freshwater sources and lack of evaporation, qanats suffer little of the salinity problems associated with diverted surface water supplies in arid regions (Hussain, Abu-Rizaiza, Habib, & Asfaq, 2008; Jomehpour, 2009). The qanat system might be even more widely developed today except for over-drafting of source-well aquifers by mechanized pumping, high human labor costs, the challenge of maintenance and repairs from seismic damage and ground subsidence, and various modern socioeconomic pressures.
Abundance of cheap energy and high capital investments have also permitted new technologies to tap aquifers in arid marginal landscapes, as expressed by clusters of deep-well center pivot fields in areas previously inaccessible even to aqueducts and qanat-based irrigation schemes (Figure 10). These remote agricultural enterprises generally exploit large supplies of stored groundwater for the most part inherited from wetter Pleistocene times.
Agricultural Erosion and Its Control
Plowing of fields, harvesting of crops, and overgrazing expose soils to wind and water erosion, processes that can be especially damaging depending upon slopes, time of year, climate, degrees of compaction, exposure, tilth, and many other factors. Montgomery (2007, p. 13268) states, “erosion rates from conventionally plowed fields average 1–2 orders of magnitude greater than rates of soil production, erosion under native vegetation, and long term geological erosion” [italics added]. With field losses averaging ~ ½ mm/yr in fertile soil thickness, this rate of soil loss is simply unsustainable and, globally considered, a definite threat to the long-term viability of civilization. And despite new technologies and improving knowledge, civilization remains as dependent upon food surpluses as it did when it first arose thousands of years ago. The impacts of erosion as seen from space are especially visible in semi-arid and arid marginal lands (Figure 11 and Figure 12). Researchers at Sheffield University (United Kingdom) conclude that as much as a third of the world’s adequate or high-quality arable land has been lost to soil erosion or pollution over the past 40 years (Cameron, Osborne, Horton, & Sinclair, 2015).
During the 1930s, the U.S. Department of Agriculture focused with urgency on issues related to the Great Dust Bowl, a severe human-induced erosional catastrophe caused by market-driven over-plowing for grain, coupled with unexpectedly severe drought conditions in the American Midwest. Concern about loss of soils related to abusive agricultural practices and grazing had impacted political and historical thinking extending back into the previous century, beginning with the pioneering environmental work of George Perkins Marsh (Lowenthal, 2003). As the Dust Bowl catastrophe began in 1933, the U.S. government established the Soil Conservation Service, the name of which has since changed to the Natural Resources Conservation Service (1994). Working with the broader Department of Agriculture, the Department of Interior, and other federal agencies, conservation management encouraged the development of new agricultural land-use practices intended to combat what many regarded as irreplaceable losses of topsoil (Montgomery, 2012). These practices focused upon reducing soil losses due to runoff and the development of natural drainage networks across overworked fields, as well as reduction of saltation and the potential for destructive dust storms from strong winds blowing across loosely compacted, plowed fields. Large-scale features visible from space pertaining to these generally successful initiatives include shelterbelts (windbreaks), contour plowing, strip farming, and terracing.
Shelterbelts consist of rows of trees growing along the borders outlining or separating fields, with the intention of reducing air flow (Figure 13). Where h represents the mean height of a well-constructed shelterbelt, reductions in wind speeds to a horizontal distance leeward of as much as 50h have been reported, sufficient in many cases to protect entire fields (DeWalle & Heisler, 1988). These trees may also provide resources including fruit, nuts, or wood to local farmers. Where shelterbelts alternate with strips or “alleys” of companion crops sown in between, the resulting pattern is called “alley cropping.”
Many fields traditionally have been plowed with furrows and rows trending up and down slopes at an angle. Runoff channeled into slanting furrows can gain speed and capacity to rapidly degrade cropland. Contour farming involves plowing at a right angle to the slope—“along the contour”—so that little or no elevation differences occur along individual furrows. Runoff is most effectively captured and infiltrated by contoured furrows, and soil erosion minimized (Van Oost, Govers, De Alba, & Quine, 2006; see Figure 14). Crop yields also increase, in some cases by tens of percent (Traoré, Gigou, Coulibaly, & Doumbia, 2004). Contour farming is only practical on fairly gentle slopes, however, generally not to exceed 2 to 3o (NRCS, 2017).
Contour farming may be modified by a related form of plowing and planting called strip farming. In this practice, crops are planted in narrow bands, parallel to contour, with alternating strips left fallow to stimulate natural soil rejuvenation. Forage crops and grains are commonly cultivated this way, with the width of strips—typically 7 to 25 meters—determined by average wind velocities and slope gradients for any given locality. In some instances, alternation involves two separate crops rather than fallowing—a technique called strip intercropping. Either way, erosion from runoff and wind is greatly mitigated by this practice (Ghaffarzadeh, 1999).
Agricultural terraces—an extreme form of contour farming—reconfigure the landscape with an anthropogenic imprint. Terracing transforms a steep slope (> 10°) into a series of soil platforms constructed like a staircase straddling the side of a mountain or hill so that cultivation can take place on irregular, artificially leveled strips of ground (Figure 15). Retaining walls of stone or wood, depending upon the region, capture sediment in order to deepen the soil and control moisture within individual terraces (Treacy & Denevan, 1994; Lasanta, Arnaez, Oserin, & Ortigoza, 2001). Cultivators developed terracing in response to growing populations as early as 2500 bce in China (“Shang” times). Possibly the practice spread across the Fertile Crescent, eventually reaching Europe where it is still utilized by farmers and vintners in regions such as Tuscany. But the prehistoric origin of terracing is also evident in northern England, the Caucasus, and many other parts of the world, suggesting multiple venues of origin (e.g., Agnoletti, Frezzo, & Santiro, 2015; Borisov, Simkhova, Zenina, Bukhanov, & Demidov, 2012; see Figure 16).
Terracing almost certainly arose independently of the Old World in parts of Mexico and Peru as early as 500 to 600 bce (Denevan, 1995). Massive terraces supported by heavy stone embankments, for instance, are a distinguishing feature of the 15th-century Incan city of Machu Picchu, creating space for building homes and other dwellings as well as crops. Much as far-reaching aqueducts were a keystone engineering feature of the Roman Empire, terraces enabled the Incan imperial power to flourish in its heyday.
Irrigation can be an especially sophisticated aspect of terracing depending upon crop type and climate. For instance, deliberately introduced floodwater in rice paddy terraces induces growth of nitrogen-fixing cyanobacteria, alters soil pH favorably, buffers root temperature, and hinders weed growth (Lansing & Kremer, 1993). Excavation and maintenance of drainage ditches along terrace walls is important to irrigate crops with runoff, conserve soil, and permit incorporation of mineral nutrients with less labor than would be required otherwise. The essential idea is to maximize gravity as a tool for doing the work. A well-managed network of terrace ditches also allows greater adaptive resiliency to diverse climate conditions and fluctuations, permitting sustainable exploitation of marginal lands (Koohafkan & Altieri, 2010; Altieri, Nicholls, Henao, & Lana, 2015).
Attempts to modernize terracing through mechanization have proven difficult. Mechanically cultivated terrace plots must be wider and longer than they are traditionally in order to accommodate large farm equipment, which raises challenges for irrigation and maintenance of soil quality circumvented via more traditional, albeit more labor-intensive approaches (Stanchi, Freppaz, Agnelli, Reinsch, & Zanini, 2012). Where terracing has been abandoned due to social, demographic, and economic changes, restoration of traditional works may be too costly owing to high required human and/or animal labor inputs (Arnáez, Lana-Renault, Lasanta, Ruiz-Flano, & Castroviejo, 2015; Shi, Cai, Ding, Wang, & Chow, 2004).
Vegetative buffer strips provide an additional control on soil erosion and help mitigate the spread of pollutants such as fertilizers and pesticides washing off from fields. The strips consist of vegetation that has no purpose other than to capture runoff and absorb or curtail its flow. Some fields include permanent vegetative buffers planted in contoured geometries. Many buffers simply protect stream banksides next to fields, helping to protect local biodiversity. Some consist of steep slopes left uncultivated and covered with natural vegetation, separating patches of flatter arable terrain (Figure 17).
Many of the environmental impacts associated with agriculture have already been introduced, including deforestation and groundwater overdrafts, leading to significant changes in regional hydrological conditions. Figure 18 and Figure 19 provide additional images illustrating these impacts.
The potential global environmental impacts of deforestation presently underway are difficult to overestimate. Given that the world population is expected to reach 11.2 billion by the end of the 21st century, and considering the concomitant 50%–70% increase in global food supply required to feed this number of persons, ongoing agricultural expansion has been termed an “agriculture bomb” ready to explode in the forested tropics (Laurance, Sayer, & Cassman, 2014). This is because the tropical climate allows for year-round growth of crops and biofuel production, and potential new farmlands are less expensive for foreign investment than in more developed temperate countries (Laurence, Sayer, & Cassman, 2014). Food supply alone of course is not the only issue; land-use change from deforestation worldwide contributes 10% of the 10 billion metric tons of carbon released into the atmosphere every year (Quéré et al., 2015). Loss of capacity for nature to store carbon in biomass and soils, and alteration of vital rainfall patterns are further concerns (e.g., Butt, DeOliveira, & Costa, 2011; Coe et al., 2013).
Further significant environmental impacts from agriculture visible from space include overgrazing, eutrophication (and associated marine “dead zones”), erosion related to soil compaction from mechanized farming, and soil salinization.
Roughly a quarter of the Earth’s ice-free land surface is dedicated to livestock grazing. Of this, about 20% of the land surface is degraded owing to overgrazing and compaction (Steinfeld, Gerber, Wassenar, Castel, & de Haan, 2006). Treading from the hooves of too many livestock compacts soils, creating networks of trails on hillsides, potential erosion runnels, seedbeds for opportunistic weeds, and increased sediment runoff, which ad extremum can lead to the development of badland topography (Drewry, Cameron, & Buchan, 2008; see Figure 20). When cattle grazing and irrigation are combined under the same land-use practice, soil compaction and environmental impact is even greater (Houlbrooke, Paton, Littlejohn, & Morton, 2011). Indirect effects include diminished water quality, clarity, and habitat for fish in streams downslope and a general decline in landscape biodiversity, especially along stream and river banks where cattle seek water (Bilotta, Brazier, & Haygarth, 2007).
A further strain on the quality of natural waters comes from agricultural runoff laden with nutrients from fertilizers, including manures. The introduction of large concentrations of nitrate and phosphate to streams and rivers promotes algal blooms—a process called eutrophication—which deprives water of oxygen required to support many other forms of aquatic life, including fish. The collapse of fisheries from eutrophication in turn negatively impacts many species of birds and mammals dependent on fish as a food source. The clear blue water of a healthy stream or lake turns pale green as eutrophication sets in—a transition visible all the way from space (Figure 21). Where nutrient-overloaded waters reach the ocean, similar exclusionary patches of algal growth may degrade or destroy marine habitat across thousands of square kilometers. The Baltic Sea is the most polluted large water body in the world owing to its narrow mouth and pattern of sluggish currents. It receives tremendous nutrient loading from rivers that enter from surrounding countries, especially those lining the southern rim.
Salinization is the accumulation of soluble salts—including chlorides, halides, and sulfates—in soils. While this can occur naturally, the phenomenon is exacerbated by excessive irrigation of poorly drained farmlands in arid and semi-arid climates where evaporation rates are also high (Gupta et al., 2008). Salts absorb much of the water that would normally be available to plant roots, and saline waters essentially lead to crops dying of thirst. Near-surface crystallization of salts also alters soil properties in ways critical for plants, including hardness, roughness, cracking, permeability, and porosity—all of which can contribute to erosion (Metternicht & Zinck, 2003; Rengasamy, 2014). Salt-related landscape degradation impacts more than 800 million hectares of land worldwide at an increasing rate—as much as 10% faster each year (Munns & Tester, 2008; Shrivastava & Kumar, 2015; see Figure 22). The potential role of salinization as a contributor to climate change owing to changes in vegetative cover and albedo, certainly at the regional level, is considerable (e.g., Jesus, Castro, Niemela, Borges, & Danko, 2015).
Growing Urbanization and Agricultural Lands
As global populations expand and pressures to mechanize food production increase, so too does the movement of rural populations to urban settings. While this is partly reflected in build-up and densification rather than the build-out of some cities, the expansion of urban footprints is significant, and poses a growing threat to many of the fertile landscapes supporting the world’s most productive agriculture—an important trade-off for humanity (Figure 23).
In the western United States where urbanization is currently strongest, the total area of urban sprawl increased by a third between 1982 and 1997, with over 4% of the 11 western states covered by cities and their suburbs by the year 2000 (Wilshire, Neilson, & Hazlett, 2008). That is about one-tenth the area of farm and pasture land for the entire United States (USDA, 2007). Between 1950 and 2004, the average size of the typical American home grew from 983 ft2 to 2,349 ft2, contributing to this rapid expansion and showing how prosperity in developed countries plays a significant role in land-use conversion (National Association of Home Builders, 2006). The rural-urban transformation in China with its rapidly growing economy is arguably even more spectacular (e.g., Xu, Chan, & Young, 2015; Yu, Verberg, Liu, & Eitelberg, 2016).
Farmlands thrive on level landscapes with well-watered or irrigated, nutrient rich soils. This terrain, together with proximity to food supplies, also favors the development of cities at various strategic nodes favorable for trade, supply, and communication. The process of urbanization at the expense of fertile farmland has been ongoing since late medieval times. Introduction of new practices and technologies—such as the Norfolk four-course system and Jethro Tull’s seed drill—greatly accelerated the process: increasing food supplies enabled larger numbers of people to move to cities. This provided in no small part a foundation for the Industrial Revolution (Sayre, 2010). Today, the total footprint of continuous urban surface, with 75,000 distinct settlements, is equivalent to almost a third of the total area of crops worldwide (GRUMP, 2018). Despite this correlation and current economic trends, no threat appears on the horizon for global food production. But the picture may be different in terms of developing sustainability at local and regional scales.
Agriculture is the human enterprise transforming the Earth’s surface to a greater degree than any other major category of human activity, thanks to Holocene climate stabilization favoring long-term cultivation and grazing in many regions. Patterns of agriculture represent an amalgamation of factors quite apart from crop selection, many of which continue to evolve. For any given region these include history and longevity of agricultural practices; topographic, climatic, hydrological and soil conditions; need for controlling erosion and to irrigate; methods of surveying; economic prosperity and market integrations; spreading of particular political systems and policies; among other factors. The fate of Earth’s environment is strongly linked to the future of agriculture, which can be monitored tellingly by viewing our planet from high above.
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