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The driving forces of environmental change

The rapid rise and the enormous power of the Aztec state were based on the control of much of Mesoamerica and on the subordination of hundreds of different groups that paid tribute to the emperor. Aztec wealth depended to some extent on the concentration of high-quality goods (e.g. metals, obsidian, tropical fruits, high protein food) and labour collected as tribute from conquered groups. Even with the highly productive chinampa system and dry-field and irrigated terraces throughout the slopes, the Basin of Mexico probably had to subsidize a sizeable portion of its food and fibre during Aztec times from elsewhere and took food from considerable distances during times of famine (Williams 1989).

This tradition of concentration persists today. Intensified by the Spaniards and during independence, it has now reached staggering proportions. Approaching 20 million residents and with most of its agricultural lands and water taken over by settlement, the basin generates only a small fraction of its resource needs (table 7.16) and must import vast amounts of food, energy, wood, water, building materials, and many other products. In effect, other ecosystems, often to their own economic and environmental detriment, subsidize the water and energy flows of the basin. This subsidization is possible because Mexico City is the hub of an immense concentration of economic and political power that permits it to concentrate resources as well.

Critical to understanding the driving forces of environmental change, then, is a system of subsidies, in terms of both natural (ecological subsidies) and financial resources (economic subsidies), underpinned by social institutions and organizations that legitimate and implement them. This system represents the "how" of sustaining the process of concentration, but it is its forces of demand and production that ultimately drive environmental changes in the basin. These integrative forces are population change, governmental policy, and technological capacity.

Table 7.16 Energy consumption in the Mexican Republic and in the Basin of Mexico, 1970-1975

  Consumption (m³)a
1970 1975
Mexico Basin of Mexico Mexico Basin of Mexico
Oil 34,060,003 9,215,600 (27.1%) 48,081,005 13,202,132 (27.5%)
Electricity 2,320,482 753,874 (32.5%) 3,708,698 1,065,554 (28.7%)
Coal 1,470,000 0 2,616,000 0
Total 37,850,485 9,969,474 (26.3%) 54,405,703 14,267,686 (26.2%)

Source: Ibarra, Saavedra, Fuente, and Schteingart (1986).
a. The numbers in parentheses indicate the proportion used by the Basin of Mexico with respect to the whole country.

We are unable here to demonstrate quantitatively the linkage of each of these proposed driving forces to environmental change per se. We are stymied by the paucity of quantitative measures of these forces, save for population change. Our claims, therefore, reflect qualitative judgements regarding the major forces that shape demand for natural resources in the basin, including those that exacerbate demand by not dealing adequately with its environmental impacts.

Population change

The Mexican model of development has given priority to improving the quality of life in the large cities, in which demand and political power are more concentrated, at the expense of the rural areas, which have become comparatively poorer. As a result, from 1950 to 1980 the basin experienced marked improvements in demographic and domestic indicators of quality of life (table 7.17), exceeding those for the country as a whole and especially those of impoverished rural areas. Public services such as education, potable water, and sewerage are underdeveloped in the poorer areas of central and southern Mexico. In the Mixtec region south of the basin, for example, the proportion of houses with drinking water is less than 4 per cent, and most of the towns do not have sewerage systems.

Table 7.17 Evolution of the quality of life indicators for the Basin of Mexico, 1950-1980

Indicator Basin of Mexico Mexican Republica1980
1950 1960 1970 1980
Life expectancy at 1 year of age (years) 55.0 60.8 63.2 65.2 64.4
Infant mortality (%) 12.0 7.9 7.4 4.3 7.1
Adult literacy (%) 83.8 88.4 92.6 95.6 83.0
% of houses with running water n.a. 35.0 53.0 67.0 n.a.
% of houses with drainage n.a. 33.0 63.0 81.3 n.a.
% of houses owned by their residents n.a. 34.0 50.0 52.7 n.a.

Source: Ibarra, Saavedra, Fuente, and Schteingart (1986). n.a. = not available
a. Values for the Mexican Republic are given for comparison purposes.

The rural poor, then, view the basin not only as a concentration of potential employment but as an improvement in the quality of life, at least in terms of education, health services, and so on. These push-pull factors drive rural migrants to the basin, generating most of the enormous population growth there.

Population growth cannot be ignored as a major driver of environmental change in the basin because of the overwhelming demands for resources and generation of effluents that follow from it. At the current growth rate, estimated at approximately 4 per cent, the population doubles every 15-18 years, and with it the demand for water, electricity, transportation, and housing more than doubles. For example, the number of automobiles grows at an annual rate of about 8 per cent, and doubles every decade. In turn, this increase in cars has enormous impacts on air quality in the basin. If the population levels of the basin were lower and growth rates smaller, the magnitude of environmental changes would surely have been less; comparisons with environmental conditions in the 1950s illustrate this claim.

Governmental policy

Governmental policy has acted as a driving force in at least three ways: it has promoted the concentration of employment and services in the basin, subsidized resources that sustain this employment, and aggravated the environmental impacts of demand through environmental policies.

Since Aztec times, the basin has been a "primate place" where all political and economic power rested. This primacy has fostered decisions to promote industrial, governmental, and service activities in the basin, providing employment and attracting population. Services are, in effect, underwritten by the country at large. For example, buses, trolley-buses, and the metro train now cost approximately US$0.10 per trip, a fixed tariff that is independent of the distance travelled. The metro train, which transports 3 million passengers per day (Bravo Alvarez 1986), generates a revenue of US$300,000 per day, but the real cost of operating the system is in the order of US$1.5 million per day (Bazdresch 1986). The difference is ultimately met by all taxpayers, many of whom reap no benefit from the service in any way. Other services, such as electricity, gas, garbage collection, and road maintenance, are subsidized for the whole country and not only for the Basin of Mexico. Because the city receives these services in a higher proportion than the rest of the nation, however, it also receives a higher share of the subsidy, as in the case of energy.

Having exceeded the resource base of the basin itself, policy makers have turned to subsidies from elsewhere. These need not be reiterated here, save for the example of water, noted above. Unable to meet the demand for water, officials have met chronic shortages in the basin by importing water from the Lerma and Cutzamala basins (Bazdresch 1986). At the same time, the export of sewage water into the Tula basin is contaminating that system. Water costs around US$0.10/m³ to distribute in Mexico City, largely owing to the high costs of pumping water into the basin (Bazdresch 1986). The government spends approximately US$150 million per year to supply water to the Basin of Mexico, but the revenue collected is in the order of US$42 million, less than 30 per cent of the total cost.

The official response to environmental deterioration in the Basin of Mexico has largely helped to sustain the problems. Solid waste, for example, was disposed of in open dump-yards surrounded by poor neighbourhoods until 1986 (Ezcurra 1990b). Only when government officials realized that the yards were infiltrating toxic leachates into the aquifer and contaminating the air thus affecting the entire basin, not just the immediate neighbourhoods - did they make a decision to dump waste in confined trenches designed expressly for the purpose.

In another example, the Mexican Index of Air Quality (IMECA) has been copied almost verbatim from the National Ambient Air Quality Standard (NAAQS) of the United States (SEDUE 1985), which in turn was derived from an index proposed by Thom and Ott (1975). Table 7.18, which offers a comparison of the three indexes, reveals that IMECA minimizes the risks involved in different levels of atmospheric pollution. That is, for a similar concentration of air pollutants, the Mexican standard informs the population of risks much milder than those really involved. Similarly, during the winter season when atmospheric thermal inversions abound and render the air quality unacceptably poor almost every day, the health authorities inform the public to take care against influenza and other respiratory diseases, thereby implying that these diseases are related to seasonal ills rather than to pollution. In short, governmental policies have ignored environmental problems until they become dangerous to human occupation. Such disregard actually promotes the drivers of environmental change by not controlling them.

Table 7.18 Comparison of the Mexican air-quality index (IMECA), against Ott and Thom's index and the National Ambient Air Quality Standard (NAAQS) of the United States, for similar pollution levels

Index IMECA description Ott and Thom NAAQS
0-50 Favourable environment for all sorts of physical activities Good Below NAAQS
51-100 Favourable environment for all sorts of physical activities Satisfactory Below NAAQS
101-200 Slight reaction in sensitive persons Unhealthy Above NAAQS
201-300 Reaction and relative in- tolerance towards physical exercise in persons with breathing or cardiovascular problems. Slight reaction in the population in general. Dangerous Alert
301-400 Diverse symptoms and in- tolerance towards physical exercise in healthy people. Dangerous Warning
401-500 Diverse symptoms and in- tolerance towards physical exercise in healthy people. Dangerous Emergency
501 + (Not described) Significant harm Significant harm

Source: Thom and Ott (1975).

Technological capacity

Environmental problems have not been seen as such in part because of a "technological-fix" mentality that seeks solutions through technological change, largely on the supply side. After all, did not economic and technological growth from 1950 to 1980, on average, lead to increases in the material standard of living for the people of the basin during a time of major environmental change? This mind-set prevails in the case of coping with water shortages by enlisting the requisite technology for long-distance transport of the water. A plan developed in the early 1970s sought to serve the immediate water needs of the basin by using an even more extensive pumping system to import water from the states of Morelos, Tlaxcala, and Puebla (Guerrero, Moreno, and Garduño 1982). An extreme faith in the technological capacity to handle the impacts of human actions on the environment has bred complacency and thwarted inclinations to deal with those actions themselves.

In another vein, technological capacity has acted as a direct agent of environmental change by enhancing people's access to and use of the resources of the basin. Increasing numbers of people are acquiring more and more ability to consume resources, be it through electrical and water systems or automobiles. Simply stated, the concentration of technology in the basin increases resource use and damage, especially compared with the country as a whole (but not with other major urban centres).

In summary, the driving forces of environmental change in the basin reflect a systemic relationship among long-term policies (institutions and organizations) to promote growth and concentrate wealth, the demographic and technological responses to these policies, and the specific mitigations to counter the impacts brought on by the "concentration" ethic. The entire system of causes, extending back at least to Aztec times, has been predicated on external subsidies of resources to the basin. These subsidies have subsequently grown in scale and expanded in kind to the point that the basin provides little more than a physical locale for a huge metropolitan complex.

The vulnerability of the basin

On the basis of open space, water availability, water and air quality, and sewage treatment, the Basin of Mexico may well be approaching the limits beyond which it will be economically infeasible to continue subsidizing these resources and facets of the environment at current levels. In this case, the likelihood of sustaining the current quality of life in the basin is questionable. Current indicators provide strong reasons to believe that these limits are being approached rapidly. Projections of present trends (table 7.19) show that Mexico City may well spread over 2,700 km² by the year 2000, possibly spilling over the boundaries of the basin into the adjacent basins and drainages of Toluca, Querétaro, Cuernavaca, and Puebla. Perhaps 30 million people will live in the basin, which will have changed from the patchy mixture of urban and rural environments typical of the first half of this century to a massive and overcrowded urban sprawl.

By that time, around 100 m³ per second of water will have to be extracted from inside and outside sources, unless new and more efficient wastewater treatment and recycling methods are adopted soon. The source for this future hydrological subsidy from other watersheds into the Basin of Mexico is not clear yet. Other basins have been mentioned as potential water sources, but it will require an enormous expenditure of energy (in the order of 3 million MW hours per year) to pump such huge volumes of water, as well as significant ecological alterations and water shortages in the supplier basins.

One cannot be optimistic about these prospects. The current system is viable only by enormous levels of subsidies, which would have to increase significantly to sustain continued growth under current resource uses. In this sense, the environmental and economic systems in the basin appear to be in a state of high vulnerability to perturbations of several kinds, all of which would affect the quality of life of the basin's inhabitants.

Factors of environmental endangerment

Vegetation and open space

The settlement and paving of the basin's last open lands is occurring at a rapid rate, creating conditions of exceptional crowding and little open space. At the current pace of change, 92 per cent of the basin will soon be houses and roads. Each person will have only 5 m² of green area (table 7.19), and in extremely poor areas this space will be reduced to 1 m². The few patches of "preserved" vegetation (e.g. reserves) will also be consumed. Hence, open spaces and "natural" vegetation are highly endangered in the basin.

Table 7.19 Population, total urban area, and vegetated areas per person for the Metropolitan Area of Mexico City, 1950 and 1980, and projected values for the year 2000

  1950 1980 2000
Population (million) 3.0 13.8 32.7
Total urban area (km²) 215 980 2,700
Total urban green areas (m²/person) 29.0 9.9 5.6
Parks, gardens, and recreational areas(m²/person) 9.0 5.9 5.0

Water quality and quantity

During this century, the Basin of Mexico has evolved from a high level of self-sufficiency in natural resources to a complete dependence on imports from other regions. Most of the lacustrine area, which represents the best soils of the basin, is now occupied by buildings; the aquifer level has been drawn down by more than 10 m in some areas, and water quality is degraded. This is particularly noticeable in the south in Xochimilco and Chalco. Chinampa agriculture still exists, but is fast disappearing owing to the declining water table and the inadequate recharge of canal waters with partially treated wastewater. Therefore, most of the vegetables consumed within the basin are imported from outside its boundaries. The main problems associated with water quality are its bacterial contamination and chemical pollution as well as the contamination of crops irrigated with wastewater.

The second important problem associated with water is the availability and renewability of the resource. If a daily quota of 300 litres per capita is considered, then the recharge rate of around 27 m³ per second can supply water for 7-8 million people. In theory, this is a reasonable carrying capacity for the population of the basin. Any population density above this threshold will necessarily mean, in the long term, obtaining extra water from the catchment of rainwater during the wet season, recycling and treating wastewater, or importing water into the basin at very high cost. The basin surpassed this supply level in the 1960s.

Pumping from external sources into the Basin of Mexico is inefficient and expensive. The pumping system from the Cutzamala basin, for example, must elevate water flow some 1,100 m, and requires a constant energy input of 190 MW (SARH 1985). The whole water system in the basin, which includes the Cutzamala and Lerma basins, the deep-well pumps, and the drainage system, requires around 400 MW on average (i.e. the system uses 3-4 million MW hours per year; Castillo 1991). For comparison, this energy input is 53 per cent of the energy produced at peak operation (750 MW) by the thermoelectric generating plant of the Basin of Mexico which supplies energy to the city, 33 per cent of the energy produced at peak operation by Chicoasén, the largest (1,200 MW) hydroelectric dam in Mexico (García de Miranda and Falcon de Gyves 1972), and 31 per cent of the energy that the two nuclear generators at Laguna Verde will produce when they finally start full operations (1,280 MW). The energy costs involved in pumping water into, within, and out of the Basin of Mexico appear at present as one of the most limiting factors to the growth of Mexico City.


At present waste is not classified in Mexico City, and very little recycling occurs. Most of the waste that is recycled is processed by pepenadores (i.e. people who gather waste), but no official effort has been made to reduce the amount of waste dumped by the city. Most experts believe that solid waste disposal will remain a problem until a better disposal service is implemented and more regulations are applied (Aguilar Sahagún 1984) and, above all, until waste-processing plants are built to cope with the large output of garbage produced by the city (Monroy Hermosillo 1987; Trejo Vázquez 1987). Owing to its high content in organic residues, the garbage generated by Mexico City could be used for making compost at a relatively low cost if litter were classified according to its recycling characteristics. Solid waste of industrial origin, which forms 48 per cent of all the city waste, represents a potential problem since toxic waste may be combined with other types of refuse.

Air quality

As noted above, air quality is literally dangerous in some sections of the basin and at certain times of the year when thermal inversions trap pollutants within the basin. Automobile emissions, which account for 85 per cent of all atmospheric pollutants, cause much of the pollution. Automobiles are increasing at a 7 per cent annual rate of growth. Given a 10-year average life-span of a car in the basin, a 1991 measure requiring catalytic converters in new cars may not have a significant effect on air quality until the late 1990s. In addition, the Mexican standard for maximum allowable exhaust emissions in new vehicles is two to three times more permissive than that Of, for example, the United States (tables 7.18, 7.20, and 7.21).

Despite a series of controls inserted to combat air pollution (see below), air quality has not improved much. Not only have some individual pollutants frequently exceeded what are considered to be healthy limits, as is the case with ozone, but globally the air quality index (IMECA) has surpassed 200 points during most of the recent years, indicating that at least one pollutant is well above the healthy limit. These levels of pollution represent a serious danger to human health.

Table 7.20 Emission standards for new vehicles in Mexico and in the United States (g/km per vehicle)

  1975 1985
  Mexico USA Mexico USA
Hydrocarbons 2.5 0.9 2.6 0.5
Carbon monoxide 29.2 9.4 24.2 9.4
Nitrogen oxides (no standard) 1.93 2.2 0.62

Source: Bravo Alvarez (1987).

Table 7.21 Lead content in regular gasoline for different countries, compared with Mexico (ml of lead tetra-ethyl per litre of gasoline)

Country 1980 1987 1991
England 0.28 zero zero
Germany 0.15 zero zero
Mexico 0.77 0.14 0.14 (leaded)
      zero (unleaded)

Source: Bravo Alvarez (1987).

If the growth in automobiles continues, then the overall impacts on air quality may remain more or less the same, even with the increased use of catalytic converters. Mass transit, responsible for about 23-30 per cent of all vehicular emissions, already transports 81 per cent of all passengers in the basin (Bravo Alvarez 1986); it is doubtful, however, that major improvements will follow from major shifts in ridership on mass transit. The average emission of pollutants per passenger using private vehicles is 10 times higher than that of passengers using public transportation. Vulnerability associated with air quality, then, very much hinges on sustained controls on automobile exhaust and on reducing the number of cars on the road.

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