opulation, consumption and technology impact on the environment by way of two major types of human activity. First, we use resources. We occupy or pre-empt the use of space, and so modify or remove entirely the habitats of many wild species. We extract resources -- growing food, catching fish, mining minerals, pumping groundwater or oil. This affects the stock of resources available for humans and for other species in the future.
Resources fall into two main categories. Renewable resources like water or fish are replenished naturally. Non-renewable resources like oil or iron ore have a limited stock that is not replenished, except on geological timescales of millions of years.
Second, we dump wastes -- not just those that consumers throw away, but all the waste solids, liquids and gases that are generated from raw material to final product. These affect the state of land, groundwater, rivers, oceans, atmosphere and climate.
Resources have traditionally been the main focus of concern about the impact of population and consumption on the environment. Frequent warnings were issued that we faced massive famines, or that we would "run out" of essential fuels and minerals. More recently it has become apparent that more serious, more immediate and more intractable problems lie in the global threats that derive from our wastes. [Add]
Ultimately, all non-renewable resources on Earth are limited: if used constantly they must sooner or later run out. So far, however, the threatened exhaustion of non-renewable resources has not happened, thanks to market mechanisms which have ensured successful adaptation.
When shortages of any mineral resource begin to be felt, prices rise. This stimulates more exploration and research, and makes it economical to develop more expensive technology, and to exploit reserves that are more costly to work. Manufacturers find ways of making do with less, recycling increases, and cheaper substitutes are found.
Due to these mechanisms, the projected lifespan of many minerals has remained more or less level or in some cases grown with time, despite dramatic increases in use. In 1989, for example, recoverable reserves of oil and natural gas liquids were enough to cover 41 years of production at current rates. Nine years later they were enough to cover 43 years. Recoverable reserves of natural gas were enough to cover only 23 years of production in 1989; by 1998 this had grown to 57 years. Recoverable reserves of coal did fall, but were still sufficient for more than two centuries of production .
Prices are a good indication of impending shortage, and the prices of minerals have declined in real terms over the past four decades. In constant prices, between 1980 and the price of metals and minerals fell by an average 41 percent, while that of oil fell by 65 percent .
Of course, this conjuring trick cannot go on forever. But in modern times the human race has not run into shortages of any key non-renewable resource that has actually constrained the end use to which that resource was put. The mechanism of adaptation, based on free markets, resourceful companies, continual research and canny consumers, has worked very well in this sphere, and there is no strong reason to believe it will not continue to do so.
This is not to say that our use of non-renewable resources is problem-free, but the major difficulties arise from the wastes created in producing and consuming these resources. Extracting and processing fossil fuels and other mineral resources on an increasing scale produces water and air pollution as well as solid wastes. [Add]
Renewable resources like freshwater, soil or wild fish stocks are much more problematic than non-renewable resources, because most of them are vulnerable to human overuse or pollution.
By definition, renewable resources are replenishable by nature -- yet replenishment is not guaranteed. Renewal occurs only if they are given the chance to renew. If we exploit them faster than they can renew themselves, they deplete or degrade. The majority of renewable resources, including the most basic ones needed for human survival -- land, food and water -- are now affected by human overexploitation or pollution. [Add]
Food and landThe oldest question about human population and the environment was posed by Malthus. Can agricultural production keep up with potential human population growth? Malthus' answer was no: agricultural production can only increase arithmetically (3+3+3=9) whereas population can increase geometrically (3x3x3=27).
It followed, Malthus argued, that the human population would always be kept in check by the food supply. In reality, the reverse has usually been the case: market mechanisms have worked to expand the food supply in line with demand, and this expansion has more than matched the growth of the human population. [Add]
Malthus' basic outlook still dominates the popular view, and some recent trends provide material for renewed concern.
In most parts of the world, cultivated land has not been expanding in line with population growth, so the amount of farmland per person has been declining. The area per person has declined only slowly in developed countries, from 0.65 hectares in 1965 to 0.51 hectares 30 years later. In developing countries, where population growth is faster, the area per person fell from 0.3 to 0.19 hectares over this same period .
The steepest fall was in Africa, where the extension of the farmed area has lagged far behind population growth. In 1965, Africa had half a hectare of cultivated land per person, but this dropped dramatically to a mere 0.28 hectares in 1995. If expansion continues at the same rate as it did between 1965 and 1995, and the UN's medium population projection is realized, then by the year 2040 Africa will have only 0.15 hectares of farmland per person. This is less than Asia had in 1995, and Asia has fewer problem soils and climates, and far more potential for irrigation. Many parts of Central, Southern-Central and West Africa still have abundant land, but much of this is subject to severe soil, climate or disease constraints. It seems likely that many African countries will run into serious land shortages . [Add]
Overall, cereal production has not been keeping pace with population growth for the past decade. The amount of grain available per person rose fairly steadily from 135 kilos in 1961 to 161 kilos in 1989, but since then has dropped to about 157 kilos .
We must be cautious before concluding that we are seeing the harbingers of a coming global food crisis. If these developments were really reducing the ability of farmers to meet market demand then we would expect to see rising food prices and declining food intakes.
Nor was there any overall decline in average food intakes per person. Average daily calorie intake in 1998 was 2 790 calories , the highest on record, following fairly steady growth from 2 295 in 1963. Average daily protein intake was 74.9 grams, again the highest on record, up from 63 grams in 1961 .
Improvements were notched up in all developing areas, most rapidly in Asia, but also in Africa, even though average calorie and protein intakes remained low. [Add]
How can we explain the simultaneous drop in cereal production per person and the rise in dietary intakes? The simple answer is that people do not live by bread alone: calorie intakes from cereals have been more or less static since 1984 . The increase has come rather from meat and fish, oils and other vegetable products. Global meat intake per person grew steadily from 24 kilos per person in 1963 to 37 kilos in 1998 .
Within the cereal sector it is likely that cereals are being used more efficiently at every stage, with lower losses in storage and processing between harvest and table. Cereals used for livestock feed are increasingly being replaced by soybeans, and soybean production has been growing rapidly .
Moreover, people's need for dietary energy is, on average, declining. Farming has the highest calorie requirements, followed in turn by heavy industry, light industry, services, and then non-employment. The general trend in all societies is to have higher and higher percentages of people represented in sectors with lower food energy needs.
Barring severe climatic change, it is very unlikely that we face catastrophic food shortages at global level. Research has shown that with relatively modest improvement in regionally specific agricultural practices, the world could feed 10 billion people with current land and technology levels . [Add]
The agricultural sector has been very successful in raising food production since 1945 to meet growing populations and consumption levels, but this has often been at the cost of exporting problems to other ecosystems. High levels of fertilizer application have caused water pollution and eutrophication. The expansion of farmland has been to the detriment of wildlife habitat and biodiversity, which has been further harmed by pesticide use.
Population growth is directly implicated in all of these trends. For example, the area of land needed for any given crop is the product of population, multiplied by consumption per person, multiplied by the area needed to produce each unit of consumption. This latter element is the result of farming technology. Where this has been able to increase yield faster than the growth in population multiplied by consumption, the area needed for farming has fallen over time. But where yield has not kept pace, the area of farms and pastures has increased at the expense of forests and other wild habitats.
Unsustainable soil and water management practices have caused land degradation. A major assessment found that by 1990 soils had degraded on 38 percent of the world's cropland, 21 percent of pasture and 18 percent of forests . Productivity has declined significantly on 16 percent of agricultural land in developing countries . One recent estimate suggested that cropland productivity is 12.7 percent lower than it would have been without human-induced soil degradation . [Add]
Serious problems of food production will also continue in localized areas and in individual countries. These include many countries in sub-Saharan Africa and some individual countries outside Africa such as Bolivia, Haiti and Afghanistan. Many countries that cannot produce enough food for their own needs can pay for food imports by exporting manufactured goods or services. But many marginal areas, and many poor food-deficit or landlocked countries, especially in Africa, are badly placed to develop competitive industries or services.
There are millions of people who do not get enough food for a healthy, active and productive life. The estimated incidence of malnutrition in developing countries has halved from 35 percent of the population in 1969-71 to 17 percent in 1995-97, but because of the growth in population the absolute drop in numbers, from 917 million to 790 million, has been much more modest .
The numbers of malnourished will probably continue to decline slowly, while still remaining unacceptably high. However, malnutrition is not a sign that not enough food is available at global or even national level. It is a symptom of poverty and inequality -- the poor lack enough money to buy, or enough good land to grow, sufficient food for the needs of their families.
Reducing the number of malnourished means taking measures to create jobs, redistribute land more equitably, and increase the productivity of small and marginal farms through targeted agricultural training, crop breeding, and soil and water conservation programs. Once the poor's own resources have been boosted, they themselves will grow, or the world market will produce, enough food to meet their effective demand. [Add]
Most freshwater is locked up in ice and snow and in aquifers too deep to tap, and the rest is very unevenly distributed. Equatorial regions and some northern latitudes have a surfeit. Dry areas in between, including much of Africa, have supplies that are too scarce or too uncertain.
Freshwater is crucial for survival, for health, for agriculture, for industry, and for comfort and leisure. But the freshwater resources of any country are limited. There is only so much to go round: the larger the population, the less there is for each person.
In some countries, shortages are already biting. According to Swedish hydrologist Malin Falkenmark, a minimum of 1 700 cubic meters of renewable freshwater is needed per person per year to avoid serious problems. Below this level, a country is in a situation of water stress, when water supply problems may become chronic and widespread. There may be a need for long-distance water transfers, reuse of treated waste water, or supply interruptions in dry periods.
Where supplies fall below 1 000 cubic meters per person per year, a situation of water scarcity applies, and a society will face difficult choices between agriculture, industry, personal health and convenience which will hamper development .
In 1995 some 436 million people were already suffering water scarcity or stress. Even these levels of water shortage are causing severe development problems in some areas. There are conflicts among farmers and between farming and urban needs, and heightening tensions between countries dependent on the same resources, such as Israel and Jordan; Turkey, Syria and Iraq; India and Bangladesh; Sudan and Egypt . Saudi Arabia, Israel and the whole of North Africa from Egypt to Mauritania are already withdrawing groundwater faster than it can replenish itself. Yet these countries face population increases of between 52 and 152 percent over the next 50 years .
Different population futures make a considerable difference to water futures. An analysis of the UN's 1996 population projections has estimated numbers likely to be suffering water shortage in the future. By 2050, on the medium projection, the number of people in countries suffering water stress or scarcity will have risen to 4 billion . If the UN's low population projection could be achieved, then the total population in countries facing water scarcity or stress would amount to only 2 billion. By contrast, if the world were to hit the high projection, this total would be 6.8 billion. [Add]
Perhaps the most intractable threats to the globe today relate as much to what we waste as to what we consume. Pollution places a mounting burden on local and planetary ecosystems. Ultimately it is exported to the global commons: the oceans and atmosphere, where our understanding of interactions is still inadequate. Sustainable management strategies are complex to devise and politically difficult to introduce.
In the process of making the end products we actually use, our machines dig up, churn over, swallow up and spew out gigatons of material. One study found that some 93 percent of materials used in production do not end up in saleable products but in waste, while 80 percent of products are discarded after a single use .
The result is a veritable avalanche of materials. In 1995, for example, the world produced 1.42 billion tons of cement -- about a quarter of a ton for every man, woman and child on Earth. Some 2.57 billion tons of sand and gravel were produced in the 52 countries for which data are available .
Figures on carbon dioxide (CO2) illustrate how the waste deluge has grown. Back in 1750, the human race produced only 11 million tons of CO2 from fossil-fuel burning and cement production. A century later this had grown 18-fold to 198 million tons, and in another century a further 30-fold to around 6 billion tons. By 1995 our annual CO2 output had multiplied by another four times to reach almost 24 billion tons .
These material flows have left deepening scars on the planet. The solid wastes that are not incinerated deface or pollute localized areas and water courses. Liquid and gaseous pollutants are more insidious and spread invisibly across the whole globe. [Add]
Humans raised the level of CO2 in the air from 280 parts per million in pre-industrial times to 363 parts per million in 1996. Over this same period we raised methane concentrations by 145 percent. There were no gaseous chlorines in the atmosphere before industrial times. By 1996 there were 2 731 parts per trillion, most of these produced in the 20th century .
Significant traces of organic and metallic pollutants are now found in the deepest marine sediments, in the remotest glaciers and icecaps, and in the fat of arctic mammals. Studies of human breast milk have found traces of more than 350 contaminants, including 87 dioxin and dioxin-like compounds and 190 volatile compounds  [Add].
The rise of pollution and waste is not inexorable. Water and air pollution usually increase in the early stages of economic development, but once a certain income threshold has passed, people tend to value environmental quality more highly and have the resources to pay for protectionmeasures. In most developed countries there have been significant reductions in emissions of lead, sulfur dioxide (SO2) and particulates (smoke), and widespread improvements in water quality in rivers and around beaches. These are cases of immediate hazard, or easily noticeable local problems, or substances that have been the subject of intense media publicity, where political pressure for change is strong .
But even in rich countries waste emissions with less immediate, less visible or less dramatic effects have not been the subject of effective controls. The same is true where the costs are exported over a vast area or over the whole globe, or where remedial action would be costly and might affect powerful business interests or important groups of voters. These include, for example, emissions of the greenhouse gases CO2 and methane. [Add]
Population is always a factor in waste and pollution, along with consumption and technology. The level of production of wastes or pollutants is the product of the number of people, the amount each person consumes, and the amount of waste created for each unit of consumption in the whole process from production and packaging to the consumer and his or her dustbin or sewage outlet.
Several efforts have been made to identify the relative shares of responsibility for rising pollution. Environmentalist Barry Commoner studied examples from the United States between 1946 and 1968. Population growth accounted for only 14 to 18 percent of the increase in synthetic organic pesticides, in nitrogen oxides and in tetraethyl lead from vehicles. It was responsible for only 7 percent of the increase in non-returnable beer bottles and a mere 3 percent of the increase in phosphorus from detergents. In almost every case, technology was the dominant factor. A later study by Commoner of nitrates, cars and electricity in 65 developing countries came to similar conclusions .
Clearly, technology is always implicated, and in many cases it may be the prime culprit. However, Commoner chose only cases where technological change was rapid. There are other cases where population or consumption are dominant, such as increased methane emissions from livestock or paddy fields. In more and more cases, technological change is a downward pressure, working to reduce our output of wastes, while growth in population and consumption continues to gear it upwards.
Studies of changes in air pollutants (SO2, nitrogen oxides, smoke and CO2) in countries of the Organisation for Economic Co-operation and Development (OECD) between 1970 and 1988 showed technology as a downward pressure in all four cases -- mainly through increased energy efficiency in the case of CO2 and nitrogen oxides, and through cleaner technology in the case of SO2 and smoke. Population growth was responsible for a quarter of the upward pressure on emissions, while consumption was responsible for three quarters . [Add]