In every human interaction with the environment – even in the simplest societies – the three major elements are in play. They can be linked in the famous formula introduced by Ehrlich and Holdren:

I = P x A x T, or

Impact = Population x Affluence x Technology¹

More explicitly, environmental impact is the product of population, multiplied by consumption per person, multiplied by the amount of resources needed, or wastes created, while producing each unit of consumption.

Ehrlich used the formula to show that population growth was the dominant factor in environmental damage. In reality, at various historical times, different elements have been uppermost. The increase in arable area in many parts of Africa up to around 1980, and the deforestation that went with it, was mainly driven by population growth. There was little rise in consumption of agricultural products per person, and little improvement in yields. By contrast, the dramatic rise in human output of chlorofluorocarbons from the 1940s onwards was due overwhelmingly to the introduction of a new technology.

Of course, the impact also depends on the sensitivity of the environment, and this is not always predictable. It has certain thresholds which, if crossed, lead to rapid depletion and degradation. Resources such as fisheries, forests and groundwater have a maximum sustainable yield, beyond which they will be unable to replenish themselves. Sinks for our wastes, such as soils, rivers, lakes, oceans and atmosphere, have critical loads for various pollutants, beyond which important aspects of their productivity will degrade.

Sometimes the environment may successfully evolve as human pressures increase. At other times it may change suddenly when human pressure exceeds certain thresholds. When erosion strips soil close to bedrock, yields fall abruptly. If global warming melts the polar icecaps, major ocean currents may cease or shift direction, producing faster climate change.

Although the IPAT model as it is written assumes independence of each of the PAT elements, the authors recognize that these are not independent in the real world, rather they interact with each other. In the 1980s, for example, slower population growth appeared to facilitate faster growth of consumption in developing countries. Higher income levels tend to improve environmental technology – wealthier nations have a greater willingness and ability to pay for environmental quality.

The systems approach has two key differences from conventional approaches. It does not focus on a single factor, but instead builds in as many potential factors as possible; and it does not see human impact on the environment simply as a one-way street. There is feedback. Changes in the environment have an impact on human welfare. This is primarily as a result of:

• resource depletion or degradation and the resulting shortages and scarcities;
• loss of a valued amenity such as natural wilderness areas or beautiful landscapes;
• impacts on human health and fertility.

These environmental problems in turn produce a human response – often driving us to alter our behavior so as to reduce the problems.

CRISIS VERSUS ADAPTATION
The systems approach helps resolve another conflict in the theory of population-environment linkages, over the way in which humans respond to environmental problems of their own making.

On one side is the “Malthusian crisis” approach, exemplified by Ehrlich and the Beyond the Limits studies. In this approach, the pressures of resource demands and pollution loads can build up and are predicted to reach crisis level if business continues as usual. Unless drastic action is taken, catastrophe follows: economy and society collapse, death rates rise and populations fall. We do not achieve adaptation by choice or plan – it is forced on us by nature. However, Malthusian scenarios usually suggest that catastrophe can be avoided – as long as humanity heeds the warning signs and takes the necessary steps in time⁵.

On the opposing side is what might be called the “economic adaptation approach”, fervently championed by economist Julian Simon. In this scenario, humans adapt to the problems that our development produces, for the most part smoothly and without grave setbacks. In the process we gain increased productivity and efficiency, and improved human welfare. Simon saw population growth as an asset, producing more brainpower to deal with any specific problem⁶.

A more sophisticated adaptation approach was put forward by Ester Boserup in her classic book The Conditions of Agricultural Growth. Boserup suggested that population growth was the principal force driving societies to find new agricultural technologies⁷.

Unlike Simon, Boserup did not claim that the process ran smoothly. She Acknowledged that population pressure could cause serious resource shortages and environmental problems, and it was these problems that drove people to find solutions. Nor did she claim that things were always better after the adaptation.

They could often be worse. For example, when hunter-gatherers with growing populations depleted the stocks of game and wild foods across the Near East, they were forced to introduce agriculture. But agriculture brought much longer hours of work and a less rich diet than hunter-gatherers enjoyed. Further population growth among shifting slash-and-burn farmers led to shorter fallow periods, falling yields and soil erosion. Plowing and fertilizers were introduced to deal with these problems – but once again involved longer hours of work⁸.

The major flaw with both the adaptationist and the Malthusian approaches lies in their claim to universality. In reality, both may be true of different civilizations at different historical periods, and a comprehensive theory must be able to account for both approaches.

HUMAN ADAPTABILITY AND ITS LIMITATIONS
Humans, by nature, are adaptable. That is why we have been so successful as a species. Through the course of our history, we have radically changed our cultures, our technologies, our consumption patterns, and the number of children we have. In modern times we have changed all of these at unprecedented rates.

Adaptation is possible even in apparently extreme situations. In the 1930s the Machakos area of Kenya had almost no tree cover and rapid soil erosion. Enforced colonial soil conservation programs achieved little. But from the late 1970s onwards indigenous methods of soil conservation were introduced, and there was a massive wave of tree planting. Soil erosion diminished, crop yields rose and fuelwood availability increased – all this at the same time as population continued to grow rapidly. This success story depended on farmers being able to respond freely to market demand, and having de facto ownership of their land so that they could benefit from their own tree planting and terracing efforts⁹.

Nonetheless, much of sub-Saharan Africa, at least during the period from 1970 to 1990, was trapped in a Malthusian-type scenario. Rapid population growth was increasing pressure on the land, yet agricultural technology was not adapting fast enough, leading to deforestation, soil erosion and, in many places, stagnant or falling yields. For dryland Africa the technologies and crop varieties still do not exist to allow crop yields to keep pace with population growth, leaving migration as the only way to relieve the pressure for many marginal groups. Of course in many countries inadequate governance, market imperfections and endemic conflict made the task of adaptation all the more difficult.

A systems approach sees our interactions with the environment in terms of pressure, state, feedback and response. The pressure is the particular human activity, such as carbon dioxide (CO2) emissions or fish catches, causing an impact. The level of pressure is determined by population, consumption and technology, and by the level of resource use and waste output these generate. The state is the resulting condition of the environment – in these cases the atmospheric concentration of CO2 and global mean temperature, and the size of fish stocks.

Changes in the environment act as feedback when we notice a problem – a resource shortage, an effect on human health, a new hazard or the loss of an amenity, such as the disappearance of a species or wilderness area.

How we respond to the feedback depends on various filters through which we process environmental information: our level of monitoring and scientific understanding, the form of ownership or management of the resource, the freedom of the market to respond to scarcities, and the biases in the political or legal system that determine an adequate response.

The response is the policy or action taken to deal with the environmental problem, such as regulations regarding fuel efficiency, carbon taxes or fishing quotas. Our responses, in turn, change the pressures we load onto the environment, completing the cycle¹⁰.

Failures of adaptation can occur at many points in the cycle. In general, feedback works very well in free markets with privately controlled resources like mines or land. In these cases the people affected are able to take direct action to remedy their problem. Where farmers and entrepreneurs have the freedom and incentive to respond to shortages, they can shift to other resources or change their technology very swiftly. This is why the world, by and large, has not faced any constraining scarcity of key inputs – the raw materials required for producing energy or food, for example.

Even in these situations, technology does not automatically keep pace with growing population and consumption pressures so as to reduce environmental impact or keep it constant. Fuel efficiency is constantly improving in cars, for example, but not fast enough to counteract the growth in car ownership and the mileage driven.

Feedback does not work at all well in the case of commonly owned or non-owned resources – such as groundwater, fishing stocks, and the oceans and atmosphere which we use as sinks for our liquid and gaseous wastes. As Garrett Hardin pointed out, lack of ownership or management arrangements encourages individuals to overuse commons for their own private advantage, even if this means degrading the resource. Each user gains the full advantage of their overuse, but suffers only a very small share of the losses it causes. This is the well-known “tragedy of the commons”¹¹.

In traditional societies, if shared resources seemed to be under threat, users often agreed on rules for their management. But in modern societies the people affected are often unable to take direct action, and must channel their demands through the political and legal systems.

Effective feedback depends on information about the environmental change and its effects passing a long sequence of filters, any of which can act as a blockage or a bottleneck. An emerging problem must first be recognized as a problem, yet many environmental changes are slow, hard to identify and require monitoring. The people affected must be able to make their voices heard in the political and legal system. Yet large sections of society may be unable to do so in countries without effective democracy, a free and investigative press, high levels of literacy and access to an affordable legal system.

Finally, there must be a good scientific understanding of how to remedy the problem. All environmental impacts are extremely complex. They involve the interaction of many different agents and elements, and often the outcome can be unpredictable. Oversimplistic solutions can often lead to further problems. The systems approach allows us to see the Malthusian and adaptationist outcomes as special cases occurring under different conditions.

The human response to environmental change can be effective and timely when:

• the impacts are perceived and properly understood;
• those affected can act directly or compel the political and legal system to act;
• the science is good and the measures are well-chosen.

Under these circumstances, the economic adaptation model applies. However, our responses may be delayed, inadequate or misguided when:

• we do not perceive the problem or properly understand its causes;
• blockages and bottlenecks prevent the smooth flow of information;
• market or democratic imperfections prevent appropriate action;
• the appropriate technology or management techniques or institutions have not yet been properly developed.

When the delays and mistakes are serious enough, severe environmental damage can occur. Sometimes, if a critical resource is involved, this can lead to the collapse of societies and steep population drops. In these situations the Malthusian model applies. However, such cases happen rarely and only under extreme circumstances.

In today’s world we have a mixture of these two patterns. In our use of privately controlled resources, we generally adapt well and the supply of key inputs like food, minerals or energy is maintained. In our use of commonly owned or non-owned resources or waste sinks, we do not as a rule adapt well.

In one area after another, problems have not been effectively dealt with until they have become highly visible and had a strong impact on very large numbers of people. In one area after another we have exceeded the maximum sustainable yield or the critical load of pollutants: marine fish, coral reefs, acid deposition, greenhouse gases, the ozone layer.

The oceans and atmosphere are crucial to the stability of all ecosystems on Earth, and determine the productivity of all the natural resources on which humans depend. Yet these are non-owned resources, and we have been in the habit of allowing problems to mount to critical levels before acting. They are also resources where gradual change can suddenly become catastrophic. It is possible that in this case a Malthusian situation – a situation of failed adaptation – could arise for the whole human race.