What does the word "sustainability" mean?
The United Nations Brundtland Commission published Our Common Future in 1987, in which, "sustainable development" is defined as:

“development that meets the needs of the present without compromising the ability of future generations to meet their own needs.”

Achieving sustainability requires that we strive to achieve continued social and economic development, closing the gap between the rich and the impoverished in such a way that does not jeopardize the ability of future generations to meet their needs. Understanding sustainability means recognizing that unlimited growth is impossible in a world with finite limits, and that the current economic model has both physical and ecological limits.
"The Four System Conditions" advocated by the Natural Step, and Herman Daly's three "Daly Principles" explain sustainability in terms of these physical and ecological limits, providing a conceptual framework for the establishment of a sustainable society.

In 1998, Swedish cancer scientist, Dr. Karl Henrik Robe`rt, sought the support of a range of scientists, resulting in a proposed definition of sustainability based on scientific principles. These principles, referred to as "The Natural Step Four System Conditions"^[1], are the conditions that must be kept if the natural environment and society, as a whole, are to move toward a "sustainable state".

1. Nature is not subject to systematically increasing concentrations of substances extracted from the Earth’s crust;

(While resource extraction is possible on a short term basis, it cannot be sustained long-term )

2. Nature is not subject to systematically increasing concentrations of substances produced by society; (Materials produced by human society, -chemicals, pesticides, plastics, etc., must not be introduced into natural systems at a rate faster than that which they can be broken down);
3. Nature is not subject to systematically increasing degradation by physical means (Destruction of the ecosystem through human-causes, such as soil erosion, asphaltification, deforestation, salinization, agricultural run-off, and over fishing cannot continue indefinitely);
4. People's ability to meet their needs is not systematically undermined (Resources are used efficiently and fairly, unfair distribution of resources between rich and poor countries must be avoided).

American economist Herman Daly proposes three rules^[2] laying out the conditions for the sustainable use of resources and waste disposal:

1. Renewable resources such as fish, soil, and groundwater must be used no faster than the rate at which they regenerate. (For example, fish stocks may not be depleted faster than the fish can reproduce);
2. Nonrenewable resources such as minerals and fossil fuels must be used no faster than renewable substitutes for them can be put into place. (For example, reinvesting profits from fossil fuel powered processes into renewables such as solar or bio-fuel so that there will be adequate energy available if oil is no longer available);
3. Pollution and wastes must be emitted no faster than natural systems can absorb them, recycle them, or render them harmless. (For example if sewage is released into a lake or river, unless it is at a pace such that aquatic life can absorb the nutrients, it cannot be considered sustainable).

As we have seen above, intergenerational responsibility is inherent in the concept of sustainability. The Brundtland Commission refers to "future generations" in the plural form and, if we consider each generation to span about twenty-five years, one may argue that the minimum time frame for dealing with the issue of sustainability is fifty years. This seems a very short timeframe considering the 3.8 billion year history of life on Earth, but for practical purposes, this report will deal with sustainability in a fifty-year perspective.

How will the preconditions for life in our children's and grandchildren's world differ from those of the present? Will such future changes result in new restrictions on social and economic development? This report aims to deal with these key issues from as neutral a stance as possible. Looking back at our actions as a species in the last fifty years, and ahead at the key trends emerging for the coming fifty, it is our hope to create a basis for a broad social debate on how to maintain the natural life support systems upon which our survival and prosperity depend.

According to United Nations statistics, global population has increased about 2.4 times between 1950 and 2000 (from 2.5 billion to just over 6 billion people). While it took from the dawn of humanity until 1950 for human population to exceed 2.5 billion, the next increase of 2.5 billion occurred in just forty years as the earth's population reached 5.28 billion in 1990.

Population growth peaked between 1965 and 1970, increasing an average of 2.04% each year. Between 2000 and 2005, this growth rate declined to 1.21%. Between 1985 and 1990, population grew by 87.1 million people annually, dropping to 75.8 million people annually between 2000 and 2005, revealing a trend toward deceleration of population growth.

FIgure 1: United Nations World Population Projections, 1950 - 2050
Source: World Population Prospects^[3]

The United Nations revised population projections in 2002, indicating new low, medium, and high range estimates in Figure 1. The most conservative estimate predicts 7.68 billion people by the year 2050. The mid-range estimate is set at 9.08 billion people, and the highest estimate is that 10.65 billion people will inhabit the earth by 2050. At the medium estimate, there will be an increase of about 2.6 billion people between 2005 and 2050 (from 6.46 billion to 9.08 billion).

In the latter of the 20th century, resource consumption rates surpassed population growth rates. Table 1 shows the changing consumption rates of various resources for the period between 1950 and 2000.

Table 1 Population and Consumption Trends, 1950-2000
^{Source: adapted from Limits to Growth: The 30-Year Update[4]}

Between 1950 and 2000, human population increased to 247% of the 1950 level, while oil consumption increased to 727% over the same period. Natural gas consumption increased to 1,454%, coal to 364%, and electricity generation to 2,104%. Other minerals and natural resource consumption rates also overtook population growth as, led by the rich industrialized countries, lifestyles became increasingly affluent in the latter half of the 20th century.

From such data covering the latter half of the 20th century, at least two key questions emerge as we look toward the coming fifty years:

• How large is the proportion of global population that has truly enjoyed the fruits of this extraordinary growth?
• What must we do over the coming fifty years to enable the drive toward more prosperous lifestyles across the Globe?

According to the International Monetary Fund (IMF), the gap between per capita GDP of the world's twenty most affluent and twenty most impoverished countries widened in the two decades between 1980 and 2000 (Figure 2). Per capita GDP of the twenty richest countries grew 2.6 times whereas the poorest twenty countries' average was reduced by almost half. Even with the economic growth and increased material consumption experienced at the end of the 20th century, poverty and hunger remain major problems in the early 21st century. International mal-distribution of wealth continues, and even within those countries that have the most, the gap between rich and poor is widening.

Figure 2 Global Per Capita GDP, 1980-2000 (US Dollars)
Source'Compiled from World Economic Outlook 2000 ^[5]

For future development to be sustainable on a global scale, it is imperative that the most impoverished countries realize economic growth. Yet, one potentially negative effect of large-scale economic development, in both developed and developing countries, is a tremendous increase in environmental impact. Growth that undermines natural life support systems cannot be called "sustainable" from an ecological and resource perspective. Therein lies one of the major dilemmas of sustainable development. Overcoming it will require development that decouples economic growth from increased environmental impact.

Economic growth in the last half of the 20th century until now has almost invariably caused increased environmental impact through the continued growth in energy and natural resource consumption and waste outputs. If sustainable development is to become a reality, this connection must be severed.

Due, in part, to increasing energy efficiency and resource productivity, the environmental impact of most developed nations has decreased relative to economic output. However, it is rare to find a case where a country's impacts have decreased on an absolute basis.
Herman Daly described the decoupling of economic growth and environmental impacts in an article in the September 2005 issue of Scientific American, "Crossroads for the Planet Earth".

“[T]rying to define sustainability in terms of constant GDP is problematic because GDP conflates qualitative improvement (development) with quantitative increase (growth). The sustainable economy must at some point stop growing, but it need not stop developing. There is no reason to limit the qualitative improvement in design of products, which can increase GDP without increasing the amount of resources used.”^[6]

The economic growth of the last half of the 20th century has helped to alleviate poverty, but has failed to eradicate it. In the same way, energy and resource efficiency has increased in developed countries, but the correlation between economic growth and increased environmental degradation has not been overcome.

Although population growth is expected to slow in the 21st century, a considerable increase in global population will take place, mainly driven by an increase in the developing world, virtually guaranteeing increased energy and resource consumption in the first half of the 21st century.

Can we satisfy the needs of the current generation, provide people around the world with a higher quality of life and, at the same time, use resources responsibly, enabling future generations to continue to develop? A "sustainable" world is one where all people can benefit from what the earth provides - both today and tomorrow. This is not just a material-related, but one based on ethics - a shared goal of humanity.

1. The Natural Step. http://www.naturalstep.org (accessed November, 2005)
2. Meadows, Donella H., Meadows, Dennis L., and Randers, Jorgen. Beyond the Limits: Confronting Global Collapse, Envisioning a Sustainable Future. (Post Mills, Vermont: Chelsea Green, 1992)
3. United Nations, World Population Prospects: The 2002 Revision (New York: 2003)
4. Meadows, Donella H., Meadows, Dennis L., and Randers, Jorgen, Limits to Growth: The 30-Year Update, (Post Mills, Vermont: Chelsea Green, 2004)
5. International Monetary Fund (IMF), World Economic Outlook. (Washington DC: 2000)
6. Daly, Herman E. "Economics in a Full World", Scientific American Special Issue, September, 2005, 81