The bright promise of a new century is clouded by unprecedented threats to the stability of the natural world. "In a globally interconnected economy, rapid deforestation, falling water tables, and accelerating climate change could undermine economies around the world in the decades ahead," say Lester Brown and Christopher Flavin, lead authors of the 1999 State of the World report brought out by the premier environmental communications organisation, Worldwatch Institute.

During the past century, world population grew by more than 4 billion - three times the number of people when the century began. At the same time, the use of energy and raw materials grew more than ten times. These trends cannot continue for many more years. As the 21st century approaches, the big question is whether we can muster the ingenuity to change - and do so rapidly enough to stave off environmentally-based economic decline. The one thing we can say for sure is that the 21st century will be as different from the 20th as that one was from the 19th.

Since our emergence as a species, human societies have continually run up against local environmental limits that have caused them to collapse, as local forests and cropland were overstressed. But the advances in technology that have allowed us to surmount these local limits have transferred the problem of environmental limits to the global level, where human activities now threaten planetary systems.

World energy needs are projected to double in the next several decades, but no credible geologist foresees a doubling of world oil production, which is projected to peak within the next few decades. Meeting the needs of the 2 billion people who do not have modern fuels or electricity might become a new social imperative. However, the key to a reliable, diversified energy system will be the use of hydrogen as a major energy carrier and storage medium.

In 'Reinventing the Energy System', researchers Christopher Flavin and Seth Dunn say, "Designing a new energy system suitable for the 21^st century may help re-establish the positive but too often neglected connections between energy, human well-being, and the environment. Rather than treat energy as a commodity to be consumed without regard for its consequences, we might instead recover a much older notion of energy as something to be valued, saved, and used to meet our needs in ways that respect the realities of the natural world - thereby avoiding any kind of ecological catastrophe that has befallen civilisations that overdrew their environmental endowments. The sooner we can bring the fleeting hydrocarbon era to a close and accomplish the historic shift to a civilisation based on the efficient use of renewable energy and hydrogen, the sooner we can stop drawing down the natural inheritance of future generations and begin investing in a liveable plant.

While protein demands are projected to double in the century ahead, no respected marine biologist expects the oceanic fish catch, which has plateaued over the last decade, to double. Eleven of the 15 most important oceanic fisheries and 70 percent of the major fish species are now fully or over-exploited, according to experts. And more than half the world's coral reefs are now sick or dying. Anne Platt McGinn points out at the sea of problems in 'Charting a New Course for Oceans' when she mentions the study conducted by Food and Agricultural Organization in 1992 that the global fishing industry was losing $54billion a year.

Growing stress can also be seen in the world's woodlands, where the clearing of tropical forests has contributed recently to unprecedented fires across large areas of Southeast Asia, the Amazon, and Central America. The value of the wood trade (legal and illegal) makes this sector a potent economic force, say Janet Abramovitz and Ashley Mattoon in 'Reorienting the Forest Products Economy.'. "More and more wood products enter the international market every year, reflecting a general trend toward trade globalisation.

Environmental deterioration is taking a growing toll on a wide range of living organisms. Of the 242,000 plant species surveyed by the World Conservation Union in 1997, some 33,000, or 14 percent, are threatened with extinction-mainly as a result of massive land clearing for housing, roads, and industries. John Tuxill, in 'Appreciating the Benefits of Plant Biodiversity', feels governments can begin to chart a new course by resolving the most prominent policy issue affecting plant biodiversity today: how to distribute biodiversity's economic benefits fairly and equitably. Establishing a system of intellectual property rights to plant resources has proved contentious because of a simple pattern - plant diversity (both wild and cultivated) is held mostly by developing countries, but the economic benefits it generates are disproportionately captured by industrial nations.

The atmosphere is also under assault. The billions of tons of carbon that have been released since the Industrial Revolution have pushed atmospheric concentrations of carbon dioxide to their highest level in 160,000 years-a level that continues to rise each year. As scientists predicted, temperatures are rising along with the concentration of carbon dioxide. The latest jump in 1998 left the global temperature at its highest level since record-keeping began in the mid-19th century. The early costs of climate change may already be evident: weather-related economic damages of $89 billion in 1998 exceeded losses for the decade of the 1980s. In Central America, 11,000 people were killed by Hurricane Mitch, and Honduras suffered losses equivalent to one-third of its annual GDP.

Human societies may also face growing stress in the new century. In Africa, for example, where populations have doubled in the last three decades, economic growth is already failing to keep up with human needs. Several African countries, including Botswana, Zambia, and Zimbabwe, where 20-25 percent of the adult population is now HIV-positive, are expected to lose one-fifth or more of their people within the next few decades. This could undermine their societies in the same way the plague did those of Europe in the Middle Ages.

"Our analysis shows that we are entering a new century with an economy that cannot take us where we want to go," says Brown. "Satisfying the projected needs of 8 billion or more people with the economy we now have is simply not possible. The western industrial model-the fossil-fuel-based, automobile-centred, throwaway economy that so dramatically raised living standards in this century-is in trouble."

The shift to an environmentally sustainable economy may be as profound a transition as the Industrial Revolution. The foundation of such a system is a new design principle-one that shifts from the one-time depletion of natural resources to an economy that is based on renewable energy and that continually reuses and recycles materials. A sustainable economy will be a solar-powered, bicycle/rail-based, reuse/recycle economy, one that uses energy, water, land, and materials much more efficiently and wisely than we do today.

In the absence of a concerted effort by the wealthy to address the problems of poverty and deprivation, building a sustainable future may not be possible. Growing poverty, and the political and economic chaos that can be provoked by it, reverberate around the world, as was seen in 1998 with the Asian economic meltdown, which pushed tens of millions of people below the poverty line in just a few months.

There is a problem with global cooperation. David Malin Roodman says in 'Building a Sustainable Society' that governments have ratified more than 200 international environmental treaties. Most of these treaties and agreements have been inadequate to the problems at hand, either in design or in implementation and enforcement. The institutions they have created have typically been given ambitious mandates in principle, but minimal authority and funding.

One key to reversing environmental degradation is to tax the activities that cause it, according to the report. By putting a price on these activities, the market can be harnessed to spur progress. If coal burning is taxed, solar energy becomes more economically competitive. If auto emissions are taxed, cleaner forms of transportation become more affordable. The effort to replace today's unsustainable economy with one that is suited to the demands of the 21st century will create some of the new century's largest investment opportunities.

Roodman is optimistic though, when he points out that recent trends are positive. Polls show the global public is becoming more worried about environmental problems. According to a 1998 survey by Environics International covering 30 nations as different as China and Italy, the majority in 28 felt that their governments needed to do more to protect the environment. During the 1990s, there has been a halting but global shift toward democracy and space for civil society. Increasingly, public concern about the inadequacy of governmental action on the environment is voiced, and is heard.

"No challenge is greater, or more satisfying, than building an environmentally sustainable global economy, one where economic and social progress can continue, not only in the 21st century, but for many centuries beyond," the authors conclude.