Our twenty-first century civilization is not the first to move onto an economic path that was environmentally unsustainable. Many earlier civilizations also found themselves in environmental trouble. As Jared Diamond notes in Collapse: How Societies Choose to Fail or Succeed, some were able to change course and avoid economic decline. Others were not. We study the archeological sites of Sumerians, the Mayans, Easter Islanders, and other early civilizations that were not able to make the needed adjustments in time.

Fortunately, there is a consensus emerging among scientists on the broad outlines of the changes needed. If economic progress is to be sustained, we need to replace the fossil-fuelbased, automobile-centered, throwaway economy with a new economic model. Instead of being based on fossil fuels, the new economy will be powered by abundant sources of renewable energy: wind, solar, geothermal, hydropower, and biofuels.

Instead of being centered around automobiles, future transportation systems will be far more diverse, widely employing light rail, buses, and bicycles as well as cars. The goal will be to maximize mobility, not automobile ownership.

The throwaway economy will be replaced by a comprehensive reuse/recycle economy. Consumer products from cars to computers will be designed so that they can be disassembled into their component parts and completely recycled. Throwaway products such as single-use beverage containers will be phased out.

The good news is that we can already see glimpses here and there of what this new economy looks like. We have the technologies to build it—including, for example, gas-electric hybrid cars, advanced-design wind turbines, highly efficient refrigerators, and water-efficient irrigation systems.

We can see how to build the new economy brick by brick. With each wind farm, rooftop solar panel, paper recycling facility, bicycle path, and reforestation program, we move closer to an economy that can sustain economic progress. If, instead, we continue on the current economic path, the question is not whether environmental deterioration will lead to economic decline, but when. No economy, however technologically advanced, can survive the collapse of its environmental support systems.

The Nature of the New World

We recently entered a new century, but we are also entering a new world, one where the collisions between our demands and the earth’s capacity to satisfy them are becoming daily events. It may be another crop-withering heat wave, another village abandoned because of invading sand dunes, or another aquifer pumped dry. If we do not act quickly to reverse the trends, these seemingly isolated events will come more and more frequently, accumulating and combining to determine our future.

Resources that accumulated over eons of geological time are being consumed in a single human lifespan. We are crossing natural thresholds that we cannot see and violating deadlines that we do not recognize. These deadlines, determined by nature, are not politically negotiable.

Nature has many thresholds that we discover only when it is too late. In our fast-forward world, we learn that we have crossed them only after the fact, leaving little time to adjust. For example, when we exceed the sustainable catch of a fishery, the stocks begin to shrink. Once this threshold is crossed, we have a limited time in which to back off and lighten the catch. If we fail to meet this deadline, breeding populations shrink to where the fishery is no longer viable, and it collapses.

We know from earlier civilizations that the lead indicators of economic decline were environmental, not economic. The trees went first, then the soil, and finally the civilization itself. To archeologists, the sequence is all too familiar. Our situation today is far more challenging because in addition to shrinking forests and eroding soils, we must deal with falling water tables, more frequent crop-withering heat waves, collapsing fisheries, expanding deserts, deteriorating rangelands, dying coral reefs, melting glaciers, rising seas, more-powerful storms, disappearing species, and, soon, shrinking oil supplies. Although these ecologically destructive trends have been evident for some time, and some have been reversed at the
national level, not one has been reversed at the global level.

The bottom line is that the world is in what ecologists call an "overshoot-and-collapse" mode. Demand has exceeded the sustainable yield of natural systems at the local level countless times in the past. Now, for the first time, it is doing so at the global level. Forests are shrinking for the world as a whole. Fishery collapses are widespread. Grasslands are deteriorating on every continent. Water tables are falling in many countries. Carbon dioxide (CO2) emissions exceed CO2 fixation everywhere.

In 2002, a team of scientists led by Mathis Wackernagel, who now heads the Global Footprint Network, concluded that humanity’s collective demands first surpassed the earth’s regenerative capacity around 1980. Their study, published by the U.S. National Academy of Sciences, estimated that global demands in 1999 exceeded that capacity by 20 percent. The gap, growing by 1 percent or so a year, is now much wider. We are meeting current demands by consuming the earth’s natural assets, setting the stage for decline and collapse.

In a rather ingenious approach to calculating the human physical presence on the planet, Paul MacCready, the founder and Chairman of AeroVironment and designer of the first solar-powered aircraft, has calculated the weight of all vertebrates on the land and in the air. He notes that when agriculture began, humans, their livestock, and pets together accounted for less than 0.1 percent of the total. Today, he estimates, this group accounts for 98 percent of the earth’s total vertebrate biomass, leaving only 2 percent for the wild portion, the latter including all the deer, wildebeests, elephants, great cats, birds, small mammals, and so forth.

Ecologists are intimately familiar with the overshoot-andcollapse phenomenon. One of their favorite examples began in 1944, when the Coast Guard introduced 29 reindeer on remote St. Matthew Island in the Bering Sea to serve as the backup food source for the 19 men operating a station there.

After World War II ended a year later, the base was closed and the men left the island. When U.S. Fish and Wildlife Service biologist David Kline visited St. Matthew in 1957, he discovered a thriving population of 1,350 reindeer feeding on the four-inch-thick mat of lichen that covered the 332-square-kilometer (128-square-mile) island. In the absence of any predators, the population was exploding. By 1963, it had reached 6,000. He returned to St. Matthew in 1966 and discovered an island strewn with reindeer skeletons and not much lichen. Only 42 of the reindeer survived: 41 females and 1 not entirely healthy male. There were no fawns. By 1980 or so, the remaining reindeer had died off.

Like the deer on St. Matthew Island, we too are overconsuming our natural resources. Overshoot leads sometimes to decline and sometimes to a complete collapse. It is not always clear which it will be. In the former, a remnant of the population or economic activity survives in a resource-depleted environment. For example, as the environmental resource base of Easter Island in the South Pacific deteriorated, its population declined from a peak of 20,000 several centuries ago to today’s population of fewer than 4,000. In contrast, the 500-year-old Norse settlement in Greenland collapsed during the 1400s, disappearing entirely in the face of environmental adversity.

As of 2005, some 42 countries have populations that are stable or declining slightly in size as a result of falling birth rates. But now for the first time ever, demographers are projecting population declines in some countries because of rising death rates, among them Botswana, Lesotho, Namibia, and Swaziland. In the absence of an accelerated shift to smaller families, this list of countries is likely to grow much longer in the years immediately ahead.

The most recent mid-level U.N. demographic projections show world population increasing from 6.1 billion in 2000 to 9.1 billion in 2050. But such an increase seems highly unlikely, considering the deterioration in life-support systems now under way in much of the world. Will we not reach 9.1 billion because we quickly eradicate global poverty and lower birth rates? Or because we fail to do so and death rates begin to rise, as they are already doing in many African countries? We thus face two urgent major challenges: restructuring the global economy and stabilizing world population.