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The World at 7 Billion People: How Much More Growth Can the Planet Support?

With global population expected to surpass 7 billion people this year, the staggering impact on an overtaxed planet is becoming more and more evident.

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This, in fact, is the story of China, often seen not as an example of population's impact on the environment but that of rapid industrialization alone. Yet this one country, having grown demographically for millennia, is home to 1.34 billion people. One reason the growth even of low-consuming populations is hazardous is that bursts of per-capita consumption have typically followed decades of rapid demographic growth that occurred while per-capita consumption rates were low. Examples include the United States in the 19th and 20th centuries, China at the turn of the 21st, and India possibly in the coming decade. More immediately worrisome from an environmental perspective, of course, is that the United States and the industrialized world as a whole still have growing populations, despite recent slowdowns in the growth rate, while already living high up on the per-capita consumption ladder.

Many of the impacts of this ubiquitous multiplication of per-capita resource consumption by the number of individuals are by now well documented. Humanity started to overwhelm the atmosphere with greenhouse gases not long after the Industrial Revolution began, a process that accelerated along with population and consumption growth in the 20th century. Fresh water is now shared so thinly that the United Nations Environment Program (UNEP) projects that in just 14 years two thirds of the world's population will be living in countries facing water scarcity or stress. Half of the world's original forests have been cleared for human land use, and UNEP warns that the world's fisheries will be effectively depleted by mid-century. The world's area of cultivated land has expanded by about 13 percent since its measurement began in 1961, but the doubling of world population since then means that each of us can count on just half as much land as in 1961 to produce the food we eat.

For the rest of life on Earth, the implications of all this are obvious. Where we go, nature retreats. We are entering an epoch scientists have begun calling the Anthropocene, a break with the geologic past marked by humanity's long-term alteration of the natural world and its biota. We are inadvertently bringing on the sixth mass extinction not just because our appetites are vast and our technologies powerful, but because we occupy or manipulate most of the land in every continent except Antarctica. We appropriate anywhere from 24 percent to nearly 40 percent of the photosynthetic output of the planet for our food and other purposes, and more than half of its accessible renewable freshwater runoff.

Given these facts, it's hardly surprising that wildlife conservation faces an uphill battle globally and in every nation, while ambitious concepts like the creation of wildlife corridors to help species escape the ravages of development and climate change proliferate despite their impracticality in a world of growing human impacts.

So should we be afraid on the day we gain a 7 billionth living human being, especially considering UN demographers are now projecting anywhere between 6.2 billion and 15.8 billion people at the end of the century? Fear is not a particularly productive response — courage and a determination to act in the face of risk are the answer. And in this case, there is so much to be done to heal and make sustainable a world of 7 billion breathing human beings that cowering would be not just fatalistic but stupid.

Action means doing a lot of different things right now. We can't stop the growth of our numbers in any acceptable way immediately. But we can put in place conditions that will support an early end to growth, possibly making this year's the last billion-population day we ever mark. We can elevate the autonomy of women to make life-changing decisions for themselves. We can lower birth rates by assuring that women become pregnant only when they themselves decide to bear a child.