It is unpleasant and unprofitable to talk about limits to the human enterprise. Yet in principle, the argument for eventual limits to growth is comprehensible by nearly anyone.
Simple arithmetic growth is easy to understand. Imagine starting with $100 in a piggy bank and adding to it $10 every year—that's arithmetic growth. By the end of 50 years you will have $600. A debt or a problem that grows arithmetically is much simpler to deal with than one that grows exponentially—that's where the quantity expands by a certain percentage per unit of time. Start again with $100 in a piggy bank, but let it somehow magically grow by 10 percent per year, compounded, and the results are quite different: At the end of 50 years, you will have nearly $12,000, or over 20 times as much as yielded by arithmetic growth (figure 1.1). When discussing investments, exponential growth sounds like a very good thing, but when debts or problems grow in this way, calamity has a way of sneaking up on us.
If any quantity grows steadily by a certain fixed percentage per year, this implies that it will double in size every so many years; the higher the percentage growth rate, the quicker the doubling. A rough method of figuring doubling times is known as the “rule of 70”: Dividing the percentage growth rate into 70 gives the approximate time required for the initial quantity to double. If a quantity is growing at 1 percent per year, it will double in 70 years; at growth of 2 percent per year, it will double in 35 years; at 5 percent growth, it will double in only 14 years; and so on. If you want to be more precise, you can use the Y^x button on your calculator, but the rule of 70 works fine for most purposes.
Here's a real-world example: Over the past two centuries, human population has grown at rates ranging from less than 1 percent to more than 2 percent per year. In 1800, world population stood at about 1 billion; by 1930 it had doubled to 2 billion. Only 40 years later (in 1975) it had doubled again to 4 billion; currently we are on track to achieve a third doubling, to 8 billion humans, around 2025. No one seriously expects human population to continue growing for centuries into the future.
In nature, growth always slams up against nonnegotiable constraints sooner or later. If a species finds that its food source has expanded, its numbers will increase to take advantage of those surplus calories—but then its food source will become depleted as more mouths consume it, and its predators will likewise become more numerous (more tasty meals for them!). Population “blooms” (that is, periods of rapid growth) are always followed by crashes and die-offs. Always.
Here is another real-world example. In recent years China's economy has been growing at 8 percent or more per year; that means it is more than doubling in size about every 9 years. Indeed, China consumes more than twice as much coal as it did a decade ago—the same with iron ore and oil. The nation now has four times as many highways as it did, and almost five times as many cars. How long can this go on? How many more doublings can occur before China has used up its key resources—or has simply decided that enough is enough and has stopped growing?