In the last few centuries, the number of people living on Earth has increased many times over. By the year 2000, there will be 10 times more people on Earth than there were 300 years ago.
How can population grow so fast? Think of a family tree. At the top are 2 parents, and beneath them the children they had. Listed beneath those children are the children they had, and so on and so on, down through each generation. As long as the family members continue to reproduce, the family tree continues to increase in size, getting larger with each passing generation. This same basic idea applies to the world's population.
Population grows in the same way that money grows when it's left to compound interest in a bank. With money, growth comes through accumulating interest upon interest. The interest payments you accumulate eventually earn interest, increasing your money. With population growth, new members of the population eventually produce other new members of the population. The population increases exponentially as time passes.
A crucial difference between money and population is that money can increase without limits while population can't. Any population of living creatures is constrained by the availability of food, water, land, or other important resources. Once those resources are depleted, a population won't continue to grow exponentially. It will plateau, or even decline, as a result of disease or malnutrition. Unlike calculating interest, calculating population growth is an imprecise business.
How fast will it grow?
To arrive at a reasonable estimate of how the world's population will grow in the next 50 years, you need to look at birth and death rates (the rates at which people are being born and dying in any given period). If birth and death rates stayed the same across the years in all parts of the world, population growth could be figured with a fairly simple formula much like the one used to figure compound interest. Birth and death rates aren't constant across countries and through time, however. Disease or disaster can cause death rates to increase for a certain period. A booming economy might mean higher birth rates for a given year.
The rate of Earth's population growth is slowing down. Throughout the 1960s, the world's population was growing at a rate of about 2% per year. By 1990, that rate was down to 1.5%, and by the year 2015, it's expected to drop to 1%. Family planning initiatives, an aging population, and the effects of diseases such as AIDS are some of the factors behind this rate decrease.
Even at these very low rates of population growth, the numbers are staggering. By 2015, despite a low expected 1% growth rate, experts estimate there will be 7 billion people on the planet. By 2050, there may be as many as 10 billion people living on Earth. Can the planet support this population? When will we reach the limit of our resources?