In Example 2, keep in mind that the populations shown in the graph are estimates and may differ slightly from populations listed by other sources.

In the solution to Example 2, note that the ratios are all approximately equal to 1.40. Recall from Section 1.3 that you can use a percent to represent change. So, you can write 1.40 as 140% and say that the 4500 B.C. world population was 140% of the 5000 B.C. world population. Or, you can say that the world population increased by 40%. In general, you can say that the world population was increasing by about 40% every 500 years from 5000 B.C. through 1500 A.D.

Are you curious about what the world's population is at this very moment? Check out census.gov to view a world population clock, where you can see the estimated population of the world or the United States. The clock is updated once per minute, and features an up to the minute estimate of the world's current population.

No, if the population had followed the pattern, there would have been about million people in the world. Instead the population increased by over 1300% to about 6.07 billion. The agricultural and industrial revolutions of the 17^th, 18^th and 19^th centuries caused life expectancy to increase dramatically. Improvements in health care from the 19^th century through modern times have caused continued increases in life expectancy. These increases caused the population to grow much faster than predicted by the exponential pattern it followed up to 1500.