The Basics of Population Change

To understand how a population changes, geographers look at two main 'valves' in the human bathtub: people being born and people dying. We measure these using the Crude Birth Rate (CBR) and the Crude Death Rate (CDR). These aren't just total numbers; they are calculated as the number of births or deaths for every 1,000 people in a population per year. By using a 'per 1,000' scale, we can fairly compare a small country like Iceland to a giant like India. If a country has a CBR of 20, it means for every 1,000 residents, 20 babies were born that year. If the 'water' coming in (births) is faster than the 'water' draining out (deaths), the population grows.

For most of human history, population grew very slowly because the 'drain' (death rate) was almost as wide as the 'faucet' (birth rate). This changed with three major revolutions. First, Medical Advances like vaccines and antibiotics stopped millions of premature deaths. Second, Sanitation Improvements—specifically clean running water and sewers—prevented the spread of diseases like cholera. Third, the Green Revolution introduced high-yield crops and better farming technology, ensuring more people had enough to eat. These changes didn't necessarily make people have more babies; instead, they drastically slowed down the rate at which people were dying, causing the population to skyrocket.