What's happening?

It took 300,000 years for the number of humans on Planet Earth to reach one billion in 1804. But it only took another 123 years for the number of humans to reach two billion. Now, there are more than eight billion people on the planet.

But now scientists think the number of humans is going to start shrinking. By 2400, they think, we might be back to two billion.

To make sure that a country's populationAll of the people that live in a place. does not fall, each couple needs to have 2.1 children on averageTypical or usual. . This replaces each parent - and then the .1 makes up for anybody that does not have children. It is called the replacement rateThe rate of births needed for babies to replace their parents. .

But most of the world is below that. Even in Africa, which is still at 4.5, each couple is starting to have fewer children.

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This is a huge change for the world. The population has always grown. There have always been more people to have more ideas and discover new things. Some people worry about the future.

So why are people having fewer children? Some think it is because people do not want lots of children. Today, most people can choose not to have children if they do not want to.

Others think there is a different reason. Having children can cost a lot of money, and many people think they simply do not have enough money to give children a good life. Some say we need to change how we live now - so people have enough money for children again.

Yes! We need to get used to the idea of a world with fewer, but older, people. And we should change how we live so that people have enough money to have children.

No! Humans are very good at changing so they fit in with what is happening around them. We should not worry too much about the future.