Over the last 100 years, the world's population has exponentially grown from 1.6 billion people in 1920 to an estimated 7.8 billion people today. The shift has been driven by rising birthrates, improved healthcare, and higher life expectancies in many regions. There has also been significant migration, with many countries seeing an influx of new immigrants.
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### Instruction:
Describe the changes in the world's population over the last 100 years.

### Response:
Over the last 100 years, the world's population has exponentially grown from 1.6 billion people in 1920 to an estimated 7.8 billion people today. The shift has been driven by rising birthrates, improved healthcare, and higher life expectancies in many regions. There has also been significant migration, with many countries seeing an influx of new immigrants.
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Over the last 100 years, the world's population has undergone significant changes, driven by various factors such as technological advancements, economic growth, and improvements in healthcare. Here are some key changes in the world's population:

1. **Rapid urbanization**: The world's population has shifted from rural to urban areas, with cities accounting for a larger share of the global population. According to the United Nations, the urban population grew from 46% in 1920 to 57% in 2010, and it is expected to continue growing.
2. **Increased life expectancy**: Advances in healthcare have led to a significant increase in life expectancy, with the average global lifespan increasing from 31.5 years in 1920 to 72.5 years in 2020. This has contributed to a decline in mortality rates and an increase in the population.
3. **Decline in mortality rates among children**: The number of children dying in infancy and early childhood has decreased dramatically, from an estimated 50 million in 1920 to 8.3 million in 2020, according to the World Health Organization (WHO).
4. **Shift from rural to urban populations**: The proportion of the population living in urban areas has increased, with cities becoming the primary centers of economic activity, culture, and innovation.
5. **Growing disparities in population**: The population has become increasingly unequal, with the wealthiest countries accounting for the majority of the population, while the poorest countries have seen their populations decline or stabilize.
6. **Rise of the middle class**: The number of people in the middle class, defined as those with an annual income between 50% and 100% of the median income, has increased significantly, particularly in developed countries.
7. **Increased migration and urbanization**: As people move from rural areas to cities in search of better economic opportunities, the population has become increasingly urbanized, leading to changes in urban planning, infrastructure, and social structures.
8. **Environmental concerns**: The population growth has also led to increased environmental concerns, such as pollution, climate change, and resource depletion, as the demand for resources and energy continues to grow.

Overall, the world's population has undergone significant changes over the last 100 years, with significant increases in urbanization, life expectancy, and disparities in population. However, these changes also raise important questions about the sustainability of population growth and the need for sustainable development strategies.

**Key statistics:**

* Population (2020): 7.92 billion
* Urban population (2020): 54.5%
* Infant mortality rate (2020): 33.4 per 1,000 live births
* Life expectancy (2020): 72.5 years
* Median income (2020): $10,449 per year

**Sources:**

* United Nations Department of Economic and Social Affairs (2020). World Population Prospects 2019.
* World Health Organization (2019). Global Health Observatory.
* United Nations Department of Economic and Social Affairs (2019). World Population Prospects 2019.