Pope Francis often makes shocking statements. This time was no different.

Speaking to the public, he attacked pet owners who refuse to have multiple children. He called them "selfish".

Francis' concerns reflect a global issue. Although the world population is rising, birth rates in many countries are falling. This decline will soon become global. 

By 2100, Italy's population is expected to shrink from 61 to 28 million. By 2050, the Italian death rate will be twice its birth rate.

There is a falling global fertility rate. Between 1950 and 2020, this sank from 4.7 to 2.4. In the same timeframe life expectancy has increased from 45.7 to 72.6 years. Fewer people are being born but those alive are living longer.

Many agree with Francis that these two trends threaten a crisis. As the number of old people grows, the young will have to work harder and pay more to care for them. In the future, the young might become the servants of the old.

Writer and researcher Robert Colvile says it is happening already: "Britain is turning into an elderly care system with a state attached." 

Others disagree. Better healthcare might allow elderly people to remain active for longer. And a smaller population could make everyone richer. 

David Attenborough, Miley Cyrus and Prince Harry have all argued that stopping further growth will help save the planet.

Others claim that we should control and reduce the world population. The Earth is past full. Any growth will make our lives worse.

Improvements have often come along with a growing population. Poverty has decreased as populations grow. 

For the whole of human history, our population has got bigger. We have never had an overall decline before. The results, whatever they are, will be huge.

Yes, say some. For centuries, population growth has walked in step with better lives. A population decline might lead to the opposite.

No, say others. A smaller population will give each person more space, food and water, and will reduce our effect on the environment.

Or, says a third view, population decline is something to take as it comes. Whether it changes things for the good or the bad is up to us.