The figures on the World Population Clock yesterday were mesmerising. Births, deaths and the population growth for the year were increasing by the second. Watchers could not help noticing that the births (around 116,000) were almost exactly double the number of deaths. But one statistic in particular could not be ignored. For the first time, the people on our planet exceeded 8,000,000,000.

There are plenty of other startling figures. It is only 11 years since the world's population reached 7 billion. According to the UN's calculations, it will hit 9 billion by 2037 and 10.4 billion by the 2080s.

India has 180 million more people than it did a decade ago, taking it to nearly 1.42 billion. It is expected to replace China (now on 1.45 billion) as the most populous country in the world next year, and reach 1.67 billion by 2050.  

There are fears that the number of people in sub-Saharan Africa46 countries lie south of the Sahara desert. 95% of all deaths from malaria occur here in this region. will increase by 95% over the next 30 years.¹ Lagos in Nigeria is projected to be the world's largest city by 2100.

The picture is not clear-cut, however. The biggest population boom came in the 1960s; since then, the rate of growth has slowed, thanks to the increased availability of contraception, and of education and jobs for girls and women.

To sustain a population, women need to have an average of 2.1 children. In India, the figure has fallen from 3.4 to 2 over the last 30 years. In China, it is down to 1.16.

Because of this, some very different outcomes have been predicted. Researchers at Washington University believe that the world's population will peak at 10 billion in 2064 and drop to 9 billion by 2100.

But some countries see the decline in fertility as bad news. People are generally living longer after they retire - which means that more young people are needed to look after them, and to pay taxes so that the government can support them.

China, which restricted families to one child between 1980 and 2015, is now encouraging them to have three children.

Climate campaigners, however, argue that every extra person puts a further strain on Earth's resources. They say that the biggest single thing you can do for the planet is not have children.

How many people it can sustain is much disputed. Some experts argue that this is simply a question of how many can be fed, and that from that point of view humanity is doing well. Between 1961 and 2011, food production per head increased by 46%.

But others point out that this is not evenly distributed. The UN estimates that 9% of the world does not have enough to eat, and the figure has increased over the last eight years.  

Then there is the pressure food production puts on the environment. Almost 40% of the world's land surface is used to meet the needs of humans and livestock.²

In 1994 researchers from Stanford University in California calculated that the ideal size of our species would be between 1.5 and 2 billion people.

Yes: Many live in overcrowded places without enough to eat. And the demands on the planet are huge: between 1970 and 2020 the wildlife population fell by two thirds, while the human population doubled.  

No: The problem is not the number of people but the resources they use up. If we can all learn to live more frugally - particularly the richest people, who consume most - the planet need not suffer.

Or... We should not be focusing on the world, but on the whole solar system, as Jeff Bezos has suggested. If we can colonise Mars and other planets, there will be plenty of room for everybody.