7.5 billion and counting—how many humans can the Earth support?

July 9, 2018 by Andrew D. Hwang, The Conversation
Slums in Caracas, Venezuela. Credit: Wikimedia

Humans are the most populous large mammal on Earth today, and probably in all of geological history. This World Population Day, humans number in the vicinity of 7.5 to 7.6 billion individuals.

Can the Earth support this many people indefinitely? What will happen if we do nothing to manage future and total resource use? These complex questions are ecological, political, ethical – and urgent. Simple mathematics shows why, shedding light on our species' ecological footprint.

The mathematics of population growth

In an environment with unlimited natural resources, size grows exponentially. One characteristic feature of is the time a population takes to double in size.

Exponential growth tends to start slowly, sneaking up before ballooning in just a few doublings.

To illustrate, suppose Jeff Bezos agreed to give you one penny on Jan. 1, 2019, two pennies on Feb. 1, four on March 1, and so forth, with the payment doubling each month. How long would his $100 billion fortune uphold the contract? Take a moment to ponder and guess.

After one year, or 12 payments, your total contract receipts come to US$40.95, equivalent to a night at the movies. After two years, $167,772.15 – substantial, but paltry to a billionaire. After three years, $687,194,767.35, or about one week of Bezos' 2017 income.

The 43rd payment, on July 1, 2022, just short of $88 billion and equal to all the preceding payments together (plus one penny), breaks the bank.

Real population growth

For real populations, doubling time is not constant. Humans reached 1 billion around 1800, a doubling time of about 300 years; 2 billion in 1927, a doubling time of 127 years; and 4 billion in 1974, a doubling time of 47 years.

On the other hand, world numbers are projected to reach 8 billion around 2023, a doubling time of 49 years, and barring the unforeseen, expected to level off around 10 to 12 billion by 2100.

This anticipated leveling off signals a harsh biological reality: Human population is being curtailed by the Earth's carrying capacity, the population at which premature death by starvation and disease balances the birth rate.

7.5 billion and counting—how many humans can the Earth support?
Credit: The Conversation

Ecological implications

Humans are consuming and polluting resources – aquifers and ice caps, fertile soil, forests, fisheries and oceans – accumulated over geological time, tens of thousands of years or longer.

Wealthy countries consume out of proportion to their populations. As a fiscal analogy, we live as if our savings account balance were steady income.

According to the Worldwatch Institute, an environmental think tank, the Earth has 1.9 hectares of land per person for growing food and textiles for clothing, supplying wood and absorbing waste. The average American uses about 9.7 hectares.

These data alone suggest the Earth can support at most one-fifth of the present population, 1.5 billion people, at an American standard of living.

Water is vital. Biologically, an adult human needs less than 1 gallon of water daily. In 2010, the U.S. used 355 billion gallons of freshwater, over 1,000 gallons (4,000 liters) per person per day. Half was used to generate electricity, one-third for irrigation, and roughly one-tenth for household use: flushing toilets, washing clothes and dishes, and watering lawns.

If 7.5 billion people consumed water at American levels, world usage would top 10,000 cubic kilometers per year. Total world supply – freshwater lakes and rivers – is about 91,000 cubic kilometers.

World Health Organization figures show 2.1 billion people lack ready access to safe drinking water, and 4.5 billion lack managed sanitation. Even in industrialized countries, water sources can be contaminated with pathogens, fertilizer and insecticide runoff, heavy metals and fracking effluent.

Freedom to choose

Though the detailed future of the human species is impossible to predict, basic facts are certain. Water and food are immediate human necessities. Doubling food production would defer the problems of present-day birth rates by at most a few decades. The Earth supports industrialized standards of living only because we are drawing down the "savings account" of non-renewable resources, including fertile topsoil, drinkable water, forests, fisheries and petroleum.

The drive to reproduce is among the strongest desires, both for couples and for societies. How will humans reshape one of our most cherished expectations – "Be fruitful and multiply" – in the span of one generation? What will happen if present-day birth rates continue?

Population stays constant when couples have about two children who survive to reproductive age. In some parts of the developing world today, couples average three to six children.

We cannot wish natural resources into existence. Couples, however, have the freedom to choose how many children to have. Improvements in women's rights, education and self-determination generally lead to lower birth rates.

As a mathematician, I believe reducing substantially is our best prospect for raising global standards of living. As a citizen, I believe nudging human behavior, by encouraging smaller families, is our most humane hope.

Explore further: UN says world population will reach 9.8 billion in 2050

Related Stories

UN says world population will reach 9.8 billion in 2050

June 22, 2017

India's population is expected to surpass China's in about seven years and Nigeria is projected to overtake the United States and become the third most populous country in the world shortly before 2050, a U.N. report said ...

Is the Earth over-populated?

October 31, 2017

In 1800 the world's population was around 1 billion people. Since then it has increased more than sevenfold to reach over 7.5 billion in 2017 (see figure 1), and is forecast to top 10 billion by 2050. Will population growth ...

Fair sharing of water resources is key, says expert

March 22, 2018

To really do something about our massive overconsumption of water, we should limit the strain we put on rivers and freshwater basins. Information on the water use of our products should be transparent and clear. And in the ...

Recommended for you

Extreme weather events rarely occur in isolation

September 25, 2018

The end of the baking hot summer is a forceful reminder of what the climate could hold in store for us in the future. Between April and August of this year, rainfall in Eastern Switzerland was lower than has ever been recorded. ...

10 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

doogsnova
2 / 5 (4) Jul 09, 2018
THIS IS CRIMINALLY WRONG INFORMATION. THE WORLD'S POPULATION IS NOW OVER 8.85 BILLION. IT WILL BE 9 BILLION IN 2020.
https://ca.figu.o...ics.html
The sustainable population level for this planet, according to the Plejaren, is 529 million.
Overpopulation will destroy us all, unless we limit births and provide birth control and education so the population naturally reduces to a more sustainable level. Huge disasters are coming because of the ignorance of this topic, including the ignorance of the author of this article. While the author points out problems and suggests reducing birth rates, he has no clue as to the extent of the problem as it is right now.
aksdad
1 / 5 (3) Jul 09, 2018
Sigh, another article by a depressed and agitated, fact-challenged, Malthusian population crank. They think there are just too many people. Interestingly, none of them ever volunteer to off themselves to reduce the population.

How many people can the earth sustain? At least 7.5 billion so far. Earth can likely sustain many more. Technology and agricultural methods continue to improve and make it possible. See the fascinating graphs in this article:

https://ourworldi...rishment

Half the population lives in countries with sub-replacement fertility rates and in many countries population is in decline. Since the global trend is to have fewer children as living standards improve, it's doubtful that "overpopulation" will ever be a problem.

https://en.wikipe...ertility

And lucky for us inventive humans, we keep developing more efficient ways to use and recycle resources. The future is bright. Except for Malthusians.
humy
3.7 / 5 (3) Jul 09, 2018
THE WORLD'S POPULATION IS NOW OVER 8.85 BILLION.

Strange. Not according to the Wikipedia website;

https://en.wikipe...pulation
"...In demographics, the world population is the total number of humans currently living, and was estimated to have reached 7.6 billion people as of May 2018...."

Which website is right?
TheGhostofOtto1923
5 / 5 (1) Jul 09, 2018
"7.5 billion and counting—how many humans can the Earth support?"

-They forgot a component: for how long?
Sigh, another article by a depressed and agitated, fact-challenged, Malthusian population crank
Easy to pretend when you dont include all the facts isnt it? Like the ONE BILLION ABORTIONS since roe v wade. Plus the 100s of millions more never conceived due to contraception, family planning, and the emancipation of women.

These things only happened in the secular world. Elsewhere, religions are pumping at near max and the result is wave after wave of refugees flooding northward.

And then there's notable events like the Arab spring and the Rwandan massacre, CAUSED by explosive, religion-fueled growth.

The only reason Pakistan stayed stable? Taliban and bin laden funneled all those excess, idle, disaffected fighting age males westward and into the guns of coalition forces.

A little taste of reality brought to you by - otto.

I'm sorry - did I wake you up?
Cusco
1 / 5 (1) Jul 09, 2018
I encourage people to look up Hans Rosling's Ted talks on YouTube, very enlightening analysis of statistics on world population growth. One of the most interesting points he raises is that the country with the largest drop in fertility, 100% voluntary, is the desperately poor Muslim country of Bangladesh. Even with the flood of refugees from Myanmar their population is dropping.

Now the bad news. No matter what we do, barring nuclear war, we're going to hit 11 billion people by the 2030s even if families worldwide stabilize at only 2 children. This is because of the increase in the average life expectancy. Population will probably either stabilize there, or go into slow decline.

Now more good news. Modern genetic technologies are creating a new Green Revolution without the huge burden of petrochemical-based fertilizers and pesticides so we'll probably be able to feed them all without too much trouble.
Ojorf
2.6 / 5 (5) Jul 10, 2018
Sigh, another article by a depressed and agitated, fact-challenged, Malthusian population crank. They think there are just too many people. Interestingly, none of them ever volunteer to off themselves to reduce the population.


Sigh², another comment by a deluded and dishonest, intelligence-challenged, politically motivated crank. They don't understand even simple mathematics. Interestingly, he never responds directly to the article, but posts links to justify his fallacious reasoning. Sigh³.
Beethoven
5 / 5 (2) Jul 10, 2018
With the amount of pollution and waste humanity is generating and the rate our most precious irreplaceable resource- nature, is dying. I don't get how anyone can think this population level is sustainable. we haven't even endured a century at current levels, and most of the damage comes from production. it's a simple math, more population = need for more production = more strain on the environment, wildlife extinctions, pollution litter and waste.
No matter how efficient you get, it will always have an impact, and nature is a limited resource the depletion of which isn't expected to "stabilize" anytime soon.
TheGhostofOtto1923
5 / 5 (1) Jul 10, 2018
Now more good news. Modern genetic blah
The bad news is that cuzcos good news is just wishful thinking.

"Researchers say producing that much food for the growing population by 2050 could significantly increase the levels of carbon dioxide and nitrogen in the environment, which can cause the extinction of numerous species.

"Tilman pointed out that the agricultural sector currently accounts for a third of all the greenhouse gas emissions around the world, and that along with the demand for food, these emissions could double.

"Not only that, but in order to keep up with the demand, poor nations with their current practices will need agricultural land larger than the United States - around 2.5 billion acres."
TheGhostofOtto1923
5 / 5 (1) Jul 10, 2018
Population will probably either stabilize there, or go into slow decline
Delusion is always bad news...

"Speckhardt discovered causation: "religious emphasis on copious reproduction and common teachings against the use of contraception are having a direct impact on population growth" and "religious belief increases the number of children an average woman will have."

and

"as Pope Francis has said, fundamentalism is "a disease of all religions". Kaufmann warns that fundamentalist groups have high birth rates, are growing in number, and some are advocating endogenous growth as a strategy. The book's chapters introduce examples: Mormons and Quiverfull in the USA, Salafists in the Islamic world, and Haredi in Israel. Kaufmann's illuminating work has done more than any other to highlight the role of fundamentalism in global demography."

-Outgrow and overwhelm... the religionist formula for success.
TheGhostofOtto1923
5 / 5 (1) Jul 10, 2018
The bleat of the shofar...

"Kiryas Joel has by far the youngest median age population... and the youngest, at 13.2 years old, of any population center of over 5,000 residents in the US... More than five-eighths of Kiryas Joel residents live below the federal poverty line and more than 40 percent receive food stamps..."

"There are three religious tenets that drive our growth: our women don't use birth control, they get married young and after they get married, they stay in Kiryas Joel and start a family. Our growth comes simply from the fact that our families have a lot of babies, and we need to build homes to respond to the needs of our community."

— Gedalye Szegedin, village administrator

"In 1990, there were 7,400 people in Kiryas Joel; in 2000, 13,100, nearly doubling the population. In 2005, the population had risen to 18,300."

-Dozens of US enclaves with similar demographics
https://en.wikipe...d_States

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.