(bright music) - Hey, guys, Joe here.

I've been thinking, if you sang "Happy Birthday" to every person who will be born today, you'd be singing for nearly six and a half weeks without stopping.

That's adding 386,000 more birthday cakes and humans every day.

It's estimated that 1 in 15 humans ever born is alive right now.

When I was growing up, there were only 5 billion of us.

But now, Earth is home to 7.6 billion people.

And some folks are worried what will happen if things continue at this rate.

Space isn't the problem.

If we all lived as densely as people in Manhattan, every human could fit inside Norway, with a few other to spare.

But our species true footprint is much, much larger.

For many, today's climate and ecological imbalances are proof there's simply too many people on this planet.

But are there really?

What is overpopulation and how did it get this way?

(lighthearted music) (spaceship booming) No one really asked these questions until recently.

Because for tens of thousands of years, our specie's numbers only hovered in the millions.

But by the year 1800 or so, there were finally 1 billion of us.

And then things really started to change.

Well, it took tens of thousands of years for the human population to hit 1 billion.

It only took 123 years to double that and just 47 to double again.

Since the 1970s, the fifth, sixth, and seventh billion have arrived every 12 years.

So when does population growth become overpopulation?

The answer, it depends.

Way back in 1798, Thomas Malthus warned that unchecked population growth would, as a rule, outpace food supply, leading to global mass starvation and violent conflict.

Many environmentalists in the 20th century predicted the incoming population bomb would send shockwaves of disease, poverty, and environmental destruction rippling around the globe, basically ruining everything.

This hasn't proven to be the case.

Malthus underestimated humanity's ability to increase Earth's capacity using good old science.

But one thing is true.

The way we feed 7 billion people today won't scale to feed 10 billion tomorrow.

I don't know if you've noticed, but many parts of the world have too much food.

Maybe we aren't getting it where it's needed because we let imaginary lines on maps make too many rules.

A lot of what we grow doesn't go into our own stomachs.

A third of crops go to feeding livestock.

Animals raised for food occupy 80% of Earth's agricultural land, yet provide only 20% of our calories.

Beef alone requires 10 times as much land per unit protein as produce from grains, even eggs.

We've been able to scale up food production so far, but there is some upper limit to how many people Earth can feed no matter how we grow it.

And we can't exactly eat the moon.

But what makes populations grow or not grow?

The reality for most of history was that many children would die before adulthood, so you better have plenty.

Lots of births balanced by lots of deaths kept populations low but steady for a long time.

But beginning in the 1700s, advances in agriculture and transportation meant fewer people starved.

Later, during the Industrial Revolution, public health and economic advancements translated into less disease and higher living standards.

Death rates went down, but old habits die hard, so people continued to have lots of babies, which led to rapid population growth.

Eventually, people caught on that more of their kids were going to survive.

And as education and opportunities for women improved, families started having fewer children and population growth slowed.

Eventually, when birth and death rates remain low, populations level off, and become stable, or even start shrinking.

Things had gotten a lot better for a lot of people, if they lived in Europe or North America.

Not every country moves through these demographic transitions at the same time.

So while population growth was slowing or coming to an end in many developed countries, it was just picking up in other places.

But developing nations are moving more rapidly through these transitions.

It took the United Kingdom 95 years to halve birth rates, while Brazil did it in 26, and Iran just 10.

Today, birth rates are falling almost everywhere.

The less time a country spends in stages of rapid growth, the quicker Earth's population stops increasing.

It's unlikely that the 12 billionth human will ever be born.

And by 2100, our population will most likely peak between 9 and 12 billion.

Instead of one big population bomb, the challenge today is diffusing a few population cluster bombs in pockets of the developing world.

And there are two big ways to accomplish this.

Increasing women's access to education is the most effective way to lower birth rates.

It improves children's health and leads to better family planning.

Empowering women leads to slower population growth.

This alone could reduce greenhouse gas emissions as much as all wind energy by 2050.

Today, the richest 10% of humans are responsible for almost half of climate emissions, while the poorest half of people are only responsible for a 10th.

Developed nations will have to reduce their impact and meet developing nations in a cleaner middle.

Populations can't grow forever without consequences.

But under the right circumstances, populations control themselves.

But 10 billion people is still a lot of mouths to feed, and doing it without ruining nature or anything like that won't be easy.

But it's not impossible.

And it won't take some apocalyptic robot army of forced population control to do it.

While history's taught us that population growth has natural checks and balances, we have yet to find a limit when it comes to creating new ways to live.

Stay curious.

Hey, guys, I hope you enjoyed our video.

Population growth is a big challenge, and I know overpopulation is something a lot of you are thinking about.

It's one thing to look at history or some statistics, but like me, you probably wanna know what's actually happening in the real world.

That's why we partnered with Bill and Melinda Gates to bring you this video.

I got the chance to ask Bill how he stays optimistic about population growth.

And this is what he told me.

- We are worried that improving health might exacerbate population growth and therefore the challenges of feeding people and preserving the environment.

And so, it was very non-intuitive when we learned that as you improve health, particularly child survival, then families choose to have less children.

And the average family size has gone down everywhere we've improved health.

And so, you don't have many countries that have good health and high population growth.

This repeats in society after society.

And so, saving lives not only is great for those kids but also for the impact it has.

The world at large has actually peaked in terms of the number of children born.

So all the population growth from this point forward is because the number of years that people live is going up, you know, which is absolutely a good thing.

But then we do get to the peak population because we've already had the peak number of babies.

- Hearing about that, about the work people like Bill and Melinda support around the world, makes me optimistic too.

Humans' impact on Earth is complicated.

And the answers for how we reduce that are kind of unexpected.

But the biggest surprise is that what scientists, and doctors, and educators are doing for women and children, it's working.