Original subtitles

This is the Earth, our planet.

Home to millions of different species.

But only one species dominates everything.

Human beings.

There are nearly seven billion of us living on the Earth.

And the human population is increasing by more than two people every second.

200,000 people every day.

Nearly 80 million people every year.

Each additional life needs food, energy, water, shelter

and hopefully a whole lot more.

Today we're living in an era

in which the biggest threat to human wellbeing, to other species,

and to the Earth as we know it, might well be ourselves.

The issue of population size is always controversial

because it touches on the most personal decisions we make.

But we ignore it at our peril.

There's absolutely no doubt at all that the world's population will continue to grow.

The only question is by how much.

More than a billion people on the planet already lack access to safe, clean drinking water.

And we know things are going to get more difficult as the population continues to grow.

We need to double the amount of food that we have available to us as soon as possible.

Such a scale of change will leave no-one untouched.

Keep in mind that when the Titanic sank, the first-class cabins

went to the bottom just as quickly as the steerage.

I was born into a world of just under two billion people.

Today there are nearly seven billion of us.

Whenever I hear those numbers I can honestly say I find it incredible.

Triple the number of human beings in what seems like the blink of an eye,

and the world transformed utterly.

Human population density is a factor

in every environmental problem I've ever encountered,

from urban sprawl to urban overcrowding,

disappearing tropical forests to ugly sinks of plastic waste.

And now the relentless increase of atmospheric pollution.

I've spent much of the last 50 years seeking wilderness,

filming animals in their natural habitat and to some extent avoiding humans.

But over the years, true wilderness has become harder to find.

I can't pretend that I got involved with filming the natural world 50 years ago

because I had some great banner to carry about conservation. Not at all.

I've always had huge pleasure in just watching the natural world and seeing what happens.

I made those films because it was a hugely enjoyable thing to do.

But as I went on making them, it became more and more apparent

that the creatures which were giving me so much joy

were under threat.

The fun is in delighting in the animals.

But if you do that you owe them something,

and so you have an obligation to speak out

and do what you can to help protect them.

I support a group called the Optimum Population Trust

which campaigns to reduce birth rates.

Because I think if we keep on going,

we're not only going to damage nature,

we're likely to see more and more inequality and human suffering.

In this programme I want to see how population growth

will affect our ability to obtain our most basic needs -

water, food, and energy.

And to see if it's possible to answer the question,

how many people can live on Planet Earth?

Human beings are good at many things.

But thinking about our species as a whole is not one of our strong points.

I don't even think I could tell you how many people live in this country.

A googol? Yeah, I would say a googol.

TRANSLATION: I know India's population is 1.1 billion

but I don't know the population of the world.

I'd say six billion off the top of my head.

TRANSLATION: I've got no idea how many people live on the planet, no idea!

Luckily, the size of the human population is studied very closely.

By and large, every human birth and death throughout the world

has been recorded for the last 60 years.

The data is kept here in New York City, at the United Nations.

Hania Zlotnik, head of the UN Population Division, is in charge of those precious numbers.

This was the old type of working, when I arrived at the UN.

I worked with these types of files.

They are very well-organised but they look old.

Now we do it via computer and it's somehow not the same thing as feeling the data.

I am a numbers person, yes, definitely.

'My mission is to be the bean counter.'

That means we are the thermometer telling you

that the planet is getting hot or cold in terms of population change.

The UN do much more than just keep records.

They make projections into the future.

And their figures are staggering.

The human population is still growing.

One expects that at the very least it's likely to add

about 2.3 billion people by middle of the century.

We have 6.8 billion today.

We're expecting to get the seventh billion in the next three to four years.

And then that by mid-century we'll have something like nine billion.

In the next 40 years, the Earth will need to accommodate

nearly three billion more people.

That's more than the current population of the whole of Europe,

the whole of Africa, North and South America combined.

How can we be so sure of this prediction?

Well, we know that there are more than a billion teenagers alive today

and most of those teenagers will have children of their own

and live long enough to become grandparents.

And that's all that needs to happen

for there to be nine billion humans alive in 2050.

It's not people having huge families.

It's just a lot of people doing what humans naturally do.

We also have a good idea of where these additional people will live.

There are likely to be ten million more people in Britain.

100 million more in the USA.

India will overtake China to become the most populous country in the world.

The population of some countries will shrink -

Japan, Russia, Germany, and much of Eastern Europe.

The places that will experience the most rapid growth

are also the least developed countries in the world.

Afghanistan's population will double.

Most of Sub-Saharan Africa will double.

Niger's population is predicted to more than triple.

I think everyone living through the next 50 years is going to be

affected by these demographic changes, wherever they are.

For most of human existence, our population size

was kept in check by nature, just as it is for other animals.

If there's plenty of water, food and materials for shelter,

a population will thrive.

But when disease, famine or drought strike, life can be cut short.

The history of humanity is one of overcoming these environmental limits,

but it took us a very long time to achieve.

On the horizontal axis here we have time over the last 10,000 years.

On the vertical axis here we have

the size of the human population in billions of people.

Over the last 10,000 years,

in general there's been very little change.

It's a very boring picture.

But from about the year 1800 onwards you have a major increase,

a very large increase in the world's population

from about 1 billion up to 7 billion today.

Basically what this increase in population represents is control of death rates.

Death rates have been reduced because infectious diseases -

cholera, smallpox, malaria, measles, those sorts of things -

have been massively reduced.

On average for almost all of human history, a man and a woman

were only survived into adulthood by two of their children

and that's why the world's human population didn't increase.

Extending life by controlling disease

is perhaps one of the greatest achievements of humanity.

I was born into a world of 2.5 billion

and I'm seeing it almost triple in my lifetime.

And life has not gotten worse.

In fact for most of the population of the world,

life has gotten better in these 50 years.

Living healthily and long has consequences - population growth.

Just as the human population was starting its unprecedented growth spurt

in the late 18th century, this was published.

It's a first edition of An Essay on Population

by the English clergyman Thomas Malthus.

Malthus made a very simple observation about the relationship

between humans and resources and used it to look into the future.

He pointed out that "the power of population is indefinitely greater

"than the power in the Earth to produce subsistence for man."

Food production can't increase as rapidly as human reproduction.

Demand will eventually outstrip supply.

Malthus goes on to say, if we don't control human reproduction voluntarily,

life could end in misery, which earned him a reputation as a bit of a pessimist.

But Malthus's principle remains true.

The productive capacity of the Earth has physical limits

and those limits will ultimately determine how many human beings it can support.

To help answer that question, we need to have an idea of what human beings need.

And the people who calculate this more precisely than most

are the people who are more interested in leaving the planet than staying on it.

Astronauts.

One of the people in charge of the wellbeing of astronauts

on the International Space Station is Doug Hamilton.

NASA, we calculate and simulate everything.

If you are going to plan a rocket launch, you have to know how much

food and water and equipment you need to bring into space.

As well as working out how much space the astronauts need,

Doug and his team have to calculate their daily requirements

for food, water and breathable air.

They typically need about 820 grams of oxygen, which is

just a really large, large balloon, really.

We need about 4,000 to 5,000 calories of food

which is about 820 grams dry,

and they need about 3.52 litres of water,

of which 2.5 litres is just consumed daily.

We want them to drink a lot of water - it's very good for them.

And then we urinate out and put that into our processing system and we make it into drinkable water,

so you might be drinking the same water molecule hundreds and hundreds of times

on the space station, because we recycle.

NASA's calculations are tailored for space,

but they're the same ingredients each and every one of us needs.

When you see how hard it is to reproduce

what Mother Nature does every day for all of us,

you begin to really appreciate the world that you have.

Whatever our technological achievements,

we're still utterly reliant on the natural systems of the Earth for our very survival.

By and large the planet has provided for the human race, so far.

As the population has increased, people, through agriculture and industry,

have exploited those resources ever more effectively.

But increasingly, we're seeing signs of strain.

We're reaching the limits of our environment.

Perhaps most alarmingly with that fundamental ingredient for life - water.

We call our Earth the Blue Planet

because about 70% of the Earth's surface is covered in water.

But most of that is sea - just 2.5% is fresh water.

And of that tiny fraction, just 1% is available for human use.

The rest is locked up in mountain glaciers

and the Earth's polar ice caps.

But there's another fact we need to understand about water.

Well, there's no more water on the planet than there was when life first appeared on Earth.

It changes its distribution.

There's more water in different parts of the world.

But its still the same amount of water that's been here always.

We appropriate over half of all the available fresh water in the world to serve our needs.

To transform deserts into fields.

To generate energy from rivers.

And to build cities in some of the most arid regions on the planet.

But despite our ingenuity, there are many who struggle to get enough of this basic resource.

More than a billion people on the planet already lack access to safe, clean, drinking water.

And we know that things are going to get more difficult as the population continues to grow.

Within the next 20 years as much as half of the world's population will live in areas of water stress.

Chronic water shortages are often the result of poor infrastructure,

politics, poverty, or simply living in an arid part of the world.

But increasingly the pressures of population are to blame.

Mexico City is ranked as the eighth-richest city in the world,

ahead of Moscow, Hong Kong and Washington DC.

It also benefits from heavy annual rainfall.

But its water system is buckling under the pressure

of supplying water to its 20 million inhabitants.

And every day at least a million people are affected by the shortages.

Enrique Vazquez is a water truck driver for the government.

And the number of people relying on this emergency service is growing daily.

Today he's heading for a poor district in the city's south-west, where he's a regular visitor.

TRANSLATION: At some time in the future, wars are going to be fought over water, not oil.

But people don't seem to understand.

Instead of conserving it, we just waste it.

The problem is a combination of leaks in the system,

and back-up reservoirs that are running dry.

The city authorities predict that these reservoirs

may be completely emptied within a matter of months.

TRANSLATION: Look - the tap's on but there's no water coming out.

The people living here have had to adapt their lifestyles to an erratic water supply.

We only have half a bucket of water to wash ourselves with.

And we can't flush the toilet until two or three people have used it.

TRANSLATION: Unfortunately, I think there's going to be water shortages

all over the world, not just in Mexico City.

I think everyone needs to take water more seriously.

The few people who have water should conserve it better,

or there'll come a time when the shortages are global,

and there's little left for anyone.

In Mexico City, shops which sell water to meet people's daily needs

are becoming ever more common.

But the water we use at home is only a fraction of the water we actually consume.

And that's because we use colossal quantities in industry and agriculture.

We may know where the water out of our tap comes from,

but we seldom know where the water that went into our can of cola

or into the shirt we're wearing,

where those goods were produced and how much water it required,

what the consequences were for the natural systems

and local communities that are dependant on that same water.

So for example the cup of coffee you may have in the morning

requires on the order of 120 litres

just to produce the coffee and bring it to your table.

A can of beer, 150 litres.

A hamburger, 8,000 litres of water.

To produce enough water to grow the cotton in my shirt

is 3,000 litres, as well.

The impact of human demands on the world's freshwater systems

are so massive, they can be seen from space.

The Aral Sea, a freshwater lake in Central Asia,

once covered 65,000 square kilometres.

In the last 40 years it has lost 90% of its water,

the rivers that feed it diverted to irrigate cotton.

Lake Chad on the southern edge of the Sahara

has also been drained to a tenth of its former size

by drought and overuse. Yet 30 million people depend on it.

It is possible to distil fresh water from the sea.

And in the last 20 years, more and more countries have turned to desalination.

But with current technology desalination plants

are often extremely expensive, use an enormous amount of energy

and their by-products can be damaging to our seas.

With groundwater levels declining across the world from North Africa to China,

Pollution of rivers and wetlands on the increase,

and already today more than 1.2 billion people living with water scarcity,

our prospects for providing water to nearly three billion more people do not look good.

But in many ways, supplying water to people is the least of our worries.

As we've seen, the lion's share of the water we use goes into agriculture.

And that means any water shortages we face in the future

will affect our ability to provide that other staple of life - food.

When it comes to the world's food supply,

some of the most accurate information comes from space.

Geographer Molly Brown monitors food production on Earth

using data from NASA's satellites.

This is a ecosystem in Thailand, where they do rice agriculture,

and it's extraordinarily productive

and in one of the most highly productive agricultural regions.

Now she's beginning to see global agriculture hit a natural limit.

One of the things that all these different landscapes really show us

is how we're using almost all the land that's available to us

that's really highly productive,

that has great agricultural potential. So we know

that there isn't a lot of extra land.

I mean, we're using 30 or 40% of the entire land surface.

As the world's population increases,

the urgency with which we're going to have to increase the amount of food we produce will increase.

So we need to double the amount of food that we have available to us, as soon as possible.

How we're going to do that is through raising productivity,

because there's really no more land with which to expand to.

A doubling of productivity sounds ambitious,

but we've done even better than that in the past.

In the 20th century, the industrialised nations managed to triple their farming yields

with the invention of synthetic fertilisers

and then by the introduction of mechanised processes.

The less developed parts of the world continued using traditional farming methods

into the 1960s, until an Iowan farmer

decided to do something about it.

Norman Borlaug, who died this year aged 95,

is credited with saving millions of lives in what's become known as the Green Revolution.

So he was a very unpretentious man.

You can see from his office.

Small but very functional.

And he had some of his awards on the wall.

But also, in particular, I always thought this picture

which he kept on the wall was quite typical of

the kind of person he was.

His interactions with the next generation of scientists

around the world and his enthusiasm for getting out into the field

and showing people what could be done with the science,

in improving agricultural productivity.

Borlaug developed high-yielding, disease-resistant crops

and taught Indian and Mexican farmers

how to get the most out of them with modern farming methods.

The astonishing five-fold increases in yield that they achieved

allowed many countries to become self-sufficient in food.

In 1970, Borlaug received the Nobel Peace Prize

for his work in alleviating world hunger.

He was able to get his wheat, his new varieties, delivered to India,

and within a few years, it was really astounding.

He showed me pictures of the mounds of wheat,

the surplus that had been produced within a few years of introducing these new varieties.

And in fact that's the seminal event, that's the Green Revolution.

Thanks in part to Borlaug, much of the world is now fed,

but globally we're beginning to see a levelling off of agricultural yields.

This is leading to a worrying new trend.

To maintain their own food supplies,

some of the richest and most powerful countries in the world

are acquiring large tracts of land from some of the very poorest.

Olivier De Schutter is a human rights lawyer

who's been monitoring these land deals for the United Nations.

Arable land suitable for cultivation is becoming a scarce commodity

and countries find it more and more difficult

to produce enough food to feed their populations.

So they are now scrambling in a global competition

to achieve food security by buying land abroad.

International corporations and increasingly governments

are leasing some of the last remaining areas

of un-developed farmland in the world.

Their aim is to introduce intensive farming methods

and export the food back to their home countries.

The problem is that in most cases

these deals are not sufficiently well monitored.

They are not transparent, and we are not certain that local communities will benefit from these investments.

These deals are often controversial and shrouded in secrecy.

But according to local media reports,

Chinese investors are negotiating land deals

throughout Africa, as well as with Kazakhstan, Mexico and Brazil.

Saudi Arabian firms have leased farmland in Sudan.

And several British investment funds are reported to be raising capital

to buy farmland in Angola, Malawi and Ukraine.

Most of the target countries for foreign investors are in Africa,

some of which already struggle to feed their own people.

When we see paradoxical situations

such as foreign investors producing food in Ethiopia,

shipping this food back to the home country,

or selling it on the international markets

when Ethiopia is still a country which is heavily dependent on international food aid.

So this is a country which is at the same time producing food for export markets

and depending on international aid in order to feed its population.

The future is going to be particularly challenging for the countries of sub-Saharan Africa.

With many of their populations projected to double,

there's going to be increasing pressure for a limited supply of land.

There are few nations as acutely aware of

how destabilising these kinds of pressures can be as Rwanda.

Our land is not growing

and yet our population is.

We estimate that it will be double in 26 years,

so in 26 years we will probably be 20 million.

Rwandans consider land a vital resource.

But they also see it as a resource for primarily their own use,

for their own security, for their own food security.

Martin Seturinka grows bananas and maize on three acres of farmland.

Like 80% of Rwandans, his family subsist on what they can grow.

Land is an issue all over Rwanda.

There isn't enough land to go around

and people find it hard to grow enough food to survive.

In Rwanda, children inherit land from their parents,

but in a country where the average family has more than five children,

that can only mean one thing.

Smaller parcels of land to live off.

I don't know what will happen to my children,

or how they'll cope, I honestly don't.

It's already impossible for me to provide enough food for them.

Only God knows.

Martin is father to 15 children.

But they aren't all his own.

Five of them are adopted, orphans whose parents

were brutally murdered in Rwanda's devastating genocide.

In 1994, the two major tribes in Rwanda,

the Hutus and Tutsis, embarked on a mutual slaughter

that left almost a million dead in just three months.

Amongst the many causes of that conflict,

competition for scarce resources was an added pressure.

Poverty became a mobilising tool, the poor unemployed youth,

some of them were encouraged to kill their neighbours...

..with the hope they'd either inherit their piece of land,

or their house, or their livestock.

If we cannot grow the economy fast enough to meet this growth,

and can't slow it down, then there will be increased competition

for resources which are finite.

So our forests are likely to go, our swamps will be overused.

Therefore this will also have an effect on the climate,

climatic changes which will further exacerbate

the negative effects of this growth.

It's a bit of a vicious cycle and we must find a way of breaking it.

In Rwanda, the government can already foresee the impact

population growth is likely to have on their immediate environment.

Across the world, population growth is likely to take

an even greater toll because of our ever-increasing demands

for a resource we've come to depend on,

but which may be causing us the biggest damage of all.

Of all the resources that humans have harnessed from the Earth,

the one that has transformed everything is energy.

Fossil fuels are the remains of plants and animals

that lived perhaps 350 million years ago

and later became buried in the Earth's crust.

With the technologies of the industrial age,

we liberated this energy

and used it to get more from nature than had ever been possible before.

Our favourite fossil fuel is oil.

Our demand for it increases every year.

Today we use 85 million barrels a day.

Oil provides the fertiliser, pesticide and mechanisation that has

allowed us so far to produce enough food for our expanding population.

But just as we're realising how much we depend on it,

it's getting harder to find.

Houston, Texas.

One of the richest places in the world,

thanks to its vast reserves of oil and gas.

Danny Davis is an independent oil producer.

This is our office, our base of operations and what we do.

Our little company sign, which we're very proud of.

Danny has been drilling oil in Texas since the early 1980s.

This is a collection of jars of oil from all the wells

we've found over the years, I guess over the last 15 years.

One of them I kind of like the most,

is this one. This was discovered about 30 minutes

outside of Houston on the Brookshire Dome.

This came out at 1,000 barrels of oil a day from 2500 feet.

High gravity sweet crude. It smells great.

When it comes out its so fresh you can put it on your salad,

little oil and vinegar, it's good stuff.

This is why we do it, this is what it's all about,

it's an exciting business.

There's a fortune to be made treating these reservoirs.

These days, oil in Texas is getting harder to find.

Danny's looking much further afield, to Alaska.

He's been granted a rare license from the government

to drill offshore. But before he can get started,

Danny needs to raise millions of dollars of investment.

Let me ask you a question, how many years you been doing this,

about 40 or 50?

If his plans are successful, the figures are truly staggering.

You look at a billion of barrels of oil and oil's 70 a barrel

and you got two billion barrels, in gross numbers,

200 billion dollars, probably.

I don't know, I couldn't predict that.

You can only go on the value today,

you don't know what it's is going to be tomorrow.

Yeah, I'll call him and let him know.

Thanks for everything. All right, guys.

We'll see y'all soon.

Danny won't be short of customers for his oil because energy demand

is predicted to increase by 40% over the next two decades.

The Alaskan fields may make him a very wealthy man.

But the fossil fuels that have helped to bring great wealth

to many nations as well as individuals are proving to be a double-edged sword.

Not just because of their contribution to climate change.

What cheap energy has allowed us to do fundamentally

is to appropriate the Earth's natural systems to serve our needs,

without paying too much attention to the long-term effects

on the environment and other species.

It seems we're just beginning to realise the full impact

that our industrialisation is having upon the natural world.

In the oceans we've depleted fish stocks massively.

10% of the world's coral reefs

are estimated to be degraded beyond recovery.

A third of the world's amphibians, a fifth of all mammals and 70%

of all plants are currently under threat of extinction.

When it comes to conserving our natural world,

there are two arguments to contend with.

On the one hand, there's a sense of our moral obligation,

as the most intelligent species on the planet, to protect

the marvellous variety of species that have evolved alongside us.

On the other, there's self-interest. The more we damage the environment,

the more we threaten our own survival.

Perhaps self-interest is the more powerful argument

because how we treat our environment certainly determines

how many people the Earth can sustain.

There's a concept in ecology called "carrying capacity".

It's a calculation of how large a population

any given environment can support.

William Rees is a human ecologist who's taken the concept

and applied it to ourselves and our environment, the Earth.

The simple fact of the matter is

the Earth can accommodate so much consumption.

You might have ten billion people at one level of living

and a billion at a more comfortable level of living.

So carrying capacity is a very flexible idea.

You simply divide the total productivity of the Earth

by the number of people and that gives you some idea

of how many people the Earth can support.

Rees has estimated what he calls

the productive bio-capacity of the Earth.

This is made up of all the food, water and energy produced across

the world each year, and measured in units called global hectares.

He's worked out that if we were to share the Earth's

productive bio-capacity fairly, there'd be two global hectares each.

But the reality tells a very different story.

According to Rees's data, most of Africa use little more than

half of their share of the Earth's productive capacity.

The average Indian uses less than half.

The Chinese use their fair allocation of two hectares each.

But Europeans use much more with the British on average

using over five global hectares.

And the average American, using more than four times their fair share.

So how many people can the Earth sustain?

Well, according to these calculations,

if all humans consumed the same as the average Indian does today,

the Earth could sustain as many as 15 billion people.

If we consumed as little as the average Rwandan,

this would go up to 18 billion.

But our planet can only sustain 2.5 billion people

living as we do in Britain.

And only 1.5 billion living in the lifestyle

of those in the United States.

But the picture may even be worse than this.

These figures are based on rates of consumption

that many think are already unsustainable.

There's plenty of evidence right now

that we are already in the state of what we call overshoot.

Each year the human population at current average levels of consumption,

which most of us in Europe and North America

would consider to be inadequate, is already exceeding

the productive capacity of the planet.

Not only in terms of its ability to produce,

but also in terms of its capacity to assimilate our wastes.

Rees believes that today's population requires the equivalent

of 1.5 Earths to support our current way of life.

We're simply living beyond the means of our environment to sustain us.

To have a state of sustainability where we remain

within the productive capacity of the planet,

means that people in industrialised countries

are going to have to give up consumption of a great deal in order

to create the ecological space for needed growth in the third world.

If we don't make those kinds of compromises,

then we're going to continue to erode the resource base

of the planet to the point where we all suffer.

As I see it, humanity needs to reduce its impact on the Earth

urgently and there are three ways to achieve this.

We can stop consuming so many resources.

We can change our technology

and we can reduce the growth of our population.

We probably need to do all three.

For most people, the idea of someone else telling them

how many children they should have is simply unacceptable.

So when governments attempt to do exactly that,

it always causes controversy.

In 1979, the Chinese government introduced its infamous

one child policy, changing family life in China forever.

Families were encouraged to have fewer children,

those that didn't were fined.

The policy was a direct response

to the preceding decades of famine and starvation.

It's still in place today.

According to official figures, without the one child policy,

there'd be 400 million more people in China -

that's more than the entire population of the USA.

It's unlikely that other governments could undertake

such an extreme path without major civil opposition.

In the 1970s, the Indian government

also sought to bring down its birth rate.

To start with, it took a less aggressive path,

setting up festivals around the country

where vasectomies were offered in return for small incentives.

In those days, in those festivals,

they have done in a week something like 80,000 sterilisations.

The incentive was some cash,

some money, nothing much.

The problem was the festivals were attracting the wrong customers,

people who already had large families.

That is the weakness of incentivisation - they could not

attract the couples with two children,

they attract couples with five children, six children.

It's like closing the door after the horse has gone.

But in some areas, politicians took the sterilisation drive a step too far.

In 1977, when Indira Gandhi was introduced the emergency programme.

What they did,

the punishment for every crime in those days were sterilisation.

For example, if a person travels in a train,

he has no ticket,

what is the punishment? He was taken for sterilisation.

In 1977 alone, around eight million people were sterilised.

And the public outcry was so great

that it helped to bring down the government.

Hopefully these kind of coercive policies are a thing of the past.

Because we're beginning to realise that birth rates fall,

provided the conditions are right.

In the south-west of India lies the long narrow coastal state of Kerala.

Most of its 32 million inhabitants live off the land and the ocean,

a rich tropical ecosystem watered by two monsoons a year.

It's also one of India's most crowded states.

But the population is stable because nearly everybody has small families.

How many of you have only one child in the house? Raise your hands.

Only one. You are the only one in the house.

Only one? Only one?

I think today almost 30 to 40% of couples

in Kerala have just one child.

How many of you have two in the house two? Two.

Two in the house.

Today on average, Kerala women produce only 1.5 children.

How many of you three in the house?

Three, three, three.

No problem, brother or sister?

Two brothers. They wanted a girl. That's why they got three. Otherwise no.

You will rarely see a couple with now three children, very rarely.

At the root of it all is education.

Thanks to a long tradition of compulsory schooling for boys

and girls, Kerala has one of the highest literacy rates in the world.

Even too-young children are coming to school.

See, they are carrying bags bigger than them.

Where women are well-educated,

they tend to choose to have smaller families.

When all girls goes to school,

automatically they will marry very late.

For example, today in Kerala average woman marries at the age of 28.

Whereas a state like Uttar Pradesh, Rajasthan, Bihar,

the girl marries at the age of 18.

So, at 28, these women in Uttar Pradesh and Bihar

have already four children, where Kerala girl is even not married.

How many children do you want to have?

ALL: One.

What Kerala shows is that you don't need aggressive policies

or government incentives for birth rates to fall.

Everywhere in the world where women have access to education,

and have the freedom to run their own lives,

on the whole, they and their partners

have been choosing to have smaller families than their parents did.

But reducing birth rates is very difficult to achieve

without a simple piece of medical technology - contraception.

We can think of modern contraception as a crucial technology

for the sustainability of the planet because it's the element

that has allowed the populations of many developing countries

to reduce their fertility as rapidly as they have done.

Despite a recent history that makes population a particularly delicate subject in Rwanda,

the government here is one of the few in Africa

to have made universal access to contraception

a national priority in recent years.

Console Mukanyarwaya is one of hundreds of family planning officers

who give contraceptive advice to local communities.

Since the year 2000, family planning education

has been provided for everyone in the country.

Rwandans understand that while it's wonderful to have children,

you've got to be able to look after them as well as you can.

We try to get people who use contraception

to teach their neighbours so they can see for themselves

the advantages of having fewer children.

Since it has become freely available,

the uptake for contraception has been huge in Rwanda, with many women

opting for injections or even five-year hormone implants.

While Rwanda is addressing its population growth,

it's estimated that a quarter of married women

in sub-Saharan Africa still don't have any access to contraception.

And across the world, over 80 million births are unplanned.

In my view it's a basic human right,

that everyone should have access to contraception.

All the evidence is that people take advantage of this

once they have the possibility and they reduce their fertility.

If that happens, then, amongst other things, the world's population

growth will eventually level out at a lower rather than a higher number.

And that's a good thing.

When it comes to other ways of reducing human impact on the Earth,

there are a few glimmers of hope emerging.

Governments across the world are beginning to recognise

that the life-support services provided by our ecosystems

are in need of repair, and they're doing something about it.

Often it takes individuals with vision

to lead the process of change.

Valente Souza is an urban planner and a committed environmentalist

with a lot of responsibility.

The government of Mexico City have employed him to find

a sustainable solution to their water shortages.

And he's convinced the local ecosystem holds the answers.

The solution is at hand and the solution is called the rain.

Because we are at the top of the mountain

and the only source of water is rain, not rivers.

We have to re-establish what we call the hydrological cycle.

This cycle relies on ancient forests that used to surround the city.

But as the city's grown they've all but disappeared.

And here you can see a water truck coming up.

Souza's mission is to protect the remaining forests.

Part of that is building walls to prevent soil erosion.

Mexico City is surrounded by a rock like this with a forest on top.

It rains, the soil prevents it from running fast.

It trickles inside all of these holes and the water comes out here,

on the valley of Mexico.

And that's how Mexico City gets its water from, from this rock,

which is like a doughnut around it.

For this natural process to work, it relies on a rich layer of topsoil.

My hand is moist, because this is saturated with water.

If, when it rains, this gets saturated with water then the rocks

have the time to get saturated with water, because they have...

They're slower at having water inside, so you need this.

The only way for us to have water down there, is to catch it up here.

If we lose the forest, we lose the water.

Souza is drawing up plans to conserve,

protect and replant the forests,

working with the local communities who own them.

These people are the owners of this particular forest.

It's private property. And instead of being farmers

cultivating corn, they cultivate trees.

They call this a water forest.

We're responsible for the forest. We must look after it.

We make sure there's no illegal developments or logging.

No pollution, no rubbish.

It's both our role and our duty.

Even in the heart of a vast urban metropolis like Mexico City,

the intimate relationship between humans and the natural world endures.

It seems to me that an understanding

of the natural world is crucial for all of us.

After all, we depend upon it for our food, for the air we breathe,

and some would say, for our very sanity.

It's a relationship that we're stretching to breaking point

as we continue to grow in numbers.

Within the course of this programme,

the human population has increased by another 9,000 people.

Each one of them will be making their own demands on the Earth.

We have to be using water and all of the other natural resources

in a much more sustainable fashion.

We have to quit wasting so much, we have to quit polluting so much,

and if we do those things and if we put the science and the technology

that's already available to us into play, into implementation today,

then we have a chance to make it into the next

30 or 40 or 50 years, and into a population of eight or nine billion.

But if we don't start doing those things immediately,

we don't stand a chance.

If current trends unfold the way some scientists think they will,

it will be a very different planet by the middle of this century.

The temperature may be up to two or three degrees warmer.

If that's the case, food and most other resources

are going to be scarcer.

There will be eight or nine billion people here

and the question our children are going to ask us is,

"If you saw this coming, why weren't you able to do anything about it?"

I'm very aware that this film could be seen as bleak and depressing.

An increasing population with an ever-decreasing supply of resources.

But humans have capabilities that animals don't -

to think rationally, to study and to plan ahead.

The number of people on the planet in the future depends on

the personal decisions we each make about how many children we have.

Even setting aside the moral responsibility we have

to protect other species, if we continue to damage our ecosystems,

we damage ourselves.

It's clear that we'll have to change the way we live

and use our resources.

We're at a crossroads where we can choose

to cooperate or carry on regardless.

Can our intelligence save us?

I hope so.

Subtitles by artistharry

E-mail subtitling@bbc.co.uk