When astronaut Jim Irwin stood on the Moon in 1971, it's reported that he held out his arm, closed one eye and put up his thumb.

His thumbnail blacked out the entire Earth from space, and with it about 3.9 billion earthlings.

If he did the same thing today, his thumb would eclipse 7.7 billion people.

In the time it's taken to read this sentence (about five seconds), 24 people have been born.

If you make it to the end of this article (about five minutes), more than 1,400 people will have been born.

The world's population began increasingly exponentially in the 1900s, and is predicted to pass 11 billion by 2100. ( Supplied: Worldometers )

So far today, Earth's population — that is, births minus deaths — is about 140,000 people more than yesterday.

So, is there a maximum number of people the planet can hold, and how would we know if we've already passed it?

Earth Overshoot Day set for July 29

Wealthy countries tend to consume far more resources per person than developing nations. ( Getty Images: Volker Mohrke )

The annual Earth Overshoot Day is set to occur on July 29 this year, three days earlier than last year.

That's the day in 2019 when we will have used up all the resources that Earth's natural systems can provide and replace in a year, according to the Global Footprint Network.

Or another way to think of it is like this: Let's say we've got a certain amount of food that has to last us all year. We'll have eaten all of it by July 29. Everything we're eating after that, we're taking from next year's supply, and the year after and so on.

But it's not just food we're doing it with.

Qatar used up their per-capita annual resources by February 11. ( Supplied: The Global Footprint Network )

The Global Footprint Network bases its calculations primarily on United Nations data, and consider consumption of things like crops for food and fibres, as well as livestock, seafood, timber and forestry harvest, urban infrastructure development, and preservation of carbon sinks like forests.

They use that data to calculate the average consumption or "footprint" per capita for more than 200 countries, and the Earth.

And while the world will make it to July this year, Australia's Earth Overshoot Day flew by on March 31. If everyone on the globe lived like us, we'd have broken the bank in 90 days.

Not great, but it could be worse. The United States overshot on March 15. The UAE on March 8. And if we all lived like Qataris, who overshot on February 11, we'd need 8.7 Earths to provide the goods.

While Kyrgyzstan makes it to Boxing Day, no country's people consume resources at a slower rate than they can be replaced.

So if we're consuming Earth's resources faster than they're replenishing, why haven't we run out yet?

We ain't nothin' but animals

Animal populations tend to boom beyond a system's carrying capacity, then "correct". ( ABC Local: Nicole Bond )

Population-dynamics science tells us two things broadly about invasive animal population trends. Typically, the population of an invasive species that moves into a new area and is freed from predation will boom — it expands beyond what is called the "carrying capacity", and diminishes the food and resources it needs to sustain itself in the process.

In the case of humans, modern medicine and technology have facilitated that boom. Things that used to kill us don't, and we can exploit resources much further afield and faster than during all of our history.

But, in animals at least, then comes the crash — where the ratio of animals to resources becomes so skewed that a period of fierce competition for limited resources breaks out, and only the fittest survive.

After the crash, populations tend to slowly recover, but never reach as high as they were during the first boom.

People started getting concerned in the 1970s that the number of humans on the planet was increasing too fast, according to evolutionary biologist Ben Phillips of the University of Melbourne.

"Since then we've seen astonishing technological innovations that have increased our ability to obtain resources, so the [Earth's] carrying capacity has actually gone up," Dr Phillips said.

"But to some extent that's just delayed us reaching the crisis situation."

Declining resources, climate change, and global extinction rates unprecedented in modern history all suggest we're in the later stage of the boom.

So is population or consumption the problem?

If Australians want to continue living as we do without making any changes, and as a planet we want to meet our footprint, then the number of humans Earth can sustain long term is around 1.9 billion people, which was roughly the global population 100 years ago in 1919.

Alternatively, we'll have to find three more Earth-like planets to support us.

Even if the global population stabilised today, the rate of consumption in wealthy nations is much higher than the world can sustain.

The problem is what can we do about it before we hit the crash?

Reducing consumption?

We need to cut down on what we consume, according to waste management expert Gayle Sloan. ( ABC News: Patrick Martin )

The good news is that the things we can do to reduce our footprints have a lot of other benefits too.

As individuals, cutting down on meat, particularly red meat, can significantly reduce consumption of resources.

And research shows that rehabilitating degraded habitats and increasing tree cover can have great restorative benefits to our environment.

We should aim to practice "mindful consumption", said Gayle Sloan, CEO of the Waste Management and Resource Association of Australia.

"Think about 'Do I need that?' 'Am I going to use that?' We can avoid mindless consuming," Ms Sloan said.

"The public is way beyond straws and plastic bags — we need to be reusing."

But the biggest changes that need to be made are at a systemic level, and they need to be led by government and industry.

Making companies responsible for the end of life of their products is a way to start cutting down on waste. ( Getty images: Daniela Dirscherl )

We need cleaner transport and power-generation industries, and we need to transition to a circular economy, according to Ms Sloan.

That means getting rid of single-use products and packaging.

To do this, the onus of responsibility for single-use items needs to be put back onto producers.

For instance, if a soft-drink company wants to sell their drink in a plastic container, they need to recoup that bottle after it's been used and ensure it is reused or recycled.

"We need a lot more end-of-life extension of responsibility on companies," Ms Sloan said.

"It's not enough that they bring it to market, they need to be responsible for it through to end-of-life and governments need to be a lot stronger on mandating that."

Reducing population?

Educating and empowering women is key to lowering birth-rates in developing countries. ( AFP: Adek Berry )

When it comes to population, the highest rates of growth tend to be in developing nations.

Educating and empowering women is key to changing this trend, according to Climate Council CEO Amanda McKenzie.

Research shows that birth rates tend to decrease as women have more financial security.

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"There are a few factors, one is having access to birth control and the second is education and having economic opportunities," Ms McKenzie said.

"And then there's empowerment [of women] to make their own choices."

Improving living standards in developing countries can also improve resilience to climate change, she added.

"The poorer you are, generally the more exposed you are when an extreme weather event occurs — so in extreme heatwaves there's no air-conditioning, and houses are less likely to stand up to cyclones."

On current trend we are on track for a population of more than 11 billion people by the end of the century.

And — at least for now — we've only got one Earth to sustain us.