BY 2050, there could be 10 billion humans living on Earth.

It's a prediction Chris Packham - who's dedicated his life to championing the natural world - is gravely concerned about.

We're all aware of the climate and environment emergency, suggests the Southampton-born broadcaster, 58, and there are plenty of conversations about biodiversity loss taking place.

But the massive threat posed by human population growth is the "elephant in the room".

So, he decided to make a documentary for BBC Two, titled Chris Packham: 7.7 Billion People & Counting.

"If people watch this and they don't think, then we are doomed..." muses the naturalist, who's known for fronting The Really Wild Show on CBBC, plus Springwatch and Autumnwatch.

"We did everything we could to make this a programme which would prick up people's ears, because it confronts them with any number of different issues - ethical, moral and biological."

Packham travels around the globe in the one-off film to see the impact that our rapidly growing population is having.

He visits Brazil, investigating how Sao Paulo is a mega-city on the verge of running out of water. It's shocking to watch.

"The human species is not particularly good at change, but we've reached a point in our history when, confronted with a number of serious problems, people are still putting their heads in the sand.

"My duty is to pull people's heads out of the sand and ask them to look, listen, think about and to come up with ways in their own lives where they can make some positive progress.

"There's an ongoing raft of programmes that are trying to generate awareness, but I think what is missing is the urgency that is required, and the one that is lagging is the conversation about population growth. It's a very controversial subject."

He also heads to Lagos in Nigeria (a country set to become the third most populous nation on Earth by 2050, overtaking the United States).

"Unfortunately, when it comes to addressing these issues, the finger is often pointed at sub-Saharan Africa, because that is where the human population is growing most rapidly at the moment, and most rapidly in Nigeria - hence our visit to Lagos," he explains.

"We were very keen to address the fact that you can't point the finger at large families with poor black children as being the problem.

"Actually, at this point, we [in the UK] are the problem, because, when it comes to exacerbating climate change and biodiversity loss, we are the principal consumers."

In exploring how the world can re-balance its consumption to accommodate the needs of more than two billion more people, Packham even analyses his own lifestyle choices.

"One of the crucial points in the film is where I honestly, and embarrassingly, point out that I've got 10 hoovering devices," he admits.

"That is exactly the root of the problem. Because I've got no kids, but I've got 10 hoovers, and the cost of producing those, and the batteries that are in them, and replacing them... That's the damage that we are doing, in this point of time."

On the topic of Packham having no children, a particularly memorable segment of the documentary sees him interview a couple going through IVF treatment.

Did that meeting have any impact on him, considering his decision not to be a father?

"Not personally; I'm 58," says the presenter, who's stepdad to 24-year-old Megan (her mum is a former girlfriend; he's now in a relationship with Charlotte Corney, owner of Isle of Wight Zoo).

"Would it have made a difference when I was 25, when I might have been thinking about that? Maybe.

"When I was 25 years old I was terrified of the concept; you have to have the confidence to want to reproduce a part of yourself and, at that part of my life, I didn't like myself in any form, I didn't understand myself. So, the thought of making another little part of me was abhorrent.

"But moving forward - and certainly when I met Megan - I was faced with another question of investing a considerable amount of time and money into another organism. Are they going to have a secure future? What is the future going to be like for them? Do they have the potential to be happy, comfortable and healthy?

"I've always been worried about that, and I think that's because I've been drawing my fear from the declines that I've been seeing in the environment and the natural world."

However, he adds candidly, witnessing the couple's drive and profound desire to go through the birth process was "actually quite awesome".

"It was alien to me, but nevertheless awesome. And I genuinely hope that they succeed because it was something that they wanted so badly.

"You can't undermine or fail to understand the biological desire to have a baby, but I hope at the same time they are thinking about how they foresee that child's future."

When he was in his 40s, Packham was diagnosed with Asperger's, a lifelong disability that affects how a person makes sense of the world, processes information and relates to other people.

It's something he's very open about; he made documentary, Chris Packham: Asperger's And Me, for the BBC in 2017.

The condition, which is a form of autism, wasn't really an issue when filming this most recent show, he reflects.

"It's something I can almost invariably cope with, if I know what I'm going to be doing. If you're going to go into a situation that you know it's not your comfort zone, then you can prepare yourself and manage yourself in that situation.

"What was an issue was this programme was bound to expose me to the worst excesses of the things that I fear most."

"I'm a pragmatist," the talkative star continues. "I have to remain in a position where I'm not going to give up. Because I'm determined to make a last stand, if that's what it comes to, for our environment.

"And to do that, I need to remain active and I need to remain capable about thinking about things, about communicating as best I can to try and make people change their minds in the right direction. So, not being overwhelmed and swamped by pessimism is something that I have to constantly guard."

Chris Packham: 7.7 Billion People & Counting airs on BBC Two on Tuesday January 21