The threat of climate change is endlessly discussed, but the ballooning growth of the world’s population may be the most critical issue facing humankind. Chris Packham certainly thinks so.

“It’s undeniably the elephant in the room,” he says, though lack of elephants is one of its many alarming symptoms – and in 7.7 Billion People and Counting, he showed us why.

The Earth’s population is about 7.7 billion now and could reach 10 billion by 2050. Packham touched down in São Paolo, Brazil, where the population is five times greater than London’s and the inhabitants are having to pay to drill their own private wells to find a water supply. He visited formerly forested areas of Brazil where the natural animal and plant life has been obliterated to grow soya (to feed animals to provide more food for the expanding masses of people) or eucalyptus trees (imported from Australia to make toilet paper). In Lagos, he found a pop-up city built on mounds of compacted garbage, its two million inhabitants scratching out a nightmarish existence.

Packham often seems as if he’s slightly irritated at having to explain the obvious to a bunch of dunderheads. He talked about his experiences with Asperger’s in a previous documentary, and it is this, perhaps, that gives him a kind of tunnel vision which lets him cut to the core of an argument without worrying too much about crushing anybody’s toes. In one scene, he roped in his unfortunate father to illustrate how scientific progress isn’t always beneficial.

Medical advances have allowed Packham Snr to reach an age that wouldn’t have happened in previous centuries. This was “anti-nature”, Packham suggested, since evolution’s original plan was that people were supposed to die younger and be replaced by new generations. Now, the ever-ageing population is threatening economies, labour markets and healthcare systems.

But it wasn’t quite all apocalyptic gloom. India and China have both demonstrated (albeit by force) that population control is possible, and the birth rate in Africa is falling. Nonetheless, Packham remains fearful. “We’ve got to look after this planet,” he insisted. “There is no Planet B, and this one is beautiful.”