The flood of several hundred thousand refugees to Europe from civil war and crop failure in Syria and North Africa had toppled liberal European governments and "sent us back to the Dark Ages", said Cameron.
"So what happens to us globally when it's millions, then tens of millions, and eventually hundreds of millions of people, as is being predicted, fleeing from farms that have become deserts, fleeing coasts and rising seas that are devouring their fertile deltas and their coastal cities?
"The handwriting is on the wall about this kind of dark political scenario."
However, "sane" countries like New Zealand with small populations and the ability to adapt quickly had an opportunity to show global leadership on climate change, Cameron suggested.
"The elephant in the room here, the cow in the room here, is obviously animal agriculture," he said.
Expressing empathy for fellow farmers, Cameron acknowledged the importance of the dairy industry both to New Zealand and the Taranaki region, where the government's two-day, low-carbon summit is being held in response to last year's decision to stop issuing new permits for offshore oil and gas exploration.
"This is where the rubber's going to meet the road and it's going to affect a lot of people. There are alternatives to the way things are done and hopefully they can be as lucrative, if not more so if we're smart about it," he said. "But we have to change."
He suggested that if New Zealand moved away from animal agriculture, there would be a public health "win-win", since New Zealand was also among the most at-risk populations for "diseases that are known to be the result of eating meat and dairy", citing heart disease, diabetes, osteoporosis and bowel cancer.
With farmers around the country reeling from the government's announcement yesterday of a target to reduce methane emissions from cows, sheep and beef cattle by as much as 47 per cent by 2050, Cameron said those targets were "quite ferocious" but that one way to reduce methane was to move to shed-based farming.
"But is that what New Zealand really wants to do?" he asked. "Do we really want to shatter that bucolic image of the cows grazing naturally so we can lock them up in these filthy pens next to vast waste lagoons?
"That seems like such a huge step backwards to me and something that's really going to undermine that very, very important clean image of NZ's image worldwide."
However, Prime Minister Jacinda Ardern stopped well short of endorsing a post-animal agriculture future for New Zealand, saying she was "from the Waikato", loved cheese and worried she might not be able to go home if she adopted such a stance.
"Ultimately those will be land use decisions for those who are already in the sector," she told journalists after the Camerons' keynote presentation. "So it's not about the government dictating the way that those in the agricultural sector use their land now and in the future.
"There will continue to be consumer demand for products NZ's really good at producing. That includes dairy products. What we need to be able to do when we promote our dairy products to the world, we've got to stamp that under the banner of being environmentally friendly and sustainable."
However, Cameron used a film industry analogy for the choices New Zealand agriculture faces.
"If we don't adapt to change then we're going to be like Kodak," he said. "Kodak refused to accept that the world was going to change away from film and here we are, 20 years later, with all movies being made digitally, all television and Kodak is long dead and the movie industry is doing just fine."
He urged the government to incentivise a shift in agricultural practice, citing the success of New Zealand government incentive schemes that had underpinned the growth of a New Zealand film sector.
