So does family size matter?
Scientists indeed say human population growth is intangibly linked to the mass-extinction event the group mentions.
In fact, it's one of the largest ever recorded in the Earth's history, threatening a million species of plants and animals, and often discussed by heavyweight naturalists such as Dame Jane Goodall and Sir David Attenborough.
The idea gained further weight when influential US congresswoman Alexandria Ocasio-Cortez told her Instagram followers in February 2019: "There's a scientific consensus that the lives of children are going to be very difficult — is it still ok to have children?"
In the last major assessment by the United Nations' Intergovernmental Panel on Climate Change (IPCC), it was estimated that global carbon dioxide emissions might be lowered by nearly a third if contraception was available to all women who expressed a need for it.
Another study, published by Sweden's University of Lund and Canada's University of British Columbia in 2017, found that the single most effective measure an individual in the developed world could take to cut their carbon emissions over the long term was to have one less child.
In fact, their study established that this step was 25 times more effective than the next most effective measure — living without a car.
Dramatically, a group of environmental scientists have argued that societies should embrace population ageing, as is being experienced here, and even decline.
They cited multiple reports of the socio-economic and environmental benefits of population ageing, while pointing out that smaller populations made for more sustainable societies.
That sentiment also rings true in the IPCC's most recent report, which warns that high population growth will be a "key impediment" to hitting the critical target of limiting global warming to 1.5C.
Those nations with massive populations, such as India and China, are among the most significant contributors to climate change overall, despite relatively low impacts from each individual.
What's your carbon footprint? Try this five-minute FutureFit survey, supported by Auckland Council, to find out.
While India and China have reasonably low population growth, it's expected that people born today in countries whose populations are still expanding rapidly will have a climate impact for generations to come.
It might be argued that slowing population growth is already happening here in New Zealand — but not for environmental reasons.
Population growth since 2013 has been dominated by net migration, rather than the number of births, which, before the Covid-19 pandemic, had been running relatively steady at about 60,000 a year despite a decline in birth rates.
In other words, the number of births for every 1000 people is falling — but the growing population means total births remain at relatively high levels, reaching a recent peak of almost 65,000 per year in the period from 2007 to 2010.
However, New Zealand's total fertility rate in 2017 was down to 1.8 births per woman, its lowest recorded level.
In any case, many environmentalists argue that focusing too much on population distracts us from tackling the root causes of the ecological crisis we've created.
The global network behind the most commonly used indicator of our impact on the world, the ecological footprint, calculates that humans are chewing up natural resources about 1.7 times faster than they can be regenerated.
Even if everyone lived like people in supposedly "clean and green" New Zealand, which in 2012 had the thirty-first highest ecological footprint out of 188 countries, we'd need about 2.8 Earths to sustain our consumption.
As Climate Change Minister James Shaw has pointed out, 18 countries managed to slash their emissions between 2005 and 2015, even while their populations and economies grew over the same period of time.
"So while there's obviously a correlation between population growth and use of resources, particularly in wealthier countries which, like New Zealand, have the highest per capita emissions, the evidence shows that it is possible to decouple emissions growth from population and economic growth," he said.
"The main challenge lies in adopting new technologies and business models and in being far more efficient with resources, which New Zealand has, so far, been slow to get started on."
Put another way: people might be the problem, but we're also the solution.
• This extract series continues tomorrow, as part of the Herald's Covering Climate Now coverage, and looks at whether focus on personal action distracts from corporate responsibility.
• Text extracted from Climate Aotearoa: What's happening and what we can do about it, a new book from a range of leading New Zealand climate scientists and commentators, edited by Helen Clark. Published by Allen & Unwin NZ. RRP$36.99. Available in stores from Monday, April 19
