With new evidence that catastrophic climate-change "tipping points" are nearing - from surging sea levels as polar ice melts to spiking temperatures as methane escapes thawing permafrost - scientists are quietly planning for the unthinkable.
Irreversible and abrupt shifts likely above 1.5C of warming
Scientists, policymakers ponder how to warn of and avert worst
Positive tipping points for a resilient future also possible
"Extreme climate change risks are under-explored," Luke Kemp, a researcher with the Centre for the Study of Existential Risk at the University of Cambridge, warned at a pioneering conference on the theme at the University of Exeter this week.
"Climate scholars have strong incentives to err on the side of least drama," he noted. "You don't want to be branded an alarmist."
But with fossil fuel emissions still going up and climate-fuelled disasters multiplying, it's time for an "honest assessment of the risks and what can be done", he told an audience stunned into silence by frank assessments of looming threats.
A study published last week in the journal Science found that four dangerous planetary tipping points are "likely" above 1.5 degrees Celsius (2.7F) of warming above preindustrial temperatures - a level that could be passed within a decade.
Tipping points happen when a small change - such as an incremental increase in global temperature - sparks a rapid, often irreversible transformation, scientists say.
One - accelerating melting leading to the eventual collapse of the Greenland ice sheet - may have already been triggered, some believe, setting in motion 7 metres (23 feet) of sea level rise over time, enough to swamp key coastal cities.
David King, former chief scientific advisor for Britain and founder of the Climate Crisis Advisory Group, an expert panel, said he thinks "the Arctic circle tipping points are now passed".
With the Arctic having warmed 3C - well above the global average, which is already about 1.3C, he said - risks are also growing that large amounts of methane trapped in thawing permafrost could be released.
Adding much more of that potent greenhouse gas to the atmosphere could drive an unstoppable cycle of higher global temperatures and more melting, King said.
"If all of that is released, we'll see temperatures rise 5-8C (8-14F) over 20 years," he said, adding this would be "extraordinarily destructive to the future of humanity", likely causing food system collapse and displacing billions of people.
United Nations' chief António Guterres on Wednesday called the devastating floods covering a third of Pakistan a "window into the future".
"What is happening in Pakistan demonstrates the sheer inadequacy of the global response to the climate crisis, and the betrayal and injustice at the heart of it," he told the UN General Assembly in New York.
"If one-third of G20 countries were under water today, as (they) could be tomorrow, perhaps they would find it easier to agree on drastic cuts to emissions," he added.
'POSITIVE' TIPPING POINTS
Scientists at the Exeter conference emphasised that channeling money and attention to rapidly scale up renewable energy - already as cheap as fossil fuels in most places - along with better nature protection could still hold off many disastrous shifts.
They pointed to early signs of "positive" tipping points that could also be approaching as some societies and economies push toward a safer and more sustainable path.
Most major car and truck manufacturers, for instance, now plan to stop producing fossil fuel vehicles.
And in many countries meat-eating - a major driver of emissions and nature loss - is falling, even if global demand is still rising, the scientists said.
Tasty, high-quality and increasingly cheap meat substitutes could prove "a potential tipping point that could take the carpet right out from under livestock farming", which is a major driver of deforestation, said Tim Lenton, director of the Global Systems Institute at Exeter and an organiser of the conference.
Similarly, the cost of renewable energy is expected to plunge so fast in coming decades that using anything else will soon be uncompetitive, said Doyne Farmer, director of the complexity economics programme at the Oxford Martin School.
"Even if you're a climate denier, you should be behind making the green energy transition quickly" just to save cash, he added.
Social tipping points are also emerging, such as many new graduates refusing jobs at unsustainable companies, social scientists said.
Economist Kate Raworth said her students at Oxford University's Environmental Change Institute now consider the traditional push for endless growth more radical and risky than her "doughnut" economics model which seeks a safe space between planetary boundaries and human needs.
From neighbours installing solar panels to Greta Thunberg launching her lone climate strike and cities like Harlem in the Netherlands banning ads for fossil fuels and, soon, meat, "behavior is contagious", noted Dutch social campaigner Femke Sleegers.
Johan Rockström, director of the Potsdam Institute for Climate Impact Research and another conference convener, said that increasingly self-reinforcing green switches would be crucial to hold off catastrophic climate impacts.
"Nothing less than positive social tipping points will take us to a safe landing," he said. "The radical suggestion is to continue business as usual. That's the really nightmarish dead end."
An array of new efforts to reduce catastrophic risk - or to deal with the consequences of failing - are starting to appear.
Efforts to pass a fossil fuel "non-proliferation" treaty - designed to end new oil, gas and coal exploration and production - are gathering steam, while the Bezos Earth Fund this week put $1.15 million into efforts to "activate" positive tipping points.
Scientists also want a special report from the Intergovernmental Panel on Climate Change on catastrophic climate change and tipping points, in part to help raise the profile of the threats.
In New York, members of a new Climate Overshoot Commission are meeting on Friday and Saturday to discuss potential options - including a controversial proposal to spray sun-blocking chemicals into the sky - to limit runaway heating of the planet.
"Unfortunately we know 1.5C (of warming) has a very high likelihood of being overshot, and this necessitates a review of the action," Pascal Lamy, former World Trade Organization head and co-chair of the commission, said in a phone interview. "We are here to leave no stone unturned."
As they grapple with ever-clearer data about how close the planet may be to irreversible tipping points, scientists say they are struggling to deliver clear and realistic warnings about the unthinkable, without undermining hope.
"It's scary and it's real - and these are the futures that are going to be opening up to us if we don't act strongly now," warned Laura Pereira of the Global Change Institute at South Africa's Witwatersrand University.
But as worst-case threats loom closer, "I don't think that's cause for complete despair or people saying, 'we give up'," said Rockström. "We have more empirical evidence that the reverse happens - people get angry."