As fossil fuel use continues apace and a hotter planet edges close to passing safety limits, some scientists are exploring a controversial technological stopgap: spraying chemicals into the atmosphere to reflect away some of the sun's warmth.
Deploying the technology, using special planes, would be relatively cheap and simple, costing a few billion dollars a year, its backers say.
And it could - if maintained - hold down global average temperatures, potentially staving off increasingly deadly climate-change impacts such as heatwaves, they argue.
"I do see it as a likely option" if plans to cut emissions fall short and dangers grow, said Emmi Yonekura, a researcher on the risks of climate "geoengineering" at RAND Corporation, a military-focused policy think-tank, during an online event, reports Reuters.
But the technology, which mimics the sky-darkening effect of volcanic eruptions, also carries serious and unpredictable risks, critics say - with some scientists so worried that they believe research should stop and outdoor tests be banned.
Threats range from potential shifts in rainfall patterns that could spur worsening hunger to rapid, uncontrollable temperature rise if the technology's use is suddenly stopped.
The availability of such a planet-cooling option could also give climate polluters an unwarranted green-light to carry on - even though "stratospheric aerosol injection" (SAI) would only mask the problem, not solve it.
Early efforts to create rules to govern its use show signs of stalling, critics warn, making it more likely that one powerful state or even individual could go it alone, potentially to the detriment of others, sparking conflict.
"The more we see extreme events like hurricanes, wildfires, also just heatwaves that have acute impacts - those may motivate key actors to try to protect themselves," said Jonathan Wiener, co-director of the Duke Center on Risk.
Unilateral use of the technology "might be very difficult to deal with geopolitically", said Wiener, a law and environmental policy professor at Duke University in the United States.
Where scientists and policy experts agree is that, as the world speeds towards the lower 1.5 degree-Celsius warming limit set in the Paris Agreement, serious thinking about what happens if the Earth's climate breaks down must happen - fast.
"We are now entering a situation where the likelihood of overshooting 1.5 degrees is higher than not overshooting it," said Janos Pasztor, executive director of the Carnegie Climate Governance Initiative (C2G), a think-tank focused on how to manage "climate-altering technologies".
With the world still "very far" from taking the aggressive steps needed to limit climate change, the impacts of a warming planet will worsen - and in some cases be "catastrophic", he warned.
That means more extreme ways to cool the climate are now on the table.
Pasztor told an online event it was important to weigh up the risks of taking such measures against not taking them.
"Simply not doing either anything or enough ... in itself has huge risks," he said.
TIPPING POINTS
To effectively limit warming, climate-heating emissions - largely from burning fossil fuels - need to fall 45 per cent globally by 2030, scientists say.
Instead, they are still rising, as oil and gas use continues to grow and investment in renewable energy alternatives lags.
Scientists with the Intergovernmental Panel on Climate Change (IPCC) have warned that if global average temperatures exceed 1.5C above pre-industrial times, the world could see changes that will be hard to adapt to.
Those could include surging hunger as crops fail, as well as growing water shortages, migration and conflict.
Deaths and financial losses from worsening heatwaves, wildfires, droughts, floods, hurricanes and sea level rise could also increase, affecting rich countries as well as poor.
More heat could melt Arctic and ocean permafrost that holds climate-heating methane, turbo-charging temperature rise and launching the planet into a vicious heating cycle that would be hard to reverse, scientists say.
Some impacts of passing 1.5C - such as likely losses of many of the world's coral reefs - "will be irreversible, even if global warming is reduced", warned Thelma Krug, a climate scientist and vice chair of the IPCC.
With the World Meteorological Organization projecting that the 1.5C threshold could be passed, at least temporarily, within five years, a Climate Overshoot Commission of 16 world leaders was launched in May.
It will look at controversial sun-dimming technologies, alongside efforts to remove carbon dioxide from the atmosphere and to adapt to new conditions.
SAI research has won some powerful financial backers, including Bill Gates and a range of venture capitalists and philanthropists, including at least one former oil executive.
But some experts think dimming the sun by spraying particles into the upper atmosphere should be ruled out as an option to deal with an international failure to reduce emissions.
Wil Burns, a longtime policy and governance expert on climate technologies, who teaches at Northwestern University, is one signatory of a proposed "non-use agreement" for sun-dimming.
He believes SAI is ungovernable, with nations unlikely ever to agree on whether the planet-altering tech should be used.
Sweden last year blocked a small-scale test by a Harvard University team of equipment designed to be deployed later to release small amounts of sun-dimming material in an outdoor experiment, after indigenous people objected to the launch.
Åsa Larsson Blind, vice president of the Saami Council, said such technological fixes are "completely against what we need to do now - transform to zero-carbon societies in harmony with nature".
'MAGIC PILL'?
Backers of the technology emphasise it should be used only temporarily and alongside steep emissions cuts, to buy time and stave off potential catastrophe as those reductions take effect.
But with climate change still far from a top priority for many people and politicians, Burns fears the availability of an easy tech option to temporarily lower temperatures and short-term risks would undermine the will to phase out fossil fuels.
"My fear is if we had this 'magic pill', the public wouldn't care that much" if it were used, he said, adding fossil fuel firms might then argue against a rush to renewable energy.
That could create a "sword of Damacles" hanging over future generations, he said, who would then find it hard to stop using the technology without disastrous global consequences.
Another concern is that the technology is simple and inexpensive enough to be deployed by one powerful nation - such as China, India or the United States - or even a wealthy tech enthusiast, irrespective of whether others object.
That could spark conflicts as countries try to stop deployment or fight over where the planet's new "thermostat" should be set, researchers warn.
Burns said a technology that is both hard to govern and carries substantial risks - including to the world's most vulnerable - "is not one we should be looking at in earnest".
But Yonekura, of RAND Corporation, questioned whether use of a technology likely to be seen as an attractive option by some in the event of climate breakdown could be stopped.
"Is it ever possible to rule a technology out... once you've invested in it?" she asked.
