In 1991, Mount Pinatubo in the Philippines erupted. The volcano sent 17 million tonnes of sulphur dioxide (SO2) into the atmosphere.

The gas formed a fine mist in the ozone layer, reflecting the sun's rays into space. The next year, global temperatures fell by about 0.5C. Now, scientists want to copy this cooling effect to tackle climate changeLong-term shifts in temperatures and weather patterns. Human action is a major cause of climate change..

The technology is called solar radiation management (SRM) and has funding from billionaire Bill GatesFounder and former CEO of Microsoft, the company responsible for popularising household computers in the 80s (and still one of the major global tech companies). His house on the shores of Lake Washington is called Xanadu 2.0. It took seven years to build and is seen as an early example of a smart home.. Planes injecting reflective particles into the stratosphereA layer of the atmosphere which extends for 31 miles above the Earth's surface. could create a "global thermostat" to control global temperatures.

In California, the tech startup Make Sunsets has begun experiments with small balloons. Company founder Luke Iseman aims "to cool the Earth as quickly as we safely can".

This year, the UNUnited Nations. An intergovernmental organisation based in New York that aims to maintain international peace and security. warned we are failing to cut CO2 emissionsWhen a gas is let off from something, like a car, it is an emission. Carbon dioxide, or CO2, is a greenhouse gas that contributes to global warming. fast enough to stop a 1.5C rise in global temperatures.¹ A UN report said SRM is the "only" way to cool the planet in the short term.

In February, hundreds of climate scientists signed two open letters supporting the research. Expert Daniele Visioni says we should not reject "an additional tool to fight global warming".

The issue divides experts. Physicist Raymond Pierrehumbert calls it a "big distraction" from cutting emissions. Activists fear it gives polluters an excuse to ignore net zeroWhen the gases going into the atmosphere are balanced by those being taken out of it. targets.

Others warn it may cause unpredictable extreme weather. But Pierrehumbert says the worst risks come when the project stops.

"You're setting the world up for a real catastrophe," he warns. Once the aerosols disappear, temperatures will surge. To avoid disaster, we would need global cooperation for hundreds of years.

Who will control the thermostat? Climate scientist Claudia Wieners says we must "stop crazy people doing crazy stuff". Acting alone, one country or company could radically change the Earth's atmosphere.

Expert Wake Smith says geoengineeringDeliberate attempts to intervene in the Earth's natural processes to counteract climate change on a large scale. is "inevitable" but will not fix climate change. It is a "fire extinguisher" for emergency use only.

July was the hottest month on record. Europe has seen the largest wildfire in its history. "The era of global boiling has arrived," says UN chief Antonio Guterres. Is it time to reach for the fire extinguisher?

Yes: Deliberately polluting the atmosphere with sulphur dioxide is a foolish act of desperation. We won't know the effects until it is too late. And meanwhile, we will grow complacent about CO2 emissions.

No: Human activity is already putting aerosols into the air. It is responsible for offsetting a third of greenhouse gas climate warming.² All SRM means is taking control of our atmosphere and our survival.

Or... All technology comes with risks. And doing nothing is also dangerous. That is why climate scientists demand further research, regulation and debate to find the best solutions to climate change.

Climate change - Long-term shifts in temperatures and weather patterns. Human action is a major cause of climate change.

Bill Gates - Founder and former CEO of Microsoft, the company responsible for popularising household computers in the 80s (and still one of the major global tech companies). His house on the shores of Lake Washington is called Xanadu 2.0. It took seven years to build and is seen as an early example of a smart home.

Stratosphere - A layer of the atmosphere which extends for 31 miles above the Earth's surface.

UN - United Nations. An intergovernmental organisation based in New York that aims to maintain international peace and security.

CO2 emissions - When a gas is let off from something, like a car, it is an emission. Carbon dioxide, or CO2, is a greenhouse gas that contributes to global warming.

Net zero - When the gases going into the atmosphere are balanced by those being taken out of it.

Geoengineering - Deliberate attempts to intervene in the Earth's natural processes to counteract climate change on a large scale.