Ideas to combat climate change by hacking Earth's climate to make it cooler - "geoengineering" - have been around for decades.
From putting physical barriers in the outer atmosphere to reflect the sun, to using chemicals such as sulphur injected into the atmosphere to mimic the cooling effect of volcanic eruptions - what are the latest theories being floated by scientists?
Two recent studies explore the most current geoengineering ideas: manipulating clouds to absorb less of the outgoing radiation from Earth's surface, and injecting sulphur into the stratosphere.
Both if these methods would alter the radiative balance of the planet - reducing warming.
Ulrike Lohmann and Blaz Gasparini headed up the team looking at ways to manipulate clouds to absorb less outgoing longwave radiation from the surface of our planet.
Wispy, thin cirrus clouds, at high altitudes, can absorb longwave radiation, creating a heating effect. Scientists are using models to explore ways in which to make artificial, low-altitude cirrus clouds that trap less heat.
The artificial clouds are made using molecules that attract moisture. The searchers note a number of caveats with widespread use of this approach - citing a potential reduction in global mean precipitation and intensification of tropical convection. If artificial creation of cirrus clouds is not done carefully, the effect could be additional warming rather than the intended cooling, they say.
Ulrike Niemeier and Simone Tilmes looked at the pros and cons of continuously injecting sulphur into the stratosphere — a technique called stratospheric aerosol modification.
Models suggest that SAM would mitigate greenhouse gas-induced changes in global temperatures and extreme precipitation. However, anticipated side effects include slowing of the hydrological cycle, which would affect water availability and reduce monsoon precipitation.
The researchers caution that the extent of injection required for a given level of cooling is uncertain, varying widely between models. Understanding the economic costs and technological requirements of SAM is also critical, they say.
Assuming a scenario in which aggressive mitigation and large-scale carbon capture and removal start as late as 2040, sulphur would need to be injected for 160 years to limit the temperature increase to 2°C above preindustrial levels – such endeavours could cost $20 billion a year.
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