A number of SRM technologies involve altering the properties of clouds, causing the clouds to reflect more solar radiation back into space. Marine cloud brightening (MCB) is one form of cloud engineering. The idea is to spray droplets of seawater into the sky, with sea-salt crystals providing additional seed nuclei for water droplets to condense.¹⁴ This would make marine clouds whiter and more reflective, sending more solar radiation back into space and cooling the surrounding region. A range of spray technologies have been considered.¹⁵

Future Horizons:

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5-yearhorizon

MCB experiments are promising

The Australian MCB experiments yield encouraging results, and result in the launch of a 10-year programme to apply MCB to contribute to the protection of the Great Barrier Reef.

10-yearhorizon

Small-scale MCB begins

The first prototypes of automated ships that can spray seawater for MCB are launched. The first official small-scale use of MCB is undertaken over endangered corals. Real-world testing of cirrus modification begins.

25-yearhorizon

MCB in regular use

MCB is in regular use over corals and other heat-sensitive ecosystems during heatwaves.

Various researchers have also proposed thinning and dispersing high-altitude cirrus clouds, which contribute to warming by trapping a disproportionately large amount of terrestrial radiation that would otherwise escape into space.¹⁶ Injecting these clouds with particles of bismuth tri-iodide allows the formation of large ice crystals within the clouds. These large ice crystals fall out more rapidly, shortening the clouds’ lifespan. Cirrus reduction can improve the transmission of long-wave terrestrial radiation into space.¹⁷

In February 2024, researchers at the US National Oceanic and Atmospheric Administration proposed injecting ice-nucleating particles into the troposphere to prevent water vapour reaching the stratosphere, where it usually absorbs outgoing radiation. This “intentional stratospheric dehydration” would help to cool the planet by increasing outbound radiation, although significant technical and practical hurdles would need to be overcome first.¹⁸

Whereas SAI has a globally uniform effect, cloud-engineering techniques produce more localised cooling, allowing them to be used in a targeted way. For instance, Australian MCB could be used to cool major coral reefs, which suffer bleaching when water temperatures become too high.¹⁹

Cloud engineering - Anticipation Scores

The Anticipation Potential of a research field is determined by the capacity for impactful action in the present, considering possible future transformative breakthroughs in a field over a 25-year outlook. A field with a high Anticipation Potential, therefore, combines the potential range of future transformative possibilities engendered by a research area with a wide field of opportunities for action in the present. We asked researchers in the field to anticipate:

The uncertainty related to future science breakthroughs in the field
The transformative effect anticipated breakthroughs may have on research and society
The scope for action in the present in relation to anticipated breakthroughs.

This chart represents a summary of their responses to each of these elements, which when combined, provide the Anticipation Potential for the topic. See methodology for more information.