A study into reversible geoengineering method to offset rising global temperatures has narrowed in on an energy efficient method of cloud brightening.

The "Rayleigh Jet" technique could help offset the effects of carbon emissions at the expense of 30 megawatts of energy.

It relies on spraying a fine jet of salt water that breaks down into small droplets into the sky. The liquid droplets evaporate quickly, leaving behind just the salt particles.

These particles could be generated from specially built ships that could travel the world's oceans spraying salt particles into the air, says Manchester University press release.

The fine mist of salt particles is propelled high into the atmosphere to settle on clouds and help reflect back more sunlight into space.

Increasing the amount of salt particles in the atmosphere allows more of these water droplets to form, making the clouds denser and therefore more reflective.

Previous studies have optimised the size of the salt particles needed to produce the best increase in cloud reflectance but did not look into the energy costs involved.

This new paper, by teams at the universities of Manchester, Washington and Edinburgh, saw researchers test energy efficiency levels of four techniques that provided for an increase in reflection of 5%.

This figure has been calculated as the optimum reflectance desired to combat the predicted effects of increased carbon dioxide levels over the rest of this century.

The Rayleigh jet method could produce the desired effect using 30 megawatts of energy, about the same energy that two large ships produce.

Dr Paul Connolly, based in the School of Earth, Atmospheric and Environmental Sciences at The University of Manchester, said: "It can be incredibly energy intensive to propel water high into the atmosphere and the energy required had never really been tested before. Our paper optimises the salt particle sizes to produce the required change in cloud reflectance for the least energy cost. It is an important finding if these techniques should be needed in the future."

Tinkering with the complex climate dynamics has in the past seen voices raised against geo-engineering. Studies have noted the inherent high costs and low benefits involved in many techniques.

But the team notes that the study and future ones will be useful in the eventuality when no progress is forthcoming on emission reductions.

Compared to pumping sulphuric acid and iodide compounds, the present method which uses salt is said to be comparatively less harmful.

Geo-engineering techniques might become necessary to avoid dangerous rises in global temperatures. The current atmospheric models predict a 3-4 degree rise in temperature unless steps are taken to drastically cut down global carbon emissions.