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Spraying seawater into clouds to cool the planet would reduce the chances of reaching most major climate 鈥渢ipping points鈥�, but would also increase the risk of East Antarctic glacier collapse and a shift in the West African monsoon.

That鈥檚 according to a study by at the University of Victoria, Canada, and his colleagues, which modelled the effects of marine cloud brightening (MCB), a form of geoengineering that has been proposed to counteract global warming.

This would involve spraying seawater into low-hanging stratocumulus clouds over the ocean, creating microscopic droplets that quickly evaporate to leave salt crystals hanging in the air. The idea is that water vapour would condense around these crystals, forming smaller droplets that reflect more sunlight back into space compared with normal cloud droplets.

An has been testing water cannons to spray this mist from ships. But critics fear that any geoengineering 鈥渜uick fix鈥� could have unexpected climate consequences.

Stratocumulus clouds are widespread over the eastern Pacific and south-eastern Atlantic oceans. If enough marine cloud brightening were deployed in these areas to keep the global average temperature at its early 20th-century level, it would have far-flung, divergent effects similar to those seen with La Ni帽a climate phase, the study found.

Much of the globe would cool, while rainfall would increase in places like Australia, India and Panama and decrease in Indonesia, the Republic of the Congo and eastern Brazil.

鈥淚 was expecting more downsides to MCB originally,鈥� says Hirasawa. 鈥淏ut this sort of work does demonstrate just how much work we need to do to properly evaluate MCB.鈥�

With the world on track to exceed 2掳C of warming above pre-industrial levels, scientists warn that such a change in the climate system could trigger sudden, self-perpetuating changes known as tipping points. Long-term marine cloud brightening would lower the risk of most of these, the study suggests, including the collapse of the Greenland and West Antarctic ice sheets, the shutdown of a vital current in the Atlantic Ocean, abrupt permafrost thawing and coral reef die-off.

But the approach wouldn鈥檛 help stave off the loss of Barents Sea ice and would encourage conditions linked to two other tipping points. In East Antarctica, it would shift a belt of strong winds southward, causing the upwelling of warmer water near floating ice shelves that protect against the collapse of glaciers.

In the Sahel region of Africa, it would increase rainfall and possibly result in the greening of the semi-arid savannah, a potentially beneficial, but still unpredictable tipping point.

The predicted impacts vary depending on where MCB is deployed. If it were done in the south-eastern Atlantic, but not the Pacific, rainfall would decrease in the Amazon, raising the threat of rainforest dieback.

at the University of Washington in Seattle cautioned that these results are from a single model and further research is needed into methods of blocking solar radiation. 鈥淭here are going to be uncertainties in how something like marine cloud brightening or stratospheric aerosol injection would play out in the climate system,鈥� she says. 鈥淭here are also uncertainties in how the climate system is going to change with increasing greenhouse gases, absent these approaches.鈥�

Attempts to avoid tipping points with MCB would need to continue for centuries, says at the University of Oxford.聽鈥淏ecause of the long residence time of CO2 in the atmosphere, you鈥檇 have to maintain the MCB for at least that long,鈥� he says.