Geoengineering is controversial: opponents claim that costs are uncertain, as are wider ramifications for the environment. But a bigger question is whether the approaches would even reduce warming as they intend, which is why Ivana Cvijanovic and Ken Caldeira at the Carnegie Institution for Science, US, together with Douglas MacMartin at the California Institute of Technology, addressed this puzzle for ocean-albedo modification.

"Before asking if such methods are radical or applicable or feasible," said Cvijanovic, who is now at Lawrence Livermore National Laboratory, "it is important to determine whether or not they would have the desired environmental impact if they were implemented."

The prospect of geoengineering the planet to mitigate climate warming has gained increasing attention as carbon-dioxide levels continue to rise. Ideas largely centre on raising the Earth's albedo, whether by injecting sunlight-reflecting aerosols into the atmosphere at high altitudes, by brightening marine clouds or by painting the roofs in cities white.

Comparatively less attention has been paid to increasing the albedo of oceans – particularly the Arctic, which has lost reflective sea ice. But there are ideas. In 2011, physicist Russell Seitz of Harvard University, US, suggested that ocean albedos could be boosted by injecting the water with stable micro-bubbles. Then in 2013, Leslie Field at SmallTech Consulting, US, and others proposed floating granular materials under the sea surface, to reduce solar heat absorption and encourage new ice to form.

Cvijanovic and colleagues constructed simulations in which ocean-albedo values are substantially increased across a large portion of the Arctic, in a world where carbon dioxide is four times that of pre-industrial levels, the Arctic Ocean is seasonally ice free, and the air above is about 10 °C hotter – in other words, one of the worst-case scenarios for the end of this century. They compared these simulations with those for the same scenario but without a boosted ocean albedo.

Some of the results were positive: an extreme albedo boost could recover 40% of the sea ice that existed pre-industrialization and cool the surface of the Arctic by some two degrees. However, the researchers found no fall in global temperatures, and predicted that the geoengineering would alter climate outside the Arctic, for example with wetter and milder winters in the southwest US.

Cvijanovic said that ocean-albedo geoengineering could be a means to restore sea ice, which is also important for biodiversity and indigenous communities. But she added that the results do not support the proposals as a means to tackle climate warming. "Large-scale sea-ice restoration could affect weather patterns worldwide, and that is an additional reason for caution," she said.

The study is published in Environmental Research Letters(ERL).

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