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Salt Spray May Prove Most Feasible Geoengineering

The geoengineering technique might have some unintended benefits, like more rainfall, but also consequences if ever interrupted



Richard Dorrell/Wikimedia Commons

Researchers have wondered for years whether we will one day be able to re-engineer the planet and slow down, perhaps even pause, global warming. A recent study out of Norway is now the first to describe how that might be done.

There are many ways to try and climate engineer the planet, but many of them are so far-fetched that scientists aren't sure if they would even be physically possible, let alone physically successful. Sea-salt climate engineering (SSCE) might be the most low-tech, and plausible, possibility.

In such a situation, specially designed unmanned boats would plow the seas, spraying salt water into the air. The water would evaporate and leave behind sea-salt particles, which may be lifted into the clouds, increasing their albedo, or reflecting power.

A study released last month in the Journal of Geophysical Research: Atmospheres used three different models to run the same SSCE scenario in which sea-salt engineering was used in the low-latitude oceans to keep top-of-atmosphere radiative forcing at the 2020 level for 50 years and was then abruptly turned off for 20 years.

It is widely believed to be the first study to use multiple models to determine the effects of a specific geoengineering experiment -- most past experiments simply assume radiative forcing has been reduced, and model the effects, rather than applying a specific experiment -- and it had one noticeable unexpected result.

Many past studies have determined that geoengineering experiments, while keeping the planet cool, also can disrupt the Earth's hydrological cycle to dangerous degrees (ClimateWire, Nov. 6). The SSCE study found that sea-salt engineering, while reducing precipitation over low-latitude oceans, also increased precipitation over low-latitude land regions.

Jón Egill Kristjánsson, a meteorologist at the University of Oslo and a co-author of the study, said he was surprised by that result.

"That's distinctly different from the simpler simulations where you just turn down the solar constant, which has been done in many studies," he said.

As the logic -- and science -- goes, a cooler, geoengineered planet means less evaporation, and thus less rainfall. But Kristjánsson said their result could be explained by a monsoon circulation that shifts to land when only ocean areas get cooled down.

"It's an interesting result, and all three models agree on that [result]," he said.

"We try to simulate in a more physically realistic manner -- to the extent one can use such a term at this stage," he added. "Right now, it's all science fiction."

A simpler, not yet doable experiment
Research into other specific geoengineering experiments is much less mature. Besides SSCE, scientists have also been investigating stratospheric sulfur injections -- firing sun-reflecting aerosols into the air, similar to the cooling effect after a volcanic eruption -- and cirrus cloud thinning, where you thin the top level of clouds, which have a warming effect on the planet.

Kristjánsson said science "doesn't really have an answer yet" for which experiments might be more effective, but SSCE is an early front-runner. Not only could it be cheaper -- you would only have to pay for the vessels and inject the seawater at a lower height than with sulfur injections or cirrus cloud thinning -- but it could also be much more localized. And if it's more localized, potential side effects may be more easily contained.

"Clouds are more spatially non-uniform, whereas if you inject something into the stratosphere, it tends to spread out rather quickly, at least over a hemisphere or maybe the whole globe," he said.

The Oslo experiment tested SSCE over the entire low-latitude ocean, and Kristjánsson said the engineering could be even more localized, though he said he wasn't sure about the effect that different magnitudes of sea-salt engineering would have on precipitation, or their "termination effect."

It's for this reason that it's important to understand the differences in responses between geoengineering experiments, said Ben Kravitz, a climate modeler at the Pacific Northwest National Laboratory who helps run the international Geoengineering Model Intercomparison Project.

"When people talk about 'geoengineering,' they're not being specific enough, because you can get a very different answer based on how you engineer," he said.

Greenhouse gas reductions remain the priority
The termination effect is one example of how geoengineering experiments can differ.

A past study that Kravitz helped run at GeoMIP found that the abrupt termination of radiative forcing would cause global warming to effectively speed up to make up for all the time it lost, cramming five decades of warming into five or 10 years (ClimateWire, Nov. 27).

Kristjánsson and his team included a similar scenario in their models. After 50 years of SSCE, they simulated abruptly ending the experiment, and watched what would happen for the next 20 years.

The result: Surface temperatures increased rapidly, especially in the Arctic, which saw its September sea ice cover shrink by 25 percent.

For now, there are many more unanswered questions about geoengineering than answered ones. Kristjánsson plans to run more modeling studies of SSCE, stratospheric sulfur injections and cirrus cloud thinning. Scientists hope to know a lot more about the effects of specific climate engineering methods in the coming years, but even that only brings us so far.

"The real problem right now is greenhouse gas emissions; they have to come down," said Kristjánsson.

"I hope we never have to do [geoengineering], because even if we find out it's possible and might work, it is still a desperate solution. It's not the way we should be going about this."

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500

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