We could slow global warming by making brighter clouds – but should we?

Human activity has been messing with the Earth's atmosphere since the Industrial Revolution, and one controversial proposal is to attempt to slow the effects of climate change by… messing with the atmosphere. This week, two independent studies have examined the idea, one running computer simulations of a "cocktail" of techniques, while the other outlined a small-scale test to figure out how practical and safe the idea might be.

Geoengineering is the blanket term for a range of methods of tinkering with the delicate processes that regulate the climate. Suggested strategies include spraying small, reflective particles into the atmosphere to reduce the amount of solar energy that gets through. These particles, it's said, would mimic the effects of those blasted into the air from volcanic eruptions and rapidly cool the planet in a matter of years.

Of course, volcanic eruptions come with their share of hazards as well, polluting the air and water and in extreme cases triggering ice ages. It's unsurprising then that geoengineering is a sensitive topic. After all, unnatural airborne particles are what got us into this mess in the first place, so is it really wise to steer into the skid?

In a 2015 report, the US National Academies of Sciences, Engineering, and Medicine concluded that climate intervention technologies "pose considerable risks and should not be deployed at this time." But they didn't rule out the idea completely, instead recommending further research into the risks, challenges and benefits, and favoring studies that could improve our understanding of the climate in general.

Over the years, various research efforts have found both pros and cons. On the plus side, the diffused light from the Sun, as well as the high CO2 concentration in the air, can improve crop yields. But on the other hand, deflecting sunlight could undermine solar power systems and wouldn't address other problems like ocean acidification. Worse, there's no way to really know what other run-on consequences it could trigger.

With further research required, a team at the University of Washington is investigating marine cloud brightening, and has outlined a test to determine how feasible it might be. By spraying saltwater into the air, the process could make the clouds over the oceans "brighter" so they reflect more of the Sun's rays and, in keeping with the NAS's guidelines, also provide data on how clouds and aerosols interact.

"A major, unsolved question in climate science is: How much do aerosol particles cool the planet?," says Rob Wood, lead author of the study. "A controlled test would measure the extent to which we are able to alter clouds, and test an important component of climate models."

The researchers are already developing a nozzle that can reduce saltwater into tiny droplets and spray them high into the atmosphere by the trillions every second. The proposal outlines a three-year plan to produce the system, then conduct tests in a lab, before moving them outdoors and eventually offshore. These would all be small scale proof-of-concept runs, but if the technology works, scaled up versions could eventually be deployed over bigger patches of the ocean.

The second study comes out of Carnegie Institution for Science. In this project, climate scientists focused on two different methods of geoengineering: thinning cirrus clouds, which act like a blanket and trap heat close to Earth, and solar geoengineering, where sunlight is scattered by aerosol particles.

While both of these techniques could be effective at cooling the Earth, they run the risk of disrupting the precipitation cycle. Solar engineering, it was found, would reduce rainfall too much, while focusing on cirrus clouds doesn't do enough to slow the increase in rain that global warming causes.

So, the team ran "cocktail shaker" simulations to see what would happen if both were deployed at the same time. Interestingly enough, the study showed that warming would drop to pre-Industrial levels, while rainfall levels would on average stay the same. But the key word there is "average": where and how much rain fell would be drastically different.

"The same amount of rain fell around the globe in our models, but it fell in different places, which could create a big mismatch between what our economic infrastructure expects and what it will get," says Ken Caldeira, lead author of the study. "More complicated geoengineering solutions would likely do a bit better, but the best solution is simply to stop adding greenhouse gases to the atmosphere."

In the end, any kind of geoengineering technique may be too risky to roll out in the near future, but they should still be investigated as an emergency backup plan, in case climate change hits harder than expected.

"If the world cannot slow emissions or the effects of climate change are more extreme or occur sooner than expected, there may be demands to pursue additional climate-intervention technologies about which scientists need a better understanding," says Ralph J. Cicerone, National Academy of Sciences President. "Although riskier ideas to lessen the amount of energy absorbed from the sun should not be considered for deployment, they should be studied so that we can provide answers if someday these ideas begin to be considered in attempts to avert catastrophe."

The University of Washington study was published in the journal Earth's Future, while the Carnegie paper was published in Geophysical Research Letters.

Sources: University of Washington, Carnegie Institution for Science