Depressing study shows a big issue with using cloud seeding to solve global warming

The clouds that hang low and thick in our sky, reflecting sunlight back out into space, are melting into thin air as the world warms.

The loss will not only trigger greater climate changes than we expected, but new research suggests it could also undermine the potential of future geoengineering solutions. 

The idea of seeding clouds with an injection of light-reflecting particles to reflect sunlight back into space – thereby 'cooling' the planet – is a controversial one, yet to be proven useful or even feasible in the real world.

Some scientists worry about the unforeseen dangers of meddling in our planet's climate any further, while others point out this climate-hacking solution does nothing to address ocean acidification or adverse ecosystem effects. 

It's also only as good as the clouds in our sky, and in the coming century, there may be a lot less of those. Even in the most ideal scenario, where solar geoengineering works with no side effects, a new model suggests it may not be enough on its own.

If the world's carbon emissions continue to rise unfettered, we will trigger a much larger cascade of warming – cloud seeding or no. 

"Hence, elevated greenhouse gas concentrations may trigger substantial global warming by reducing the cooling effect stratocumulus clouds provide," the authors of the new study write, "even when all or much of the effect of greenhouse gas at the top of the atmosphere is compensated by solar geoengineering." 

This is an extreme scenario where carbon dioxide continues to accumulate in the atmosphere at a concentration four times greater than today.

That would take about a century, researchers say, and yet it could still very well happen, even if we spent decades injecting aerosols into the atmosphere. 

Today, stratocumulus clouds cover huge swathes of the subtropical ocean, and recent evidence suggests they are responsible for reflecting about 5 degrees Celsius of global warming back out into space.

As greenhouse gases build up in the atmosphere, high-resolution simulations show these clouds will thin out and maybe even dissipate completely. Even under modest warming scenarios, the results show subtropical stratocumulus clouds gradually thin out and break apart.

Without this protective shield, all that heat from the sun can then sink freely into the lower atmosphere and our planet's oceans, raising temperatures around the globe even further. 

As such, solar geoengineering may only work up to a point, and in the long run, researchers say it's no replacement for cutting carbon emissions. 

"Solar geoengineering is not a fail-safe option to prevent global warming," the authors conclude, "because it does not mitigate risks to the climate system that arise from direct effects of greenhouse gases on cloud cover."

Their model illustrates what happens when increased CO₂ concentrations occur at the same time as increased cloud cooling. As such, it assumes that solar geoengineering actually works and that our emissions will continue unchecked.

But while growing research has tied increased greenhouse gas emissions to a potential tipping point of thinning cloud cover that's often overlooked in climate models, it's still not clear at what concentration those clouds begin to thin or how that impacts their light-reflecting abilities. 

Surface warming on Earth leads to evaporation and increased atmospheric moisture, which appears to weaken cloud-top cooling, sort of like how the ground takes longer to cool at night in humid weather as opposed to dry conditions.

The new model is an interesting exploration of what happens when emissions continue while cloud cooling improves, but it is still a relatively simple explanation that does not take into account seasonal cycles, 'noise' in meteorological data or regional differences.

"The lack of spatial heterogeneity and temporal variability in our simulations make it difficult to quantify more precisely the range of CO₂ concentrations over which stratocumulus breakup may occur, or when the clouds reform after CO₂ concentrations are lowered," the authors admit. 

"Caveats and limitations notwithstanding, the results illustrate a hitherto-unrecognised risk of solar geoengineering." 

The risks and benefits of solar geoengineering remain uncertain, and we need far more research before we can say for sure whether it's worth pursuing at all. Clouds are really hard to simulate; seeding them is a whole other issue.

For now, reducing our emissions is still undoubtedly the best option.

The study was published in PNAS. 

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