Astronomy

Solar Eclipses Provide a Rare Way to Study Cloud Formation

April 8’s North American solar eclipse is just around the corner, and it has astronomy fans and weather aficionados alike preparing for an incredible show. But it’s not just fun and games. Eclipses are rare opportunities for scientists to study phenomena that only come around once in a while.

Last week, a team of meteorological experts from the Netherlands released a paper describing how eclipses can disrupt the formation of certain types of clouds. Their findings have implications for futuristic geoengineering schemes that propose to artificially block sunlight to combat climate change.

Published in Nature Communications Earth & Environment, the paper examines satellite imagery of cloud cover during three solar eclipses between 2005 and 2016.

They found that in the wake of an eclipse, shallow cumulus clouds tend to disappear – and it doesn’t even need to be a total eclipse for this to occur – it happens when just 15% of the Sun is obscured.

The effect isn’t immediate. There’s a delay of about 20 minutes. That’s because the eclipse isn’t destroying the clouds directly. Instead, it’s cooling the land beneath, interrupting packets of warm air that race upwards in updrafts to condense into clouds. By suppressing the updrafts, the eclipse puts a pause on cumulus cloud formation.

Proposals to reduce climate change by artificially blocking the Sun work on a similar principle to an eclipse. A swarm of sun-shade spacecraft, or an injection of light-absorbing aerosols into the atmosphere, could reduce the amount of solar energy reaching the surface of the Earth, cooling the temperature back to historical norms. For a project like this to work, about 3.5% to 5% of sunlight would have to be blocked.

The cloud modeling data from this paper indicates reasons to be cautious, however. First and foremost, it suggests that blocking sunlight isn’t as effective as you might think, because while it does cool the ground initially, it also reduces cloud cover, which once again increases the amount of solar energy reaching the Earth.

The decrease in cloud cover would also have an effect on precipitation – fewer clouds means less rain – which might result in regional increases in drought and desertification.

It’s unclear whether the reduction in cumulus clouds would persist with a more permanent, artificially constructed eclipse – true solar eclipses only last a few minutes locally, after all. But the authors say the data ought to influence the design of any serious geoengineering proposals going forward. A solar shade stationed between the Sun and Earth, at Lagrange point 1, for example, might not block the Sun uniformly. If it caused either partial or intermittent local eclipses, it would be more likely to feature these cloud-destroying effects.

Atmospheric aerosol injection might seem like a more uniform method of blocking sunlight, but large-scale weather patterns actually make these methods potentially even more variable, blocking up to 45% of sunlight locally on occasion (well beyond the 15% needed to see a reduction in cloud formation).

These geoengineering projects, in other words, might solve climate change only to introduce new, unexpected challenges, and the costs might not be borne equitably across the globe.

So what’s the lesson? Well, if you’re going out to see the eclipse on April 8, and you feel a little chill in the air, you’re not imagining it. The Earth around you is cooling – and it might also get a little sunnier after it’s over, as cumulus cloud formation gets interrupted. These effects are tangible reminders that the relationship between Earth’s climate and the Sun is complex – and tinkering with it comes with a high chance of unintended consequences.

Read the Paper:

Victor Trees et al. “Clouds dissipate quickly during solar eclipses as the land surface cools.” Communications Earth and Environment. February 12, 2024.

Scott Alan Johnston

Scott Alan Johnston is a science writer/editor at the Perimeter Institute for Theoretical Physics, a contributor at Universe Today, and a historian of science. He is the author of "The Clocks are Telling Lies," which tells the story of the early days of global timekeeping, when 19th-century astronomers and engineers struggled to organize time in a newly interconnected world. You can follow Scott on Twitter @ScottyJ_PhD

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