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Honeycombs in the Sky: Marine Clouds and Climate Change

Michael D. Lemonick

By Michael D. Lemonick

The behavior of clouds in a warming climate is something scientists are still struggling to understand. Changes in clouds could either amplify or minimize warming, depending on the precise nature of the response. Clouds tend to cause cooling, by reflecting sunlight back into space, and can also cause warming by preventing heat radiation from escaping into space. The warming effect of high clouds is generally stronger, so increases in high clouds would amplify warming. With low clouds, however, the cooling effect is generally stronger, so increases in low clouds would reduce warming.

But clouds are incredibly complex creatures whose waxing and waning depend on temperature, altitude, humidity, winds and aerosols made of dust and other particles floating in the air — not just their composition, but also their size. Most climate scientists think that on balance, clouds will end up as a net positive feedback, meaning they will accelerate the warming of the planet due to increasing concentrations of greenhouse gases.

The case is hardly ironclad, though, so anything that provides even a little bit of insight into cloud behavior is welcome.

A little bit of insight is just what’s on offer in a paper published August 11 in Nature. Graham Feingold of the U.S. National Oceanic and Atmospheric Administration’s (NOAA) Earth System Research Laboratory in Boulder, Colorado, along with several colleagues, focused on a type of cloud that’s relatively common over the oceans. Known as “open-cell clouds”, they’re vast sheets of stratocumulus clouds perforated with regular holes, in a sort of irregular honeycomb pattern (if the holes are relatively big compared with the cloudy parts, they’re known as closed-cell clouds).

 

Image of open and closed-cell clouds taken by NASA's Moderate Resolution Imaging Spectroradiometer (MODIS) on the agency's Aqua satellite. According to NASA, "Open-cell clouds look like empty compartments, whereas closed cells look like compartments stuffed with cloud." Credit: NASA Earth Observatory.

Open-cell clouds are remarkably persistent over the oceans; to figure out why, Feingold and his colleagues created simulations to study their behavior in detail. It turned out that the thickest parts, naturally enough, are the most likely to produce rain. The falling rain cools the surrounding air as it falls, creating a downdraft. When the downdraft hits the surface and spreads out, it eventually runs into the spreading air from the next downdraft over, at which point the colliding streams of air rise up again, freshly charged with evaporated water from the ocean’s surface, to form new clouds. These new clouds form in places where the holes were before, while the places that started out cloudy become clear.

That’s an oversimplification, of course (to avoid this trap, we’d have to simply reprint the original paper, and where’s the fun in that?). But in general, the clouds don’t exactly persist; their configuration oscillates back and forth in such a way that the overall pattern persists. The evidence that what happens in the computer also happens in the real world: satellite movies of real clouds show just this sort of oscillation.

So how does that help with climate projections? Well…it doesn’t really at this point. What it does do is give cloud modelers confidence that they’ve got at least one important cloud-formation process right, and the more of these processes they can model successfully, the better they’ll be able to simulate cloud dynamics on a scale large enough to nail down their role in climate.