The study into Southern Ocean clouds is in.
Scientists just sampled the most pristine air on Earth. Here's what they found.
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But due to how remote the Southern Ocean is, there have been very few actual studies of the clouds there. Because of this lack of data, computer models that simulate present and future climates overpredict how much sunlight reaches the ocean surface compared to what satellites actually observe. The main reason for this inaccuracy is due to how the models simulate clouds, but nobody knew exactly why the clouds were off. For the models to run correctly, researchers needed to understand how the clouds were being formed.
To discover what is actually happening in clouds over the Southern Ocean, a small army of atmospheric scientists, including us, went to find out how and when clouds form in this remote part of the world. What we found was surprising — unlike the Northern Hemisphere oceans, the air we sampled over the Southern Ocean contained almost no particles from land. This means the clouds might be different from those above other oceans, and we can use this knowledge to help improve the climate models.
Ice clouds and liquid clouds
Clouds are made of tiny water droplets or ice crystals, or often a mixture of the two. These form on small particles in the air. The type of particle plays a big role in determining whether a liquid droplet or ice crystal forms. These particles can be natural — like sea spray, pollen, dust or even bacteria — or from human sources like cars, stoves, power plants and so on.
To the untrained eye, an ice cloud and a liquid cloud look much the same, but they have very different properties. Ice clouds reflect less sunlight, precipitate more and don't last as long as liquid clouds. It matters to the weather — and to climate models — what kinds of clouds are around.
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This was the mystery: Why are there more liquid clouds than the models think there are? To solve it, we needed to know what kinds of particles are floating around in the atmosphere around Antarctica.
Before we went down there, we had a few clues.
Previous modeling studies have suggested that the ice–forming particles found over the Southern Ocean may be very different from those found in the Northern Hemisphere. Dust is a great ice cloud seeder, but due to the lack of dusty land sources in the Southern Hemisphere, some scientists have hypothesized that other types of particles might be driving ice cloud formation over the Southern Ocean.
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Bacterial maps
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The atmosphere is full of microorganisms that are carried hundreds to thousands of kilometers on air currents before returning to Earth. These bacteria are like airborne license plates, they are unique and tell you where the car — or air — came from. Since scientists know where most bacteria live, it's possible to look at the microbes in an air sample and determine where that air came from. And once you know that, you can predict where the particles in the air came from as well - the same place the bacteria usually live.
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Ocean bacteria alone
In most ocean regions around the world, especially in the Northern Hemisphere where there is a lot of land, the air contains both marine and terrestrial particles. That's what we expected to find down south.
With the frozen filters safely back at our lab in Colorado, we extracted DNA from the bacteria and sequenced it to determine what species we had caught. Much to our surprise, the bacteria were essentially all marine species that live in the Southern Ocean. We found almost no land-based bacteria.
If the bacteria were from the ocean, then so were the cloud-forming particles. This was the answer we were looking for.
https://www.livescience.com/most-pristine-air-on-earth-bacteria.htmlAirborne bacteria confirm the pristine nature of the Southern Ocean boundary layer
We found that the summer airborne bacterial community in the marine boundary layer over the Southern Ocean directly south of Australia is dominated by marine bacteria emitted in sea spray, originating primarily from the west in a zonal band at the latitude of collection. We found that airborne communities were more diverse to the north, and much less so toward Antarctica. These results imply that sea spray sources largely control the number concentrations of nuclei for liquid cloud droplets and limit ice nucleating particle concentrations to the low values expected in nascent sea spray. In the sampled region, the sources of summer cloud-active particles therefore are unlikely to have changed in direct response to perturbations in continental anthropogenic emissions.
https://www.pnas.org/content/117/24/13275
