Atmosphere / Climate Change / Ocean

Bottom of the Food Web Brightens Tops of the Clouds

Wind and waves in the Southern Ocean launch sea spray far into the atmosphere. (Photos: NOAA/ersl)

Wind and waves in the Southern Ocean launch sea spray far into the atmosphere. (Photos: NOAA/ersl)

They may be small, just one single cell, but phytoplankton hold sway over much of the world. Not only do they prop up the food web in all the world’s oceans, but we now have evidence that phytoplankton are a major contributor to clouds and can help cool the planet.

In a recent study in the journal Nature Climate Change, an international team of researchers reveals that phytoplankton is responsible for more than half the water droplets in clouds over some parts of the Southern Ocean, and that they play a significant role in cooling climate in these areas.

So, how can those miniscule, single-celled, primitive organisms that live in the surface waters of the ocean affect what happens in the sky miles above them? It all has to do with waves, and wind and sea spray.

Ocean Clouds

Clouds are made up of millions of tiny water droplets that form when water vapor rises high into the atmosphere and condenses around particles of dust, gas molecules and other aerosols. The particles act as condensation nuclei, and the size and shape of the nuclei determines the size of the water droplets.

Satellite observations over the Southern Ocean revealed that during warm sunny periods when phytoplankton grow in abundance cloud droplets are smaller and there are more of them. But satellite observations are unable to determine the origin of cloud droplets. For that, scientists had to do a little detective work.

It’s long been known that ocean spray, whipped up by wind and waves, can loft sea salts and bits of organic phytoplankton waste into the air. Previous studies also established that phytoplankton emit dimethylsulfide (DMS), which breaks down into Sulphur dioxide, an aerosol that is also a major component of industrial air pollution.

Understanding that DMS, salt and organic particles all have the potential to become cloud condensation nuclei, the researchers ran simulated models of clouds to determine which particles could be responsible for the changes satellites observed as phytoplankton bloomed over the Southern Ocean. What they found was that phytoplankton was responsible for 60 percent of the water droplets in the clouds over the areas they studied.

Clouds as Mirrors

That’s important because each droplet acts as a tiny reflector, bouncing solar radiation back into space. The net effect of more tiny mirrors is more heat being reflected away from the earth. In fact, the study authors determined that phytoplankton in parts of the Southern Ocean are responsible for reflecting a yearly average of 4 Watts worth of solar radiation per square meter. In summer, when phytoplankton growth is most vigorous, that number can jump to 10 Watts of solar radiation reflected away from the earth.

What’s significant here is that the effect of phytoplankton on cloud reflectivity in the Southern Ocean is as strong as the effect of human induced pollution on clouds in some of the most heavily polluted regions in the northern hemisphere.

The scientists have shown that the phytoplankton and clouds are partners in an elegant feedback loop; As the bright summer sun reaches the ocean surface, it fuels the growth of phytoplankton which proliferate and in turn provide seeds for the formation of clouds which then block the sun and reflect heat, cooling the ocean surface and reducing solar energy for phytoplankton growth. Said more simply: Sun increases phytoplankton—phytoplankton increase clouds—clouds block and reflect the sun.

So, what does this new study add to the jumble of information scientists are sifting through? It tells them to keep their eyes on the tiniest players in the vast oceans far from civilization. Everyone is trying to predict what will happen as the climate warms, as the oceans rise and acidify, as dust clouds spew from parched landscapes and cars and coal plants continue to cough out carbon dioxide. But any calculation that omits the effect of phytoplankton on climate will be way off base.

Never a Simple Story

There’s something else it implies, and that is: the changesalready occurring in the oceans are be affecting climate in ways we don’t understand and can’t anticipate. This new piece of evidence just confirms the complexity climate scientists are faced with.

Increasing carbon dioxide in the atmosphere is pumping more dissolved carbon into the ocean, and that’s causing the oceans to become more acidic. Both of which have important effects on phytoplankton. Most evidence suggests that in coming years increasing water temperature and acidity are likely to boost growth rates among diatoms (one particular group of phytoplankton). But some studies suggest there’s a breaking point at which too much carbon and too much acidity can actually slow the growth of diatoms. Still other studies show that increasing ocean acidity will weaken the calcium plates diatoms excrete around themselves for protection. No one knows quite what the effect of that will be, or how these various responses will interact with one another.

What’s more, diatoms are just one of the many types of phytoplankton making up the base of the ocean food web. Just recently, researchers reported that ocean acidification will cause some species of phytoplankton to die out while others flourish. The shifting balance is bound to trickle up the food web, changing the composition of larger animals throughout the ocean. But it may also change the properties of clouds as different phytoplankton species may generate different water droplet sizes and densities.

Understanding why and how all this works is crucial to predicting and mitigating the effects of climate change. Perhaps it will also offer clues to engineers with big plans to change the course of global warming. There’s serious talk by serious people about developing mega blasters to launch cloud condensation nuclei into the atmosphere. The idea is that they will do exactly what natural cloud condensation nuclei do: gather water vapor into cloud droplets that will provide shade and reflect heat. Understanding how marine phytoplankton manage to get lofted into the air and how they behave once they get there could lead the problem solvers in the right direction.


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