The plankton-rich oceans do more than absorb CO₂: plankton produces the gas dimethylsulfide (DMS) which, after chemical reactions in the atmosphere, leads to clouds with more but smaller droplets. These clouds are therefore whiter and reflect more sunlight back into space. This way, marine life cools the Earths climate. How exactly does this work and how large is the effect?
Clouds Over the Sea
To form a cloud, you need condensation nuclei. These are suspended particles in the air (aerosols) that attract water vapor. When humidity reaches 100 percent, water vapor condenses and forms a cloud droplet. There are many types of aerosols that can serve as condensation nuclei. They can be of natural origin or result from human activities. For example, burning fossil fuels and livestock farming emit gases such as sulfur dioxide and nitrogen oxides that react in the atmosphere to form sulfate and nitrate aerosols. Over the sea, the main natural aerosols are small sea salt particles from splashing seawater, sometimes mixed with organic substances from plankton and microalgae, and aerosols formed by chemical reactions in the atmosphere from the gas dimethylsulfide.
Phytoplankton and Dimethylsulfide
Phytoplankton in the ocean produces dimethylsulfide, a sulfur-containing gas. This gas reacts in the atmosphere to form sulfate aerosols and MSA aerosols (Figure 1). Both serve as condensation nuclei and influence the properties of clouds that form over the oceans. Measurements show that the more aerosols present, the smaller the cloud droplets. The reactions are complex but can now be reasonably accurately modeled in climate models.
More Reflection of Sunlight
More than 70 percent of the Earth is covered by oceans. Ocean water absorbs most of the sunlight that falls on it, reflecting only 5 to 6 percent. A cloud reflects much more sunlight, about 50 percent. Thus, clouds over the sea have a strong cooling effect on the climate because they reflect much more sunlight than the water surface. And the more small droplets a cloud contains, the stronger the reflection. Aerosols enhance this cooling effect of clouds. This is called the Twomey effect, named after Sean Twomey who first described it in 1974 here. The effect of sulfate aerosols from DMS on reflected solar radiation can be large, especially in ocean areas with clean air where aerosols come only from the sea. For example, in the Southern Oceans, studies show that in summer this can mean 10 Watts per square meter more reflection. This is enough to raise the temperature of a 1-meter deep layer of water by 1 degree in less than 5 days.
Complex Web of Interactions
The Earths climate results from a complex web of interactions. Phytoplankton, microscopic plant-like organisms living in the upper illuminated part of the ocean, have a non-negligible cooling effect via clouds. The global cooling effect of phytoplankton through the Twomey mechanism is probably around 1 Watt per square meter, but precision is lacking and uncertainty is high.
Climate Models
Climate models generally show a too warm Southern Ocean because they reflect too little sunlight through clouds (Figure 2). This is mainly because aerosol-cloud interactions and the ratio between ice and water in clouds in this area are not realistically represented. As a result, low clouds are too dark and too transparent compared to observations. Model experiments show that better representation of aerosols over the sea (such as DMS sulfate) leads to whiter clouds and more reflection of sunlight, making the Southern Ocean temperature better match observations. This leads to more reliable projections of future climate.
