Atmospheric Chemistry

Tiny plankton make big clouds brighter


McCoy, Daniel T., et al. “Natural aerosols explain seasonal and spatial patterns of Southern Ocean cloud albedo.” Science Advances 1.6 (2015): e1500157. DOI: 10.1126/sciadv.1500157


Have you ever looked up at the clouds and wondered where they come from? What causes clouds to form? Clouds affect Earth’s climate in complex ways, but our understanding of what controls clouds and their effects on climate is limited.

Satellite image showing phytoplankton blooms and clouds in the South Atlantic Ocean (NASA Earth Observatory, December 2014)


On a hot day when the sun is bright, you might have noticed that wearing lighter clothing keeps you cool. This is because dark clothing absorbs sunlight, while light clothing reflects the sun’s energy away.  When sunlight hits the earth’s surface, it is either absorbed or reflected. The same thing happens to clouds. White clouds in our atmosphere are very good at reflecting sunlight back to space. The brighter the cloud, the more energy is reflected. Clouds are not all created equal, and some types of clouds are much brighter than others. The brightness of a cloud depends on tiny particles called aerosols which act as seeds for cloud droplets to form.

There are many types of small aerosol particles, both natural and man-made, which can act as seeds for cloud growth. One important cloud-forming aerosol is the sulfate gas aerosol. These aerosols are often produced by volcanic eruptions or pollution from human sources. Sulfates can also be formed by biological sources, particularly marine phytoplankton – tiny, photosynthesizing organisms that are abundant throughout the ocean. Phytoplankton also seed cloud droplets a second way by producing organic matter which becomes airborne through sea spray.

The Southern Ocean, which encircles Antarctica, is the cloudiest region on earth and is far away from human-created pollution, making it a great region to study how natural aerosols influence cloud formation. A group of researchers from the University of Washington analyzed data from one of NASA’s satellites to study the relationship between cloud brightness and phytoplankton growth. Scientists are able to estimate the phytoplankton growth through the green color produced by a pigment called chlorophyll-a, which is also found in land plants. The researchers found that high values of chlorophyll-a coincide with high cloud brightness, indicating that they are related.

Figure 1 from McCoy et al. 2015. Correlation between cloud droplet concentration (a measure of brightness) and chlorophyll-a concentration for different time periods.

Although cloud brightness and chlorophyll-a are correlated, the satellite data is not able to demonstrate that chlorophyll-a concentration is causing brighter clouds. To investigate this hypothesis, the scientists looked at model simulations with different combinations of aerosols to test how well they could recreate the observed cloud brightness. Sulfate and organic matter were able to explain the majority of spatial patterns and seasonal variations in cloud brightness.

The scientists also looked at some regions where sulfate was more important and others where organic matter was dominant. They found sulfates were more important further North where there is more sunlight and land sources of sulfate, but organic matter dominated in regions further South where there were large phytoplankton blooms.

Figure 4 from McCoy et al. 2015. Observed and model estimates of cloud droplet concentration, sulfate concentration, and organic matter aerosols which are related to chlorophyll-a concentration.

The model estimated that phytoplankton caused an increase in the amount of sunlight reflected by clouds of up to 10 W/m2 during summer. This is comparable to the amount of extra radiation reflected by clouds formed from man-made aerosols in highly polluted regions like China. These results suggest that natural aerosols can have a big impact on how clouds reflect the sun’s energy. While these results are exciting, models have limitations. In order to find out more about the effect of phytoplankton blooms on cloud brightness, the results of this study should be verified by direct measurements of phytoplankton aerosols in the Southern Ocean. Studies like this are useful in guiding observational scientists to decide what to focus their research on and where to go to sea.
Clouds have an enormous impact on weather and climate, by regulating how much heat is trapped at Earth’s surface. This research is demonstrates how ocean plankton can alter the effect of clouds on our climate, and this understanding will improve our ability to model and predict future climate changes.

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