Clearing up a cloudy view on cloud formation

Phytoplankton - tiny, plant-like organisms drifting through the great, vast ocean - are barely visible to the naked eye, and some are visible only through a microscope. Yet, when they are thriving, it is possible to see them from as far away as space. Their location is marked by swirling patterns of bright blues and greens that give the ocean a slick, marbled appearance, like oil on water.

Researchers Trish Quinn and Tim Bates of the US National Oceanic and Atmospheric Administration's (NOAA) Pacific Marine Environmental Laboratory, are interested in understanding how phytoplankton leave their mark on the climate system. For decades, earth scientists have hypothesized that, in addition to being the cornerstone of the ocean food web, chemicals released by phytoplankton are responsible for allowing clouds to form over the open ocean.

Because clouds play a huge role in how much sunlight reaches and warms the Earth's surface, scientists thought phytoplankton's contribution to cloud formation was a critical part of the climate system. Proposed in the 1980s, it was an elegant hypothesis that reflected scientists' growing awareness of how intimately one part of the Earth system could be connected to another.

The proposed link between phytoplankton and clouds is aerosols - tiny particles formed from a sulfur compound called dimethyl sulfide that phytoplankton emit into the atmosphere. Water vapor condenses onto some of those aerosol particles, allowing cloud droplets to form.

High concentrations of aerosols in the air can lead to clouds made of lots of small cloud droplets. These droplets function like an intricately designed crystal, reflecting sunlight off of their many surfaces. Ironically, the dense, bright white clouds are "dirtier:" they contain more aerosol particles than darker clouds that form in cleaner air, with fewer aerosols.

For more than 25 years, scientists thought that over the open ocean - far from continental sources of aerosols like dust, soot, and pollen - phytoplankton emissions of dimethyl sulfide were the most significant source of aerosol particles that provide seeds for clouds.

To evaluate this hypothesis, Quinn and Bates recently reviewed two decades of observations made from ships and remote coastal sites as well as computer simulations that estimated the impacts of dimethyl sulfide emissions on clouds and climate.

"Over the past two decades, the NOAA Climate Program Office has supported our research group's efforts to make measurements of aerosols aboard ships in many of the world's ocean regions," said Quinn. "One of our goals for these cruises was to find evidence to support this hypothesis."

What Quinn and Bates concluded was that phytoplankton emissions are not as important to the cloud-making process over remote oceans as was previously thought. Scientists are now investigating to see if other natural aerosol sources, such as sea salt and organic compounds from sea spray, show a stronger link between ocean emissions and cloud formation.

Even though the role of phytoplankton in modulating climate through cloud feedbacks is small, phytoplankton remain critical in regulating the amount of carbon dioxide in the atmosphere. Phytoplankton are responsible for nearly half of the planet's primary production, consuming carbon dioxide and releasing oxygen as they grow. Even small changes in the growth of phytoplankton could affect atmospheric carbon dioxide concentrations.

A version of this story first appeared on NOAA's website.