‘The clouds over the Southern Ocean reflect significantly more sunlight in the summertime than they would without these huge plankton blooms,’ says Daniel McCoy, atmospheric scientist and co-author of new research on plankton blooms. As a collaboration between the University of Washington and the Pacific Northwest National Laboratory, he has been investigating the link between microscopic plankton and cloud albedo.
To make clouds, airborne particles called aerosols or ‘cloud seeds’ are needed for cloud droplets to form around. Plankton in the Southern Ocean seem to be producing more aerosols for the cloud droplets to stick to. Daniel Grosvenor, co-author of the study explains that ‘the plankton actually contribute to making more “cloud seed” aerosols that are required to form cloud droplets upon’.
More aerosols in the air means brighter clouds. This is because cloud moisture can spread itself over more particles. ‘For each cloud, there is only so much liquid,’ says McCoy. ‘If you share that liquid over a higher density of aerosols, the size of the water droplets will get smaller.’ Since cloud reflectivity is mostly governed by the overall surface area of these droplets, more of them makes brighter clouds. ‘Sharing cloud moisture over many aerosol particles translates into a higher cloud albedo,’ says McCoy.
Brighter clouds mean that more sunlight is reflected back into space. ‘This is likely changing the energy balance of the region,’ says Grosvenor, ‘since more energy is being reflected as opposed to absorbed by the ocean as heat.’
How can marine life produce aerosols for clouds? One way is by emitting natural gases, such as dimethyl sulfide released by Sulfitobacter bacteria and phytoplankton. A second is through the organic matter and bubbles that collect at the water’s surface. This can get taken up by the wind and taken into the air as tiny particles of dead plant and animal material.
‘Although it is hard to be 100 per cent sure that it is plankton having this effect,’ says Grosvenor, ‘we observed strong correlations between cloud droplet numbers and chlorophyll, a good indicator of plankton. While correlation does not necessarily mean causation, some modelling suggested that the higher droplet numbers we observed was when the plankton was particularly active. They are likely to be having a significant influence on the cloud brightness.’
‘In the summer,’ adds McCoy, ‘we get about double the concentration of cloud droplets as we would if it were a biologically dead ocean.’
It is vital that this research takes places around the Southern Ocean, where there is relatively little pollution by industrial contaminants. In the northern hemisphere, it is thought that as much as half of all cloud droplets are formed by human pollutants. Unlike the northern hemisphere, the Southern Ocean isn’t near any aerosol-producing landmasses. This means the Southern Ocean can be a good indication of how the atmosphere might have looked and behaved before human activity.
‘In our estimates of the effects that man-made aerosol has had on our climate, the lack of pre-industrial, “baseline” state is actually the largest cause of uncertainty,’ says Grosvenor, ‘so an improved understanding of aerosol in remote places like the Southern Ocean is likely to help bring down that uncertainty.’