Most life in the sea ultimately depends on photosynthetic plankton. Also known as microalgae, these tiny or microscopic organisms live near the surface and take their energy from the sun and pass it on through the marine food chain.
But these plankton have a big role to play above the surface of the sea too. In new study published in the journal Science Advances, colleagues and I found that plankton help to control clouds over remote seas far from land. These clouds in turn bounce the sun’s energy back into space, regulating the Earth’s climate and keeping temperatures cooler than they would otherwise be without them.
Clouds are made up of many tiny droplets of water that have condensed from water vapour onto microscopic particles floating in the Earth’s atmosphere. These particles are known as cloud condensation nuclei. Plankton essentially help provide clouds with these nuclei to form around.
The number of these particles in a given volume helps to determine the number of droplets in a cloud, which can have a big influence on how much sunlight a cloud reflects back into space. The more droplets a given mass of cloud water is broken up into, the more sunlight is reflected, as the overall surface area of the cloud’s droplets increases. Since a significant portion of the planet’s reflectivity, or albedo, is due to clouds, this can have a major impact on the energy balance of the Earth.
To investigate the link between plankton and clouds, we looked at the Southern Ocean. This sea, encircling Antarctica, is one of the most remote places on the planet and far from any man-made sources of particles. And yet it is also one of the cloudiest places on Earth. What then are these clouds clinging on to?
Counting cloud droplets from space
We analysed satellite cloud data in a section of the Southern Ocean spanning right around the globe between the 35th parallel south (which passes through Australia and just south of South Africa) and the 55th (which just clips the bottom of South America).
We found that more cloud droplets tended to occur above patches of the ocean with more plankton, indicated by increased concentrations of a type of chlorophyll used in photosynthesis. This means plankton are likely to influence cloud albedo and the amount of energy from the sun that is reflected to space.
Most of this is down to plankton releasing gases either through cell ageing, or when they are broken open and eaten by their microscopic animal counterparts zooplankton. Some of this gas is then converted into new microscopic solid particles, or adds to existing particles, which act as extra condensation nuclei.
However, we also found that some organic material – which can come from the bodies of the plankton, other sea creatures, viruses, bacteria and so on – is emitted directly into the atmosphere through sea spray. Water can condense around these tiny particles, forming extra cloud droplets (although organic material may affect droplet numbers in other ways too – the science is still hotly debated).
Keeping the oceans cool
We also estimated how much extra solar energy was prevented from reaching the surface due to the extra cloud droplets formed by phytoplankton – up to ten watts per square metre in summer. That’s comparable to similar estimates of the annual mean effect on clouds from man-made particles downwind of highly polluted regions. Thus, in a lifeless ocean without phytoplankton it is likely that the Southern Ocean’s surface would be somewhat warmer.
Many climate models underestimate the amount of sunlight reflected back into space by clouds in the Southern Ocean. This can lead to errors in regional sea surface temperature predictions and incorrect large-scale circulation patterns both locally and in far afield regions such as the tropics, and so it is important that they are corrected.
These biases might partly be down to an unrealistic representation of the link between phytoplankton and cloud formation. Somewhat ironically, uncertainties in our knowledge of the “baseline” effect of these natural condensation nuclei are also one of the biggest causes of uncertainties in how anthropogenic aerosols are affecting the climate.
Due to its remoteness and inhospitality there is, however, very little in-situ data available from within Southern Ocean clouds with which to study what is going on. This makes satellite data very valuable, but also emphasises the need for more direct observations in this region.