Hydrosphere Contents

4. Oceans

Cloud-algae theory explains a dynamic balance between marine plankton, sunlight and cloud cover

In a cloud, every droplet starts by condensing on a microscopic particle called a cloud condensation nucleus. If there is a shortage of these nuclei, as can happen in very clean air, clouds will form with fewer but larger droplets. If there are plenty of nuclei then many, smaller droplets form. Smaller droplets make for whiter clouds, which in turn reflect more of the sun's radiation back into space. Clouds formed from more, smaller droplets will also persist for longer in the atmosphere as more time is required for the droplets to combine until they become large enough to fall as rain. The theory goes on to argue that over large parts of the oceans the main source of these cloud condensation nuclei is dimethylsulphide gas which is emitted into the atmosphere by algal plankton that live close to the surface of the sea. The number of these algae increases when there is a lot of sunlight, producing more gas which leads to more clouds which reduces the amount of sunlight which means plankton production falls. As it does, the cloud cover diminishes, allowing in more sunlight - and so on. There is consequently a self-adjusting equilibrium between plankton, sunlight and cloud cover. This, it appears, has an important impact on the world's weather.

What is the cloud-algae theory?