Around 90% of the extra heat absorbed by the Earth from climate warming has been taken up by the oceans. Today this is equivalent to 7 Hiroshima bombs exploding in them every second. However, today’s sharp rise in sea surface temperatures is of particular concern to climate scientists and oceanographers. As these warmer seas naturally flow into the polar regions they melt the ice there, sometimes from beneath the ice sheets. In addition, more evaporation puts more water vapour in the air, and water vapour itself acts as a greenhouse gas. Higher humidity in the dark polar winters also produces thick clouds that trap even more heat. This slows the rate of polar ice refreezing, accelerating sea level rise.
Around 23% of the sun’s energy is reflected away from the Earth by clouds. Typically, only around 40% of the tropical and subtropical oceans are covered by cloud at any time. Increasing cloud cover in these regions by 3-4% could provide sufficient cooling to halt today’s warming trend.
Where do marine clouds come from? Phytoplankton, seaweed, and corals produce the ‘smell of the sea’ (dimethyl sulphide, or DMS). This substance reacts and combines with airborne sea salt and mineral dust to produce microscopic particles known as cloud condensation nuclei (CCN). These CCN particles float naturally in the air, and water vapour naturally condenses onto them up high where the air is cold, forming clouds in the sky. The more DMS there is in the air over the oceans, the brighter the clouds, the longer they last before ‘burning off’ in the sun, and the greater their total cooling effect.
Clouds also naturally increase the tropospheric ‘oxidative effect’, which means they enhance the methane sink. Reduced levels of methane and other powerful warming pollutants would additionally slow the rate of climate warming.
But photosynthetic growth in much of the ocean is increasingly limited by lack of nutrients. The removal of ocean life by over-fishing and historical whaling has reduced natural nutrient levels at the surface of large ocean areas by loss of their excretion. These nutrients still exist in deeper waters, but warmer seas are slowing the rate of mixing that brings nutrients to the surface.
Addition of appropriate levels of the missing nutrients to the ocean surface could enable substantial recovery of ocean biomass. Photosynthetic growth also naturally de-acidifies the ocean surface (raises pH) which helps to prevent the escape of CO2 from the ocean.