From a climate perspective, the ocean plays a number of different and critical roles. In terms of physical processes, the ocean represents an enormous “heat sink” (store of energy) that can act to buffer temperature changes in the atmosphere and on land , as well as a source of moisture through evaporation that affects the climate both through clouds (predominantly cooling) and the greenhouse effect (warming).

However, the ocean also plays an important role in the climate system through its biogeochemical cycles. These cycles encompass the interaction between the ocean’s living systems and its geochemistry – manifested in the cycling of important chemical elements such as carbon, nitrogen, phosphorous and oxygen. Some of these elements are nutrients that are needed by living organisms, while others control processes critical for life.

The ocean’s store of carbon is more than 50 times that of the atmosphere, but this storage can be affected by changes in the climate system. The process that cycles carbon from the surface ocean to the deep ocean is known as the solubility pump. CO₂ reacts and dissolves in water to form Dissolved Inorganic Carbon (DIC), and therefore removing it from the atmosphere. The solubility pump is affected by temperature changes as CO₂ is more soluble in cooler waters such as the Arctic Ocean where the water is cooler, saltier and denser. CO₂ that is dissolved here is then transported to the deep interior of the ocean as this dense water sinks.

Almost all life in the ocean is ultimately driven by the activity of microscopic algae in the uppermost layers. These algae grow whenever local conditions of light and nutrients are sufficient, and they fuel a complex food web spanning the tiny – viruses and bacteria – through to the gargantuan – sharks and whales. Added up, the photosynthetic production by tiny algae in the ocean is almost the same as that produced by all of the plants on land.

In addition, as part of their photosynthetic growth, algae take up dissolved carbon dioxide (CO₂) from seawater. Through the food web, some of this is ultimately transported into the deep ocean by sinking particles known as “marine snow” where a significant fraction is remineralised into DIC. The net effect of this cycling is that the ocean stores more carbon than it would if it was lifeless (although it would still store a lot then). However, the vertical circulation of the ocean is weakened when the surface warms, reducing the physical flux of carbon into the deep ocean. Less growth of algae means less marine snow and decreased ocean CO₂ storage.

However, the relationship between the atmosphere, the ocean – as well as the organisms living there – is a complex one. Depending upon the precise balance of processes that transfer CO₂ between the ocean and the atmosphere, the ocean may become a larger or smaller store for CO₂ in the future, with significant implications for our climate.

Marine snow: tiny particles that sink slowly and transports organic carbon to the deep ocean and ocean floor.