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.
Read more about the role of Ocean biogeochemistry in UKESM1
Andrew is a marine systems modeller in UKESM, member of the National Oceanography Centre (NOC) based in Southampton. Andrew’s interests are on: global-scale plankton ecosystem modelling, biogeochemistry of the ocean's carbon cycle, the role of nitrification in the ocean's nitrogen cycle, geoengineering, and diatom ecology and the ocean's silicon cycle.
Lee is a marine ecosystem modeller with an interest in the development, evaluation and validation of global scale models. Lee leads the marine model evaluation toolkit development for the UK Earth System Model.
As a member of the UK Ocean Biogeochemistry Network Model Inter-comparison Experiment, iMarNet, Lee coupled the ERSEM model to NEMO in the global settings to allow the comparison of UK biogeochemical models coupled with the same underlying physics. Lee also used this model to investigate the use of emergent properties as a method to evaluate the performance of marine ecosystem models.
Outside of model development, Lee has also contributed to a range of shorter projects, including studies on the impact of leaks in carbon capture and storage sites in the North Sea, and a pilot study which pioneers the use of surfers as a platform for collecting data in the near shore environment.
UKESM: The UK Earth System Modelling project is jointly funded by the UKRI – Natural Environment Research Council (NERC) national capability grant number NE/N017951/1 and the Met Office. Copyright 2017.
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