Ocean Biogeochemistry

The ocean is part of the climate system

Although we might think of the climate as being to do with the atmosphere, the ocean is responsible for much of the storage and transport of heat so plays a key part in the whole climate system. From a local perspective, the climate of the UK and western Europe is relatively mild compared to that of North America at the same latitude because of heat transported from the tropical Atlantic by ocean currents. Evaporation of this warm tropical seawater also provides the rainfall that helps make the land in our part of the Earth productive for farming.

These movements of heat and freshwater in the surface ocean affect its temperature and salinity, and in turn lead to geographical differences in the density of seawater. These large-scale differences help drive what is known as the “thermohaline circulation”, which moves water from the ocean’s surface around its deep interior and back again. This circulation makes the ocean both a good store for heat in the Earth system and helps it play a role modulating the climate on seasonal, annual and longer timescales of decades to centuries.

This storage of heat by the ocean also helps offset extra atmospheric warming by the carbon dioxide (CO2) that human activities have put there, protecting us somewhat from more extreme climate change.

The ocean is a carbon reservoir

The atmosphere is where carbon dioxide (CO2) changes the climate, but the atmosphere only holds about 2% of the CO2 that’s actively circulated in the Earth system. By contrast, the ocean holds about 93%, primarily as dissolved CO2 that occurs in the form of bicarbonate (HCO3) and carbonate (CO32-) ions. Most of this carbon has dissolved there from the surface and is transported into the deep ocean by the ocean’s large-scale circulation. But a significant amount of CO2 is there because of algae growing in the surface ocean. This growth locks-up CO2 in the bodies of the algae, and the animals that eat them, so that when they die and sink the CO2 is transferred into the deep ocean via what’s known as the “biological pump”.

At the moment, about a quarter of our CO2 emissions are getting absorbed by the ocean, and this will be where the majority of it will eventually end up on extremely long timescales (>> 1000 years). While this absorption of our CO2 by the ocean stops it from contributing to climate change, it does affect seawater chemistry, making the ocean more acidic with consequences that are still being understood.

Figure reproduced from Khatiwala et al., doi: 10.1038/nature08526, 2009

The graph above shows several different estimates (black lines and blue crosses) of the rate at which the ocean has taken up CO2 from the atmosphere since the start of the Industrial Revolution (from around 1765).

Until about 1950, the rate of uptake grew steadily, but the rate has increased significantly since then, tracking the rise in the concentration of CO2 in the atmosphere (red line; using the right-hand y-axis). What happens in the future will depend on how well societies manage to reduce emissions and move to technologies that avoid CO2 release.

The ocean’s ecosystems

Almost all life in marine ecosystems is dependent on the growth of microscopic algae that live in the sunlit surface waters of the ocean. These algae are eaten by similarly microscopic animals, zooplankton, which in turn are eaten by slightly larger zooplankton, and so on, up to familiar large animals like fish, sharks and whales. But this doesn’t happen everywhere – like the land, the ocean is diverse, with constantly productive tropical waters, seasonally productive polar ones, through to the mid-ocean “deserts” known as “oligotrophic gyres”.

What controls these patterns is the availability of light and nutrients for growth. Tropical regions have good light all year, while polar regions are highly seasonal, with long “polar nights” that last for months during winter. Meanwhile, surface nutrients are controlled by physical processes like upwelling or mixing, which can bring them up to the surface from deeper waters where their concentrations are higher. But where light and nutrients are plentiful, algae and everything that depends on them, are also plentiful, giving us rich fishing grounds that many societies depend on.

Algal productivity at the surface also ultimately supports life deeper in the ocean as the bodies and wastes of surface dwellers sinks into the deep ocean as so-called “marine snow”. Similar to the surface, this material supports a broad range of organisms that either feed directly on this sinking material, or indirectly on those organisms which do. This  creates a diverse ecology with many unique adaptations to cope with life in the dark, cold and relatively food-poor deep ocean. These deep organisms play an important role in the ocean’s biogeochemical cycles, returning carbon and nutrients to dissolved forms, and ultimately influencing their concentrations when deep seawaters are mixed or upwelled back to the surface.

The ocean’s biological pump (A. Yool)

The ocean and climate change

The ocean is playing an important role in climate change, absorbing much of the CO2 that we are adding to the atmosphere, as well as much of the excess climate heat that it causes there. However, in doing so, the ocean is also changing, and these changes have many important consequences. Most obviously, as the climate has warmed, the Arctic Sea has steadily been losing its “permanent” cap of sea-ice. As well as being a response to warming, this also drives further warming, as the loss of sea-ice decreases the reflectivity of the Earth (its “albedo”) allowing it to warm even faster.

This warming-up also reduces the density of seawater, affecting the ocean’s circulation and mixing, which will likely slow the ocean’s uptake of both CO2 and heat from the atmosphere, again allowing the Earth to warm faster. An ocean with less mixing will also struggle to replenish the nutrients that algae are dependent on, so will potentially make more of the ocean like a “desert”, with particular consequences for fisheries and healthy marine life.