Important CO2 sink

The oceans play a vital role in our planet’s heat balance. Oceans are capable of storing a large part of the carbon dioxide that is emitted annually. In the oceans, CO2 is dissolved, distributed, and stored in the depths of the seas.

According to data from the IPCC (Intergovernmental Panel on Climate Change), which – as a United Nations institution – is often referred to as the “World Climate Council”, the pace of ocean warming and thus heat uptake has more than doubled since 1993.

The oceans can absorb roughly 50 times more carbon than the atmosphere and 12 times more than all terrestrial plants and land together. By binding huge amounts of carbon dioxide from the atmosphere, the oceans slow down anthropogenic global warming and act as a buffer. Some 90 billion tonnes of carbon are exchanged between the elements air and water annually.

Some ocean ecosystems are particularly efficient in absorbing and storing CO2 in the ground. This so-called “blue carbon”, which is accumulated in coastal and ocean ecosystems, involves coastal habitats, such as mangroves, tidal salt marshes, and seagrass meadows. According to the IPCC, over the last 100 years almost 50% of all coastal wetlands have been lost due to human interventions, rising sea levels, warming, and extreme climate events.

Acidification

The growing accumulation of CO2 in seawaters also has negative effects however, as the seawater acidifies, i.e. the pH level declines. The average pH level of seawater is 8.2 and is thus mildly alkaline. Over the past 200 years, this value has fallen to 8.1. Since pH is a logarithmic scale, this is equivalent to a decline of almost 30%. In the future, the pH value of the oceans may decline by another 0.3 to 0.4 units by the year 2100, as a result of which seawaters would acidify by another 100% to 150%. An increase in the pH value of seawater is especially detrimental for marine organisms, such as corals, mussels, and crabs. Furthermore, increasing acidification means that seawater can absorb less and less anthropogenic CO2 emissions.

Heat uptake

Global oceans are exposed to increasingly significant stresses due to anthropogenic changes and are less and less able to mitigate climate change. According to the IPCC, the oceans can absorb around 90% of the heat generated on the Earth by solar radiation. Thus, the oceans not only protect the planet from higher airborne CO2 concentrations, they also protect the Earth from higher temperatures.

Climate change impairs the effect of the world's oceans as climate stabilisers

• According to the IPCC, since 1970 the oceans have absorbed more than 90% of the additional (anthropogenic) heat from the Earth’s atmosphere, growing steadily warmer in the process.

• In the North Atlantic, surface water temperature increased from approximately 22.9 C to more than 24°C during the period 1982–2011. The IPCC also states that between 2006 and 2015 the related rise in average global sea levels amounted to 3.6 mm annually.

• During the period from 1997–2006 to 2007–2016, the pace of ice melting in the Greenland and Antarctic ice sheets accelerated by a factor of three, as a result of which the rise in sea levels has also sped up. However, the rise in sea levels is not uniform at the global level and varies from region to region.

The oceans are also a key factor in the global oxygen balance

Algae and many marine microorganisms produce around 70% of total atmospheric oxygen.

Due to climate change, the exchange of gases and nutrients is changing. Rising temperatures mean lower oxygen solubility in seawater, and less oxygen is absorbed by the upper levels of surface waters. Partially due to the inflows of freshwater in higher latitudes, warm surface waters exhibit increasingly low densities compared to the deeper parts of the ocean, leading to reduced circulation between individual seawater layers.

According to the IPCC, in a period of 40 years since 1970 the oxygen level in the upper 1,000 m of the open oceans declined by 1.9%. Parts of the ocean that are affected by a lack of oxygen are called “oxygen minimum zones”. Marine organisms that need oxygen can no longer live in these areas. These zones have expanded significantly in recent years.

At the surface, the oceans integrate with the atmosphere, and there are strong interactions between these two “climate actors”. These interactions include coupled phenomena with self-reinforcing mechanisms. One of the known phenomenon in this regard is El Niño, the El Niño–Southern Oscillation (ENSO).

Marine circulation

Global marine currents play a major role in the global heat and CO2 balance. The circulation of ocean water moves waters enriched with heat and CO2 around the planet on the one hand and to great ocean depths on the other.

As a result of this, CO2 dissolved in surface water is transported by marine currents and mixing processes to the depths of the ocean, where it can accumulate over time. This is referred to as a “physical carbon pump”.

On the other hand, CO2 is also stored by the development of marine plant and animal biomass. A large number of these organisms live in the uppermost water layers and upon dying they transport the stored carbon into the depths, in a process known as the “biological carbon pump”.

Functioning like a massive conveyor, large-scale marine currents have long since been responsible for comfortably warm and mild temperatures in higher latitudes. That said, it takes centuries for the global conveyor to mix all of the water in the oceans. Consequently, shorter-term developments on the ocean surface, such as increased absorption of CO2 or heat, only exert an effect on the oceans as a whole after a long delay. Nevertheless, the IPCC believes that the Atlantic meridional overturning circulation, commonly known as the Gulf Stream, has weakened in recent years compared to pre-industrial times.

Marine warming and the related thermal expansion of the water is also causing a continuous rise in sea levels. With temperatures also rising, there is a risk of massive melting of global ice packs. This in turn opens up the possibility that the related freshwater input into the oceans could change marine circulation patterns overall and thus influence weather developments. The reaction of the oceans to the rising temperatures on the one hand and the increase in greenhouse gas emissions on the other hand occurs very slowly. Thus, if the marine ecosystem is knocked out of balance, it is possible that the negative consequences may only become visible decades later.

Over the last twelve months, new record-setting temperatures were seen every month for the planet’s oceans. The previous pattern of the sea temperature reaching a high point in March and then cooling down significantly by June also failed to materialise.

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Round table discussion on ocean warming, the possible consequences, and responsible action