Oceans under climate change
WOR 7 The Ocean, Guarantor of Life – Sustainable Use, Effective Protection | 2021

Oceans under climate change

Oceans under climate change
> The oceans provide an invaluable service to humankind: They regulate climate and curb global warming by absorbing much of the heat that is trapped in the Earth System due to anthropogenic greenhouse gas emissions. This, however, also sets large-scale chain reactions in motion. On the one hand, water temperatures and sea levels rise. On the other, the physics and chemistry of the oceans are altered so dramatically that marine life is thrown out of sync.
The fatal consequences of heat fig. 2.9: Jes Aznar

The fatal consequences of heat

> A great tragedy is playing out in the world’s oceans. As humankind continues to release more greenhouse gases into the atmosphere every year, resulting in one high-temperature record after another, the oceans are countering the otherwise disastrous warming. They are absorbing more than 90 per cent of the excess heat and are storing it at increasingly greater depths. There is a high price for this service to the climate. The oceans themselves are warming! They are expanding and, in the process, losing their most valuable elixir of life – oxygen.

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Biodiversity under assault fig. 2.26: © Rich Reid Photo

Biodiversity under assault

> Climate-induced changes in the ocean are now affecting marine biological communities at all levels. As a result, many marine creatures are being forced to abandon their ­traditional territories. Predator-prey relationships are changing and ocean productivity is falling. More­over, the impacts of climate change are reinforcing each other and weakening the resistance of marine species to other anthropogenic stress factors. There is no longer any question that climate change is one of the driving forces behind the extinction of marine species.

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Ocean barometer

The world’s attention is currently focussed on the temperature trend in the global ocean. As long as it continues to rise, global warming will progress un­impeded. This knowledge is based on the fact that the ocean is the most effective heat-storage component in the Earth’s climate system. Since the 1970s, the ­oceans have absorbed more than 90 per cent of the heat energy trapped by anthropogenic greenhouse gas emissions and stored it at increasingly ­greater depths. The oceans have thus helped to significantly slow the rise in global surface temperatures, and to delay drastic changes in the Earth’s climate system. In the future, the ocean will also act as a ­gauge and monitoring service. Until water temperatures stop rising, or possibly even begin to drop, humankind will not be able to speak of any true progress in the struggle against climate change.
The warming of the seas and oceans is producing numerous dramatic changes. The water masses are expanding, causing sea level to rise and threatening millions of residents, especially in tropical coastal areas. At the same time, the ocean is losing oxygen, the elixir of life, because warmer water cannot store as much gas as cold water. Ocean currents are losing strength and wind-driven mixing is wea­kening because of the stronger stratification related to temperature differences. Extreme events like ­marine heatwaves are occurring more frequently. Furthermore, the chemistry of the ocean is changing. Since the onset of industrialization, the oceans have absorbed around a quarter of the anthropogenic carbon dioxide emissions. As a result, the pH value of the ocean has fallen and seawater has become more ­acidic, causing living conditions to deteriorate, especially for marine organisms with calcareous shells and skeletons.
The fatal aspect of the effects of climate change on the ocean is that they not only act to amplify their effects through feedback mechanisms, they also ­weaken the resistance of biological communities to other human interventions such as fishing, resource mining and pollution.
In response to climate stress, most animal and plant species are abandoning their traditional habitats and pursuing their necessary environmental conditions. This means that they either move poleward or they migrate down into deeper and colder water layers, if that option is available.
The major losers in this species migration driven by climate change are the cold-loving animal and plant species, because they have no other place to retreat to; organisms with calcareous shells or skeletons that are dissolved by acidification; sessile organisms like corals, whose dispersal mechanisms are too slow for them to escape the heat; and highly active predatory fish that cannot obtain the high levels of oxygen they need for respiration in the oxygen-poor water layers.
These few impressive examples suffice to illustrate that climate change is already changing the ­species structure in the oceans on a large scale. Not only is biodiversity declining, but total biomass production is decreasing as well. Marine ecosystems are losing their ability to perform the many ecosystem services utilised by humans. And with regard to the diversity of marine life, climate change is becoming the most powerful driver of species extinction, and in this respect represents an enormous challenge for sustainable ocean management.
It is crucial that, beginning immediately, the short- and long-term consequences of ocean warming, acidification and oxygen depletion are taken into account in every decision related to the use of the oceans.