The world’s oceans form a primary component of the climate system. They contribute to climate regulation by absorbing carbon dioxide (CO2) emissions and heat. They have absorbed over a quarter of human-caused CO2 and around 90% of the excess heat produced in recent decades. The oceans are by far the largest active carbon reservoir on the planet, storing about 38,000 billion tonnes of carbon. They are also a crucial source of food supply and livelihoods for billions of people.

However, climate change is causing the oceans to warm and become more acidic, which in turn may affect how the oceans absorb and store carbon. This includes the possibility that, as anthropogenic (manmade) CO2 emissions continue, some of the ocean carbon sequestration routes could change from being sink to a source in the future.

Ocean warming, melting ice and sea level rise

The excess heat taken up by the oceans has increased their mean temperature at an average rate of 0.11°C per decade since 1970. Because the oceans redistribute heat towards the poles, this warming is contributing to the melting of ice sheets and glaciers and leading to a rise in the mean sea level – currently estimated at 0.19m between 1901 and 2010. The greatest threat of future sea level rise comes from the possibility that the massive ice sheets in the Antarctic and Greenland could melt. Sea level rise is also caused by thermal expansion, whereby seawater becomes less dense and expands as it warms, and in recent decades this has been one of the major drivers in this change – responsible for over one-third of all sea level rise observed.

An additional problem is that sea ice is gradually melting as the oceans warm. Although sea ice meltdown does not directly impact sea levels, it reduces the amount of bright surfaces on the planet that can reflect sunlight back into the atmosphere (the albedo effect). Hence, more of the sun’s energy is absorbed at the surface – leading to further rises in the oceans’ temperature and a cycle of warming and melting. These changes to sea ice can also contribute to climate change by disrupting normal oceanic circulation patterns, with the potential weakening of the Atlantic Meridional Overturning Circulation (AMOC) being a major example.

If greenhouse gas emissions are reduced rapidly, in line with the Paris Agreement goals, sea level rise will still increase by 29–59cm above 1986–2005 levels by the end of the century. Sea level rise could reach 1m by 2100 compared with 1986–2005 if emissions continue as they are and ice sheets respond to this in the way scientists expect. This would present serious risks to coastal regions around the world. Sea level rise also increases the amount of water that hits the coast during a storm, which can cause more destructive flooding.

Ocean warming, weather and climatic impacts

The increase in ocean temperature is associated with more extreme weather events because with warming there is extra atmospheric energy that produces storms, hurricanes and other tropical cyclones. The warmer oceans and warmer, moister atmosphere make it likely that hurricanes will be more intense and affect areas outside of the typical hurricane zone.

Warmer ocean surface temperatures also affect weather patterns, shifting rainfall and causing some regions to experience flooding while exacerbating drought and wildfire risk in others. The oceans move water around the planet, driving evaporation and precipitation cycles. The warmer surface temperatures increase evaporation, producing more water vapour which feeds rainstorms and blizzards. Research has also linked ‘landfalling droughts’ – droughts that start over oceans and that migrate over months onto land – in the Western United States with weather patterns over the Pacific that are changing in a warming world.

Threats to food security

As the oceans take up CO2 the water becomes more acidic, due to a series of chemical reactions that result in the increased concentration of hydrogen ions. There is a relative reduction in carbonate ions, which are important to building seashells and coral. Coral bleaching occurs, too, in warming water as corals expel algae, placing the coral under stress and affecting the other marine life that depends on this ecosystem. Even if emissions are reduced sufficiently for global warming to be limited to 1.5°C, only 10–30% of all global coral reefs are expected to survive, with the figure falling to 1% under 2°C of warming. Warming is also leading to the poleward migration of species adapted to cooler water temperatures – cod is one example.

Warming and acidification are putting ever greater pressure on ocean fisheries that are already compromised by poor governance, overfishing and destructive fishing practices, and destruction of habitats such as mangroves. This is already threatening food security. Over 3 billion people across the globe, particularly coastal populations in lower-income countries, depend to some extent on ocean fisheries for protein, vitamins and other vital nutrients. In small island developing states, and some West African and Southeast Asian communities, marine fisheries provide over 50% of dietary animal protein in addition to important micronutrients including zinc, iron and omega-3 fatty acids. Warming also leads to an increase in microbial respiration, contributing to the expansion of areas of oxygen depletion in the ocean that eventually threaten the survival of other marine life.

Could the oceans become a carbon source?

Ocean warming disrupts the carbon cycle: for instance, as waters in the high latitudes (close to the poles) warm, the oceans’ solubility power decreases, which reduces the amount of carbon that can be absorbed and eventually sequestered.

As emissions continue to increase, the oceans are likely to sequester increasing quantities of carbon via biogeochemical processes. However, the latest evidence suggests the net production of micro-plants and the quantity that sink from the surface is expected to decrease by the end of the century. If an increased quantity of organic carbon (in the form of sinking particulate organic matter) makes its way to the deep ocean while the production of new micro-plants decreases, it is theoretically possible that this carbon sequestration mechanism could stall, in which case the rate of carbon leaving the deep ocean and returning to the atmosphere would be higher than the rate of carbon reaching it (i.e. its sequestration rate). The biogeochemical route for carbon sequestration would therefore become a source of carbon instead of a sink. However, there is still much uncertainty about these processes, both on how they work and on their response to climate change.

We still understand very little, too, about the ocean’s ‘twilight zone’, which lies 200–1,000m below the surface, where light only penetrates partially. This further limits assessments of the impact of climate change – for instance with respect to the proportion of carbon reaching the deep ocean rather than being released back into the atmosphere, and how this interacts with higher levels in the food chain, such as fish. This is important because the ocean’s twilight zone is home to some of the biggest fishery stocks in the world, and we do not know how fishing in this zone could impact the rest of the oceanic system, in particular the carbon biogeochemistry.

This Explainer was written by Francisco de Melo Viríssimo, Georgina Kyriacou and Elizabeth Robinson.

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