How does climate change affect the ocean?

A glacier in Iceland © MAGNIFIER/ Shutterstock

Climate change warms the ocean, causing knock-on effects such as thermal expansion - which leads to a rise in sea level - and changes in ocean currents. The melting of ice both on land and in the sea also affects the ocean, causing more sea-level rise and reducing the salinity of the ocean, respectively. Greater concentrations of carbon dioxide in the atmosphere also mean that more of it dissolves in the ocean, leading to acidification.

Each of these changes affect marine wildlife. Warming ocean temperatures can lead to coral bleaching - the sudden die-off of large parts of coral reefs - as well as cause animals such as fish to seek cooler waters, shifting their habitats north. This can have a knock-on impact on human communities that rely on those fish for food.

Why is ocean acidification a problem?

Acidification can weaken sea animals' shells and external skeletons. This includes coral exoskeletons - a further reason coral reefs are under threat worldwide.

What happens when sea ice and glaciers melt?

When ice on land - ice sheets and glaciers - melts, it adds water volume to the ocean. This increases sea levels globally, which can inundate low-lying land and important coastal environments such as mangrove forests and wetlands.

When sea ice melts, it doesn't add volume to the ocean, but it does add freshwater, locally decreasing the saltiness of the sea. Saltiness and temperature are the drivers of ocean currents that move heat and nutrients around the world. Melting sea ice and rising temperatures can disrupt these currents, not only affecting wildlife that depends on them but also, potentially, local climates.

For example, the UK is comparatively mild because of the heat brought by an ocean current from the Gulf of Mexico. Melting Arctic sea ice is weakening this current, meaning the UK could face more extreme weather as a result of climate change.

The algae found on sea ice is also essential to many species in the Arctic. In the long term, the loss of sea ice will likely have cascading effects within the food web and impact the coastal ecosystem resources on which Indigenous Peoples rely.

Sea ice decline is also linked to a loss of genetic diversity in polar bears, putting these animals at an increased risk of extinction. 

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