The ocean and cryosphere—a collective name for the frozen parts of the Earth—are essential to the climate and life-giving processes on our planet.
Changes in the ocean and cryosphere occur naturally, but the speed, magnitude, and pervasiveness of the global changes happening right now have not been observed for millennia or longer. Evidence shows that the majority of ocean and cryosphere changes observed in the past few decades are the result of human influences on Earth’s climate.
Every one of us benefits from the role of the ocean and cryosphere in regulating climate and weather. The ocean has absorbed about a third of the carbon dioxide humans have emitted from the burning of fossil fuels since the Industrial Revolution, and the majority (more than 90%) of the extra heat within the Earth system. In this way, the ocean has slowed the warming humans and ecosystems have experienced on land. The reflective surface of snow and ice reduce the amount of the sun’s energy that is absorbed on Earth. This effect diminishes as snow and ice melts, contributing to amplified temperature rise across the Arctic. The ocean and cryosphere also sustain life-giving water resources, by rain and snow that come from the ocean, and by meltwater from snow and glaciers in mountain and polar regions.
Nearly two billion people live near the coast, and around 800 million on land less than 10 m above sea level. The ocean directly supports the food, economies, cultures and well-being of coastal populations (see FAQ 1.2). The livelihoods of many more are tied closely to the ocean through food, trade, and transportation. Fish and shellfish contribute about 17% of the non-grain protein in human diets, and shipping transports at least 80% of international imports and exports. But the ocean also brings hazards to coastal populations and infrastructure, and particularly to low-lying coasts. These populations are increasingly exposed to tropical cyclones, marine heat waves, sea level rise, coastal flooding and saltwater incursion into groundwater resources.
In high mountains and the Arctic, around 700 million people live in close contact with the cryosphere. These people, including many Indigenous Peoples, depend on snow, glaciers and sea ice for their livelihoods, food and water security, travel and transport, and cultures (see FAQ 1.2). They are also exposed to hazards as the cryosphere changes, including flood outbursts, landslides and coastal erosion. Changes in the polar and high mountain regions also have far-reaching consequences for people in other parts of the world (see FAQ 3.1). Warming of the climate system leads to sea level rise. Melt from glaciers and ice sheets is adding to the amount of water in the ocean, and the heat being absorbed by the ocean is causing it to expand and take up more space. Today’s sea level is already about 20 cm higher than in 1900. Sea level will continue to rise for centuries to millennia because the ocean system reacts slowly. Even if global warming were to be halted, it would take centuries or more to halt ice sheet melt and ocean warming.
Enhanced warming in the Arctic and in high mountains is causing rapid surface melt of glaciers and the Greenland ice sheet. Thawing of permafrost is destabilising soils, human infrastructure, and Arctic coasts, and has the potential to release vast quantities of methane and carbon dioxide into the atmosphere that will further exacerbate climate change. Widespread loss of sea ice in the Arctic is opening up new routes for shipping, but at the same time is reducing habitats for key species and affecting the livelihoods of Indigenous cultures. In Antarctica, glacier and ice sheet loss is occurring particularly quickly in places where ice is in direct contact with warm ocean water, further contributing to sea level rise.
Ocean ecosystems are threatened globally by three major climate change-induced stressors: warming, loss of oxygen, and acidification. Marine heat waves are occurring everywhere across the surface ocean, and are becoming more frequent and more intense as the ocean warms. These are causing disease and mass-mortality that put, for example, coral reefs and fish populations at risk. Marine heat waves last much longer than the heat waves experienced on land, and are particularly harmful for organisms that cannot move away from areas of warm water.
Warming of the ocean reduces not only the amount of oxygen it can hold, but also tend to stratify it. As a result, less oxygen is transported to depth, where it is needed to support ocean life. Dissolved carbon dioxide that has been taken up by the ocean reacts with water molecules to increase the acidity of seawater. This makes the water more corrosive for marine organisms that build their shells and structures out of mineral carbonates, such as corals, shellfish and plankton. These climate-change stressors occur alongside other human-driven impacts, such as overfishing, excessive nutrient loads (eutrophication), and plastic pollution. If human impacts on the ocean continue unabated, declines in ocean health and services are projected to cost the global economy $428 billion per year by 2050, and $1.979 trillion per year by 2100.
The speed and intensity of the future risks and impacts from ocean and cryosphere change depend critically on future greenhouse gas emissions. The more these emissions can be curbed, the more the changes in the ocean and cryosphere can be slowed and limited, reducing future risks and impacts. But humankind is also exposed to the effects of changes triggered by past emissions, including sea level rise that will continue for centuries to come. Improving education and using scientific knowledge alongside local knowledge and Indigenous knowledge can support the development of context-specific options that help communities to adapt to inevitable changes and respond to challenges ahead.