Ocean and cryosphere in a changing climate

Over recent decades, global warming has led to significant changes in the cryosphere, with the loss of ice mass from glaciers and ice sheets, reductions in snow cover, and thinning Arctic Sea ice. Permafrost temperatures have also risen. The global ocean has warmed steadily since 1970, absorbing over 90% of excess heat, with the rate of ocean warming accelerating since 1993. Marine heatwaves have increased in frequency and intensity, and the ocean’s surface acidification has worsened due to higher CO2 absorption. Oxygen levels in the ocean have declined from the surface to 1,000 meters.

Global mean sea level (GMSL) has risen, with accelerated ice loss from Greenland and Antarctica contributing to this trend. Tropical cyclones, extreme waves, and rising sea levels have worsened coastal hazards. Changes in the cryosphere have impacted terrestrial and freshwater ecosystems, particularly in polar and mountain regions, leading to shifts in species distribution and seasonal behaviours. This has disrupted ecosystems and their functioning.

Marine species have also shifted their geographical ranges and seasonal activities in response to ocean warming, sea ice loss, and other biogeochemical changes. These shifts have affected ecosystem structure and species interactions, sometimes exacerbated by fishing pressures. Coastal ecosystems have faced impacts from ocean warming, acidification, oxygen loss, and sea level rise, which, along with human activities, harm biodiversity and ecosystem services.

The shrinking cryosphere has negatively affected food security, water resources, infrastructure, and the well-being of human societies, especially Indigenous peoples. Adaptation efforts have often incorporated local and Indigenous knowledge. Changes in ocean ecosystems have had diverse regional impacts on food security, local cultures, and tourism. Coastal communities are particularly vulnerable to multiple climate-related hazards, and global responses have mainly focused on addressing these after extreme events or anticipating future risks.

Climate-related changes in the ocean and cryosphere are increasingly challenging existing governance efforts to develop effective adaptation strategies, often pushing them to their limits. Vulnerable communities, especially those with the least capacity to respond, are disproportionately affected. Protecting, restoring, and managing ocean and cryosphere ecosystems through ecosystem-based approaches can help reduce climate risks and provide societal benefits. However, financial, institutional, and ecological constraints limit the effectiveness of these approaches, especially under higher levels of warming.

Coastal communities face difficult decisions regarding responses to sea level rise, weighing the costs, benefits, and trade-offs of various adaptation options, including protection, accommodation, and managed retreat. Integrated, context-specific solutions will be crucial.

Effective climate resilience requires urgent emissions reductions and ambitious, coordinated adaptation actions across scales. Key enablers include stronger cooperation, climate literacy, data sharing, finance, and addressing social vulnerability, all of which support capacity-building, equitable adaptation, and long-term sustainability.