The Sixth Assessment cycle of the Intergovernmental Panel on Climate Change (IPCC) has compiled evidence of worsening climate change impacts in the ocean. To reduce mounting losses, urgent action is required to make rapid, deep cuts in greenhouse gas emissions. In parallel, transformative adaptation is needed. The choices we make now will determine our future.

Ocean and coastal ecosystems support life on Earth and many aspects of human well-being. Covering more than 70% of the planet, the ocean hosts vast biodiversity and modulates the global climate system by regulating cycles of heat, water and elements including carbon. Open and coastal waters and their communities of plants and animals are central to human health, livelihoods and cultures around the globe. They provide half of the oxygen used by all life as well as food, minerals, energy and employment to billions of people. Without a healthy ocean, life on earth and our very human existence are in jeopardy. 

The Sixth Assessment cycle of the Intergovernmental Panel on Climate Change (IPCC) has compiled evidence and increased understanding of the impacts of climate change on ocean coastal and terrestrial systems, of how human communities are experiencing these impacts, and of the potential solutions for ecosystem and human adaptation. This evidence was provided by new laboratory studies, field observations and process studies, a wider range of model simulations, as well as Indigenous knowledge and local knowledge that was considered and assessed in the reports. 
The picture painted by the various sources is more alarming than ever before: Human-made climate change has exposed ocean, coastal and terrestrial ecosystems to conditions that are unprecedented in tens of thousands of years. Climate change affects organisms from microbes to mammals at individual and ecosystem level, in every region. In the ocean, fundamental increases in temperature, acidification and oxygen loss are acting individually and together, reflected in the shifting distribution and abundance of oceanic and coastal organisms and their altered interactions and timing of seasonal activities. For example, when plankton blooms occur earlier than the hatching of fish larvae that feed on the plankton, the fish offspring is doomed to starve and die, leaving fisher families with less to catch. With global warming, more frequent heatwaves on land and in the ocean lasting weeks to several months are exposing species and ecosystems to environmental conditions never experienced, exceeding tolerance and acclimation limits and causing local and regional mass mortalities and extinctions. 

These and other ecosystem responses to climate change are expected to increase with every bit of additional warming. The changes observed reflect that risks will increase sooner and reach higher levels than stated in previous IPCC reports. Marine species richness near the equator and in the Arctic is projected to continue declining, even if warming occurs by less than 2°C at the end of the century. In the deep ocean, global warming regardless of its level, will cause faster shifts of thermal niches by 2100 than those that have driven extensive reorganization of marine biodiversity at the ocean surface over the past 50 years. At warming levels beyond 2°C by 2100, risks of local extinctions, ecosystem collapse and global extinction escalate rapidly.
To reduce mounting losses, urgent action is required to make rapid, deep cuts in greenhouse gas emissions, thereby minimizing global warming levels. In parallel, human adaptation to unavoidable climate change and the changes in marine systems is essential and already setting in. Our reports highlight, however, that available adaptation options are constrained by progressive climate change and are unable to completely offset climate-change impacts on marine and other ecosystems and the services they provide. For future sustainability, transformative adaptation is needed to reduce climate risks successfully – meaning that humans need to systematically and holistically change their treatment of nature, the production and use of energy, the building design of cities, and many industrial processes. This includes establishing fairness in our local communities and on regional, national and international levels as well as changing the politics and decision making towards establishing sustainability and climate resilient development. 

Evidence from implemented adaptation measures indicates that multi-level governance, early-warning systems for climate-associated hazards, seasonal and dynamic forecasts, habitat restoration, ecosystem-based management, climate-adaptive management and sustainable harvesting of natural resources tend to be both feasible and effective in both marine and terrestrial realms. By restoring marine, coastal and terrestrial habitats, we will be able to limit climate-related losses of ecosystem services, including biodiversity, coastal protection, recreational use and tourism. Similar to reforestation efforts on land, restoring mangroves, wetlands, seagrass meadows and saltmarshes provides mitigation benefits and safeguards fish-stock production in a warmer climate. However, the effectiveness of nature-based adaptation options declines progressively with increased warming.

Healthy ecosystems including a healthy ocean are fundamental building blocks of the sustainable and climate resilient future that our reports are envisioning. Moreover, to tackle the many global challenges we face today, we have to treat climate, biodiversity and human society as three interconnected systems. Changes to one of them evoke reactions from the other two. At the same time, their interdependencies determine our solution space. 
The cumulative scientific evidence is unequivocal: Climate change is a threat to human wellbeing and the health of ecosystems and the planet. Limiting global warming close to 1.5°C, through ambitious and timely emissions reductions, is a prerequisite for a healthy future. Under those conditions, we can secure rich biodiversity and healthy ecosystems. In the long run, the ocean just like terrestrial ecosystems can help us regulate the climate and provide nutritious food for billions of people, making them less vulnerable to climate risks. However, any further delay in concerted global action on climate will miss a brief and rapidly closing window to secure a liveable future for all.
Accordingly, the choices we make now will determine our future. Climate Resilient Development is already challenging at current global warming levels. It will no longer be possible in some regions, including small islands and low-lying coastal cities and settlements, if warming exceeds 2°C. The IPCC’s Sixth Assessment Reports offer solutions, but we are running out of time to implement them. 

*Figure SPM.3: Panel (a) IPCC, 2023: Summary for Policymakers. In: Climate Change 2023: Synthesis Report. Contribution of Working Groups I, II and III to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, H. Lee and J. Romero (eds.)]. IPCC, Geneva, Switzerland, pp. 1-34, doi: 10.59327/IPCC/AR6-9789291691647.001