Abstract
The ocean plays a pivotal role in Earth’s climate system, absorbing most of the excess heat and a substantial fraction of human-emitted CO₂. Recent data confirm accelerated ocean warming, rising sea levels, and widespread ecosystem stress. For example, the global ocean reached a record surface temperature of about 21 °C in spring 2024 , and record high ocean heat content was reported in 2024 . Sea levels have risen by over 22 cm since 1900 and are currently rising at ~3–4 mm/yr on average . These changes intensify extreme events: marine heatwaves are now ~20 times more frequent than in the 1980s , fueling coral bleaching and extreme weather. Marine life is under multiple stresses: over 37% of global fish stocks are overfished , key species (e.g. sharks) have plunged 70+% since the 1970s , and ~1–2 million tonnes of plastic enter the oceans every year . These warning signals have prompted new policies: the UN’s 2023 High Seas Treaty commits to protecting international waters , and the EU now targets 30% marine protection by 2030 (10% no-take areas) . Nevertheless, as the ocean warms and ecosystems degrade, urgent action is needed to mitigate climate change and preserve marine resilience.
Introduction
The Earth has warmed sharply over recent decades, driven by greenhouse gas emissions. In 2024 the global land–ocean temperature reached ~1.3 °C above the 20th-century average (≈1.46 °C above 1850–1900 levels), making it the warmest year on record . The ocean has buffered much of this warming: it has absorbed about 90% of the excess heat from human emissions and roughly 25% of emitted CO₂ . This immense capacity moderates atmospheric warming but also leads to rapid changes in the ocean itself. Since the turn of the 21st century the upper 2000 m of the ocean have warmed steadily, setting new heat-content records in recent years . As the ocean warms, water expands and ice melts, driving sea-level rise; global mean sea level has climbed over 20 cm in the past century . These shifts in turn affect weather patterns and ecosystems worldwide. Classical sources (e.g. the 2003 report Warnsignale aus dem Meer) warned of such trends; this update reviews the latest findings (as of late 2025) on ocean warming, sea level, extreme events, marine life impacts, and the policies developed in response.
The Ocean and Climate Regulation
The ocean is Earth’s largest heat and carbon reservoir, crucially regulating climate. Covering ~71% of the surface, its vast heat capacity means it absorbs the great majority of warming. Climate scientists note that “the oceans store 90% of the excess heat in the Earth system” . Similarly, about 25–30% of anthropogenic CO₂ is taken up by the ocean . This uptake slows atmospheric warming but increases ocean acidity and stratification. Ocean circulation (e.g. the Atlantic Meridional Overturning Circulation, including the Gulf Stream) redistributes heat globally, moderating regional climates such as Western Europe’s. Biological processes also regulate climate: phytoplankton photosynthesis and the marine carbon pump transport carbon to the deep sea, where it can be stored for centuries. Coastal ecosystems (mangroves, seagrasses, salt marshes) sequester carbon (“blue carbon”) and buffer storms. In short, the ocean acts as a giant thermal and chemical buffer. Without it, land temperatures would rise much faster. For example, EPA highlights that if not for the ocean’s heat uptake, atmospheric warming from greenhouse gases would be far more extreme (not shown here). The ocean also moderates weather (through evaporation and heat release) and influences patterns like monsoons and El Niño. In summary, by absorbing heat and CO₂ the ocean has slowed climate change – but this has come at the cost of rapid changes in the ocean itself .
Ocean Warming and Sea Level Rise
Recent years have brought record ocean temperatures and accelerating sea-level rise. Satellite and in-situ observations show the upper ocean warming strongly: global average sea surface temperature (SST) has risen ~0.88 °C since the late 19th century . Regional trends vary: Copernicus reports that the Northeastern Atlantic has warmed at +0.27 °C per decade since 1982, about double the global rate . In spring 2024 the global ocean surface hit a record ~21 °C , and recent IPCC assessments find the upper 2000 m of the ocean at unprecedented heat content . As heat is absorbed, seawater expands; since 1971 the ocean has absorbed ~50% of observed sea-level rise, with the remainder from melting glaciers and ice sheets . Satellite altimetry (Figure 1) now measures global mean sea level rising by roughly 3.3 mm/year (±0.3) averaged over 1993–2023 . Crucially, this rate is accelerating: during the 1990s it was ~2.1 mm/yr, doubling to ~4.5 mm/yr by 2024 . Overall, sea level has risen about 11.1 cm since 1993 , and 22.8 cm since 1900 . The IPCC confirms that human activities (thermal expansion + melt) are the dominant cause, and projects continued rise for centuries even if emissions stabilize .
Statistics and Trends
- Ocean Heat Content: The top 2000 m warmed steadily since 1970; NOAA notes record high heat content in 2024 .
- Sea Surface Temperature: Global mean SST +0.88 °C (1850–1900 to 2011–2020) .
- Global Warming: The decade 2015–2024 had 9 of the 10 warmest years on record. In 2024 the surface temperature was ~1.46 °C above pre-industrial .
- Sea Level: +3.3 mm/yr (1993–2023; NASA) , ~+4 mm/yr (2006–2018; IPCC) . Total rise ~22–23 cm since 1900 .
- Acceleration: Current SLR rate is ~30% higher than in the 1990s , and models show rise continuing even if warming is limited .
Impacts and Examples
Thermal expansion and melting ice amplify coastal flooding and erosion. For example, coastal cities already experience higher tides and more frequent “nuisance” floods. A 30-cm rise can turn a 1-in-100-year storm surge into a much more frequent event . Regions like the western US and Pacific islands face severe risks. Polar and high-altitude regions see amplified effects: as Greenland and Antarctic ice sheets lose mass, sea level responds globally . Mountain glaciers shrink, reducing summer river flows downstream. The warming ocean also degrades coral reefs through bleaching and fuels tropical storms (addressed below). In sum, current data paint a clear picture: the ocean is warming and expanding at unprecedented rates, serving as a sentinel of ongoing climate change .
Extreme Events and Ocean Circulation Disruption
Warming oceans intensify extreme weather and can disrupt major currents. Marine heatwaves – prolonged anomalous ocean warmth – have surged in frequency and severity. IPCC reports that since the early 1980s these events are now “more than twenty-fold” more frequent due to anthropogenic warming . Notably, the Great Barrier Reef experienced three catastrophic bleaching episodes in just five years (2016, 2017, 2020) after 35 years with none . Widespread bleaching and coral die-offs illustrate how even small temperature rises (1–2 °C) during heatwaves can collapse reef ecosystems. Other marine systems show damage: kelp forests in California have declined due to recurrent heatwaves, and cold-water species in Arctic seas face shrinking habitat.
On weather, a record-warm ocean surface adds energy to the atmosphere. For instance, NOAA reported 85 named tropical storms globally in 2024 (42 typhoons, 23 hurricanes), including 4 Category-5 cyclones – well above the long-term average . The North Atlantic saw 18 storms (11 became hurricanes, 5 major) in 2024 , related to elevated ocean heat content. Warm oceans also contribute to heavier rainfall in cyclones and extended droughts elsewhere. While the total number of storms may not rise dramatically, theory and observations suggest the strongest storms will become more frequent and intense, as warmer water provides more evaporation and fuel.
Global warming can also alter ocean circulation. The Atlantic Meridional Overturning Circulation (AMOC, which includes the Gulf Stream) is projected to slow. IPCC AR6 states a full collapse this century is “very unlikely” under current scenarios , but models agree the AMOC will weaken, reducing northward heat transport. Evidence hints at recent slowdown in the North Atlantic, though data are limited . Even modest weakening could cool parts of Northern Europe and shift storm tracks, while contributing to regional sea-level changes. Other gyres and upwelling systems will also shift: for example, models project a weakening of equatorial Pacific trade winds and altering of the Walker Circulation , which can influence El Niño patterns. In summary, the ocean’s changing circulation is itself a potential feedback on climate, and one more reason to monitor marine changes closely.
Marine Biodiversity Under Climate Stress
Marine life is under unprecedented stress from combined climate and human pressures. Heat stress, acidification, and deoxygenation – all consequences of warming – are altering habitats. Coral reefs have fared worst: rising temperatures cause bleaching and mortality. As noted, Australia’s Great Barrier Reef saw three back-to-back mass bleachings within five years . Globally, entire reef systems (Caribbean, Pacific, Indian Ocean) suffer repeated events that threaten collapse. Warming is also shifting species’ ranges: many fish and plankton species are moving poleward or to deeper, cooler waters. A NOAA indicator shows widespread northward shifts of coastal species due to rising sea temperatures (e.g. Atlantic cod moving north) . These shifts disrupt fisheries and ecosystems; some tropical fish now appear in temperate zones.
Ocean acidification – caused by CO₂ dissolving in seawater – lowers pH and erodes shells. Ocean pH has already dropped from ~8.2 to 8.1 (a ~25% increase in acidity since 1750). IPCC notes the ocean will continue acidifying under all scenarios . Critically, Copernicus finds that over 10% of marine biodiversity “hotspots” are acidifying faster than average, threatening organisms from corals to plankton . Recent assessments warn that a large fraction of endangered corals (over 15%) and most critically endangered corals (≈30%) are simultaneously hit by warming and acidification . Shell-building species (oysters, mussels, pteropods) are similarly at risk, which can ripple up the food chain.
Human exploitation compounds climate stress. Overfishing has depleted stocks worldwide: 37.7% of marine fish stocks are now overfished , down from 10% in the 1970s. Predatory fish (sharks and rays) have suffered ~71% declines since the 1970s, mainly due to fishing . As big fish vanish, ecosystems become unbalanced (e.g. trophic cascades). Bycatch and habitat damage from trawling further reduce biodiversity. Pollution is another major threat: about 1–2 million tonnes of plastic are dumped into the oceans annually . Marine animals ingest or entangle in plastic, which also transports toxic chemicals and invasive species. Nutrient runoff from agriculture causes coastal “dead zones” (hypoxic waters), killing local life. In sum, climate change is a force-multiplier on existing impacts. A marine environment already stressed by overfishing and pollution is now less resilient to warming, leading to biodiversity loss and ecosystem shifts.
Real-world example: The unprecedented 2023 heatwave in the North Atlantic Tropical Ocean affected over 99% of the region simultaneously. It lasted hundreds of days in places, devastating plankton blooms and fish stocks. Coral reefs in the Eastern Pacific and Central Atlantic have also recently suffered bleaching events that scientists directly attribute to human-driven warming. These events illustrate that the frequency and magnitude of marine extremes have escalated dramatically .
Policy and Protection Measures
Governments and organizations have begun responding with new policies and agreements. Internationally, the United Nations Sustainable Development Goal 14 (“Life Below Water”) sets targets to protect the ocean. For example, SDG14 calls for preventing marine pollution and conserving at least 10% of marine areas (initially by 2020) . It also mandates ending harmful fisheries subsidies by 2020 . In 2023 the UN concluded a landmark High Seas Treaty, formally adopted in June, to create marine protected areas in international waters . Once ratified by enough countries (60 required), this treaty will fill a major governance gap beyond national jurisdictions.
At the EU level, the European Green Deal and Biodiversity Strategy for 2030 contain ambitious ocean measures. The EU has pledged to protect 30% of its marine and coastal areas by 2030, with one-third of that in no-take reserves . (In practice, only ~2% of EU seas currently have fully effective protections , so implementation is urgent.) The EU’s Marine Strategy Framework Directive mandates “Good Environmental Status” for European seas, including targets on biodiversity, pollution, and fish stocks. The Common Fisheries Policy aims to end overfishing and IUU (illegal, unreported) fishing, with rules on catch limits and discards. The EU also regulates maritime pollution (e.g. ship emissions and plastic) under the Marine Pollution Convention (MARPOL). In mid-2023, the European Commission launched an Ocean Action Plan and an Ocean Pact to coordinate marine protection with climate goals.
Regionally, conventions like OSPAR (North-East Atlantic) and HELCOM (Baltic) set shared objectives. For instance, OSPAR’s 2020 strategy requires addressing climate change and biodiversity together. Many countries have declared targets (e.g. 30% by 2030 under 30×30 commitments). Global stock-take reports (IPCC, IPBES) and initiatives like the Global Ocean Alliance continue to pressure action.
Overall, the policy landscape is richer than 20 years ago, reflecting the ocean’s recognized role in climate. However, there remain large gaps between goals and implementation. Recent analyses note that despite high-level targets, many MPAs are “paper parks” lacking enforcement . Integration of climate and ocean policy is still evolving: fora like COP28 and the UN Ocean Conference (2025) are starting to focus more on ocean issues. In summary, governments have pledged significant protections (e.g. SDG14 targets, EU 30% pledge, UN high-seas treaty ), but turning those pledges into robust, enforced action is the critical next step.
Conclusion
The ocean is both a victim and a regulator of climate change. It has mitigated global warming by absorbing heat and carbon, but it is now clearly showing warning signals of anthropogenic stress. Satellite and observational data confirm record ocean temperatures, accelerating sea levels, and unprecedented marine extremes. Marine ecosystems are under siege from warming, acidification, overfishing, and pollution. These changes threaten vital services – climate regulation, fisheries, coastal protection – that human societies depend on. Encouragingly, international science (IPCC, Copernicus) and policy actors (UN, EU, regional bodies) are mobilizing to address these threats. Effective solutions will require rapid reductions in CO₂ and other emissions, strong protection of marine areas, sustainable fisheries management, and pollution controls. The latest findings make it clear: the ocean’s health is integral to the climate we live in, and protecting it is essential to ensuring a habitable planet. As John F. Kennedy once said of the Moon mission, meeting this challenge will require “boldness, imagination, and diligence.” The time to act on the warning signals from the sea is now.
References
- NOAA National Centers for Environmental Information. Assessing the Global Climate in 2024. (Data cited: global surface temp anomalies) .
- EPA. Climate Change Indicators: Ocean Heat Content. U.S. Environmental Protection Agency (2024) .
- IPCC AR6 WGI Chapter 9, Ocean, Cryosphere and Sea Level Change. (2021) – Key findings on sea surface warming and ocean heat content .
- NOAA Ocean & Atmospheric Administration. Ocean-Atmosphere CO₂ Exchange. Science on a Sphere (NOAA) .
- NASA JPL PO.DAAC. Data in Action: Sea Level Rise. (2025) – Global mean sea level trends via satellite altimetry .
- IPCC AR6 WGII Chapter 3, Oceans and Coastal Ecosystems. (2022) – Marine heatwave and coral impacts .
- Marine Stewardship Council (MSC). Biodiversity and Fishing. (2021) – Overfishing and shark population data .
- Ritchie, H., et al. Plastic Pollution. Our World in Data (2024) – Global estimates of plastic entering oceans .
- High Seas Alliance. High Seas Treaty (BBNJ Agreement). (2023) – UN agreement on high seas biodiversity protection .
- WWF Europe. Protecting and restoring our seas: Europe’s challenge to meet 2030 targets. (2025) – EU marine protection statistics .
- United Nations – Sustainable Development Goals. Goal 14: Life Below Water (SDG 14 targets) .
- von Schuckmann, K., et al. Copernicus Ocean State Report 9: Summary. (2025) – Key ocean climate indicators for 2023–2024 .
