Atmospheric carbon dioxide, methane, and nitrous oxide all reached their highest recorded concentrations during the year. The globally averaged carbon dioxide concentration rose to 425.6 parts per million, with an uncertainty of approximately 0.1 parts per million. That level is about 53% higher than the estimated preindustrial concentration of 278 parts per million. Fossil fuel emissions also reached a new record, estimated at 10.3 petagrams of carbon per year, more than three times the annual level recorded during the 1960s. These gases trap outgoing infrared radiation, increasing the energy retained within the atmosphere, oceans, land, and ice.

Global temperatures remained exceptionally high even though the tropical Pacific did not experience a conventional El Niño event. The average temperature across Earth’s land and ocean surfaces ranked among the three warmest in records extending back to the mid-1800s. All five major global temperature datasets examined by the report agreed that the period from 2015 through 2025 contained the 11 warmest years ever measured. Europe experienced its warmest year on record, while Russia, China, the Republic of Korea, and Argentina each recorded their second-warmest year. The result makes 2025 the warmest year observed without El Niño conditions dominating the Pacific.

Sea surface temperatures were similarly unusual. Although cool ENSO conditions and weak La Niña patterns affected the equatorial Pacific during parts of the year, the global annual sea surface temperature was the third highest in the 172-year observational record. It stood approximately 0.38 degrees Celsius above the 1991–2020 average. Marine heat waves affected about 87% of the world’s ocean surface at least once during 2025, while only 26% experienced a marine cold spell. Such heat extremes can disrupt fisheries, damage coral reefs, alter marine food webs, and intensify evaporation, providing additional moisture and energy for some storms.

The ocean continued to act as Earth’s largest absorber of excess heat. Over roughly the past 50 years, the oceans have taken up nearly 90% of the additional energy trapped by greenhouse gases and other climate influences. Measurements extending from the surface to a depth of 2,000 meters showed that global ocean heat content reached another record in 2025. As seawater warms, it expands through thermal expansion, while melting glaciers and ice sheets add water to the ocean. Together, these processes pushed global mean sea level to a record for the 14th consecutive year, approximately 111.2 millimeters above the 1993 satellite-altimetry baseline.

The polar regions again showed some of the clearest signs of accelerated warming. The Arctic experienced its second-warmest year in a 126-year record, with surface air temperatures rising at roughly three times the global average rate. Increasing warmth and precipitation contributed to greater tundra vegetation, and Arctic greenness reached the third-highest level in the record. Yet sea ice continued to shrink. The Arctic’s maximum annual sea-ice extent was the lowest in 47 years of satellite observations, while the minimum was the 11th lowest. Ice older than four years, which is generally thicker and more resistant to melting, has nearly disappeared. In September 2025, only about 95,000 square kilometers of such ice remained, compared with approximately 1.5 million square kilometers during the 1980s.

Antarctica also recorded its warmest year since continuous records began in 1979. Surface melting exceeded average levels across most Antarctic ice shelves, and melting on the Antarctic Peninsula approached record values in early January. Sea ice surrounding the continent remained below average, continuing a nearly decade-long period of unusually low coverage. The annual maximum and minimum sea-ice extents ranked as the third and fourth lowest, respectively. Meanwhile, land-based glaciers worldwide lost ice for the 38th consecutive year. Their average loss exceeded one meter of water equivalent for the fourth year in a row, and approximately 41% of the total ice loss recorded since 1976 occurred during the last decade.

The atmosphere’s increasing heat and moisture were accompanied by an active tropical cyclone year. Ninety-seven named tropical cyclones formed across the Northern and Southern Hemisphere storm seasons, compared with a 1991–2020 average of 87. Five reached Category 5 intensity. Hurricane Melissa became one of the strongest Atlantic hurricanes ever recorded, reaching maximum winds of 190 miles per hour and a minimum central pressure of 892 hectopascals on October 28. It struck Jamaica as a Category 5 storm, causing severe destruction in the western part of the island, 95 reported deaths, and at least $12.2 billion in damage. In the Australian basin, 12 named storms formed during the 2024–25 season, the highest seasonal total since 2005–06. Tropical Cyclone Zelia reached Category 5 strength over the ocean before making landfall in Western Australia as a Category 4 storm.

The report’s findings do not describe isolated changes in a single part of the planet, but a connected shift across the climate system. Greenhouse gases are increasing the atmosphere’s heat-trapping capacity; the ocean is accumulating energy; sea level is rising; glaciers and polar ice are shrinking; and extreme heat and powerful storms are affecting ecosystems and communities. Because the State of the Climate is peer-reviewed and assembled from independent monitoring systems around the world, its value lies not only in individual records but in the consistency of the overall signal. The measurements show that Earth’s climate continued moving into unfamiliar territory in 2025, even during a year without a strong El Niño.

Subject of Research: Earth’s climate system, including greenhouse gases, global temperatures, oceans, sea level, polar regions, glaciers, and tropical cyclones.

Web References: State of the Climate report

References: State of the Climate in 2025, special supplement to the Bulletin of the American Meteorological Society, Vol. 107, No. 8, August 2026.

Image Credits: Figure 1.1 in the State of the Climate in 2025, special supplement to the Bulletin of the American Meteorological Society, Vol. 107, No. 8, August 2026.

Keywords: Climate change, global warming, greenhouse gases, carbon dioxide, ocean heat, sea-level rise, Arctic, Antarctica, glaciers, sea ice, marine heat waves, tropical cyclones, La Niña, climate monitoring.

For several decades, scientists have been closely monitoring the characteristics of AABW, revealing alarming trends. Since the mid-1980s, ocean heat content in regions below 4,000 decibars has surged, with estimates suggesting an increase of approximately 12.9 trillion watts. This influx of heat is altering the thermal and density structure of the ocean depths. The warmer temperatures are affecting the rate and volume of AABW production, with consequences that extend to the entire oceanic and climatic systems. As AABW absorbs more heat, it experiences significant changes that could lead to long-term repercussions for the global ocean.

One of the critical transformations associated with AABW is its thinning, which has been documented to exceed 50 decibars per decade. Thinning is particularly pronounced in regions closer to the sources of AABW, where freshwater input from melting glaciers is contributing to the destabilization of dense water masses. This phenomenon of thinning not only alters AABW dynamics but also impacts the larger framework of the global overturning circulation. The gravitational balance that drives the sinking of AABW is becoming increasingly compromised as lighter, less dense waters replace them in the deep ocean.

In addition to the physical changes in AABW, the composition of the waters surrounding Antarctica is evolving due to glacial melt and fluctuations in sea ice formation. The influx of freshwater from melting ice shelves is causing a reduction in salinity, which in turn disrupts the stratification of ocean layers. As the salinity of surface waters changes, the ability of these waters to sink and contribute to AABW formation is diminished, creating a feedback loop that exacerbates the conditions of climate change. Freshening of the shelf waters is particularly concerning as it denotes a shift in the delicate balance that maintains the deep ocean’s structure.

This modification of AABW is impacting various ecological processes within the deep ocean. As the overturning circulation slows, there is a reduction in the vertical mixing of waters, which plays a vital role in distributing oxygen and nutrients throughout the marine ecosystem. This change can have cascading effects on marine life, particularly species that depend on these resources for survival. The more gradual mixing processes may create less favorable conditions for fish and other marine organisms, leading to shifts in species distributions and overall biodiversity.

Models predicting the future trajectory of AABW suggest even more drastic changes as ocean temperatures continue to rise. The potential for accelerated meltwater input from Antarctica signals that we may witness an increase in the current patterns and rates of freshwater influx into the ocean. Numerical simulations indicate that as meltwater intensifies, the thinning of AABW will not only continue but very likely intensify, leading to a more pronounced slowdown in the abyssal overturning circulation. Such outcomes could alter global ocean dynamics significantly and reshape our understanding of climate systems.

The implications of these changes in AABW are profound and span far beyond the Southern Ocean. The deep ocean’s heat and carbon content are essential for moderating global temperatures and regulating carbon cycles. Disruptions in AABW and its associated processes could influence climate feedbacks, destabilizing the current equilibrium that governs our environmental systems. AABW serves as a significant mechanism for carbon sequestration; hence, alterations in its flow could have direct and long-lasting effects on both terrestrial and marine carbon cycles.

Moreover, shifts in AABW dynamics are intertwined with sea ice dynamics and glacial behaviors. As warmer waters penetrate beneath ice shelves, they can accelerate melting processes, further contributing to the influx of freshwater into surrounding oceanic systems. This cycle not only highlights the interconnectedness of climate phenomena but also underscores the urgency of addressing these changes at multiple levels. Our understanding of how AABW interacts with sea ice and glacier systems remains limited, necessitating a robust research initiative focused on these interactions.

Future research endeavors must prioritize sustained observational efforts in the deep ocean and along the Antarctic continental shelf. Improved understanding of ocean circulation processes is essential for predicting future changes and their potential impacts. Additionally, a concerted effort is needed to explore feedback mechanisms between AABW, sea ice, dense water formation, and ice shelf melt. This multifaceted approach will enhance predictive modeling, allowing us to better represent AABW in oceanic and climate models.

Ultimately, the accelerating changes in AABW underscore the urgent need for comprehensive monitoring and robust climate action. By focusing on observational data and advancing our understanding of the Antarctic regions, we can gain invaluable insights into future climate scenarios. Recognizing the role of AABW in the geophysical system cannot be understated; it is a vital component of our Earth’s climate machinery, and understanding its trajectory will be crucial as we navigate the implications of climate change.

In conclusion, the changing dynamics of Antarctic Bottom Water reveal critical insights into our planet’s future environment. The thinning of AABW, influenced by increasing ocean heat content and freshwater influxes, poses risks to global ocean circulation and climate stability. Without immediate attention to these shifts and the feedback mechanisms at play, the ramifications for marine ecosystems and the Earth’s climate may be dire. Collaborative global efforts to monitor, understand, and mitigate these changes are essential for preserving the integrity of our ocean systems and, by extension, the health of our planet.

Subject of Research: Antarctic Bottom Water dynamics and their implications in a changing climate.

Article Title: Antarctic Bottom Water in a changing climate.

Article References:

Rintoul, S.R., Stewart, A.L., Johnson, G.C. et al. Antarctic Bottom Water in a changing climate.
Nat Rev Earth Environ (2025). https://doi.org/10.1038/s43017-025-00750-2

Image Credits: AI Generated

DOI: 10.1038/s43017-025-00750-2

Keywords: Antarctic Bottom Water, ocean circulation, climate change, freshwater influx, sea ice, glacial melt, thermohaline circulation, marine ecosystem.

22 of the planet’s 34 vital signs are at record levels, with many of them continuing to trend sharply in the wrong direction. This is the message of the sixth issue of the annual “State of the climate” report. The report was prepared by an international coalition with contribution from the Potsdam Institute for Climate Impact Research (PIK) and led by Oregon State University scientists. Published today in BioScience, it cites global data from the Intergovernmental Panel on Climate Change (IPCC) in proposing “high-impact” strategies.

“The last few years have seen vital signs breaking their records by extraordinary margins, like surface temperature, ocean heat content, sea ice loss and fire-related tree cover loss,” says PIK Director Johan Rockström, a co-author of the report. “The accelerating climate crisis presents a range of deeply interconnected risks to the planet’s essential operating systems – from critical tipping elements such as the ocean current system AMOC, to the integrity of Earth’s living biosphere, to the stability of global water resources. But our report also shows how this unprecedented threat to the Earth system – and society – can be mitigated.”

The authors note that 2024 was the hottest year on record and likely the hottest in at least the last 125,000 years. “Climate mitigation strategies are available, cost-effective and urgently needed, and we can still limit warming if we act boldly and quickly,” said William Ripple, professor at Oregon State University and co-lead author. “But the window is closing. Without effective strategies, we will rapidly encounter escalating risks that threaten to overwhelm systems of peace, governance, and public and ecosystem health.”

The research team reviews high-impact groups of actions involving strategies around different sectors, including energy, nature and the global food system:

Energy: Renewable energy sources such as solar and wind have the potential to supply up to 70 percent of global electricity by 2050, the report notes. A rapid phaseout of fossil fuels would yield one of the largest contributions to climate mitigation. 

Ecosystems: Protecting and restoring ecosystems such as forests, wetlands, mangroves and peatlands could remove or avoid around 10 gigatonnes of carbon dioxide emissions per year by 2050, which is equivalent to roughly 25 percent of current annual emissions, while also supporting biodiversity and water security.

Food systems: Reducing food loss and waste, which currently accounts for roughly 8 to 10 percent of global greenhouse gas emissions, and shifting toward more plant-rich diets can substantially lower emissions. These strategies also promote human health and food security, according to the report.

The report warns that every fraction of a degree of avoided warming matters for human and ecological well-being. Small reductions in temperature rise can significantly reduce the risk from extreme weather, biodiversity loss, food and water insecurity as well as risks posed from crossing major tipping points. The authors emphasise that delaying action will lock in higher costs and more severe impacts, while swift, coordinated measures can yield immediate benefits for communities and ecosystems worldwide.

Journal

BioScience

DOI

10.1093/biosci/biaf149