Greenhouse gas concentrations, global sea levels and ocean heat all reached record highs in 2025, according to a landmark climate study published by the American Meteorological Society (AMS). The international annual review offers the most comprehensive assessment of Earth's climate indicators, compiling data gathered across land, ocean, ice and space.
The findings present a stark portrait of a warming planet. Despite the absence of an El Nino climate pattern during 2025, Earth remained exceptionally hot, with global fossil fuel emissions and atmospheric greenhouse gas levels climbing to unprecedented peaks.
The American Meteorological Society, founded in 1919 and headquartered in Boston, Massachusetts, is one of the leading scientific organizations dedicated to advancing atmospheric, oceanic and hydrologic sciences. Its annual State of the Climate report brings together findings from 625 scientists operating across 60 countries, relying on vast arrays of environmental monitoring stations, ocean buoys, polar ice sensors and satellite remote-sensing platforms.
Record greenhouse gas emissions and atmospheric levels
Atmospheric concentrations of carbon dioxide reached a global average of 425.6 parts per million (ppm) in 2025. That level is approximately 53 per cent higher than estimated concentrations prior to the Industrial Revolution in the mid-19th century, when widespread burning of coal and other fossil fuels began altering the atmosphere.
Other major heat-trapping greenhouse gases also hit record concentrations last year. Both methane, which is released through agriculture, fossil fuel extraction and landfills, and nitrous oxide, emitted primarily from fertilizers and industrial processes, climbed to their highest levels since modern measurements began.
Meanwhile, global fossil fuel emissions reached an estimated record high of 10.3 petagrams of carbon in 2025. A petagram is equivalent to one billion metric tons. Current carbon emissions are now more than three times higher than the annual levels recorded during the 1960s.
Greenhouse gases function by allowing solar radiation to pass through the atmosphere while absorbing outbound thermal energy emitted from Earth's surface. This process traps heat within the lower atmosphere, steadily elevating global surface temperatures and driving long-term shifts in weather patterns.
Unprecedented ocean warming and rising sea levels
Much of the excess thermal energy generated by greenhouse gas emissions is absorbed by Earth's oceans, which have taken up nearly 90 per cent of the additional heat trapped in the climate system. As a result, global ocean heat content measured from the surface down to a depth of 2,000 metres (6,500 feet) set a new record high in 2025 for the ninth consecutive year.
Extreme ocean warming triggered widespread ecological strain across marine environments. Approximately 87 per cent of the world's ocean surface experienced at least one marine heatwave during 2025, compared with just 26 per cent of the surface experiencing a marine cold spell. Overall sea surface temperatures ranked as the third-highest in the 172-year record, even though cool or weak La Nina-like conditions persisted across the Pacific Ocean.
Global sea levels also hit a record high for the 14th consecutive year, standing roughly 111 millimetres above the 1993 average, when satellite altimetry measurements first began. Ocean thermal expansion, caused by water expanding as it heats up, has contributed approximately 1.6 millimetres per year to sea-level rise since 2005. Melting land ice, including glaciers and ice sheets, added an average of 2.0 millimetres per year over the same period.
Record heat at the polar caps and across Europe
On land, 2025 ranked among the three warmest years since global temperature tracking began more than 175 years ago. According to all five major global temperature datasets, the 11-year period spanning from 2015 to 2025 represents the 11 warmest years ever recorded in human history.
Extreme warmth was particularly pronounced in the polar regions. Antarctica recorded its warmest year since record-keeping began in 1979. In the northern polar region, the Arctic experienced its second-warmest year since records began in 1900.
"The extreme warmth extended to the polar regions, with Antarctica experiencing its warmest year in the record that began in 1979, and the Arctic experiencing its second-warmest year since the start of its record in 1900," the report noted.
Regional climate records tumbled across multiple continents during 2025. Europe recorded its hottest year in history, while Russia, China, South Korea and Argentina each documented their second-warmest year on record.
Accelerating loss of Arctic sea ice and mountain glaciers
Rapid warming at high latitudes has continued to degrade the structural integrity of polar ice cover. In the Arctic, multi-year sea ice that has survived for more than four years has almost completely disappeared, contrasting sharply with the expansive multi-year ice fields that covered the region during the 1980s. Antarctic sea ice extent also suffered dramatic declines throughout the year.
Mountain glaciers and land-based ice fields around the world experienced net ice loss for the 38th consecutive year. Over the past four years, glaciers have surrendered the equivalent of more than one metre of liquid water on average each year.
Storm frequency rises as Super El Nino threatens 2026
The warming climate backdrop coincided with a surge in severe weather events globally. Environmental monitoring stations logged 97 named tropical cyclones during the 2025 storm seasons, noticeably higher than the 1991-2020 annual average of 87 storms.
Scientists warn that climate conditions could deteriorate further due to shifts in Pacific Ocean temperatures. Robert Dunn, a climate scientist at the UK Met Office and one of the lead authors of the report, noted that strong ocean warming trends are developing.
The UK Met Office, established in 1854 as the Meteorological Department of the Board of Trade, serves as the United Kingdom's national weather service and operates one of the world's premier climate research centers.
"At the time of writing, halfway through 2026, there are indications of an extremely strong El Nino forming," Dunn said. "The climate in 2025, being only slightly 'below' 2023 and 2024, has potentially set some concerning precursor conditions for 2026."
