Climate change

Warmest year on record

2024, about 1.55°C above 1850 to 1900^[5]

2025 global temperature

1.44°C (±0.13°C) above 1850 to 1900, second or third warmest depending on the dataset^[4]

Three-year average, 2023 to 2025

1.48°C (±0.13°C) above 1850 to 1900^[4]

Global CO2, annual average for 2025

426.8 parts per million, 153% of the 1750 level^[6]

CO2 at Mauna Loa, latest month

426.07 parts per million in September 2026, up 1.70 from a year earlier (preliminary)^[3]

Fossil CO2 emissions

38.1 billion tonnes projected for 2025, a record^[13]

Warming projected this century

2.8°C under current policies; 2.3 to 2.5°C if national pledges are fully met^[14]

What it is

Earth stays habitable because gases in the air absorb most of the infrared heat rising from the sun-warmed surface, about 90% of it, and send part of that back down.^[12] Adding more of those gases slows the escape of heat to space. Carbon dioxide comes mainly from burning fossil fuels and clearing forests; methane from wetlands, landfills, rice paddies, livestock and leaking fossil fuel systems; nitrous oxide largely from fertilizer and combustion.^[12] Water vapor is the most plentiful greenhouse gas, but its amount follows the temperature, so it amplifies warming and does not start it.^[12]

The climate has always varied on its own, which is why scientists separate the possible causes. The IPCC's best estimate is that people caused 1.07°C of warming between 1850 to 1900 and 2010 to 2019. Over the same span natural drivers such as the Sun and volcanoes moved the temperature by no more than about 0.1°C either way, and random internal variability by no more than about 0.2°C.^[1] Aerosol pollution masked part of the greenhouse effect, cooling the planet by up to 0.8°C.^[1] NASA adds two further checks: solar output shows no net rise since 1880, and the upper atmosphere has cooled while the lower layers warmed, which a brighter Sun would not produce.^[12]

How the science developed

The idea is old. Joseph Fourier described the atmosphere as a kind of insulation in 1824, Eunice Foote showed in 1856 that carbon dioxide and water vapor hold heat, and John Tyndall recognized the natural greenhouse effect in the 1860s.^[11] Svante Arrhenius predicted in 1896 that changing carbon dioxide levels could substantially shift surface temperature, and Guy Callendar tied rising carbon dioxide to warming in 1938.^[11] Charles David Keeling started continuous measurements at Mauna Loa in Hawaii in 1958, producing the record now called the Keeling Curve, a steady climb with a yearly rise and fall set by plant growth in the Northern Hemisphere.^[19]

The IPCC assesses this research in periodic reports. Its sixth assessment, completed with a synthesis in 2023, stated that human activities have unequivocally caused global warming.^[1] It also narrowed the climate sensitivity, the eventual warming from a doubling of carbon dioxide: a best estimate of 3°C and a likely range of 2.5 to 4°C, against 1.5 to 4.5°C in the previous assessment.^[2]

Warming to date

The IPCC's reference figure is 1.09°C of warming in 2011 to 2020 compared with 1850 to 1900, with land warming nearly twice as much as the ocean (1.59°C against 0.88°C). It found the rise since 1970 faster than in any other 50-year stretch of at least 2,000 years.^[1] Recent years have run well above that decade's average. The World Meteorological Organization (WMO) puts 2024 at about 1.55°C, the record, and 2025 at 1.44°C by its January 2026 analysis of eight datasets; its March 2026 report gave about 1.43°C.^[4][5] Two of the datasets rank 2025 second and six rank it third, and the 11 years from 2015 to 2025 are the 11 warmest in all eight.^[4]

0:00 142 years of warming on one map NASA's map of temperature anomalies turns from blue to deep red between 1880 and 2022.

Figures differ by dataset. Europe's Copernicus service, using the ERA5 reanalysis, reports 1.60°C for 2024 and 1.47°C for 2025, and says 2023 to 2025 was the first three-year period to average more than 1.5°C.^[7] The WMO's combined average for those three years is lower, at 1.48°C.^[4] A single hot year or even several do not settle whether the Paris Agreement's 1.5°C limit has been crossed: the UN Environment Program's 2025 report still described exceeding it as very likely within the next decade.^[14]

An El Niño, a natural warming of the tropical Pacific that lifts global temperatures on top of the trend, developed in 2026. The WMO said on Sept. 3, 2026 that it was firmly established and heading for a very strong peak late in the year, with the effect on global temperature likely most pronounced in 2027.^[10] Copernicus measured September 2026 at 1.63°C above pre-industrial levels, the second warmest September in its record, and the Niño 3.4 region of the Pacific reached an all-time monthly high of 29.70°C.^[8] The U.S. National Oceanic and Atmospheric Administration (NOAA) ranked January to September 2026 second warmest behind 2024 and said second place is the most likely finish for the full year.^[9]

Greenhouse gases and emissions

Before industrialization carbon dioxide held near 280 parts per million for almost 6,000 years.^[19] The WMO's latest bulletin, released on Oct. 27, 2026, gives a 2025 global average of 426.8 parts per million, up 2.8 in one year, with methane at 1,948 parts per billion (267% of the 1750 level) and nitrous oxide at 339.1 parts per billion.^[6] The IPCC concluded that carbon dioxide was already higher in 2019 than at any time in at least 2 million years, and the other two higher than at any time in at least 800,000 years.^[1] Mauna Loa, one station with its own seasonal cycle, read 426.07 parts per million in September 2026.^[3]

The Global Carbon Budget project estimated fossil emissions of 38.1 billion tonnes of carbon dioxide in 2025, a 1.1% rise and a record, with coal, oil and gas all growing.^[13] It projected increases for China (0.4%), India (1.4%), the United States (1.9%) and the European Union (0.4%), and a 2.2% fall for Japan.^[13] Land-use emissions of about 4.1 billion tonnes declined enough that total carbon dioxide emissions came in slightly below 2024.^[13] Looking back, 42% of the net carbon dioxide emitted from 1850 to 2019 was released in just the three decades after 1990.^[1]

Effects already observed

2:40 Why warmer air brings heavier rain Vox, in 2017, on the physics behind heavier downpours: warmer air holds more water.

• Ocean: roughly 91% or more of Earth's excess energy goes into the sea, and ocean heat content hit a record in 2025, the ninth record year in a row. Warming of the ocean from 2005 to 2025 ran at more than twice the 1960 to 2005 rate.^[5]
• Sea level: it rose about 0.20 meters between 1901 and 2018, at a rate climbing from 1.3 millimeters a year early in that period to 3.7 millimeters a year from 2006 to 2018.^[1] By 2025 it stood about 11 centimeters above 1993.^[5]
• Ice: Greenland shed an average of 279 billion tons a year from 1993 to 2019 and Antarctica about 148 billion tons.^[11] Almost all the world's glaciers are retreating together, which the IPCC finds unmatched in at least 2,000 years.^[2] September Arctic sea ice area fell about 40% between 1979 to 1988 and 2010 to 2019, and the 2025 winter maximum was the lowest in the satellite record.^[2][5]
• Extremes: hot extremes have become more frequent and intense across most land since the 1950s, and marine heatwaves have roughly doubled in frequency since the 1980s.^[2]
• Ocean chemistry: human carbon dioxide is the main cause of the acidification of the surface open ocean, which took up about 29% of human emissions in 2015 to 2024.^[2][5]
• People: about 3.3 to 3.6 billion people live in settings highly vulnerable to climate change, and those who contributed least to the problem are hit hardest.^[1] The WMO says more than a third of the global workforce, some 1.2 billion people, faces heat risk at work each year.^[5]

Projections

How much warmer the world gets depends largely on future emissions. For 2081 to 2100 the IPCC's very likely ranges above 1850 to 1900 run from 1.0 to 1.8°C under a very low emissions scenario, through 2.1 to 3.5°C under an intermediate one, to 3.3 to 5.7°C under a very high one.^[2] Each 1,000 billion tonnes of carbon dioxide adds about 0.45°C.^[1] Sea level is projected to rise 0.28 to 0.55 meters by 2100 under the lowest scenario and 0.63 to 1.01 meters under the highest, compared with 1995 to 2014, and it will keep rising for centuries whatever happens to emissions.^[1][2]

1:06 What the models say about hurricanes Vox reads IPCC model results: fewer tropical cyclones overall but more of the strongest, with messy regional forecasts.

Some changes could prove effectively permanent. The IPCC warns that at a sustained 2 to 3°C of warming, the Greenland and West Antarctic ice sheets would be lost almost entirely over many thousands of years, and that the odds of abrupt shifts grow with each increment of warming.^[1] It also expects natural land and ocean sinks to absorb a shrinking share of emissions if those emissions keep growing.^[1]

Limiting it

Stabilizing the temperature requires net zero carbon dioxide emissions.^[1] In the IPCC's modeled pathways for 1.5°C, greenhouse gases fall 43% by 2030 and 60% by 2035 from 2019 levels, and carbon dioxide reaches net zero in the early 2050s; pathways for 2°C reach it around the early 2070s.^[1] The UN Environment Program's 2025 figures for 2035 are similar: cuts of 55% for 1.5°C and 35% for 2°C.^[14] The carbon budget is the sharpest way of stating the problem. The Global Carbon Budget team reckons about 170 billion tonnes of carbon dioxide remain for 1.5°C, roughly four years at the 2025 rate.^[13]

Adaptation is the other half of the response. The IPCC reports progress in every sector and region, but also growing gaps, insufficient finance and limits already reached in some ecosystems and regions.^[1] It says options for cutting emissions and adapting are available now, some at low cost, and that enough capital exists worldwide, though barriers keep it from reaching climate action.^[1] UNEP notes that wind and solar are expanding quickly and getting cheaper.^[14]

International politics and law

The Paris Agreement, adopted on Dec. 12, 2015 and in force since Nov. 4, 2016, commits its parties to hold warming well below 2°C and try to stop at 1.5°C, with each updating a national plan every five years.^[15] UNEP's 2025 assessment puts the outcome so far at 2.8°C under policies in place and 2.3 to 2.5°C if the pledges are fully carried out. Those compare with 3 to 3.5°C expected when the agreement was signed.^[14] Of the improvement since UNEP's previous report, 0.1°C came from changes in method, and the American departure is expected to erase another 0.1°C.^[14]

President Donald Trump ordered the United States out of the agreement on his first day back in office in January 2025, and the exit took effect at the end of January 2026.^[16][17] A memorandum on Jan. 7, 2026 went further, announcing withdrawal from the UN climate convention and from the IPCC itself; the convention exit is due in February 2027.^[16][17] Legal experts disagree on whether a president can leave a Senate-ratified treaty unilaterally.^[16] In July 2025 the International Court of Justice advised that states have legal duties to protect the climate from greenhouse gases, and may have to make reparation if they breach them.^[18]

What remains uncertain

The cause of the warming is not in scientific dispute within these assessments, but its details carry real uncertainty. Climate sensitivity still spans a range, and the very likely band reaches from 2 to 5°C.^[2] Antarctic sea ice showed no significant trend from 1979 to 2020, owing to opposing regional changes and large natural swings, though recent years brought the four lowest minimums on record.^[2][5] The IPCC finds only limited evidence of a human role in Antarctic ice sheet loss, unlike the very likely human contribution to Greenland's surface melt, and where and how often extremes of rain and drought will strike depends on shifts in regional atmospheric circulation.^[2]

Timeline

1. 1856 Eunice Foote shows that carbon dioxide and water vapor trap heat.^[11]
2. 1896 Svante Arrhenius predicts that carbon dioxide changes could substantially alter surface temperature.^[11]
3. 1958 Charles David Keeling begins carbon dioxide measurements at Mauna Loa.^[19]
4. Dec. 12, 2015 The Paris Agreement is adopted.^[15]
5. Aug. 2021 The IPCC publishes the physical science report of its sixth assessment.^[2]
6. 2023 The IPCC completes the assessment with its synthesis report.^[1]
7. 2024 The warmest year measured, at about 1.55°C above 1850 to 1900.^[5]
8. July 23, 2025 The International Court of Justice gives its advisory opinion on states' climate duties.^[18]
9. Jan. 2026 The American exit from the Paris Agreement takes effect.^[17]
10. Sept. 2026 Copernicus records the second warmest September, as a very strong El Niño builds.^[8][10]
11. Oct. 27, 2026 The WMO reports a 2025 global carbon dioxide average of 426.8 parts per million.^[6]