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    How global warming is measured: temperature and key indicators

    Global warming is measured by comparing today's average surface temperature with the 1850–1900 average, using several independent datasets plus indicators such as ocean heat, sea level and greenhouse gas levels.[1][2] WMO put 2025 at 1.44 °C above that baseline, and Copernicus found that 2023–2025 was the first three-year period to average more than 1.5 °C.[1][3]

    Editor reviewedUpdated Climate science and carbon removalEnergy and climateScience

    The short answer

    Scientists measure global warming by working out the average temperature of the whole planet’s surface for a year and comparing it with the average for 1850–1900, which serves as the pre-industrial baseline.[1] By that measure, 2025 was about 1.44 °C warmer, and the eleven years from 2015 to 2025 were the eleven warmest on record.[1][4]

    Temperature is only one sign. Scientists also watch how much heat the ocean is storing, how fast the sea is rising, how much Arctic sea ice is left and how much carbon dioxide is in the air.[5][6][7]

    The World Meteorological Organization (WMO) consolidates eight global temperature datasets. For 2025 it gave 1.44 °C (±0.13 °C) above 1850–1900; two datasets ranked the year second warmest and six ranked it third.[1] The European Copernicus Climate Change Service uses ERA5, a global climate reanalysis produced by ECMWF that covers 1940 to the present.[8] ERA5 put 2025 at 1.47 °C and 2024, the record year, at 1.60 °C.[9]

    The annual Indicators of Global Climate Change study separates out the human-induced part, which it estimated at 1.37 °C in 2025 (likely range 1.1–1.7 °C), compared with observed warming of 1.39 ± 0.13 °C.[10][11]

    Why 1.5 °C is measured over decades

    A single year can pass 1.5 °C while the long-term, human-induced level stays below it: 2024 reached 1.60 °C in ERA5, yet human-induced warming was estimated at 1.37 °C in 2025.[9][10] Copernicus found that 2023–2025 was the first three-year period to average more than 1.5 °C, and WMO’s consolidated three-year average was 1.48 °C (±0.13 °C).[3][12] Human-induced warming is rising by about 0.27 °C per decade, and Copernicus said it is projected to pass 1.5 °C in about four years.[10][13]

    Following the heat: ocean and energy imbalance

    The ocean takes up about 90% of the extra heat trapped by greenhouse gases.[14] WMO reported that ocean heat content in the top 2,000 metres reached its highest level since records began in 1960, and that the ocean has absorbed the equivalent of about eighteen times humanity’s annual energy use every year for two decades.[5][15]

    Earth’s energy imbalance measures how much more energy the planet takes in than it gives off. WMO said it reached a new high in 2025, and the indicators study found it has more than doubled since 1976–1995.[16][17]

    Other indicators

    • Greenhouse gases. In 2025 carbon dioxide averaged 425.6 ppm, methane 1936.3 ppb and nitrous oxide 339.4 ppb.[2] WMO says CO2 in 2024 was at its highest level in 2 million years.[18]
    • Sea level. Sea level rose about 228 mm between 1900 and 2024, and it has risen faster since 2012 than in 1993–2011.[19][6]
    • Sea ice. The 2025 annual average Arctic sea-ice extent was the lowest or second lowest in the satellite record, which began in 1979.[7]

    What to watch

    WMO’s State of the Global Climate 2025 appeared in March 2026 and the 2025 indicators study in June 2026.[16][10] The next key question is when the long-term, human-induced warming level passes 1.5 °C. For what that means for emissions, see the carbon budgets explainer.

    Questions readers ask

    How much has the world warmed?

    WMO's consolidated estimate put 2025 at 1.44 °C (±0.13 °C) above the 1850–1900 average, and the 2023–2025 average at 1.48 °C.[1][12]

    Has the world already passed 1.5 °C?

    Single years and one three-year period have. Copernicus found 2023–2025 averaged more than 1.5 °C, but the long-term human-induced warming level was estimated at 1.37 °C in 2025.[3][10]

    Why do scientists track ocean heat?

    The ocean absorbs about 90% of the excess heat trapped by greenhouse gases, and its heat content to 2,000 metres hit a record in 2025.[14][5]

    Why do different agencies give slightly different numbers?

    They use different datasets. Copernicus's ERA5 put 2025 at 1.47 °C and ranked it third warmest, while WMO combines eight datasets and gives a range.[9][1]

    Sources

    Each numbered claim is a statement we checked against the sources listed with it. Status shows how well established it is.

    1. [1]

      WMO's consolidation of eight datasets put 2025 at 1.44 °C (±0.13 °C) above the 1850–1900 average, the second or third warmest year on record depending on the dataset. confirmedas of 2026-01-14

    2. [2]

      Global mean surface concentrations in 2025 were 425.6 ppm for carbon dioxide, 1936.3 ppb for methane and 339.4 ppb for nitrous oxide. confirmedas of 2026-06-11

    3. [3]

      According to Copernicus, 2023–2025 was the first three-year period whose average temperature exceeded 1.5 °C above the 1850–1900 level. confirmedas of 2026-01-14

    4. [4]

      The eleven years 2015–2025 are the eleven warmest on record in all eight datasets WMO uses. confirmedas of 2026-01-14

    5. [5]

      Ocean heat content in the upper 2,000 metres reached its highest level since records began in 1960, according to WMO. confirmedas of 2026-03-23

    6. [6]

      Global mean sea level has risen faster since 2012 than in the earlier part of the satellite record (1993–2011). confirmedas of 2026-03-23

    7. [7]

      The 2025 annual average Arctic sea-ice extent was the lowest or second lowest in the satellite record, which began in 1979. confirmedas of 2026-03-23

    8. [8]

      ERA5 is a global climate reanalysis dataset produced by ECMWF that covers January 1940 to the present. confirmedas of 2026-01-14

    9. [9]

      In the Copernicus ERA5 dataset, 2025 was 1.47 °C above pre-industrial, the third warmest year, after 2024 at 1.60 °C, the warmest on record. confirmedas of 2026-01-14

    10. [10]

      The Indicators of Global Climate Change 2025 study estimated human-induced warming at 1.37 °C above 1850–1900 in 2025, rising at about 0.27 °C per decade over 2016–2025. confirmedas of 2026-06-11

    11. [11]

      The same study put observed global surface warming in 2025 at 1.39 ± 0.13 °C above 1850–1900. confirmedas of 2026-06-11

    12. [12]

      WMO put the consolidated 2023–2025 three-year average at 1.48 °C (±0.13 °C) above the pre-industrial era. confirmedas of 2026-01-14

    13. [13]

      Copernicus, summarising the 2025 indicators study, said human-induced warming is projected to pass 1.5 °C in about four years. reportedas of 2026-06-11· forecast

    14. [14]

      About 90% of the excess heat trapped by greenhouse gases is absorbed by the ocean, which makes ocean heat content a key indicator of climate change. confirmedas of 2026-01-14

    15. [15]

      WMO says the ocean has absorbed the equivalent of about eighteen times annual human energy use each year for the past two decades. confirmedas of 2026-03-23

    16. [16]

      WMO's State of the Global Climate 2025 reported that Earth's energy imbalance, which has increased since its observational record began in 1960, reached a new high in 2025. confirmedas of 2026-03-23

    17. [17]

      Earth's energy imbalance has more than doubled since the 1976–1995 period, according to the Indicators of Global Climate Change 2025 study. confirmedas of 2026-06-11

    18. [18]

      WMO reports that in 2024 the atmospheric concentration of carbon dioxide reached its highest level in the last 2 million years. confirmedas of 2026-03-23

    19. [19]

      The Indicators of Global Climate Change 2024 study found global mean sea level had risen about 228 mm since 1900, with about 26 mm of that between 2019 and 2024. confirmedas of 2025-06-19

    Revision history (1)
    1. Page created.

    Created Oct 10, 2026. Last reviewed by an editor on Oct 10, 2026. Next scheduled review: Jan 10, 2027.

    Cite this page

    "How global warming is measured: temperature and key indicators." ContentLora, updated Oct 10, 2026. https://contentlora.com/explain/how-global-warming-is-measured

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