Global surface temperature

• 1. Introduction

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  Global surface temperature is one of the most closely watched numbers in science. It is the average temperature of Earth's surface at any given moment, blending the warmth of ocean water and the air over land into a single figure. Right now, that figure sits at roughly 15 degrees Celsius. But compared to what it was in the mid-1800s, it is at least 1.1 degrees higher. And the rate of rise since 1975 has been unlike anything in at least 2,000 years of recorded and reconstructed history. How do scientists actually measure the temperature of an entire planet? What can tree rings and bubbles of ancient air tell us about climates that no thermometer ever recorded? And what does the warmest decade in 11,700 years mean for the decades ahead?

• 2. Measuring the Planet

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  The longest continuous temperature record in the world is the Central England temperature series, which stretches back to 1659. It is a single-location dataset, a useful baseline but hardly global. Quasi-global records did not begin until around 1850, when networks of land stations and ships started logging temperatures with enough coverage to be meaningful.

  On land, temperatures are captured either with electronic sensors or with mercury and alcohol thermometers, sheltered from direct sunlight inside enclosures called Stevenson screens. At sea, ships take readings from hull-mounted sensors, engine inlets, and buckets, while moored and drifting buoys add more data points across open ocean. Standardization of these methods falls to the World Meteorological Organization, which inherited that role from its predecessor, the International Meteorological Organization.

  Areas with dense populations tend to have dense networks of measurement stations. Polar regions, deserts, and large parts of Africa and South America have far fewer. Rather than reporting raw temperatures, scientists typically present surface temperature data as anomalies, the difference from a reference baseline, usually a 30-year average. A commonly used baseline is the period from 1951 to 1980.

  On the 15th of December 2025, the Copernicus Programme released the GloSAT reference analysis, a gridded dataset extending back to the 1780s and using marine air temperature observations rather than sea surface readings. Before GloSAT, the standard starting point for quasi-global instrumental records was around 1850.

• 3. From Balloons to Satellites

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  Weather balloon radiosonde measurements of atmospheric temperature at various altitudes began approaching global coverage in the 1950s. Since December 1978, microwave sounding units aboard satellites have produced data for inferring temperatures in the troposphere.

  Two major groups have worked through the satellite record. The University of Alabama in Huntsville calculated a global average warming trend of 0.130 degrees Celsius per decade for the lower troposphere between 1978 and 2019. The private, NASA-funded corporation Remote Sensing Systems found a trend of 0.148 degrees Celsius per decade up to January 2011. Early satellite analyses showed much smaller warming trends than climate models predicted, but after revisions to the satellite records, the trends aligned more closely.

  For the ocean depths, temperature readings at various levels feed into calculations of the ocean heat content, an additional dimension of the global energy budget that surface numbers alone cannot capture.

• 4. The Major Datasets

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  Several independent research groups maintain the principal global surface temperature records, and they rely on methods that are largely independent from one another. The National Oceanic and Atmospheric Administration maintains the Global Historical Climatology Network and a global temperature database that extends back to 1880. HadCRUT is a collaboration between the University of East Anglia's Climatic Research Unit and the Hadley Centre for Climate Prediction and Research. NASA's Goddard Institute for Space Studies maintains GISTEMP.

  The Berkeley Earth Surface Temperature dataset, a more recent addition, is now among the datasets used by the IPCC and the World Meteorological Organization. In October 2011, Berkeley Earth published preliminary results showing that land surfaces warmed by 0.911 degrees Celsius over the past 50 years, matching the earlier findings of NOAA, the Hadley Centre, and NASA's GISS. That study directly addressed concerns raised by skeptics, including urban heat island effects, station quality, and data selection bias, and found that none of these effects biased the earlier results.

  The datasets are updated frequently and generally agree closely with one another, which strengthens the confidence scientists have in the overall trend.

• 5. The Temperature Record Since 1850

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  From 1850-1900 to 2011-2020, the combined land and ocean surface temperature warmed by 1.09 degrees Celsius, with a range of 0.95 to 1.20 degrees across multiple independent datasets. Land surfaces warmed faster than the oceans: land air temperature rose 1.59 degrees Celsius over that same span, while sea surface temperature rose 0.88 degrees.

  The warming did not proceed at a steady pace. Most observed warming occurred in two bursts, one from around 1900 to around 1940, and another from around 1970 onward. The plateau between 1940 and 1970 has been largely attributed to sulfate aerosols, particles that scatter incoming sunlight and cool the surface. Since 1975, the rate of warming has been roughly 0.15 to 0.20 degrees Celsius per decade.

  The period from 2015 through 2025 contained all eleven of the warmest years on record since 1850. The year 2023 alone was 1.48 degrees Celsius hotter than the 1850-1900 average, according to the Copernicus Climate Change Service, and was declared the warmest on record almost immediately after it ended. The most recent decade, 2011-2020, was warmer than any multi-centennial period in the past 11,700 years.

  Not every year sets a record. Strong El Nino events typically push global temperatures 0.1 to 0.2 degrees Celsius above the surrounding years, while strong La Nina events push them down by a similar margin. The 1998 El Nino was among the strongest of the 20th century, making 1998 the warmest year on record at the time by a substantial margin, which contributed to a media narrative that warming had stopped. The slowdown in warming rates from 1998 to 2012 was in fact less pronounced in later, improved datasets than in those available in 2012.

• 6. What Drives the Changes

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  Greenhouse gases trap outgoing radiation, warming the atmosphere, which in turn warms land and sea. On top of that long-term driver sit several shorter-term influences. Volcanic eruptions can inject sulfur dioxide into the stratosphere, creating aerosols that cool the planet for one to three years. The effect is strongest for tropical volcanoes, whose aerosols can spread across both hemispheres. The Mount Pinatubo eruption in 1991 and the Mount Agung eruption in 1963-1964 were each followed by years with global mean temperatures 0.1 to 0.2 degrees Celsius below the long-term trend at the time.

  Solar output also varies, following an approximately 11-year magnetic activity cycle, though the variation is slight. Aerosols from human activities, primarily from burning fossil fuels, diffuse incoming radiation and have a net cooling effect, but a single large volcanic eruption can temporarily exceed those anthropogenic aerosol levels. Land use change, including deforestation through burning biomass, releases additional greenhouse gases and can alter how much sunlight the surface reflects.

  Urbanization has raised a question about data bias: could the heat island effect around cities inflate the land temperature record? A study concluded in 2006 that existing techniques can identify and remove such biases. A further study in 2013 found that when all available station data is divided into rural and urban subsets, both temperature sets are broadly consistent, and urban bias can be accounted for.

• 7. Proxy Records Before 1850

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  Tree ring widths, coral growth, isotope ratios in ice cores, lake and ocean sediments, cave deposits, borehole temperatures, and glacier lengths all encode information about past climates. Proxy reconstructions extending back 2,000 years have been carried out for the Northern Hemisphere, and over shorter time spans for the Southern Hemisphere and tropics.

  Those reconstructions point to several findings. Global mean surface temperatures over the last 25 years have been higher than any comparable period since AD 1600, and probably since AD 900. A Little Ice Age was centered on AD 1700. A Medieval Warm Period was centered on AD 1000, but the evidence suggests it was not a global phenomenon.

  Going further back, ice cores from Antarctica's Dome Concordia provide the longest available Antarctic record, reaching 800,000 years and covering eight glacial and interglacial cycles. The Guliya ice core from a low-latitude, high-altitude region spans more than 700,000 years and helped confirm that the Last Glacial Maximum was colder in the tropics and subtropics than previously believed. Records from Greenland's NGRIP core stretch more than 100,000 years, including 5,000 years in the Eemian interglacial, and carry some of the clearest evidence of abrupt climate shifts ever found.

  Historical human records, including reports of frost fairs on the Thames, dates of spring blossoms and lambing, and accounts of unusual floods or droughts, provide qualitative temperature evidence alongside the numerical proxies. One striking finding from this indirect evidence: a sudden, short-lived climatic shift between 2200 and 2100 BCE, with cooling and reduced precipitation across the region between Tibet and Iceland, is considered a primary cause of the collapse of the Old Kingdom of Egypt.

• 8. Projections and What Comes Next

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  Climate model projections based on greenhouse gas emission scenarios span a wide range of outcomes. Under a very low emissions scenario, global warming would likely reach 1.0 to 1.8 degrees Celsius by the late 21st century. An intermediate scenario points to 2.1 to 3.5 degrees, and a very high emissions scenario to 3.3 to 5.7 degrees above pre-industrial levels.

  The changes are not expected to be uniform. Land areas warm faster than oceans, and northern high latitudes warm faster than the tropics. Three major pathways will drive regional shifts: melting ice, changes to the hydrological cycle of evaporation and precipitation, and altered ocean currents.

  A decadal forecast issued by the World Meteorological Organization in 2021 estimated a 40 percent probability of at least one year exceeding 1.5 degrees Celsius above pre-industrial levels in the 2021-2025 period. 2023's anomaly of 1.48 degrees Celsius came close. Warming rates are expected to keep fluctuating on decadal timescales through the 21st century, driven by the same natural variability, from El Nino cycles to volcanic eruptions, that has always shaped the record, but now layered on top of a human-driven trend that ice cores and tree rings confirm has no parallel in at least two millennia.