Global surface temperature

• 1. Defining Global Surface Temperature

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  The blue line on a graph represents global surface temperature reconstructed over the last 2,000 years using proxy data from tree rings, corals, and ice cores. The red line shows direct surface temperature measurements since 1880. This visual distinction separates historical estimates from modern instrumental records. Scientists define global surface temperature as the average temperature of Earth's surface at a given time. It combines sea surface temperature with near-surface air temperature over land. These two components are weighted by their respective areas to create a single metric. Alternative terms for this concept include global mean surface temperature or global average surface temperature. The Intergovernmental Panel on Climate Change defines it as the estimated global average of near-surface air temperatures over land and sea ice. They also calculate sea surface temperature over ice-free ocean regions. Changes in this value are normally expressed as departures from a specified reference period.

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• 2. Instrumental Measurement History

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  Series of reliable temperature measurements in some regions began in the 1850 to 1880 time frame. This era marks the start of what scientists call the instrumental temperature record. The longest-running temperature record is the Central England temperature data series which starts in 1659. Before 1850, reasonably reliable instrumental records existed but had sparser coverage largely confined to the Northern Hemisphere. Temperature data comes mainly from weather stations and satellites. On land, instruments use electronics sensors or mercury thermometers read manually. These devices sit inside shelters like Stevenson screens to avoid direct sunlight. Sea records consist of ships taking measurements mostly from hull-mounted sensors or engine inlets. Modern buoys now provide additional data points across the oceans. Areas that are densely populated tend to have a high density of measurement points. In contrast, observations spread out more thinly in polar regions and deserts. Standardization of methods is organized through the World Meteorological Organization. Most meteorological observations serve weather forecasts rather than long-term climate studies alone.

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• 3. Proxy Data Reconstruction Methods

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  To estimate data in the distant past, proxy data can be used for example from tree rings, corals, and ice cores. Quantities such as tree ring widths and coral growth correlate with climatic fluctuations. Isotope variations in ice cores also provide evidence about global temperature from 1,000 to 2,000 years before present. A study of paleoclimate covers the time period from 12,000 years ago. The Antarctic EPICA core reaches 800 kyr while many others reach more than 100,000 years. Even longer term records exist for few sites. The NGRIP core from Greenland stretches back more than 100 kyr. As well as natural numerical proxies there exist records from the human historical period. These include reports of frost fairs on the Thames and dates of spring blossom or lambing. Proxy reconstructions extending back 2,000 years have been performed but reconstructions for the last 1,000 years are supported by more independent data sets. Geographic coverage by these proxies is necessarily sparse and various proxies show variation on an annual time scale. Connecting measured proxies to the variable of interest remains highly non-trivial.

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• 4. Decadal Warming Trends Analysis

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  Through 1940, the average annual GMST increased but was relatively stable between 1940 and 1975. Since 1975, it has increased by roughly 0.15 °C to 0.20 °C per decade. The global average and combined land and ocean surface temperature show a warming of 1.09 °C from 1850, 1900 to 2011, 2020. Land air temperatures are rising faster than sea surface temperatures. Land temperatures have warmed by 1.59 °C while sea surface temperatures have warmed by 0.88 °C over the same period. Most of the observed warming occurred in two periods around 1900 to around 1940 and around 1970 onwards. The cooling or plateau from 1940 to 1970 has been mostly attributed to sulfate aerosol. Each of the last four decades has been successively warmer at the Earth's surface than any preceding decade since 1850. The most recent decade 2011-2020 was warmer than any multi-centennial period in the past 11,700 years.

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• 5. Natural Variability Drivers

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  Some of the temperature variations over this time period may also be due to ocean circulation patterns. Internal climate variability is a result of complex interactions between components of the climate system. An example of internal climate variability is the El Niño, Southern Oscillation. Strong El Niño years are typically 0.1 °C to 0.2 °C warmer than the years immediately preceding and following them. La Niña usually causes years which are cooler than the short-term average. Aerosols diffuse incoming radiation generally cooling the planet. Major volcanic eruptions can produce quantities of aerosols which exceed those from anthropogenic sources over periods up to a few years. Volcanic eruptions sufficiently large to inject significant quantities of sulfur dioxide into the stratosphere have a global cooling effect for one to three years after the eruption. The largest eruptions of the last 100 years include Mount Pinatubo in 1991 and Mount Agung in 1963-1964. These events were followed by years with global mean temperatures 0.1 °C to 0.2 °C below long-term trends at the time.

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• 6. Scientific Consensus And Robustness

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  Observing the rising GST over time is one of the many lines of evidence supporting the scientific consensus on climate change. There is a scientific consensus that climate is changing and that greenhouse gases emitted by human activities are the primary driver. Joint statements come from leaders of 18 scientific organizations including the American Association for the Advancement of Science and the American Geophysical Union. The Intergovernmental Panel on Climate Change summarizes existing science while the U.S. Global Change Research Program provides further analysis. Multiple independently produced datasets show substantial agreement concerning the progress and extent of global warming. Pairwise correlations of four longer-term datasets reach at least 99.29%. The Berkeley Earth Surface Temperature dataset mirrors results obtained from earlier studies carried out by NOAA, the Hadley Centre and NASA's GISS. A study concluded in 2006 that existing empirical techniques validate local and regional consistency of temperature data adequately. Urban bias can be accounted for when all available station data is divided into rural and urban sets.

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