Global surface temperature

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Global surface temperature reconstruction over the last 2000 years using proxy data from tree rings, corals, and ice cores in blue.[1] Directly observed data is in red.[2]

In earth science, global surface temperature (GST; sometimes referred to as global mean surface temperature, GMST, or global average surface temperature) is calculated by averaging the temperatures over sea (sea surface temperature) and land (surface air temperature).

Series of reliable global temperature measurements began in the 1850—1880 time frame (this is called the instrumental temperature record). Through 1940, the average annual temperature 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, to at least 1.1 °C (1.9 °F) above 1880 levels.[3] The current annual GMST is about 15 °C (59 °F),[4] though monthly temperatures can vary almost 2 °C (4 °F) above or below this figure.[5]

Definition

The IPCC Sixth Assessment Report defines global mean surface temperature (GMST) as follows: GMST is the "estimated global average of near-surface air temperatures over land and sea ice, and sea surface temperature (SST) over ice-free ocean regions, with changes normally expressed as departures from a value over a specified reference period".[6]: 2231 

In comparison, the global mean surface air temperature (GSAT) is the "global average of near-surface air temperatures over land, oceans and sea ice. Changes in GSAT are often used as a measure of global temperature change in climate models."[6]: 2231 

Relevance

Changes in global temperatures over the past century provide evidence for the effects of increasing greenhouse gasses. When the climate system reacts to such changes, climate change follows. Measurement of the GST(global surface temperature) is one of the many lines of evidence supporting the scientific consensus on climate change, which is that humans are causing warming of Earth's climate system.

Projected global surface temperature changes relative to 1850–1900, based on CMIP6 multi-model mean changes

Measurement and calculation

The global surface temperature (GST) is calculated by averaging the temperatures over sea (sea surface temperature) and land (surface air temperature).

Surface air temperature change over the past 50 years.[7]

Instrumental temperature records are based on direct, instrument-based measurements of air temperature and ocean temperature, unlike indirect reconstructions using climate proxy data such as from tree rings and ocean sediments.[8] The longest-running temperature record is the Central England temperature data series, which starts in 1659. The longest-running quasi-global records start in 1850.[9] Temperatures on other time scales are explained in global temperature record.

"Global temperature" can have different definitions. There is a small difference between air and surface temperatures.[10]: 12 

Observations

Surface air temperature change over the past 50 years.[11]

Global warming affects all parts of Earth's climate system.[12] Global surface temperatures have risen by 1.1 °C (2.0 °F). Scientists say they will rise further in the future.[13][14] The changes in climate are not uniform across the Earth. In particular, most land areas have warmed faster than most ocean areas. The Arctic is warming faster than most other regions.[15] Night-time temperatures have increased faster than daytime temperatures.[16] The impact on nature and people depends on how much more the Earth warms.[17]: 787 

Scientists use several methods to predict the effects of human-caused climate change. One is to investigate past natural changes in climate.[18] To assess changes in Earth's past climate scientists have studied tree rings, ice cores, corals, and ocean and lake sediments.[19] These show that recent temperatures have surpassed anything in the last 2,000 years.[20] By the end of the 21st century, temperatures may increase to a level last seen in the mid-Pliocene. This was around 3 million years ago.[21]: 322  At that time, mean global temperatures were about 2–4 °C (3.6–7.2 °F) warmer than pre-industrial temperatures. The global mean sea level was up to 25 metres (82 ft) higher than it is today.[22]: 323  The modern observed rise in temperature and CO2 concentrations has been rapid. even abrupt geophysical events in Earth's history do not approach current rates.[23]: 54 

Effects

Thick orange-brown smoke blocks half a blue sky, with conifers in the foreground
A few grey fish swim over grey coral with white spikes
Desert sand half covers a village of small flat-roofed houses with scattered green trees
large areas of still water behind riverside buildings
Some climate change effects: wildfire caused by heat and dryness, bleached coral caused by ocean acidification and heating, environmental migration caused by desertification, and coastal flooding caused by storms and sea level rise.

Climate change affects the physical environment, ecosystems and human societies. Changes in the climate system include an overall warming trend, more extreme weather and rising sea levels. These in turn impact nature and wildlife, as well as human settlements and societies.[24] The effects of human-caused climate change are broad and far-reaching. This is especially so if there is no significant climate action. Experts sometimes describe the projected and observed negative impacts of climate change as the climate crisis.

The changes in climate are not uniform across the Earth. In particular, most land areas have warmed faster than most ocean areas. The Arctic is warming faster than most other regions.[15] There are many effects of climate change on oceans. These include an increase in ocean temperatures, a rise in sea level from ocean warming and ice sheet melting. They include increased ocean stratification. They also include changes to ocean currents including a weakening of the Atlantic meridional overturning circulation.[25]: 10  Carbon dioxide from the atmosphere is acidifiying the ocean.[26]

Recent warming has had a big effect on natural biological systems.[27]: 81  It has degraded land by raising temperatures, drying soils and increasing wildfire risk.[28]: 9  Species all over the world are migrating towards the poles to colder areas. On land, many species move to higher ground, whereas marine species seek colder water at greater depths.[29] At 2 °C (3.6 °F) of warming, around 10% of species on land would become critically endangered.[30]: 259 

See also

References

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