Temperature record of the last 2,000 years (Chart showing the so-called Medieval Warm Period and Little Ice Age were not planet-wide phenomena)
The global temperature record shows the fluctuations of the temperature of the atmosphere and the oceans through various spans of time. There are numerous estimates of temperatures since the end of the Pleistoceneglaciation, particularly during the current Holocene epoch. Some temperature information is available through geologic evidence, going back millions of years. More recently, information from ice cores covers the period from 800,000 years before the present time until now. A study of the paleoclimate covers the time period from 12,000 years ago to the present. Tree rings and measurements from ice cores can give evidence about the global temperature from 1,000-2,000 years before the present until now. The most detailed information exists since 1850, when methodical thermometer-based records began. Modifications on the Stevenson-type screen were made for uniform instrument measurements around 1880.[1]
Geologic evidence (millions of years)
Reconstruction of the past 5 million years of climate history, based on oxygen isotope fractionation in deep sea sediment cores (serving as a proxy for the total global mass of glacial ice sheets), fitted to a model of orbital forcing (Lisiecki and Raymo 2005)[2] and to the temperature scale derived from Vostok ice cores following Petit et al. (1999).[3]
On longer time scales, sediment cores show that the cycles of glacials and interglacials are part of a deepening phase within a prolonged ice age that began with the glaciation of Antarctica approximately 40 million years ago. This deepening phase, and the accompanying cycles, largely began approximately 3 million years ago with the growth of continental ice sheets in the Northern Hemisphere. Gradual changes in Earth's climate of this kind have been frequent during the Earth's 4,540 million year existence and most often are attributed to changes in the configuration of continents and ocean sea ways.[citation needed]
Ice cores (from 800,000 years before present)
Temperature estimates over 800,000 years of the EPICA ice cores in Antarctica. Temperatures are in Celsius relative to the average of the most recent 1,000 years; year 0 is 1950.
Even longer term records exist for few sites: the recent Antarctic EPICA core reaches 800 kyr; many others reach more than 100,000 years. The EPICA core covers eight glacial/interglacial cycles. The NGRIP core from Greenland stretches back more than 100 kyr, with 5 kyr in the Eemian interglacial. Whilst the large-scale signals from the cores are clear, there are problems interpreting the detail, and connecting the isotopic variation to the temperature signal.
The World Paleoclimatology Data Center (WDC) maintains the ice core data files of glaciers and ice caps in polar and low latitude mountains all over the world.
Ice core records from Greenland
As a paleothermometry, the ice core in central Greenland showed consistent records on the surface-temperature changes.[5] According to the records, changes in global climate are rapid and widespread. Warming phase only needs simple steps, however, the cooling process requires more prerequisites and bases.[6] Also, Greenland has the clearest record of abrupt climate changes in the ice core, and there are no other records that can show the same time interval with equally high time resolution.[5]
When scientists explored the trapped gas in the ice core bubbles, they found that the methane concentration in Greenland ice core is significantly higher than that in Antarctic samples of similar age, the records of changes of concentration difference between Greenland and Antarctic reveal variation of latitudinal distribution of methane sources.[7] Increase in methane concentration shown by Greenland ice core records implies that the global wetland area has changed greatly over past years.[8] As a component of greenhouse gases, methane plays an important role in global warming. The variation of methane from Greenland records makes a unique contribution for global temperature records undoubtedly.
Ice core records from Antarctica
The Antarctic ice sheet originated in the late Eocene, the drilling has restored a record of 800,000 years in Dome Concordia, and it is the longest available ice core in Antarctica. In recent years, more and more new studies have provided older but discrete records.[9] Due to the uniqueness of the Antarctic ice sheet, the Antarctic ice core not only records the global temperature changes, but also contains huge quantities of information about the global biogeochemical cycles, climate dynamics and abrupt changes in global climate.[10]
By comparing with current climate records, the ice core records in Antarctica further confirm that polar amplification.[11] Although Antarctica is covered by the ice core records, the density is rather low considering the area of Antarctica. Exploring more drilling stations is the primary goal for current research institutions.
Ice core records from low-latitude regions
The ice core records from low-latitude regions are not as common as records from polar regions, however, these records still provide much useful information for scientists. Ice cores in low-latitude regions usually locates in high altitude areas. The Guliya record is the longest record from low-latitude, high altitude regions, which spans over 700,000 years.[12] According to these records, scientists found the evidence which can prove the Last Glacial Maximum (LGM) was colder in the tropics and subtropics than previously believed.[13] Also, the records from low-latitude regions helped scientists confirm that the 20th century was the warmest period in the last 1000 years.[12]
Paleoclimate (from 12,000 years before present)
Plot showing the variations, and relative stability, of climate during the last 12000 years.
Many estimates of past temperatures have been made over Earth's history. The field of paleoclimatology includes ancient temperature records. As the present article is oriented toward recent temperatures, there is a focus here on events since the retreat of the Pleistoceneglaciers. The 10,000 years of the Holocene epoch covers most of this period, since the end of the Northern Hemisphere's Younger Dryas millennium-long cooling. The Holocene Climatic Optimum was generally warmer than the 20th century, but numerous regional variations have been noted since the start of the Younger Dryas.
Tree rings and ice cores (from 1,000–2,000 years before present)
Proxy measurements can be used to reconstruct the temperature record before the historical period. Quantities such as tree ring widths, coral growth, isotope variations in ice cores, ocean and lake sediments, cave deposits, fossils, ice cores, borehole temperatures, and glacier length records are correlated with climatic fluctuations. From these, proxy temperature reconstructions of the last 2000 years have been performed for the northern hemisphere, and over shorter time scales for the southern hemisphere and tropics.[14][15][16]
Geographic coverage by these proxies is necessarily sparse, and various proxies are more sensitive to faster fluctuations. For example, tree rings, ice cores, and corals generally show variation on an annual time scale, but borehole reconstructions rely on rates of thermal diffusion, and small scale fluctuations are washed out. Even the best proxy records contain far fewer observations than the worst periods of the observational record, and the spatial and temporal resolution of the resulting reconstructions is correspondingly coarse. Connecting the measured proxies to the variable of interest, such as temperature or rainfall, is highly non-trivial. Data sets from multiple complementary proxies covering overlapping time periods and areas are reconciled to produce the final reconstructions.[16][17]
Proxy reconstructions extending back 2,000 years have been performed, but reconstructions for the last 1,000 years are supported by more and higher quality independent data sets. These reconstructions indicate:[16]
global mean surface temperatures over the last 25 years have been higher than any comparable period since AD 1600, and probably since AD 900
As well as natural, numerical proxies (tree-ring widths, for example) there exist records from the human historical period that can be used to infer climate variations, including: reports of frost fairs on the Thames; records of good and bad harvests; dates of spring blossom or lambing; extraordinary falls of rain and snow; and unusual floods or droughts.[19] Such records can be used to infer historical temperatures, but generally in a more qualitative manner than natural proxies.
Recent evidence suggests that a sudden and short-lived climatic shift between 2200 and 2100 BCE occurred in the region between Tibet and Iceland, with some evidence suggesting a global change. The result was a cooling and reduction in precipitation. This is believed to be a primary cause of the collapse of the Old Kingdom of Egypt.[20]
Satellite and balloon (1950s–present)
A climate spiral depicting monthly anomalies in global temperature from 1880 till 2021.
Several groups have analyzed the satellite data to calculate temperature trends in the troposphere. Both the University of Alabama in Huntsville (UAH) and the private, NASA funded, corporation Remote Sensing Systems (RSS) find an upward trend.
For the lower troposphere, UAH found a global average trend between 1978 and 2019 of 0.130 degrees Celsius per decade.[21]
RSS found a trend of 0.148 degrees Celsius per decade, to January 2011.[22]
In 2004 scientists found trends of +0.19 degrees Celsius per decade when applied to the RSS dataset.[23] Others found 0.20 degrees Celsius per decade up between 1978 and 2005, since which the dataset has not been updated.[24]
Thermometers (1850–present)
Global average temperature datasets from various scientific organizations show substantial agreement concerning the progress and extent of global warming: pairwise correlations of 1850+/1880+ datasets exceed 99.1%.In recent decades, new high temperature records have substantially outpaced new low temperature records on a growing portion of Earth's surface.[25] Comparison shows seasonal variability.
The instrumental temperature record is a record of temperatures within Earth's climate based on direct measurement of air temperature and ocean temperature, using thermometers and other thermometry devices. Instrumental temperature records are distinguished from indirect reconstructions using climate proxy data such as from tree rings and ocean sediments.[26] Instrument-based data are collected from thousands of meteorological stations, buoys and ships around the globe. Whilst many heavily-populated areas have a high density of measurements, observations are more widely spread in sparsely populated areas such as polar regions and deserts, as well as over many parts of Africa and South America.[27] Measurements were historically made using mercury or alcohol thermometers which were read manually, but are increasingly made using electronic sensors which transmit data automatically. Records of global average surface temperature are usually presented as anomalies rather than as absolute temperatures. A temperature anomaly is measured against a reference value (also called baseline period or long-term average). For example, a commonly used baseline period is the time period 1951-1980.
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.[28] Temperatures are also measured in the upper atmosphere using a variety of methods, including radiosondes launched using weather balloons, a variety of satellites, and aircraft.[29] Satellites are used extensively to monitor temperatures in the upper atmosphere but to date have generally not been used to assess temperature change at the surface. In recent decades, global surface temperature datasets have been supplemented by extensive sampling of ocean temperatures at various depths, allowing estimates of ocean heat content.
The record shows a rising trend in global average surface temperatures (i.e. global warming) driven by human-induced emissions of greenhouse gases. The global average and combined land and ocean surface temperature show a warming of 1.09 °C (range: 0.95 to 1.20 °C) from 1850–1900 to 2011–2020, based on multiple independently produced datasets.[30]: 5 The trend is faster since 1970s than in any other 50-year period over at least the last 2000 years.[30]: 8 Within this long-term upward trend, there is short-term variability because of natural internal variability (e.g. ENSO, volcanic eruption), but record highs have been occurring regularly.
Supplement: Lisiecki, L. E.; Raymo, M. E. (2005). "Pliocene-Pleistocene stack of globally distributed benthic stable oxygen isotope records". Pangaea. doi:10.1594/PANGAEA.704257.
^Webb, Robert S.; Clark, Peter U.; Keigwin, Lloyd D. (1999), "Preface", Mechanisms of Global Climate Change at Millennial Time Scales, Washington, D. C.: American Geophysical Union, vol. 112, pp. vii–viii, Bibcode:1999GMS...112D...7W, doi:10.1029/gm112p0vii (inactive 2024-02-13), ISBN0-87590-095-X, retrieved 2021-04-18((citation)): CS1 maint: DOI inactive as of February 2024 (link)
^ abThompson, L. G. (2004), "High Altitude, Mid- and Low-Latitude Ice Core Records: Implications for Our Future", Earth Paleoenvironments: Records Preserved in Mid- and Low-Latitude Glaciers, Developments in Paleoenvironmental Research, Dordrecht: Kluwer Academic Publishers, vol. 9, pp. 3–15, doi:10.1007/1-4020-2146-1_1, ISBN1-4020-2145-3
^J.T. Houghton; et al., eds. (2001). "Figure 1: Variations of the Earth's surface temperature over the last 140 years and the last millennium.". Summary for policy makers. IPCC Third Assessment Report - Climate Change 2001 Contribution of Working Group I. Intergovernmental Panel on Climate Change. Archived from the original on November 13, 2016. Retrieved May 12, 2011.
^J.T. Houghton; et al., eds. (2001). Chapter 2. Observed climate variability and change. Climate Change 2001: Working Group I The Scientific Basis. Intergovernmental Panel on Climate Change. Archived from the original on March 9, 2016. Retrieved May 12, 2011.
^ abcNational Research Council (U.S.). Committee on Surface Temperature Reconstructions for the Last 2,000 Years Surface temperature reconstructions for the last 2,000 years (2006), National Academies Press ISBN978-0-309-10225-4
^O.Muszkat, The outline of the problems and methods used for research of the history of the climate in the Middle Ages, (in polish), Przemyśl 2014, ISSN1232-7263
^"What Are "Proxy" Data?". NCDC.NOAA.gov. National Climatic Data Center, later called the National Centers for Environmental Information, part of the National Oceanic and Atmospheric Administration. 2014. Archived from the original on 10 October 2014.
