Editing IPCC:Wg1:Chapter:Chapter-1-comments (section)

=== 1.3.1 Lines of Evidence: Instrumental Observations ===

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Instrumental observations of the atmosphere, ocean, land, biosphere and cryosphere underpin all understanding of the climate system. This section describes the evolution of instrumental data for major climate variables at Earth’s land and ocean surfaces, at altitude in the atmosphere, and at depth in the ocean. Many data records exist, of varying length, continuity and spatial distribution; Figure 1.7 gives a schematic overview of temporal coverage.

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'''Figure 1.7 |''' '''Schematic of temporal coverage of (a) selected instrumental climate observations and (b) selected paleoclimate archives.''' The satellite era began in 1979 CE. The width of the taper gives an indication of the amount of available records.
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Instrumental weather observation at the Earth’s surface dates to the invention of thermometers and barometers in the 17th century. National and colonial weather services built networks of surface stations in the 19th century. By the mid-19th century, semi-standardized naval weather logs recorded winds, currents, precipitation, air pressure, and temperature at sea, initiating the longest continuous quasi-global instrumental record ( [[#Maury--1849|Maury, 1849]] , 1855, 1860). Because the ocean covers over 70% of global surface area and constantly exchanges energy with the atmosphere, both air and sea surface temperatures (SST) recorded in these naval logs are crucial variables in climate studies. [[#Dove--1853|Dove (1853)]] mapped seasonal isotherms over most of the globe. By 1900, a patchy weather data-sharing system reached all continents except Antarctica. Regular compilation of climatological data for the world began in 1905 with the Réseau Mondial (Air Ministry – Meteorological Office, 1921), and similar compilations – the World Weather Records ( [[#Clayton--1927|Clayton, 1927]] ) and Monthly Climatic Data for the World (est. 1948) – have been published continuously since their founding.

Landand ocean surface temperature data have been repeatedly evaluated, refined and extended ( [[#1.5.1|Section 1.5.1]] ). As computer power increased and older data were recovered from handwritten records, the number of surface station records used in published global land temperature time series grew. A pioneering study for 1880–1935 used fewer than 150 stations ( [[#Callendar--1938|Callendar, 1938]] ). A benchmark study of 1880–2005 incorporated 4300 stations ( [[#Brohan--2006|Brohan et al., 2006]] ). A study of the 1753–2011 period included previously unused station data, for a total of 36,000 stations ( [[#Rohde--2013|Rohde et al., 2013]] ); recent versions of this dataset comprise over 40,000 land stations ( [[#Rohde--2020|Rohde and Hausfather, 2020]] ). Several centres, including the National Oceanic and Atmospheric Administration (NOAA), Hadley, and Japan Meteorological Agency (JMA), produce SST datasets independently calculated from instrumental records. In the 2000s, adjustments for bias due to different measurement methods (buckets, engine intake thermometers, moored and drifting buoys) resulted in major improvements of SST data ( [[#Thompson--2008|Thompson et al., 2008]] ), and these improvements continue ( [[#Huang--2017|Huang et al., 2017]] ; [[#Kennedy--2019|Kennedy et al., 2019]] ). SST and land-based data are incorporated into global surface temperature datasets calculated independently by multiple research groups, including NOAA, NASA, Berkeley Earth, Hadley-CRU, JMA, and China Meteorological Administration (CMA). Each group aggregates the raw measurement data, applies various adjustments for non-climatic biases such as urban heat-island effects, and addresses unevenness in geospatial and temporal sampling with various techniques (see ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.1.1.3|Section 2.3.1.1.3]] and Table 2.4 for references). Other research groups provide alternative interpolations of these datasets using different methods (e.g., [[#Cowtan--2014|Cowtan and Way, 2014]] ; [[#Kadow--2020|Kadow et al., 2020]] ). Using the then available global surface temperature datasets, AR5 WGI assessed that the GMST increased by 0.85°C from 1880 to 2012 and found that each of the three decades following 1980 was successively warmer at the Earth’s surface than any preceding decade since 1850 ( [[#IPCC--2013b|IPCC, 2013b]] ). Marine air temperatures, especially those measured during nighttime, are increasingly also used to examine variability and long-term trends (e.g., [[#Rayner--2006|Rayner et al., 2006]] ; [[#Kent--2013|Kent et al., 2013]] ; [[#Cornes--2020|Cornes et al., 2020]] ; [[#Junod--2020|Junod and Christy, 2020]] ). Cross-Chapter Box 2.3 discusses updates to the global temperature datasets, provides revised estimates for the observed changes and considers whether marine air temperatures are changing at the same rate as SSTs.

Data at altitude came initially from scattered mountain summits, balloons and kites, but the upper troposphere and stratosphere were not systematically observed until radiosonde (weather balloon) networks emerged in the 1940s and 1950s. These provide the longest continuous quasi-global record of the atmosphere’s vertical dimension ( [[#Stickler--2010|Stickler et al., 2010]] ). New methods for spatial and temporal homogenisation (intercalibration and quality control) of radiosonde records were introduced in the 2000s ( [[#Sherwood--2008|Sherwood et al., 2008]] , 2015; [[#Haimberger--2012|Haimberger et al., 2012]] ). Since 1978, Microwave Sounding Units (MSU) mounted on Earth-orbiting satellites have provided a second high-altitude data source, measuring temperature, humidity, ozone, and liquid water throughout the atmosphere. Over time, these satellite data have required numerous adjustments to account for such factors as orbital precession and decay ( [[#Edwards--2010|Edwards, 2010]] ). Despite repeated adjustments, however, marked differences remain in the temperature trends from surface, radiosonde, and satellite observations; between the results from three research groups that analyse satellite data (University of Alabama in Huntsville (UAH), Remote Sensing Systems (RSS), and NOAA); and between modelled and satellite-derived tropospheric warming trends ( [[#Thorne--2011|Thorne et al., 2011]] ; [[#Santer--2017|Santer et al., 2017]] ). These differences are the subject of ongoing research ( [[#Maycock--2018|Maycock et al., 2018]] ). In the 2000s, Atmospheric Infrared Sounder (AIRS) and radio occultation (GNSS-RO) measurements provided new ways to measure temperature at altitude, complementing data from the MSU. GNSS-RO is a new independent, absolutely calibrated source, using the refraction of radio-frequency signals from the Global Navigation Satellite System (GNSS) to measure temperature, pressure and water vapour ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.1.2.1|Section 2.3.1.2.1]] ; [[#Foelsche--2008|Foelsche et al., 2008]] ; [[#Anthes--2011|Anthes, 2011]] ).

Heat-retaining properties of the atmosphere’s constituent gases were closely investigated in the 19th century. [[#Foote--1856|Foote (1856)]] measured solar heating of CO <sub>2</sub> experimentally and argued that higher concentrations in the atmosphere would increase Earth’s temperature. Water vapour, ozone, CO <sub>2</sub> and certain hydrocarbons were found to absorb longwave (infrared) radiation, the principal mechanism of the greenhouse effect ( [[#Tyndall--1861|Tyndall, 1861]] ). Nineteenth-century investigators also established the existence of a natural biogeochemical carbon cycle. Carbon dioxide emitted by volcanoes is removed from the atmosphere through a combination of silicate rock weathering, deep-sea sedimentation, oceanic absorption, and biological storage in plants, shellfish, and other organisms. On multi-million-year time scales, the compression of fossil organic matter is stored as carbon as coal, oil and natural gas ( [[#Chamberlin--1897|Chamberlin, 1897]] , 1898; [[#Ekholm--1901|Ekholm, 1901]] ).

Arrhenius (1896) calculated that a doubling of atmospheric CO <sub>2</sub> would produce warming of 5°C–6°C, but in 1900 new measurements seemed to rule out CO <sub>2</sub> as a greenhouse gas due to overlap with the absorption bands of water vapour ( [[#Ångström--1900|Ångström, 1900]] ; [[#Very--1901|Very and Abbe, 1901]] ). Further investigation and more sensitive instruments later overturned Ångström’s conclusion ( [[#Fowle--1917|Fowle, 1917]] ; [[#Callendar--1938|Callendar, 1938]] ). Nonetheless, the major role of CO <sub>2</sub> in the energy balance of the atmosphere was not widely accepted until the 1950s ( [[#Callendar--1949|Callendar, 1949]] ; [[#Plass--1956|Plass, 1956]] , 1961; [[#Manabe--1961|Manabe and Möller, 1961]] ; [[#Weart--2008|Weart, 2008]] ; [[#Edwards--2010|Edwards, 2010]] ). Revelle and Keeling established CO <sub>2</sub> monitoring stations in Antarctica and Hawaii during the 1957–1958 International Geophysical Year ( [[#Revelle--1957|Revelle and Suess, 1957]] ; [[#Keeling--1960|Keeling, 1960]] ). These stations have tracked rising atmospheric CO <sub>2</sub> concentrations from 315 ppm in 1958 to 414 ppm in 2020. Ground-based monitoring of other GHGs followed. The Greenhouse Gases Observing Satellite (GOSat) was launched in 2009, and two Orbiting Carbon Observatory satellite instruments have been in orbit since 2014.

The AR5 WGI highlighted ‘the other CO <sub>2</sub> problem’ ( [[#Doney--2009|Doney et al., 2009]] ), that is, ocean acidification caused by the absorption of some 20–30% of anthropogenic CO <sub>2</sub> from the atmosphere and its conversion to carbonic acid in seawater. The AR5 WGI assessed that the pH of ocean surface water has decreased by 0.1 since the beginning of the industrial era ( ''high confidence'' ), indicating approximately a 30% increase in acidity ( [[#IPCC--2013b|IPCC, 2013b]] ).

With a heat capacity about 1000 times greater than that of the atmosphere, Earth’s ocean stores the vast majority of energy retained by the planet. Ocean currents transport the stored heat around the globe and, over decades to centuries, from the surface to its greatest depths. The ocean’s thermal inertia moderates faster changes in radiative forcing on land and in the atmosphere, reaching full equilibrium with the atmosphere only after hundreds to thousands of years ( [[#Yang--2011|Yang and Zhu, 2011]] ). The earliest subsurface measurements in the open ocean date to the 1770s ( [[#Abraham--2013|Abraham et al., 2013]] ). From 1872–76, the research ship ''HMS Challenger'' measured global ocean temperature profiles at depths up to 1700 m along its cruise track. By 1900, research ships were deploying instruments such as Nansen bottles and mechanical bathythermographs (MBTs) to develop profiles of the upper 150 m in areas of interest to navies and commercial shipping ( [[#Abraham--2013|Abraham et al., 2013]] ). Starting in 1967, eXpendable BathyThermographs (XBTs) were deployed by scientific and commercial ships along repeated transects to measure temperature to 700 m ( [[#Goni--2019|Goni et al., 2019]] ). Ocean data collection expanded in the 1980s with the Tropical Ocean Global Experiment (TOGA; [[#Gould--2003|Gould, 2003]] ). Marine surface observations for the globe, assembled in the mid-1980s in the International Comprehensive Ocean-Atmosphere Data Set (ICOADS; [[#Woodruff--1987|Woodruff et al., 1987]] , 2005), were extended to 1662–2014 using newly recovered marine records and metadata ( [[#Woodruff--1998|Woodruff et al., 1998]] ; [[#Freeman--2017|Freeman et al., 2017]] ). The Argo submersible float network, developed in the early 2000s, provided the first systematic global measurements of the 700–2000 m layer. Comparing the ''HMS Challenger'' data to data from Argo submersible floats revealed global subsurface ocean warming on the centennial scale ( [[#Roemmich--2012|Roemmich et al., 2012]] ). The AR5 WGI assessed with ''high confidence'' that ocean warming accounted for more than 90% of the additional energy accumulated by the climate system between 1971 and 2010 ( [[#IPCC--2013b|IPCC, 2013b]] ). In comparison, warming of the atmosphere corresponds to only about 1% of the additional energy accumulated over that period ( [[#IPCC--2013a|IPCC, 2013a]] ). [[IPCC:Wg1:Chapter:Chapter-2|Chapter 2]] summarizes the ocean heat content datasets used in AR6 ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.1|Section 2.3.3.1]] and Table 2.7).

Water expands as it warms. This thermal expansion, along with glacier mass loss, were the dominant contributors to GMSL rise during the 20th century ( ''high confidence'' ) according to AR5 ( [[#IPCC--2013b|IPCC, 2013b]] ). Sea level can be measured by averaging across tide gauges, some of which date to the 18th century. However, translating tide gauge readings into GMSL is challenging, since their spatial distribution is limited to continental coasts and islands, and their readings are relative to local coastal conditions that may shift vertically over time. Satellite radar altimetry, introduced operationally in the 1990s, complements the tide gauge record with geocentric measurements of GMSL at much greater spatial coverage ( [[#Katsaros--1991|Katsaros and Brown, 1991]] ; [[#Fu--1994|Fu et al., 1994]] ). The AR5 WGI assessed that GMSL rose by 0.19 [0.17 to 0.21] m over the period 1901–2010, and that the rate of sea level rise increased from 2.0 [1.7 to 2.3] mm yr <sup>–1</sup> in 1971–2010 to 3.2 [2.8 to 3.6] mm yr <sup>–1</sup> from 1993–2010. Warming of the ocean ''very likely'' contributed 0.8 [0.5 to 1.1] mm yr <sup>–1</sup> of sea level change during 1971–2010, with the majority of that contribution coming from the upper 700 m ( [[#IPCC--2013b|IPCC, 2013b]] ). [[IPCC:Wg1:Chapter:Chapter-2|Chapter 2]] ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.3.3|Section 2.3.3.3]] ) assesses current understanding of the extent and rate of sea level rise, past and present.

Satellite remote sensing alsorevolutionized studies of the cryosphere (Sections 2.3.2 and 9.3–9.5), particularly near the poles, where conditions make surface observations very difficult. Satellite mapping and measurement of snow cover began in 1966, with land and sea ice observations following in the mid-1970s. Yet prior to the Third Assessment Report, researchers lacked sufficient data to tell whether the Greenland and Antarctic ice sheets were shrinking or growing. Through a combination of satellite and airborne altimetry and gravity measurements, and improved knowledge of surface mass balance and perimeter fluxes, a consistent signal of ice loss for both ice sheets was established by the time of AR5 ( [[#Shepherd--2012|Shepherd et al., 2012]] ). After 2000, satellite radar interferometry revealed rapid changes in surface velocity at ice-sheet margins, often linked to reduction or loss of ice shelves ( [[#Scambos--2004|Scambos et al., 2004]] ; [[#Rignot--2006|Rignot and Kanagaratnam, 2006]] ). Whereas sea ice area and concentration have been continuously monitored since 1979 via microwave imagery, datasets for ice thickness emerged later from upward sonar profiling by submarines ( [[#Rothrock--1999|Rothrock et al., 1999]] ) and radar altimetry of sea ice freeboards ( [[#Laxon--2003|Laxon et al., 2003]] ). A recent reconstruction of Arctic sea ice extent back to 1850 found no historical precedent for the Arctic sea ice minima of the 21st century ( [[#Walsh--2017|Walsh et al., 2017]] ). Glacier length has been monitored for decades to centuries; internationally coordinated activities now compile worldwide glacier length and mass balance observations (World Glacier Monitoring Service, [[#Zemp--2015|Zemp et al., 2015]] ), global glacier outlines (Randolph Glacier Inventory, [[#Pfeffer--2014|Pfeffer et al., 2014]] ), and ice thickness data for about 1100 glaciers (Glacier Thickness Database (GlaThiDa), [[#Gärtner-Roer--2014|Gärtner-Roer et al., 2014]] ). In summary, these data allowed AR5 WGI to assess that over the last two decades, the Greenland and Antarctic ice sheets have been losing mass, glaciers have continued to shrink almost worldwide, and Arctic sea ice and Northern Hemisphere spring snow cover have continued to decrease in extent ( ''high confidence'' ) ( [[#IPCC--2013b|IPCC, 2013b]] ).

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