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

===== ''1.5.1.1.1 Atmosphere, land and hyd'' ''rological cycle'' =====

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Satellites provide observations of a large number of key atmospheric and land-surface variables, ensuringsustained observations over wide areas. Since AR5, such observations have expanded to include satellite retrievals of atmospheric CO <sub>2</sub> via the NASA Orbiting Carbon Observatory satellites (OCO-2 and OCO-3; [[#Eldering--2017|Eldering et al., 2017]] ), following on from similar efforts employing the Greenhouse Gases Observing Satellite (GOSat; [[#Yokota--2009|Yokota et al., 2009]] ; [[#Inoue--2016|Inoue et al., 2016]] ). By combining remote sensing and in situ measurements, knowledge of fluxes between the atmosphere and land surface has improved ( [[#Rebmann--2018|Rebmann et al., 2018]] ). FLUXNET ( https://fluxnet.org/ ) has been providing eddy covariance measurements of carbon, water, and energy fluxes between the land and the atmosphere, with some of the stations operating for over 20 years ( [[#Pastorello--2017|Pastorello et al., 2017]] ), while the Baseline Surface Radiation Network (BSRN) has been maintaining high-quality radiation observations since the 1990s ( [[#Ohmura--1998|Ohmura et al., 1998]] ; [[#Driemel--2018|Driemel et al., 2018]] ).

Observations of the composition of the atmosphere have been further improved through expansions of existing surface observation networks ( [[#Bodeker--2016|Bodeker et al., 2016]] ; [[#De%20Mazière--2018|De Mazière et al., 2018]] ) and through in situ measurements such as aircraft campaigns (Sections 2.2, 5.2 and Section 6.2). Examples of expanded networks include the Aerosols, Clouds and Trace Gases Research Infrastructure (ACTRIS; [[#Pandolfi--2018|Pandolfi et al., 2018]] ), which focuses on short-lived climate forcers, and the Integrated Carbon Observation System (ICOS), which allows scientists to study and monitor the global carbon cycle and GHG emissions ( [[#Colomb--2018|Colomb et al., 2018]] ). Examples of recent aircraft observations include the Atmospheric Tomography Mission (ATom), which has flown repeatedly along the north–south axis of both the Pacific and Atlantic oceans, and the continuation of the In-service Aircraft for a Global Observing System (IAGOS) effort, which measures atmospheric composition from commercial aircraft ( [[#Petzold--2015|Petzold et al., 2015]] ).

Two distinctly different but important remote-sensing systems can provide information about temperature and humidity since the early 2000s. Global navigation satellite systems (e.g., GPS), radio occultation and limb soundings provide information, although only data for the upper troposphere and lower stratosphere are suitable to support climate change assessments ( [[#Angerer--2017|Angerer et al., 2017]] ; [[#Scherllin-Pirscher--2017|Scherllin-Pirscher et al., 2017]] ; [[#Gleisner--2020|Gleisner et al., 2020]] ; [[#Steiner--2020|Steiner et al., 2020]] ). These measurements complement those from the Atmospheric Infrared Sounder (AIRS; [[#Chahine--2006|Chahine et al., 2006]] ). AIRS has limitations in cloudy conditions, although these limitations have been partly solved using new methods of analysis ( [[#Blackwell--2014|Blackwell and Milstein, 2014]] ; [[#Susskind--2014|Susskind et al., 2014]] ). These new data sources now have sufficiently long records to strengthen the analysis of atmospheric warming in [[IPCC:Wg1:Chapter:Chapter-2|Chapter 2]] ( [[IPCC:Wg1:Chapter:Chapter-2#2.3.1.2|Section 2.3.1.2]] ).

Assessments of the hydrological cycle in Chapters 2 and 8 are supported by longer time series and new developments. Examples are new satellites ( [[#McCabe--2017|McCabe et al., 2017]] ) and measurements of water vapour using commercial laser absorption spectrometers and water vapour isotopic composition ( [[#Steen-Larsen--2015|Steen-Larsen et al., 2015]] ; [[#Zannoni--2019|Zannoni et al., 2019]] ). Data products of higher quality have been developed since AR5, such as the multi-source weighted ensemble precipitation ( [[#Beck--2017|Beck et al., 2017]] ) and multi-satellite terrestrial evaporation products ( [[#Fisher--2017|Fisher et al., 2017]] ). Longer series are available for satellite-derived global inundation data ( [[#Prigent--2020|Prigent et al., 2020]] ). Observations of soil moisture are now available via the Soil Moisture and Ocean Salinity (SMOS) and the Soil Moisture Active Passive (SMAP) satellite retrievals, filling critical gaps in the observation of hydrological trends and variability over land ( [[#Dorigo--2017|Dorigo et al., 2017]] ). Similarly, the Gravity Recovery and Climate Experiment GRACE and GRACE-FO satellites ( [[#Tapley--2019|Tapley et al., 2019]] ) have provided key constraints on groundwater variability and trends around the world ( [[#Frappart--2018|Frappart and Ramillien, 2018]] ). The combination of new observations with other sources of information has led to updated estimates of heat storage in inland waters ( [[#Vanderkelen--2020|Vanderkelen et al., 2020]] ), contributing to revised estimates of heat storage on the continents (Section 7.2.2.3; [[#von%20Schuckmann--2020|von Schuckmann et al., 2020]] ).

The ongoing collection of information about the atmosphere as it evolves is supplemented by the reconstruction and digitization of data about past conditions. Programmes aimed at recovering information from sources such as handwritten weather journals and ships’ logs continue to make progress, and are steadily improving spatial coverage and extending our knowledge backward in time. For example, [[#Brönnimann--2019a|Brönnimann et al. (2019a)]] has recently identified several thousand sources of climate data for land areas in the pre-1890 period, with many from the 18th century. The vast majority of these data are not yet contained in international digital data archives, and substantial quantities of undigitized ships’ weather log data exist for the same period ( [[#Kaspar--2015|Kaspar et al., 2015]] ). Since AR5 there has been a growth of ‘citizen science’ activities, making use of volunteers to rapidly transcribe substantial quantities of weather observations. Examples of projects include: [http://oldWeather.org oldWeather.org] and [http://SouthernWeatherDiscovery.org SouthernWeatherDiscovery.org] (both of which used ship-based logbook sources); the DRAW project (Data Rescue: Archival and Weather, which recovered land-based station data from Canada); [http://WeatherRescue.org WeatherRescue.org] (land-based data from Europe); [http://JungleWeather.org JungleWeather.org] (data from the Congo); and the Climate History Australia project (data from Australia; e.g., [[#Park--2018|Park et al., 2018]] ; [[#Hawkins--2019|Hawkins et al., 2019]] ). Undergraduate students have also been recruited to successfully digitize rainfall data in Ireland ( [[#Ryan--2018|Ryan et al., 2018]] ). Such observations are an invaluable source of weather and climate information for the early historical period that continues to expand the digital archives (e.g., [[#Freeman--2017|Freeman et al., 2017]] ) which underpin observational datasets used across several Chapters.

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