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

==== 1.5.1.2 Threats to Observational Capacity or Continuity ====

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The lockdowns and societal outcomes arising from the COVID-19 pandemic pose a new threat to observing systems. For example, WMO and UNESCO-IOC (Intergovernmental Oceanographic Commission) published a summary of the changes to Earth system observations during COVID-19 ( [[#WMO--2020b|WMO, 2020b]] ). Fewer aircraft flights (down 75–90% in May 2020, depending on region) and ship transits (down 20% in May 2020) mean that onboard observations from those networks have reduced in number and frequency ( [[#James--2020|James et al., 2020]] ; [[#Ingleby--2021|Ingleby et al., 2021]] ). Europe has deployed more radiosonde soundings to account for the reduction in data from air traffic. Fewer ocean observing buoys were deployed during 2020, and reductions have been particularly prevalent in the tropics and Southern Hemisphere. The full consequences of the pandemic, and responses to it, will come to light over time. Estimates of the effect of the reduction in aircraft data assimilation on weather forecasting skill are small ( [[#James--2020|James et al., 2020]] ; [[#Ingleby--2021|Ingleby et al., 2021]] ), potentially alleviating concerns about veracity of future atmospheric reanalyses of the COVID-19 pandemic period.

Surface-based networks have reduced in their coverage or range of variables measured due to COVID-19 and other factors. Over land, several factors, including the ongoing transition from manual to automatic observations of weather, have reduced the spatial coverage of certain measurement types, including rainfall intensity, radiosonde launches and pan evaporation, posing unique risks to datasets used for climate assessment ( [[#WMO--2017|WMO, 2017]] ; [[#Lin--2019|Lin and Huybers, 2019]] ). Ship-based measurements, which are important for ocean climate and reanalyses through time ( [[#Smith--2019|]] [[#Smith--2019|]] [[#Smith--2019|Smith et al., 2019]] ), have been in decline due to the number of ships contributing observations. There has also been a decline in the number of variables recorded by ships, but an increase in the quality and time-resolution of others (e.g., sea level pressure, [[#Kent--2019|Kent et al., 2019]] ).

Certain satellite frequencies are used to detect meteorological features that are vital to climate change monitoring. These can be disturbed by certain radio communications ( [[#Anterrieu--2016|Anterrieu et al., 2016]] ), although scientists work to remove noise from the signal ( [[#Oliva--2016|Oliva et al., 2016]] ). For example, water vapour in the atmosphere naturally produces a weak signal at 23.8 gigahertz (GHz), which is within the range of frequencies of the 5G cellular communications network ( [[#Liu--2021|Liu et al., 2021]] ). Concern has been raised about potential leakage from 5G network transmissions into the operating frequencies of passive sensors on existing weather satellites, which could adversely influence their ability to remotely observe water vapour in the atmosphere ( [[#Yousefvand--2020|Yousefvand et al., 2020]] ).

Threats to observational capacity also include the loss of natural climate archives that are disappearing as a direct consequence of warming temperatures. Ice-core records from vulnerable alpine glaciers in the tropics ( [[#Permana--2019|Permana et al., 2019]] ) and the mid-latitudes ( [[#Gabrielli--2016|Gabrielli et al., 2016]] ; [[#Winski--2018|Winski et al., 2018]] ; [[#Moreno--2021|Moreno et al., 2021]] ) document more frequent melt layers in recent decades, with glacial retreat occurring at a rate and geographic scale that is unusual in the Holocene ( [[#Solomina--2015|Solomina et al., 2015]] ). The scope and severity of coral bleaching and mortality events have increased in recent decades ( [[#Hughes--2018|Hughes et al., 2018]] ), with profound implications for the recovery of coral climate archives from new and existing sites. An observed increase in the mortality of larger, long-lived trees over the last century is attributed to a combination of warming, land-use change, and disturbance (e.g., [[#McDowell--2020|McDowell et al., 2020]] ). The ongoing loss of these natural, high-resolution climate archives endanger an end in their coverage over recent decades, given that many of the longest monthly- to annually-resolved paleoclimate records were collected in the 1960s to 1990s (e.g., the PAGES2K database as represented in [[#PAGES%202k%20Consortium--2017|PAGES 2k Consortium, 2017]] ). This gap presents a barrier to the calibration of existing decades-to-centuries-long records needed to constrain past temperature and hydrology trends and extremes.

Historical archives of weather and climate observations contained in ships’ logs, weather diaries, observatory logbooks and other sources of documentary data also risk being lost, for example to natural disasters or accidental destruction. These archives include measurements of temperature (air and sea surface), rainfall, surface pressure, wind strength and direction, sunshine amount, and many other variables back into the 19th century. While internationally coordinated data-rescue efforts are focused on recovering documentary sources of past weather and climate data (e.g., [[#Allan--2011|Allan et al., 2011]] ), no such coordinated efforts exist for vulnerable paleoclimate archives. Furthermore, oral traditions about local and regional weather and climate from indigenous peoples represent valuable sources of information, especially when used in combination with instrumental climate data ( [[#Makondo--2018|Makondo and Thomas, 2018]] ), but are in danger of being lost as indigenous knowledge-holders pass away.

In summary, while the quantity, quality and diversity of climate system observations have grown since AR5, the loss or potential loss of several critical components of the observational network is also evident ( ''hi'' ''gh confidence'' ).

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