Editing IPCC:AR6/WGI/Chapter-1 (section)

==== 1.5.2.5 Applications of Reanalyses ====

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The developments in reanalyses described above mean that they are now used across a range of applications. In AR6, reanalyses provide information for fields and in regions where observations are limited. There is growing confidence that modern reanalyses can provide another line of evidence in describing recent temperature trends (Tables 2.4 and 2.5). As their spatial resolution increases, the exploration of fine-scale extremes in both space and time becomes possible (e.g., wind; [[#Kaiser-Weiss--2015|Kaiser-Weiss et al., 2015]] ). Longer reanalyses can be used to describe the change in the climate over the last 100 to 1000 years. Reanalyses have been used to help post-process climate model output, and drive impact models; however, they are often bias adjusted first (Cross-Chapter Box 10.2; e.g., [[#Weedon--2014|Weedon et al., 2014]] ). Copernicus Climate Change Service (C3S) provides a bias-adjusted dataset for global land areas based on ERA5 called WFDE5 ( [[#Cucchi--2020|Cucchi et al., 2020]] ) which, combined with ERA5 information over the ocean (W5E5; [[#Lange--2019|Lange, 2019]] ), is used as the AR6 Interactive [[IPCC:Wg1:Chapter:Atlas|Atlas]] reference for the bias adjustment of model output.

The growing interest in longer-term climate forecasts (from seasonal to multi-year and decadal) means that reanalyses are now more routinely being used to develop the initial state for these forecasts, such as for the Decadal Climate Prediction Project (DCPP; [[#Boer--2016|Boer et al., 2016]] ). Ocean reanalyses are now being used routinely in the context of climate monitoring, (e.g., the Copernicus Marine Environment Monitoring Service Ocean State Report; [[#von%20Schuckmann--2019|von Schuckmann et al., 2019]] ).

In summary, reanalyses have improved since AR5 and can increasingly be used as a line of evidence in assessments of the state and evolution of the climate system ( ''high confidence'' ). Reanalyses provide consistency across multiple physical quantities, and information about variables and locations that are not directly observed. Since AR5, new reanalyses have been developed with various combinations of increased resolution, extended records, more consistent data assimilation, estimation of uncertainty arising from the range of initial conditions, and an improved representation of the atmosphere or ocean system. While noting their remaining limitations, this Report uses the most recent generation of reanalysis products alongside more standard observation-based datasets.

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