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

==== 1.5.2.4 Reanalyses of the Pre-Instrumental Era ====

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Longer reanalyses that extend further back in time than the beginning of the instrumental record are being developed. They include the complete integration of paleoclimate archives and newly available early instrumental data into extended reanalysis datasets. Such integration leverages ongoing development of climate models that can simulate paleoclimate records in their units of analysis (i.e., oxygen isotope composition, tree ring width, etc.), in many cases using physical climate variables as input for so-called proxy system models ( [[#Evans--2013|Evans et al., 2013]] ; [[#Dee--2015|Dee et al., 2015]] ). Ensemble Kalman filter data assimilation approaches allow for combining paleoclimate data and climate model data to generate annually resolved fields (Last Millenium Reanalysis, [[#Hakim--2016|Hakim et al., 2016]] ; [[#Tardif--2019|Tardif et al., 2019]] ) or even monthly fields ( [[#Franke--2017|Franke et al., 2017]] ). This allows for a greater understanding of decadal variability ( [[#Parsons--2019|Parsons and Hakim, 2019]] ) and greater certainty around the full range of the frequency and severity of climate extremes. This, in turn, allows for better-defined detection of change. It also helps to identify the links between biogeochemical cycles, ecosystem structure and ecosystem functioning, and to provide initial conditions for further model experiments or downscaling (Chapter 2).

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