Editing IPCC:AR6/WGI/Chapter-2 (section)

== 2.5 Final Remarks ==

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The assessment in this chapter is based upon an ever-expanding volume of available proxy and observational records and a growing body of literature. Since AR5, improvements have been made in all aspects of data collection, data curation, data provision and data analysis permitting improved scientific insights. The chapter has also benefitted from the availability of new products from a range of emerging, mainly space-based, observing capabilities and new generations of reanalysis products. However, a number of key challenges still remain which, if addressed, would serve to strengthen future reports.

* Development of new techniques and exploitation of existing and new proxy sources may help address challenges around the low temporal resolution of most paleoclimate proxy records, particularly prior to the Common Era, and ambiguities around converting paleoclimate proxy data into estimates of climate-relevant variables. Conversions rely upon important proxy-specific assumptions and biases can be large due to limited accounting of seasonality, non-climatic effects, or the influence of multiple climate variables. These challenges currently limit the ability to ascertain the historical unusualness of recent directly observed climate changes for many indicators. {2.2, 2.3}
* Improved sharing of historical instrumental and proxy records (including metadata) along with significant efforts at data rescue of presently undigitized records would serve to significantly strengthen many aspects of the present assessment. Longer observational time series help to better understand variability, and any underlying periodicity in climate indicators and climate forcers, especially for components of the climate system where the dominant response is on multi-decadal and longer time scales. Targeted rescue of early marine data records, especially from waters of the tropical oceans and the Southern Ocean, could help constrain modes of variability and important teleconnection changes in, for example, the hydrological cycle and patterns of global temperature change. {2.2, 2.3, 2.4}
* Contemporary observing systems have limitations in parts of the Earth system with limited accessibility or coverage, for example coastal and shallow ocean, polar regions, marine biosphere, and the deep sea, leading to seasonal-dependency and regional variability in data coverage and uncertainty which serve to limit confidence in the present assessment of changes. {2.3}
* Contradictory lines of evidence exist between observations and models on the relationship between the rates of warming in GMST and GSAT, compounded by limitations in theoretical understanding. Improvements in air temperature datasets over the ocean and an improved understanding of the representation of the lowermost atmosphere over the ocean in models would reduce uncertainty in assessed changes in GSAT. {Cross-Chapter Box 2.3}

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