The overarching goal of this WP is to provide a solid evidence base to study TEs in the Earth system. We put considerable emphasis on abrupt transitions that have occurred in the long-term past, since these are the only largescale TP crossings that are evidenced in empirical data. This WP will (i) enable analysis of the interactions between TEs and of corresponding EWSs in empirical data of past climates, (ii) facilitate the evaluation of current IPCC-relevant models in collaboration with WP2, and (iii) help support a com-prehensive evaluation of climate sensitivity in WP4.

The concrete objectives of this WP are, numbered according to its 4 tasks:

  1. To provide the empirical basis to study abrupt climatic transitions that have occurred in past warm and cold climates, focusing on proxy data synthesis and synchronization of different records.
  2. To derive probabilistic time series representations of proxy records that allow for a mathematically rigorous propagation of dating uncertainties to subsequent analysis such as synchronization and dependency analyses between different records, search for EWS, but also the model evaluations planned in WP2.
  3. To assess interactions between different TEs, and the ecological and societal impacts of past abrupt climate transitions. This will provide valuable information for estimating the impacts of potentially similar events due to global warming in the future.
  4. To extend existing concepts of statistical EWSs in paleoclimatic proxy records by advancing the employed statistical estimators and taking into account associated physical mechanisms. This will provide detailed information regarding the specific subsystems and statistical characteristics in which EWSs for potential future abrupt transitions should be searched for.