community’s efforts to expand to capture the full range of variability and climate-forcing feedbacks of the global climate system, in particular for the past “extreme climate events” and warmer Earth intervals that may serve as analogues for future climate. Full testing of climate models for these time periods will require evaluation of feedback processes within models, enhanced spatial resolution, and longer simulations to better characterize climate model variability. All of these requirements, especially those for resolution and variability, will require significant computational resources.

For deep-time climate systems, the representation of paleogeographic boundary conditions can be a much greater source of uncertainty than it is for simulations based on modern geography. Furthermore, discrepancies between model outputs and paleoclimate observations may indicate the existence of additional processes, feedbacks, and/or sensitivities that are not present in the model or expressed in the modern climate system. For example, the exceptionally warm high latitudes during all past warm periods—whether transient or long term—cannot be reproduced by models without invoking unreasonable CO2 levels, revealing the inability of current models to fully capture the processes and feedbacks governing heat transport and retention or the processes that might generate heat in the polar regions under elevated atmospheric greenhouse gas levels (Covey and Barron, 1988; Rind and Chandler, 1991; Covey, 1991; Sloan and Pollard, 1998; Bice et al., 2006; Huber, 2008; Kump and Pollard, 2008; Spicer et al., 2008; Zachos et al., 2008). Thus, model development, which is based on improving specific processes and climate feedbacks and, in turn, evaluating the impact of these improvements on model simulations, depends on the availability of spatially resolved, robust, deep-time paleoclimate reconstructions of appropriate geochronological resolution and constraint. In addition, the utility of paleoclimate proxies for climate reconstruction and data-model comparisons relies on the proxies being sufficiently well preserved and the existence of an adequate understanding of the underlying processes, sensitivities, and uncertainties captured by these proxies.

Recent paleoclimate studies of deep-time successions have documented the potential of the older part of the geological record to reveal long-duration archives of forcings, responses, and long-term (centuries to tens of millennia) feedbacks that are of magnitudes and/or durations not captured by Pleistocene and Holocene paleoclimate records. Constraining the nature (e.g., rates, phasing between proxies) and origin (forcings) of climatic shifts, particularly rapid and/or transient events across climate thresholds from the deep-time record, will be greatly enhanced where orbital-scale cycles can be identified and resolved in the rock record (Box 4.1, Figures 4.1 and 4.2). Indeed, millennial to seasonal signals—at times calibrated to the orbital timescale—have been extracted from the sedimentary record spanning hundreds of millions of years (e.g., Feldman



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