Model simulations and proxy-based reconstructions are the main tools for quantifying pre-instrumental climate variations. For some metrics such as Northern Hemisphere mean temperatures, there is remarkable agreement between models and reconstructions. For other diagnostics, such as the regional response to volcanic eruptions, or hemispheric temperature differences, substantial disagreements between data and models have been reported. Here, we assess the potential sources of these discrepancies by comparing 1000-year hemispheric temperature reconstructions based on real-world paleoclimate proxies with climate-model-based pseudoproxies. These pseudoproxy experiments (PPE) indicate that noise inherent in proxy records and the unequal spatial distribution of proxy data are the key factors in explaining the data-model differences. For example, lower inter-hemispheric correlations in reconstructions can be fully accounted for by these factors in the PPE. Noise and data sampling also partly explain the reduced amplitude of the response to external forcing in reconstructions compared to models. For other metrics, such as inter-hemispheric differences, some, although reduced, discrepancy remains. Our results suggest that improving proxy data quality and spatial coverage is the key factor to increase the quality of future climate reconstructions, while the total number of proxy records and reconstruction methodology play a smaller role.
Bibliographical noteFunding Information:
This work partly resulted from contributions to the Past Global Changes (PAGES) 2k initiative. Members of the PAGES2k consortium and Rob Wilson are thanked for providing public access to proxy data and metadata from the SH and NH, respectively. R.N. is supported by the Swiss NSF (PZ00P2_154802). G.C.H., A.P.S. and R.N.’s visit in Edinburgh were partly funded by an European Research Council (ERC) Advanced Grant (EC-320691), Transition Into The ANthropocene (TITAN), G.C.H. was also supported by National Centre for Atmospheric Science (NCAS) and the Wolfson Foundation and the Royal Society as a Royal Society Wolfson Research Merit Award (WM130060) holder. G.C.H. and A.P.S. were supported by NERC under the Belmont forum, grant PacMedy (NE/P006752/1). N.J.S. is supported by the US National Science Foundation (OISE-1743738). CESM1(CAM5) Last Millennium Ensemble Community Project and supercomputing resources provided by NSF/CISL/Yellowstone are acknowledged for providing model data.
© 2018 The Author(s).