Challenges and perspectives for large‐scale temperature reconstructions of the past two millennia

Abstract

Knowledge of the temperature variability during the last one to two millennia is important for providing a perspective to present‐day climate excursions, for assessing the sensitivity of the climate to different forcings, and for providing a test bed for climate models. Since systematic instrumental temperature records only extend back to the nineteenth century, such knowledge mainly relies on climate‐sensitive proxy data. Here we critically assess some of the many challenges related to large‐scale multiproxy temperature reconstructions. We begin with a review of available large‐scale temperature reconstructions, focusing on the differences in low‐frequency variability and the response to natural forcings such as major volcanic eruptions and changes in total solar irradiance. Then, we discuss different proxy selection strategies, review previously used reconstruction methods, and discuss their ability to reconstruct the amplitude of the low‐frequency variability. To shed additional light on the challenges of large‐scale reconstructions, we investigate the spatial and temporal correlation structures in the observed temperature field and discuss the implications of these correlation structures regarding the required number and positions of proxies. We demonstrate how the unavoidable uncertainty related to noisy proxies will show up as bias and variance in the reconstruction and that the partition between these forms of errors depends on the reconstruction method. Pseudo‐proxy experiments are conducted to further discuss the influence of noise and the requirements regarding the geographical location and number of proxies necessary for reliably reconstructing the low‐frequency variability. We conclude with recommendations for future large‐scale temperature reconstructions.

Spatio-temporal variability of Arctic summer temperatures over the past two millennia: an overview of the last major climate anomalies

“…e.g. Christiansen and Ljungqvist, 2017). The reconstructed amplitude of the centennial-scale summer temperature variability was rather dampened and found to be less than 0.5°C but with large year-to-year and decadalto-decadal variability.…”

Reconstructing Late Holocene North Atlantic atmospheric circulation changes using functional paleoclimate networks

“…Note that while in general the CLD values should rather be described as a superposition of different climatic influences, we take here the opposite approach, which is potentially useful for obtaining a reconstruction of the (unknown) climate driver based upon our evolving functional paleoclimate network properties. For a detailed discussion on different (regular vs. inverse) versions of this regression problem and their implications in the context of paleoclimate reconstructions, we refer to Christiansen and Ljungqvist (2017) and Christiansen (2014) and references therein.…”

Pseudo-proxy evaluation of climate field reconstruction methods of North Atlantic climate based on an annually resolved marine proxy network

“…These studies tested the spatial skill of CFR methods using information from a composite proxy network including mostly terrestrial proxies. As CFRs depend on the characteristics of the proxy network used, such as proxy temporal resolution, growth season, character, and level of noise (Christiansen and Ljungqvist, 2016), in our analysis we test CFR methods in the context of the annually resolved marine proxy network of Arctica islandica.…”