Abstract
Global mean annual temperature has increased by more than 1 °C during the past 150 years, as documented by thermometer measurements. Such observational data are, unfortunately, not available for the pre-industrial period of the Common Era (CE), for which the climate development is reconstructed using various types of palaeoclimatological proxies. In this analysis, we compared seven prominent hemispheric and global temperature reconstructions for the past 2000 years (T2k) which differed from each other in some segments by more than 0.5 °C. Whilst some T2k show negligible pre-industrial climate variability (“hockey sticks”), others suggest significant temperature fluctuations. We discuss possible sources of error and highlight three criteria that need to be considered to increase the quality and stability of future T2k reconstructions. Temperature proxy series are to be thoroughly validated with regards to (1) reproducibility, (2) seasonal stability, and (3) areal representativeness. The T2k represents key calibration data for climate models. The models need to first reproduce the reconstructed pre-industrial climate history before being validated and cleared for climate projections of the future. Precise attribution of modern warming to anthropogenic and natural causes will not be possible until T2k composites stabilize and are truly representative for a well-defined region and season. The discrepancies between the different T2k reconstructions directly translate into a major challenge with regards to the political interpretation of the climate change risk profile. As a rule of thumb, the larger/smaller the pre-industrial temperature changes, the higher/lower the natural contribution to the current warm period (CWP) will likely be, thus, reducing/increasing the CO2 climate sensitivity and the expected warming until 2100.
Highlights
Published: 3 March 2022A good understanding of the pre-industrial temperature development is essential, as it represents crucial baseline data for modern climate change
Mann in 1999 and 2008 (“MM99”, “MM08”), (4) Fredrik Ljungqvist in 2010 (“LJU10”), (5, 6) PAGES2k in 2013 and 2019 (“PA13”, “PA19”), and (7) a group led by Ulf Büntgen in 2020 (“BÜ20”) [5,10–15] (Table 1)
Four reconstructions run over the entire Common Era, except for AR1 & MM99, and MM08
Summary
Published: 3 March 2022A good understanding of the pre-industrial temperature development is essential, as it represents crucial baseline data for modern climate change. The first trailblazing report of the IPCC was published in 1990; it contained a detailed discussion of “Observed Climate. Variations and Change” of the past (Chapter 7 of IPCC report #1) [1]. This chapter included schematic diagrams of global temperature variations since the Pleistocene on three time scales: (a) the last million years, (b) the last ten thousand years and (c) the last thousand years, shown as temperature anomalies in reference to the conditions of the beginning of the 20th century. An overview of the development, as well as key references, were given by, e.g., Frank et al [2]. A summary of key available proxies is contained in Christiansen and Ljungqvist [3], where the mathematical and statistical challenges related to large-scale multiproxy temperature reconstructions are discussed (e.g., the problem of low-frequency variability).
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