Comparisons of simulated and observed Northern Hemisphere temperature variations during the past millennium – selected lessons learned and problems encountered

Abstract

A B S T R A C T Comparison of simulated and reconstructed past climate variability within the last millennium provides an opportunity to aid the understanding and interpretation of palaeoclimate proxy data and to test hypotheses regarding external forcings, feedback mechanisms and internal climate variability under conditions close to those of the present day. Most such comparisons have been made at the Northern Hemispheric scale, of which a selection of recent results is briefly discussed here. Uncertainties in climate and forcing reconstructions, along with the simplified representations of the true climate system represented by climate models, limit our possibility to draw certain conclusions regarding the nature of forced and unforced climate variability. Additionally, hemispheric-scale temperature variations have been comparatively small, wherefore the last millennium is apparently not a particularly useful period for estimating climate sensitivity. Nevertheless, several investigators have concluded that Northern Hemispheric-scale decadal-mean temperatures in the last millennium show a significant influence from natural external forcing, where volcanic forcing is significantly detectable while solar forcing is less robustly detected. The amplitude of centennial-scale variations in solar forcing has been a subject for much debate, but current understanding of solar physics implies that these variations have been small Á similar in magnitude to those within recent sunspot cycles Á and thus they have not been a main driver of climate in the last millennium. This interpretation is supported by various comparisons between forced climate model simulations and temperature proxy data. Anthropogenic greenhouse gas and aerosol forcing has been detected by the end of Northern Hemispheric temperature reconstructions.

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

“…The influence of dating uncertainties and nonlinearities fall outside the scope of this article. 10.1002 achievements in the field [Jones and Mann, 2004;National Research Council, 2006;Mann, 2007;Wanner et al, 2008;Frank et al, 2010;Masson-Delmotte et al, 2013;Smerdon and Pollack, 2016], or been related to climate model-proxy data comparisons [Braconnot et al, 2012;Fernández-Donado et al, 2013;Moberg, 2013;Schmidt et al, 2014;Moberg et al, 2015;PAGES 2k-PMIP3 Group, 2015].…”

The PMIP4 contribution to CMIP6 – Part 2: Two interglacials, scientific objective and experimental design for Holocene and Last Interglacial simulations

“…The only exception to this is the Global Lake Status Data Base (Kohfeld and Harrison, 2000), which provides qualitative estimates of the change in lake water balance through time globally. The reliability of global temperature estimates depends on the representativeness of the data included; this point has been made abundantly clear from comparisons of records of hemispheric temperature changes during the last millennium (Fernández-Donado et al, 2013;Moberg, 2013). Currently available reconstructions of global temperature changes during the Holocene are heavily biased towards marine records, making it imperative that the reliability of these records is assessed using continental reconstructions (Davis et al, 2015;Liu et al, 2014).…”

Continental-scale temperature variability in PMIP3 simulations and PAGES 2k regional temperature reconstructions over the past millennium

“…However, obtaining unequivocal conclusions from the comparison between reconstructions and simulation results over the past millennium remains difficult due to uncertainties in climate and forcing reconstructions, the simplified world represented by climate models, and the relatively weak forced signal in the pre-industrial part of the past millennium compared to internal climate variability (e.g. Moberg, 2013). Reconstructions and simulations are two different representations of the behaviour of the actual climate system, and this creates multiple uncertainties in the task of intercomparison.…”