Relative importance of forcings and feedbacks in the Holocene temperature conundrum

• Published in: Quaternary science reviews Vol. 319; p. 108322
• Main Authors: Hopcroft, Peter O., Valdes, Paul J., Shuman, Bryan N., Toohey, Matthew, Sigl, Michael
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.11.2023
• Subjects: Holocene; Land-use; Palaeoclimate modelling; Radiative feedback; Radiative forcing; Temperature conundrum; Volcanic eruptions; Palaeoclimate modelling; Temperature conundrum; Land-use; Radiative feedback; Holocene; Radiative forcing; Volcanic eruptions
• ISSN: 0277-3791; 1873-457X
• DOI: 10.1016/j.quascirev.2023.108322

•Holocene climate simulations with HadCM3 warm too much in comparison with records even with Sahara greening.•Model-data agreement improves with the inclusion of early land-use.•Modelled warming is still greater than indicated by proxy records for Asia and Eastern North America.•Inconsistencies between different reconstructions hinder a more detailed evaluation. The pre-industrial Holocene provides the backdrop for the emergence of civilisations and the starting point of anthropogenic climate change. Several reconstructions show an early Holocene warming that was followed by cooling for several thousand years before Industrialisation. In contrast climate simulations show warming throughout the Holocene. Whilst reconstructed trends over ocean can be reconciled with warming through either seasonality or uncertainties, a consistent explanation for cooling trends over some land areas is missing. We present a suite of transient Holocene climate model simulations with a coupled general circulation model and show that a widespread mid- to late-Holocene cooling emerges over some regions of the northern hemisphere with the inclusion of anthropogenic land-use. This is mostly because in regions of prescribed late Holocene deforestation, the simulated early to mid-Holocene is characterised by a lower albedo than the late Holocene. Whilst this cooling through time can quantitatively explain some regional aspects of the reconstructions, the model-data agreement remains imperfect, and differences between reconstructions also hinders the evaluation. Moreover, model-dependency in the response of several feedbacks, particularly sea-ice, but potentially also clouds, means that it is difficult to uniquely attribute Holocene temperature evolution to specific factors. Future work should aim to derive a consensus-signal including uncertainties from the available proxy data which could be used to fingerprint simulations covering a range of plausible feedback strengths.