Relative importance of forcings and feedbacks in the Holocene temperature conundrum

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.

Automated summary

The pre-industrial Holocene experienced a warming followed by a cooling period, but the reasons behind the cooling trends in certain land areas remain unclear. In this study, climate model simulations with anthropogenic land-use show a widespread cooling in some regions of the northern hemisphere during the mid-to-late Holocene. However, the model-data agreement is not perfect and the differences between reconstructions hinder the evaluation. The response of several feedbacks, such as sea-ice and clouds, also adds complexity to attributing the temperature evolution during the Holocene.