A long-term winter warming trend was inferred at individual sites in Eurasia 10, 11, Alaska 17, and in a continental stack for North America and Europe 9 (MA18, Fig. However, the winter temperature trend during the Holocene is poorly documented and intensely debated. 1b), although long-term warming trends have been inferred for southern Europe 12, 14 and west-central North America 16. The HTM in summer season has been found across many regions of Europe 12, 13, 14, Asia 15, and North America 16, as is shown in multiproxy stacks 3, 4 (KA20, Fig. Robust seasonal, especially winter, temperature reconstructions over vast land areas can provide a new and crucial perspective on this conundrum 9, 10, 11. This Holocene temperature conundrum was suggested to result from the poor spatial representation of proxy records 8 or seasonal biases in proxies 5, 6, 7, since most of them might represent warm-season temperatures according to studies focusing on oceans 6.
By contrast, climate models simulate a long-term warming trend throughout the Holocene associated with ice-sheet retreat and rising greenhouse gas concentrations 5, 6, 7, 8. 1a) indicate an early to mid-Holocene Thermal Maximum (HTM), highlighting the crucial effect of boreal summer insolation on global temperature change 2. Multiproxy reconstructions 2, 3, 4 (MC13 and KA20, Fig. However, the long-term evolution of global temperatures over the past 11 kyr (the Holocene epoch) remains poorly constrained. Knowledge of past climatic conditions is essential to improve our understanding of the climate system with respect to the climate forcings and feedbacks, and to better constrain future climate projection 1. Our results challenge the proposal that seasonal biases in proxies are the main origin of model–data discrepancies and highlight the critical impact of insolation and associated feedbacks on temperature changes, which warrant closer attention in future climate modelling. The subsequent cooling trend is pervasively recorded except for northern Asia and southeastern North America, which may reflect the cross-seasonal impact of the decreasing summer insolation through climatic feedbacks, but the cooling in winter season is not well reproduced by climate models. Strong early Holocene warming trend occurred mainly in Europe, eastern North America and northern Asia, which can be generally captured by model simulations and is likely associated with the retreat of continental ice sheets. Our results indicate that both summer and winter temperatures warmed from the early to mid-Holocene (~11–7 ka BP) and then cooled thereafter, but with significant spatial variability.
Here we present an extensive dataset of Holocene seasonal temperatures reconstructed using 1310 pollen records covering the Northern Hemisphere landmass. The origin of the temperature divergence between Holocene proxy reconstructions and model simulations remains controversial, but it possibly results from potential biases in the seasonality of reconstructions or in the climate sensitivity of models.
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