Holocene climates of the Iberian Peninsula: pollen-based reconstructions of changes in the west–east gradient of temperature and moisture

Abstract

Abstract. The Iberian Peninsula is characterized by a steep west–east moisture gradient at present, reflecting the dominance of maritime influences along the Atlantic coast and more Mediterranean-type climate further east. Holocene pollen records from the Peninsula suggest that this gradient was less steep during the mid-Holocene, possibly reflecting the impact of orbital changes on circulation and thus regional patterns in climate. Here, we use 7214 pollen samples from 117 sites covering part or all of the last 12 000 years to reconstruct changes in seasonal temperature and in moisture across the Iberian Peninsula quantitatively. We show that there is an increasing trend in winter temperature at a regional scale, consistent with known changes in winter insolation. However, summer temperatures do not show the decreasing trend through the Holocene that would be expected if they were a direct response to insolation forcing. We show that summer temperature is strongly correlated with plant-available moisture (α), as measured by the ratio of actual evapotranspiration to equilibrium evapotranspiration, which declines through the Holocene. The reconstructions also confirm that the west–east gradient in moisture was considerably less steep during the mid-Holocene than today, indicating that atmospheric circulation changes (possibly driven by orbital changes) have been important determinants of the Holocene climate of the region.

Pollen-based reconstructions of Holocene climate trends in the eastern Mediterranean region

“…Differences in the trend of moisture availability between the models imply that the nature of the changes in circulation varies between models and thus the simulations do not provide a strong basis for explaining the observed patterns of change in moisture availability. Earlier studies, focusing on the western Mediterranean (Liu et al, 2023), Europe (Mauri et al, 2014), and central Eurasia (Bartlein et al, 2017), have shown that models have difficulty in simulating the enhanced moisture transport into the Eurasian continent shown by palaeoenvironmental data during the mid Holocene and during the late Holocene. Changes in precipitation can also affect landsurface feedbacks.…”

“…Changes in precipitation can also affect landsurface feedbacks. Liu et al (2023), for example, have argued that enhanced moisture transport into the Iberian peninsula during the mid Holocene led to more vegetation cover and increased evapotranspiration and had a significant impact on the reduction of growing season temperatures. Differences in the reconstructed trends of summer temperature and plantavailable moisture through the Holocene suggests that this land-surface feedback was not an important factor influencing summer temperatures in the EMBSeCBIO region.…”

“…Weninger et al, 2006;Drake, 2012;Kaniewski et al, 2013;Cookson et al, 2019;Weiberg et al, 2019;Palmisano et al, 2021). In addition, it would also be possible to use these data to explore the impact of climate changes on the environment, including the natural resources available for people (Harrison et al, 2023).…”

“…fxTWA-PLS reduces the known tendency of regression methods to compress climate reconstructions towards the middle of the sampled range by applying a sampling frequency correction to reduce the influence of uneven sampling of climate space and by weighting the contribution of individual taxa according to their climate tolerance (Liu et al, 2020). Version 2 of fxTWA-PLS (fxTWA-PLS2; Liu et al, 2023), applied here, uses P-spline smoothing to derive the frequency correction and also applies the correction both in estimating climate optima and in the regression itself, producing a further improvement in model performance relative to version 1, as published by Liu et al (2020).…”

“…However, the differences in the sign of the trend between the different models indicates that changes in moisture are not a straightforward consequence of the forcing but must reflect modeldependent changes in moisture supply via changes in atmospheric circulation. Reconstructions of Holocene climates in Iberia have suggested that land-surface feedbacks associated with changes in moisture availability have a strong influence on summer temperature (Liu et al, 2023). There does not seem to be strong evidence for this in the EMBSeCBIO re-gion, given the difference in the trends of α and the MTWA and the muted nature of the trend in α.…”