The forcing mechanisms responsible for the mid-Holocene onset of the Mediterranean-type climate in south-western Europe are currently unclear, but understanding these is critical for accurate climate projections under future greenhouse gas warming. Additionally, regional studies that present conflicting patterns for the onset and advancement of Mediterranean climatic conditions complicate definitively ascribing causality. Here, we use a new high resolution stalagmite density record obtained non-destructively using Computed Tomography (CT scanning) to reconstruct southern Iberian climate between 9.3 and 2.9 ka BP. By establishing correlations between stable isotopes, trace elements and stalagmite density, we demonstrate that stalagmite density can be used as a rainfall proxy, with lower densities associated with more arid conditions, consistent with expectations from previous studies of speleothem textures and crystal fabrics. Our results reveal early Holocene humid interval and mid-Holocene year-round aridity that preceded the onset of Mediterranean climate at 5.3 ka BP in southern Iberia. Using this new dataset combined with previously published results, we link the gradual advancement of the Mediterranean climate to the southward migration of the North Atlantic
Subtropical High induced by an orbitally-driven decrease in Northern Hemisphere insolation. Future anthropogenic warming could result in a reversal of this trend, a northward migration of the North
Atlantic Subtropical High, and a return to year-round aridity in south-western Europe.