The spatial and temporal complexity of the Holocene thermal maximum

The Holocene thermal maximum, a period of relatively warm climate between 11,000 and 5,000 years ago(1,2), is most clearly recorded in the middle and high latitudes(2,3) of the Northern Hemisphere, where it is generally associated with the local orbitally forced summer insolation maximum. However, proxy-based reconstructions have shown that both the timing and magnitude of the warming vary substantially between different regions(2-4), suggesting the involvement of extra feedbacks and forcings. Here, we simulate the Holocene thermal maximum in a coupled global ocean-atmosphere-vegetation model. We find that before 7,000 years ago, summers were substantially cooler over regions directly influenced by the presence of the Laurentide ice sheet, whereas other regions of the Northern Hemisphere were dominated by orbital forcing. Our simulations suggest that the cool conditions arose from a combination of the inhibition of Labrador Sea deep convection by the flux of meltwater from the ice sheet, which weakened northward heat transport by the ocean, and the high surface albedo of the ice sheet. We thus conclude that interglacial climate is highly sensitive to relatively small changes in ice-sheet configuration.