Global Temperature Modes Shed Light on the Holocene Temperature Conundrum

Reconstructions of the global mean annual temperature evolution during the Holocene yield conflicting and puzzling results. One temperature reconstruction, mainly based on ocean sediments, shows warm conditions in the Early to mid-Holocene with a following cooling trend to the late Holocene. The other reconstruction based on pollen data, however, reveals global warming until around the beginning of the common era (CE). Here we show that both a global warming mode and a cooling mode emerge when performing a spatio-temporal analysis of annual temperature variability during the Holocene using data from a transient high-resolution (about 200 km) climate model simulation. The global spatial patterns of the warming and cooling modes exhibit different centers of action. The warming mode is mainly related to the combined effect of increasing greenhouse gases and the latitude-dependent trend in annual mean insolation. The warming mode is most pronounced in the tropics. The simulated cooling mode - resembling the global cooling trend shown by one reconstruction - is forced by the effects of orbitally forced changes in the seasonal cycle of Arctic sea-ice. This mode predominantly affects the Arctic, North Atlantic and Eurasian regions. The warming mode dominates in the mid-Holocene whereas the cooling mode takes over in the late Holocene. Moreover, the weighted sum of the two modes yields the simulated global temperature trend evolution during the Holocene. These findings have strong implications for the interpretation of proxy data and the selection of proxy locations to compute global mean temperatures.