The Holocene Seasonal Temperature Conundrum

Last decade has seen greatly intensified interest in understanding the temperature evolution in the Holocene (~last 10,000 years), which provides the background climate for our ongoing anthropogenic global warming.  Much of the effort so far has focused on the mean annual temperature (MAT). The so called Holocene temperature conundrum still remains unresolved: has the global MAT exhibited a cooling trend as indicated in most proxy reconstructions, or a warming trend in response to increased concentration of greenhouse gases and retreating ice sheet as in most climate models. Here, we further point out a conundrum on the Holocene seasonal temperatures in the Northern Hemisphere extra-tropics: in comparison with a simple analogue model that predicts the seasonal cycle of temperature from insolation based on present observations, most available seasonal temperature reconstructions severely underestimate the decrease of seasonal cycle amplitude in the Holocene. Meanwhile, a newly developed set of summer and annual temperature reconstructions based on soil bacteria’s membrane lipids (branched glycerol dialkyl glycerol tetraethers (brGDGT)) exhibits an evolution pattern similar to the analogue model. Our study highlights the current uncertainty in seasonal temperature reconstructions, with implications to the MAT.