Seasonality in Holocene Temperature Reconstructions in Southwestern China

Reconstructions of global surface temperature, dominated by records from the northern extratropics, show an apparent Holocene cooling trend after the early Holocene Climatic Optimum. However, model simulations suggest a global warming Holocene tendency. This “Holocene temperature conundrum” may be caused by the seasonal bias of paleotemperature proxies. Here we report a quantitative Holocene record with ~100-year resolution based on branched glycerol dialkyl glycerol tetraethers (brGDGTs) from an alpine lake in southwestern China. Our reconstructed Holocene temperature record displays a steady long-term trend without distinct cooling or warming changes. Based on the temperature values and their evolution over time, our reconstruction is interpreted to present temperature changes in ice-free seasons from March to November. Unlike the often-documented Holocene cooling of regional summer temperatures driven by boreal summer insolation, this observed trend in our reconstructed temperatures is probably caused by slightly decreasing local ice-free season insolation and somewhat compensated by increasing atmospheric greenhouse gas concentrations. Our results demonstrate that the climatic drivers of ice-free season and summer temperature changes could be different and highlight the significance of elucidating the seasonality of proxies before using them for paleoclimate reconstructions.