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” is probably caused by the seasonal bias of paleotemperature proxies. Here we report a quantitative Holocene record with ~100-year resolution based on brGDGTs from an alpine lake in southwestern China. Our reconstructed Holocene temperature displays a steady long-term trend without distinct cooling or warming changes. Based on the temperature values and their evolution through time, our reconstruction possibly represents temperature changes in ice-free seasons from March to November. Unlike the often-documented Holocene cooling in regional summer temperatures driven by boreal summer insolation, this observed trend in our reconstructed temperature is probably caused by the slightly decreasing local ice-free season insolation and compensated somewhat increasing atmospheric greenhouse-gas concentrations. Our results demonstrate that the climate drivers of ice-free season and summer temperature changes would be quite different, and highlight the significance of elucidating the seasonality of proxies before using them for paleoclimate reconstructions.