A Chinese research team has quantified terrestrial mean annual temperature variations in East Asia over the past 2 million years, and observed a surprising Pleistocene warming trend that ran counter to the global cooling trend expected.

The researchers from Institute of Earth Environment of Chinese Academy of Sciences, in collaboration with scientists from The University of Hong Kong, Northwest University, and Xi’an Institute for Innovative Earth Environment Research, used fossil microbial lipids preserved in the Heqing paleolake to quantitatively reconstruct a 2-million-year-long, continuous, and highly-resolved terrestrial mean annual temperature timeseries in East Asia.

It is generally believed that Earth’s temperature has dropped global over the past 2 million years, based on marine records retrieved from deep sea sediments. However, this study shows a long-term warming trend in East Asia during 1.8–0.6 million years ago, challenging this traditional view.

These findings were recently published in Nature Communications.

The Pleistocene, about 2.6 million years to 11,700 years ago, is an important geological epoch that witnessed the evolution and expansion of early humans. Our knowledge on Earth′s Pleistocene temperature history mostly relies on sea surface temperature reconstructions from the marine realm. In the terrestrial realm where we inhabit, however, due to the scarcity of high-quality archives and suitable proxies, the long-term Pleistocene temperature history remains insufficiently understood. 

In 2002, an approximately 700m sedimentary core was drilled from the center of Heqing Basin, southwest China, under the support of the Chinese Environmental Scientific Drilling Program. The whole core recovery is up to 97% and most of the core is composed of typical lacustrine sediments, providing excellent materials for exploring past climatic changes in East Asia. 

Branched glycerol dialkyl glycerol tetraethers (brGDGTs), a suite of membrane-spanning lipids synthesized by bacteria, is emerging as a paleothermometer due to their sensitive to temperature changes.

“Luckily, we can quantitatively apply this paleothermometer for mean annual temperature reconstruction in the well-dated Heqing drilling core, after excluding possible non-thermal influences on it”. Said LIU Zhonghui, the study's corresponding author. “And based on our reconstruction, while the Heqing’s mean annual temperature variation generally resembles sea surface temperatures’ at glacial-interglacial timescales, its long-term warming trend diverges from the contemporaneous sea surface cooling.”

“The absence of a secular cooling trend in southwest China is consistent with some other temperature records on land, although they are generally fragmentary or argued to be affected by non-thermal factors”. Said  co-author LIU Weiguo. “The divergent Pleistocene temperature trends for land and sea, which has not attracted sufficient attention of the paleoclimate community, indicates that the Earth’s climate system has evolved into a more complex mode since 2 million years ago”. 

The long-term Pleistocene warming trend in East Asia might be dynamically linked to the growth of Antarctic Ice Sheets. “Under a relatively long-term stable global CO₂ level, Antarctic Ice Sheets expansion can transport more heat and water vapor to East Asia through changing oceanic and atmospheric circulations.” AN Zhisheng, another corresponding author explained.

The long-term warming trend in monsoonal Asia implies that extratropical Eurasia might has gradually become more suitable for the survival of hominins from the early to mid-Pleistocene. This should have directly favored the flourish of our ancestors in Eurasia, not necessarily requiring that they could adapt to climate stress such as extremely cold and arid environments. 

This work was supported by the National Natural Science Foundation of China, the Fund of Shandong Province the National Key Research and Development Program of China, and the Chinese Academy of Sciences.