A research team has quantified terrestrial mean annual temperature variations in East Asia over the past two million years, and observed a surprising Pleistocene warming trend that contradicts the long-assumed global cooling pattern, according to a recent study published in Nature Communications.

The team, from the Institute of Earth Environment of the Chinese Academy of Sciences, collaborated with international partners to quantitatively reconstruct a continuous, high-resolution two-million-year record of East Asia's terrestrial mean annual temperatures. Their work relied on fossil microbial lipids preserved in sediment from Yunnan Province's Heqing paleolake.

For decades, scientists have held that Earth's global temperatures declined over the past two million years, a conclusion drawn primarily from marine records extracted from deep-sea sediments. However, the new study challenges this paradigm: it identifies a sustained warming trend in East Asia between 1.8 million and 0.6 million years ago, a period within the Pleistocene epoch (2.6 million to 11,700 years ago).

The Pleistocene is a critical geological era marked by the evolution and spread of early humans. Yet, while most understanding of its temperature history comes from sea surface temperature data, knowledge of terrestrial conditions—where humans lived—has remained limited due to a lack of high-quality archives and reliable proxies, the researchers explained.

The study's breakthrough stemmed from a 700-meter sedimentary core drilled in 2002 from the center of the Heqing Basin in southwest China, under the support of the Chinese Environmental Scientific Drilling Program. With a 97% recovery rate, the core consists mostly of well-preserved lacustrine (lake) sediments, making it an "excellent material" for studying past climate shifts in East Asia.

To reconstruct temperatures, the researchers used branched glycerol dialkyl glycerol tetraethers (brGDGTs)—membrane lipids produced by soil bacteria. These compounds are increasingly recognized as a reliable "paleothermometer" because their chemical structure responds sensitively to temperature changes.

"Luckily, after accounting for potential non-thermal influences, we could apply this paleothermometer quantitatively to the well-dated Heqing core," said Prof. LIU Zhonghui, a corresponding author of the study. "Our reconstruction shows that while Heqing's mean annual temperatures followed sea surface temperature patterns during glacial-interglacial cycles, its long-term warming trend diverged from the simultaneous cooling of global oceans."

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. The divergent temperature trends between land and sea during the Pleistocene have not received enough attention from the paleoclimate community, suggesting Earth's climate system has operated in a more complex mode since two million years ago, the researchers noted.

The team proposed that East Asia's long-term Pleistocene warming trend may be tied to the growth of Antarctic Ice Sheets. They explained that under relatively stable long-term global CO₂ levels, the expansion of Antarctic Ice Sheets could have altered ocean and atmospheric circulation patterns, transporting more heat and water vapor to East Asia.

This sustained warming in monsoonal Asia may have had profound implications for human evolution. It suggests that extratropical Eurasia became increasingly hospitable to hominins from the early to middle Pleistocene—creating favorable conditions for the flourishing of human ancestors without requiring them to adapt to extreme cold or aridity.

This work was supported by the National Natural Science Foundation of China, and the Shandong Provincial Fund, among other sources.