Understanding how Earth’s climate naturally fluctuated during the Holocene—the current geological epoch spanning the last 11,700 years—is vital for contextualizing modern human-driven warming and improving future climate projections. Yet, tropical Australasia’s climate history has remained enigmatic, with scientists divided over interpretations of paleoclimate records.

Published in CATENA, the research led by Prof. ZHANG Enlou from the Nanjing Institute of Geography and Limnology analyzed remarkable "molecular fossils"—branched glycerol dialkyl glycerol tetraethers (brGDGTs)—preserved in Girraween Lagoon’s sediments. These organic compounds function as precise paleothermometers when properly calibrated. 

Using a specific calibration for tropical lakes, the research team reconstructed mean annual air temperatures over the past 10,400 years. The results reveal a steady warming trend of 2 °C throughout the Holocene. This warming aligns with other land temperature records, alkenone-derived ocean temperature data, and climate models for the region. Intriguingly, it diverges from estimates using magnesium-to-calcium ratios in planktonic fossils, highlighting ongoing debates about proxy reliability.

More than just temperature, the sediments recorded an environmental transformation. The lagoon grew increasingly acidic over millennia. This acidification stemmed from two key factors. First, declining rainfall in northern Australia reduced the inflow of alkaline groundwater from limestone sinkholes beneath the lake. Second, decaying organic matter in the sediments boosted acid production. Together, these processes transformed the lagoon’s chemistry.

By correlating their data with regional climate archives, researchers traced these changes to intensifying El Niño-Southern Oscillation (ENSO) activity. As ENSO variability grew, tropical Australasia shifted from cool/wet conditions to today’s warmer, drier climate with stark seasonal contrasts. 

This dual reconstruction of temperature and pH not only resolves long-standing scientific contradictions but also provides a holistic framework for understanding how ocean-atmosphere interactions, hydrology, and biogeochemical cycles jointly shaped the region’s climate history. Such insights are critical for refining models to predict how tropical climates may respond to future global warming.

"Lake sediments are like nature’s history books," said Prof. ZHANG, "By reading their chemical signatures, we’re better preparing for climate challenges ahead."