'Warm Ice Age' changed climate cycles

Geological ice ages -- called glacial periods -- are characterised by the development of large ice sheets in the Northern Hemisphere. In the past 700,000 years, phases shifted between distinct glacial and warm periods about every 100,000 years. Before then, however, the Earth's climate was governed by 40,000-year cycles with shorter and weaker glacial periods. The change in the climate cycles occurred in the Middle Pleistocene Transition period, which began approximately 1.2 million years ago and ended about 670,000 years ago. "The mechanisms responsible for this critical change in the global climate rhythm remain largely unknown. They cannot be attributed to variations in the orbital parameters governing the Earth's climate," explains Associate Professor Dr André Bahr of the Institute of Earth Sciences at Heidelberg University. "But the recently identified 'warm ice age', which caused the accumulation of excess continental ice, did play a critical role."

For their investigations, the researchers used new climate records from a drill core off Portugal and loess records from the Chinese Plateau. The data was then fed into computer simulations. The models show a long-term warming and wetting trend in both subtropical regions for the past 800,000 to 670,000 years. Contemporaneous with this last ice age in the Middle Pleistocene Transition period, the sea surface temperatures in the North Atlantic and tropical North Pacific were warmer than in the preceding interglacial, the phase between the two ice ages. This led to higher moisture production and rainfall in Southwest Europe, the expansion of Mediterranean forests, and an enhanced summer monsoon in East Asia. The moisture also reached the polar regions where it contributed to the expansion of the Northern Eurasian ice sheets. "They persisted for some time and heralded in the phase of sustained and far-reaching ice-age glaciation that lasted until the late Pleistocene. Such expansion of the continental glaciers was necessary to trigger the shift from the 40,000-year cycles to the 100,000-year cycles we experience today, which was critical for the Earth's later climate evolution," states André Bahr.

The results of this research were published in the journal Nature Communications. Scientists from Germany, France, Spain, and Portugal contributed to the research. The work was funded by the German Research Foundation.