The Earth undergoes cycles of ice ages and interglacial warm periods. Currently, we are in an interglacial period heavily impacted by anthropogenic activities. As atmospheric greenhouse gas concentrations increase, global warming accelerates, leading to the rapid melting of ice sheets, rising sea levels and posing significant threats to global ecosystems and human societies.
Recently, research has uncovered the warmest period in the last few million years, offering insights into both our current and future climate scenarios.
The warmest period within the past million years occurred approximately 400,000 years ago. During this time, the Northern Hemisphere experienced less ice cover than today, and sea levels were about 10 meters higher. Surprisingly, solar radiation, a critical driver of warm periods, was relatively weak, and greenhouse gas concentrations were lower compared to present levels.
This enigmatic period, known as the MIS 11c paradox (Marine Isotope Stage 11), has puzzled scientists for quite some time. Dr. Hsu-Ming Hu from the Department of Earth Sciences at the National Taiwan University (NTU) and an international team of researchers led by Dr. Chuan-Chou Shen from NTU have recently uncovered the solution to this mystery.
Utilizing speleothem data (cave deposits) from Mediterranean caves and records from the North Atlantic Ocean, their study published in Nature Communications identified the factors contributing to the extreme temperature rise around 400,000 years ago. Dr. Hu explains that the key to resolving the MIS 11c paradox lies in understanding the sequence of responses from Earth's ice sheets, oceans, and atmosphere to solar radiation.
Due to the lack of precise dating in most ocean and terrestrial records, this issue has confounded researchers for years. In 2014, the team from NTU, in collaboration with European colleagues, extracted a two-meter-long core sample from Bàsura Cave in northern Italy.
Using high-precision uranium-thorium dating technology, the researchers reconstructed the environmental history of Southern Europe from 480,000 to 360,000 years ago. By comparing solar radiation, global sea level changes, and various climate records, the researchers successfully resolved the MIS 11c paradox.
Dr. Shen notes that over recent years, analyses of the sample from Bàsura Cave have revealed a close connection to climate changes in the Atlantic Ocean and the Mediterranean Sea. The study demonstrates that the extreme warmth of this period resulted from a combination of factors.
Around 426,000 years ago, increased solar radiation during the Northern Hemisphere's summer months triggered extreme warming at the mid-lower Atlantic latitudes. As the oceans warmed, Earth's axial tilt gradually increased, making summers even hotter—a process that maintained elevated water temperatures for millennia.
This extended transfer of heat to higher latitudes through ocean currents resulted in unprecedented and prolonged melting of ice shelves, which subsequently led to the warmest period in Earth's history over the past several million years.
This era is a crucial reference point for understanding global warming. Similar to today, there wasn't exceptionally strong solar radiation during this period, yet it illustrates how sustained ocean warming alone can cause significant ice shelf collapse and rising sea levels, without the need for particularly high atmospheric temperatures or greenhouse gas concentrations. This research underscores the vital role oceans play in accelerating global warming and the collapse of ice shelves.