“Incoming solar radiation, commonly known as insolation, serves as the primary heat source for Earth’s climate system,” the research team wrote in a paper published in the September issue of the peer-reviewed journal Global and Planetary Change.
“We hypothesize that cumulative disturbances in solar radiation disrupted the heat balance in Earth’s climate system, contributing to long-term climate change in the Pleistocene,” said the team led by Jin Zhangdong and other researchers from the Institute of Earth Environment of the Chinese Academy of Sciences (CAS).
During the Pleistocene, a geological epoch that began about 2.6 million years ago and lasted until 11,700 years ago, extensive ice sheets and other glaciers repeatedly formed over much of the world.
Solar radiation had not been considered as a trigger for the long-term cooling trend during the Pleistocene, which included the 2.3 degree Celsius drop in sea surface temperatures that the team determined occurred between 2 million and 940,000 years ago.
The various explanations proposed for the cooling trends included changes in the carbon dioxide content of the atmosphere, the dynamics of ice sheets and ocean currents. However, the Chinese researchers said they did not adequately explain the trends.
The cooling trends eventually led to the Middle Pleistocene Transition, a major climate event that extended the duration of global ice age cycles – the periods between ice ages – from around 41,000 to 100,000 years.
The team explained in this new study that the cumulative effect of decreasing solar radiation on Earth helps reduce heat levels in the oceans, creating the necessary conditions for ice sheets to grow.
To study the impact of solar radiation on long-term climate change, the team compiled 26 sea surface temperature records from around the world and introduced a new index to quantify solar radiation anomalies.
Although the variations in solar radiation were small, the researchers said that even small changes in incoming solar radiation could affect the heat balance by altering the heat content of the oceans, and that cumulative changes could lead to a significant heat imbalance.
The researchers said that about 900,000 years ago, a “crucial reorganization of the climate system” ended a long-term cooling trend with an abrupt cooling event, leading to an increase in global ice volume.
A corresponding transition was observed in the index 935,000 years ago, which coincided with the cooling event. Up to this point, the index showed that there had been a continuous decline in cumulative solar radiation.
“Our simulation results generally support the idea that the decline in cumulative solar radiation contributed to the (cooling event),” the team said.
“Although the high-frequency variations in solar radiation cause only small changes (in the ocean’s heat content), the cumulative effect of these sustained small increases or decreases over a longer period of time can lead to significant changes,” the team said.
To gain deeper insights into long-term climate trends and a better understanding of Earth’s climate history, further simulations need to be conducted that include atmospheric carbon dioxide levels and other variables, the researchers said.
