The research team, made up of climatologists and an astronomer, used an improved computer model to reproduce the cycle of ice ages (ice periods) from 1.6 to 1.2 million years ago. The results show that the glacial cycle was fundamentally driven by astronomical forces in a very different way than it was in modern times. These findings will help us better understand the past, present and future of ice sheets and Earth’s climate.
The Earth’s orbit around the Sun and the direction of its axis of rotation slowly change over time, due to the gravitational pull of the Sun, Moon, and other planets. These astronomical forces affect the environment on Earth due to changes in the distribution of sunlight and variation between seasons. In particular, ice sheets are sensitive to these external forces leading to a cycle between glacial and interglacial periods.
The current glacial-interglacial cycle has a duration of about 100,000 years. However, the glacial cycle in the early Pleistocene (about 800,000 years ago) shifted more quickly, with a cycle of about 40,000 years. It is believed that astronomical external forces are responsible for this change, but the details of the mechanism are not understood. In recent years, it has become possible to investigate in more detail the role of astronomical forces through the refinement of geological data and the development of theoretical research.
A team led by Yasuto Watanabe at the University of Tokyo focused on the early ice age epoch from 1.6 to 1.2 million years ago using an improved climate computer model. Astronomical forces based on the latest modern theories are taken into account in this simulation. The large numerical simulations in this study reproduce the glacial cycle 40,000 years from the early Ice Age as indicated by the geological record data.
By analyzing these simulation results, the team identified three facts about the mechanisms by which astronomical forces caused changes in climate at those times. (1) The glacial cycle is determined by small differences in the amplitude of the anisotropy in the direction of the axis of rotation and the Earth’s orbit. (2) The timing of the decay is mainly determined by the position of the summer solstice over its orbit, which is at perihelion, and not just by the influence of the periodic change of tilt of the Earth’s axis. (3) The timing of the change in the direction of the rotational axis and the location of the summer solstice over its orbit determine the duration of the interglacial period.
says Takashi Ito of the Japan National Astronomical Observatory, a member of this research team that led the discussion about astronomical external forces. “The numerical simulations performed in this study not only well reproduce the Pleistocene glacial cycle, but also successfully explain the complex implications of how astronomical forcing drove the cycle at that time. We can consider this work as a starting point for studying the glacial cycles beyond.” current Earth.