Cyclical climate changes in the Earth’s history are explained by cyclical inflows of greenhouse gases into the atmosphere.

The main source of heat in the Earth's interior is the spontaneous decay of radioactive elements. An increase in the flow of heat to the surface can be caused by the nuclear chain reactions, forced decay of radioactive elements, including uranium and thorium.

A layer of actinides near a critical thickness can be formed as a result of the deposition of high-melting high-density particles of uranium and thorium oxides from the molten outer core to the solid inner core of the Earth [Mitrofanov et al., 1999]. The upward currents of mass and heat arising during nuclear chain reactions in the Earth's outer liquid core warm up the overlying layers. With the warming of the Earth's crust and the bottom of the oceans, due to the decomposition of gas hydrates, the greenhouse gas methane enters the atmosphere. With heating of the oceans due to positive feedbacks, more and more water vapor and carbon dioxide dissolved in the ocean’s water enter the atmosphere. Climate warming is initiating and accelerating.

With the dispersion of the active layer in the thermal convective flows, the stopping of nuclear reactions and a decrease in the heat flow from the interiors occurs, the methane content in the atmosphere decreases. More and more carbon dioxide is dissolved in the cooling water of the oceans. A cold snap is coming. Actinide particles begin to settle on the Earth's inner core again, with parallel reproduction of easily fissionable isotopes [Anisichkin et al., 2008]:

²³⁸U + n → ²³⁹U → ²³⁹Np → ²³⁹Pu → ²³⁵U + α (2.4´10⁴ years)

The duration of climatic cycles is determined by the time of sedimentation of actinide particles. Simulation of the process of the whole sedimentation with critical size of uranium dioxide particles and viscosity of the outer core from 10² Pa s to 10⁹ Pa s leads to a cycle duration of about 130 thousand years, which consistent with the data on climate change over the past 400 000 years obtained from ice cores in Antarctica [Gordienko et al., 1983; Petit et al., 1999; Vimeux et al., 2002].

To start nuclear chain reactions, it is enough to form a layer of actinides of critical thickness, without the sedimentation of all fissile material on the Earth’s solid inner core. But the gradual "burnout" of actinides requires more and more complete sedimentation of actinide particles. Therefore, the duration of cycles should increase over time. Indeed, over the past 400 thousand years, the duration of climatic cycles has increased from approximately 90 to 120 thousand years [Petit et al., 1999; Vimeux et al., 2002].

Approximately 1.5 million years ago, the Earth experienced a radical climate shift. The planet has already entered ice ages and emerged from them every 40 thousand years [Yuzhen et al., 2019; Voosen, 2024; An et al., 2024; Cutts, 2024]. But then the ice ages became more contrasting and longer, with an increasing duration of 90 thousand years to 120 thousand years, and the planet as a whole became colder, which cannot be explained by the changes in the level of insolation – the amount of heat coming from the Sun, Milankovitch cycles, the duration and intensity of which should be relatively constant on the such time scales. Supported nuclear hypothesis explains these rapid climate changes too. Millions of years ago, there were more easily fissile isotopes. It is possible that in the past, two georeactors periodically worked in the Earth’s interiors in different places. Therefore, the climatic cycles were approximately twice as short, less pronounced, and the climate was warmer.