d) biological organisms
Scientists constantly discuss the reasons for the existence of climatic cycles that make glaciations arise and end. The most famous theory, introduced in 1920 by the Yugoslav mathematician Milutin Milankovitch, argues that the Earth's climate is determined by the volume of energy it receives from the sun. According to Milankovitch, this insolation - incidence of solar radiation - is dictated, in turn, by three astronomical factors.
The first is the irregular nature of the Earth's orbit around the Sun. In a period of 100,000 years the planet's orbit passes from a nearly perfect circle to a slightly oval shape. As this eccentricity increases, the same occurs with the period - the minimum distance between Earth and the Sun -, causing lower insolation and lower temperatures.
The second factor is the slope of the axis of rotation of the Earth, which ranges from 21.8 to 24.4 degrees, every 40,000 years. With maximum slope, sunshine and temperatures drop. Milankovitch's third factor, precession, refers to the way the Earth oscillates on its axis, like a top. In periods of 21 thousand years, such phenomenon affects the inclination of the planet and brings colder temperatures. When the three factors reinforce each other, they plunge the Earth into a glaciation.
In addition, scientists believe that the movement of tectonic plates and volcanism are also factors of creation of the periods gcllaciais.