There seems to be several different cycles, including the Milankovitch cycles, which focus on the variations of the earths’ orbit around the sun and how the weather is affected. Also, changes in the tilt of the Earth’s axis. Currently the tilt of the axis is decreasing, and is thought to be a cause of the warming of the northern hemisphere. In addition, the amount of carbon dioxide and methane in the atmosphere and the effect on the whether has been exaggerated by many studies. Carbon dioxide promotes plant growth, which in turn produces more oxygen. There have been several times in the earth’s past when bot gases were much more prevaliant.
I remember being taught in grade school (50s-60s) that we we were in a warm period in between periods of glaciation, and that we would be re-entering a new glacial period in the next 10-20 thousand years or so.
O-D; article about Milankovitch cycles:
Science Daily Why Ice ages occur every 100,000 years or so. "The Milankovitch cycles "The explanation for the cyclical alternation of ice and warm periods stems from Serbian mathematician Milutin Milankovitch (1879-1958), who calculated the changes in Earth's orbit and the resulting insolation on Earth, thus becoming the first to describe that the cyclical changes in insolation are the result of an overlapping of a whole series of cycles: the tilt of Earth's axis fluctuates by around two degrees in a 41,000-year cycle. Moreover, Earth's axis gyrates in a cycle of 26,000 years, much like a spinning top. Finally, Earth's elliptical orbit around the sun changes in a cycle of around 100,000 years in two respects: on the one hand, it changes from a weaker elliptical (circular) form into a stronger one. On the other hand, the axis of this ellipsis turns in the plane of Earth's orbit. The spinning of Earth's axis and the elliptical rotation of the axes cause the day on which Earth is closest to the sun (perihelion) to migrate through the calendar year in a cycle of around 20,000 years: currently, it is at the beginning of January; in around 10,000 years, however, it will be at the beginning of July.
Based on his calculations, in 1941 Milankovitch postulated that insolation in the summer characterises the ice and warm periods at sixty-five degrees north, a theory that was rejected by the science community during his lifetime. From the 1970s, however, it gradually became clearer that it essentially coincides with the climate archives in marine sediments and ice cores. Nowadays, Milankovitch's theory is widely accepted. "Milankovitch's idea that insolation determines the ice ages was right in principle," says Blatter. "However, science soon recognised that additional feedback effects in the climate system were necessary to explain ice ages. We are now able to name and identify these effects accurately."
The efforts to "Stop Global Warming"-- if successful--might be more likely to propel us back into an early Ice Age, not a good thing.
In addition to specific cycles, there are also occasional major impacts from huge volcanic eruptions. The recent major eruptions by the Hunga-Tonga volcano which threw huge volumes of water into our upper atmosphere may be one reason for the pattern of droughts and floods experienced throughout the world these past two years. In a few years we will have a better analysis of those effects as the water vapor comes out of the upper atmosphere.
https://www.nasa.gov/earth/tonga-eruption-blasted-unprecedented-amount-of-water-into-stratosphere/
If you look at the blue line in the graph at comment #2, you will see that at around 75,000 years ago there was a sharp drop in the blue line, indicating a severe fall in temperature. This was caused by the volcano Toba Indonesia which left a caldera around 18 miles by 65 miles in dimension. This sharp drop in temperature apparently caused a severe decline in the human populations. Scientists have calculated that possibly only 10,000 pairs of breeding homo sapients survived the Nuclear Wintor that occurred, which can be determined by our studies of DNA. Almost no human remains have been discovered for the period between 75k and 50k years ago.
https://www.thecollector.com/mount-toba-supervolcano-wipe-out-humans/
You will note that eventually the blue line rose to continue the downward slope that was occurring before the major Toba event.