The sources of climate variability are almost beyond measure. “On average,” the Earth is approximately 93 million miles from the sun, varying by 6 million miles over the course of it’s 365.24-day trek around an elliptical orbit. Due to an average Earth tilt of approximately 23 degrees, the Northern Hemisphere is tilted away from the sun at one end of the ellipse’s axis, and the Southern Hemisphere is tilted away from the sun at the other. This is just one of the cycles that controls climate variability and it defines the Earth’s seasons as we know them. Although this is not global climate change, it nonetheless exhibits that very little change in distance or exposure from the sun can cause variability in climate.
Thus, the tilt of the Earth’s axis is responsible for one cycle of climate variability that has a period of approximately 90 days (the seasons). Because the tilt adjusts between 22.1 and 24.5 degrees, it is responsible for another cycle of climate variability that has a period of approximately 41,000 years. It is currently moving toward a lower tilt. With that comes a trend towards warmer winters, cooler summers, and global cooling, eventually bringing the next ice age.
There are other cycles of climate variability with longer periods, caused by variations in the eccentricity of the Earth’s orbit, that result from the gravitational pull of Jupiter and Saturn. The periods of these cycles range from approximately 100,000 to 400,000 years. There are also other cycles with shorter periods, ranging from 7 years to 60 years, caused by Earth’s wobble, sunspot cycles, lunar cycles, and volcanic cycles, to name a few.
Understandable then, that there were colder times in Earth’s history that occurred approximately 400 years ago, 1000 years ago, 1250 years ago, 3700 years ago, 25000 years ago, 140000 years ago, 225000 years ago, 260000 years ago, and 330000 years ago. And the warmer periods in between attained temperatures and CO2 levels much like those we are experiencing now.
In short, we are overdue for the next ice age.
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