Earth gets all its energy from the Sun and it is the Sun's energy that keeps Earth warm. But the amount of energy Earth receives is not always the same. Changes in the Sun and changes in Earth's orbit affect the amount of energy that reaches the Earth.
The 11-Year Solar Cycle
When the Sun has fewer sunspots, it gives off less energy, less energy makes its way to Earth, and our planet cools down. More than three hundred years ago, when the climate was cooler for a time called the "Little Ice Age", people noticed there were no sunspots for several decades. Over time, scientists have noticed a pattern in the number of sunspots. About every 11 years the number of sunspots reaches a high and then decreases again.
Over thousands of years, changes in Earth's orbit cause changes in the amount of the Sun's energy that gets to the planet. Over the past several million years these changes have caused cycles of global warming and cooling.
There are three ways that Earth's orbit changes over time.
Eccentricity: The shape of Earth's orbit around the Sun becomes slightly more and then less oval every 100,000 years.
Precession: Earth wobbles on it axis as it spins, completing a full wobble every 23,000 years.
Tilt: The angle of the Earth's axis relative to the plane of its orbit changes about three degrees every 41,000 years.
These two figures show former temperatures with major periods of glaciation labeled. The dashed lines are the present global average temperature of about 15° C (59° F). Thus the solid curves show small changes from this average; note that the temperature drops only about 5° C during a glaciation. This has occurred about every 100,000 years, with smaller wiggles in between. That is, there has been a 100,000 year glaciation cycle for the past million years or so, and there may be shorter cycles as well.
The most recent glaciation, 20,000 years ago, is called the Laurentide, and Earth is still recovering from it. This map from the The Illinois State Museum exhibit on ice ages shows the extent of that ice.
The most recent small drop in average temperature caused the Little Ice Age of 1500-1700 AD, which history describes. Mountain glaciers advanced in Europe and rivers like the Thames in England froze solid, which doesn't happen now.
Since the Sun is a main-sequence star, it seems logical, if not observable, that increasing solar influx would have a gradually increasing role in global warming.
Does current astrophysical thinking indicate that the Sun will gradually warm, or is it supposed that its transition will come only after an extended period of complete thermal stability?
Not the current warming trend. The current warming trend over the past 150 years -- acclerating since the mid-1980s -- is too fast to be caused by astronomical cycles. Those factors operate on time-scales of 1-10,000 years.
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