With all the discussions about the greenhouse effect, it is natural to ask the question, "What are the natural factors that affect climate change? What caused the glacial periods and other historical climate changes?"
Most changes are due to the energy of the sun fluctuating. On a short term basis, sunspots can change the energy output of the sun by about 0.4% over an eleven year cycle. Scientists believe this may have some correlation with 22 year drought patterns in the plains of the US. Large sunspot activity is also thought to be partially responsible for the 'Little Ice Age' from 1450 to 1850. This climate change is well documented in history with many impacts on civilization in Europe, including famines - yet the temperature fluctuation was only about 2 degrees F.
Changes in Earth's orbit around the sun are primarily responsible for determining times of glaciation and subsequent interglacial periods. The changes affect the intensity of sunlight striking various parts of the earth. These orbital fluctuations are called the Milankovitch cycles. The orbiting geometry of Earth and the rotation on its axis create three different long-term cycles of approximately 22,000, 41,000 and 100,000 years. The phase of the Milankovitch cycles is presently such that some natural cooling of the earth is occurring. This factor may be part of the reason why the greenhouse warming signal was hard to detect until recently, because the warming of the gases and the cooling of the Milankovitch cycles were cancelling each other out.
Volcanic eruptions can reduce the amount of solar energy reaching the earth, as the airborne ash causes the atmosphere to block some of the incoming radiation.***
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