The Milankovitch Cycles are small changes in the configuration of the Earth’s orbit around the Sun which induces slight changes in seasonal insolation (amount of radiation received per unit time at any one location). These changes initiate the 100,000 year glacial-interglacial periods. Sunspots increase the intensity of solar radiation over short periods of time. One school of thought is the current global warming is only due to the Milankovitch Cycles and sunspots. In this school of thought, anthropogenic forces (human induced) such as the release of carbon dioxide into the atmosphere from the burning of fossil fuel are not responsible for the current global warming trend.
There are three Milankovitch Cycles that alter the Earth’s orbit around the Sun. In the first cycle the Earth’s orbit around the Sun is not circular but is an elongated orbit which means the distance of the Earth from the Sun is not constant. The shape of the ellipse becomes slightly more elongated, then slightly more circular then back again. A full oscillation in the elongation takes about 100,000 years. When the Earth is closest to the Sun more solar radiation reaches the surface of the planet and the planet is warmer. When the Earth’s orbit is farthest from the Sun the planet is colder.
Tilt of the Earth axis of rotation is the second Milankovitch Cycle. The tilt varies from 21.1 to 24.5 degrees and back again every 41,000 years. The greater the tilt the more intense is the winter and summer seasons in both hemispheres.
The third cycle is the precession (rotation of a planet’s spin axis around a line drawn perpendicular to its orbital plane) of the Earth every 23,000 years. In other words the axis of the spin itself rotates around another axis (similar to the axis of a spinning top rotating around a second axis). This means that the Northern Hemisphere receives more solar radiation when precession points the North Pole toward the Sun at the same time the Earth’s orbit brings the Earth closest to the Sun.
Glacial and interglacial periods are triggered by the combined effect of the three Milankovitch Cycles. However, the Sun’s insolation from these combined cycles is too small to account for the dramatic climate changes seen between the glacial and interglacial periods. Other environmental forces must amplify the climate change the Milankovitch Cycles set in motion. Arctic feedback factors, regulating the atmospheric level of carbon dioxide by rock weathering, cloud formation and volcanic eruptions are some examples of environmental forces (my postings entitled The Arctic Feedback Factor and Climate Change, The Carbon Cycle and Clouds, Volcanoes, Water Vapor and Climate Change). The Milankovtich Cycles are the trigger of glacial-interglacial periods while environmental factors are the amplifier.
Sunspots affect the climate on a much shorter time scale than the Milankovitch Cycles. Decadal variability of solar output can be seen in the abundance of sunspots on the Sun. The strength of each cycle (11 years) is determined by the number of sunspots seen at the peak of the cycle. The more sunspots the more energy the sun radiates to the planet. Conversely, with fewer sunspots the radiation output diminishes.
What is the relationship between the climate and sunspots? The period from 1650 to 1715 known as the “Little Ice Age” saw very few sunspots. Glaciers advanced into valleys and growing seasons were shorter. The period from 1910 to 1919 saw an increase in the number of sunspots along with a 0.90F increase in the average global surface temperature. The last four decades of the 20th century saw a decline in the number of sunspots and in 2008 the number of sunspots was the lowest in the past half century. The low sunspot numbers in the last four decades of the 20th century suggest the global temperature should be cooling.
Satellites have measured the incoming solar radiation to our planet since 1978. Like the number of sunspots, satellite data demonstrates solar radiation has been declining in the last part of the 20th century and the first part of the 21st century. However, the Earth’s temperature has continued to climb over the same interval (my posting entitled The Carbon Cycle).
Clearly, the activity of the Sun is not causing current global warming trend. Anthropogenic forces (burning of fossil fuels, deforestation and methane/nitrous oxide emissions from agriculture) created the current atmospheric carbon overload (my posting entitled Fossil Fuel and Atmospheric Levels of Carbon Dioxide). This carbon overload triggers the greenhouse effect that is responsible for the current global warming trend which is amplified by environmental factors (my posting entitled Solar Activity, Greenhouse Gas Levels and Climate Change on our Earth).