Our climate has shown considerable natural variability over the course of centuries and millennia. Between the 9th and 14th century, our planet experienced the Mediaeval Warm Period. During this time, the global temperature average was higher than at present by about 1°C. Tree-ring data show that this was not a result of a natural rise in carbon dioxide, so the Sun's variability is the main suspect.
Between 1550 and 1850 AD, the trend reversed and the average temperature of the planet was colder by an estimated 1°C. This period is known as the Little Ice Age and historical data suggest that, at the same time, the Sun was almost free of sunspots. The number of such dark patches on the solar surface is a telltale signature of solar magnetic activity, because they appear where magnetic fields break out from the Sun's interior. They are also the sites of magnetic explosions that cause flares.
Historical records of sunspot numbers and the surface temperature of the Earth correlate so closely that there is little doubt that there is a link between the Sun and the Earth. The greater the number of sunspots, the hotter the surface temperature of the Earth. It is only in recent years that this correlation has been broken, prompting us to worry about the influence of industry on our climate.
On longer timescales, scientists have shown that the variable shape of Earth's orbit and our planet's orientation are the cause of the Ice Ages. There are three periodical variations that occur to the Earth and its orbit. Firstly, the Earth's orbit changes shape (known as its eccentricity) by about 5% every 100 000 years. Secondly, the inclination of Earth's rotation axis (known as its obliquity) changes between 24.5° and 21.5° gradually over 41 000 years. Thirdly, and independently of the obliquity changes, the rotation axis wobbles, like a child's spinning top, (known as precession) once every 23 000 years. Together, these cycles affect the orientation and distance of the Earth from the Sun, altering the amount of sunlight that the Earth receives. When farthest from the Sun, the Earth is not heated as much and plunges into an ice age.
During the 18th century, astronomer John Herschel first suggested that changes in the Earth's orbit could change the climate. During the 1930s, Serbian scientist Milutin Milankovitch put the theory on a firmer footing and nowadays it is the accepted model for how ice ages are triggered.