Climate change: Milankovitch cycles

The Earth orbits the sun on a slightly variable orbit. It changes from being nearly circular to highly eliptical on a cycle lasting roughly 100,000 years.

The greater the eccentricity of the orbit (the more elliptical it is), the greater the variation in solar energy recieved. Currently, the Earth is in a period of low eccentricity. 

The Earth's tilt accounts for the seasons. The tilt changes over time; over a period of 41,00 years, it goes from 22.1 degrees to 24.6 degrees. This is caused by gravitational action between Earth and the other planets.

The tilt of the Earth doesn't effect the amount of solar energy hitting it. We have seasons because the Earth's axis isn't perpendicular to the plane of its orbit. Because of the tilt, the amount of sunlight a region recieves varies throughout the year, and as the cycle progresses, this becomes more severe.

The Earth's axis precesses (wobbles like a spinning top) over the course of about 25,000 years. Precession determines the timing of the seasons relative to the position of the Earth along its orbit around the sun.