**2. Processes affecting the distribution of heat around the surface of the Earth**

The Sun is far enough away that the solar radiation arrives at the surface of the Earth as a beam. Since the Earth is round, the bulk of the energy arrives within the Tropics, 23.5° north and south of the Equator. Under present-day conditions, the Sun does not rise for at least a day in winter at latitudes higher than 66.7° north and south due to the tilt of the axis of the Earth.

**Figure 2** shows the typical fate of insolation in summer entering the atmosphere of the Earth at about 50°N latitude. In practice, the amount of insolation reaching the ground will also depend on the sum of the effects of numerous climatic cycles pulling in multiple directions [14, 18, 19]. There are a very large number of these interacting with one another, so they normally largely cancel one another out. However, they are also largely responsible for the continual noise in the climatic signal that makes it advisable to have a 30-year average of climatic data where possible.

There is one special group of cycles that are critical in determining the amount of solar radiation arriving at a given location on the surface of the Earth over long periods of time, *viz.*, the Milankovitch cycles first thought of by Adhemar (Croll [20, 21]). Milankovitch [22, 23] refined the calculations of the effect of the three types of Earth orbital movements that can alter the Sun's incoming radiation by up to 25% in the zone subtropical zones (30–60° north and south of the equator. They are the shape of the Earth's orbit (eccentricity, a 100,000 year cycle), the

#### **Figure 2.**

*Typical fate of the solar energy reaching the surface of the atmosphere and proceeding down towards the ground.*

angle of tilt of the Earth's axis relative to the Earth's orbital plane (obliquity, from 21.1–24.5°in a 41,000 year cycle), and the direction of the Earth's axis of rotation is pointing (precession, in a 23,000 year cycle).

Milankovich calculated that cold events might occur approximately every 41,000 years and subsequent research confirms that they did occur at 41,000-year intervals between one and three million years ago. About 800,000 years ago, the cycle of the Ice Ages lengthened to 100,000 years, matching Earth's eccentricity cycle [24]. However, the cycles should not affect the data for the last two decades.
