The global heat budget, the causes of the surplus of energy in low latitudes and the deficit at high latitudes, and the redistribution of energy by atmospheric circulation, ocean currents and the inter-tropical convergence zone.

What this key area is asking

The SQA wants you to explain the global heat budget: why the area between the tropics receives more energy than it loses while the poles lose more than they receive, and how the atmosphere and oceans move heat from the surplus zone to the deficit zone so that neither region keeps heating or cooling without limit. In the exam this comes up as an "explain" question worth 4 to 6 marks, where you must give reasons with development, not just describe a diagram.

Why the heating is uneven

Three things give the tropics a surplus and the poles a deficit:

• Angle of the Sun. Near the Equator the Sun is high overhead, so its energy strikes a small area and is concentrated. Near the poles the Sun is low, so the same beam is spread over a much larger surface and is weaker per unit area.
• Atmospheric thickness. At high latitudes the low-angle rays pass through more atmosphere, so more energy is absorbed, scattered or reflected before reaching the ground.
• Albedo. Ice and snow at the poles have a high albedo and reflect much of the incoming energy back to space, while the darker tropical land and sea absorb more.

The surplus and deficit by latitude

The balance crosses over at about $40$ degrees north and south. Between roughly $40$ degrees N and $40$ degrees S there is a net surplus (more incoming shortwave radiation absorbed than longwave radiation lost), and poleward of those lines there is a net deficit. Without transfer, the tropics would warm by several degrees a year and the poles cool just as fast. Because temperatures are broadly stable over time, energy must be moving polewards, and the size of that flow is greatest at about $40$ degrees, where the surplus is being handed across to the deficit.

Redistributing the surplus

If the surplus stayed at the Equator the tropics would keep heating, so energy is transferred polewards. The SQA expects three mechanisms.

The tri-cellular model

In each hemisphere three cells circulate air vertically and horizontally. In the Hadley cell, hot air rises at the Equator (low pressure, heavy rainfall), moves polewards aloft and sinks near $30$ degrees, creating the high pressure and clear skies of the hot deserts such as the Sahara. The Ferrel cell lies between roughly $30$ and $60$ degrees, with air moving polewards at the surface as the westerlies. The Polar cell carries cold sinking air at the poles. Where warm tropical air meets cold polar air at about $60$ degrees, air rises along the polar front, giving the changeable depressions of north-west Europe. Surface winds and the high-level jet streams carry the heat horizontally.

Ocean currents

Warm currents such as the North Atlantic Drift (the extension of the Gulf Stream) carry tropical heat polewards, keeping north-west Europe milder than its latitude suggests: Lerwick in Shetland (about $60$ degrees N) stays well above freezing in winter while places at the same latitude in Canada are far colder. Cold currents such as the Canary and Benguela currents return cooler water towards the Equator, completing the loop.

The inter-tropical convergence zone

The ITCZ migrates north and south with the overhead Sun through the year, reaching about $20$ degrees N over West Africa in July and swinging south of the Equator in January. This migration drives the wet and dry seasons of tropical climates such as the Sahel: rain arrives when the ITCZ is overhead and the long dry season follows when it has moved away.

Examples in context

The North Atlantic Drift and the climate of Scotland. Scotland sits at the same latitude as Labrador in Canada, yet its west coast rarely freezes hard in winter. The reason is the North Atlantic Drift, a warm surface current carrying water from the Gulf of Mexico across the Atlantic. As the prevailing south-westerly winds blow over this warm water they pick up heat and moisture, so places such as Tiree and Stornoway have mild, wet winters. This is a clear case of ocean currents redistributing the tropical surplus to a high-latitude deficit zone.

The ITCZ and the Sahel rains. The Sahel, the semi-arid belt south of the Sahara, depends entirely on the ITCZ. When it migrates north in the northern summer, convectional rain falls; when it stays south, drought follows. In years when the ITCZ failed to move far enough north, such as during the major Sahel droughts of the 1970s and 1980s, rainfall collapsed and famine resulted. The example links the global heat budget directly to human geography topics such as land degradation.

Try this

Q1. Explain why low latitudes receive a surplus of energy and high latitudes a deficit. [4 marks]

• Cue. Use the angle of the Sun (concentration over area), the greater thickness of atmosphere at high latitudes, and the high albedo of polar ice.

Q2. Explain the role of ocean currents in the redistribution of global energy. [3 marks]

• Cue. Warm currents like the North Atlantic Drift carry surplus tropical heat to higher latitudes; cold currents return cooler water, evening out the imbalance.

Q3. Describe the movement of the ITCZ through the year and explain one effect it has. [4 marks]

• Cue. It follows the overhead Sun, moving north in the northern summer and south in January; this gives tropical regions like the Sahel their single wet season followed by a long dry season.