10.2: Differential Heating

Because the Earth is round, solar radiation is not equally spread at all latitudes. Near the equator where sunlight shines directly on Earth, more solar radiation per square meter is received as compared to near the poles where sunlight shines at sharp angles to the surface (see image below). Toward Earth’s poles, the same solar radiation is spread over a larger surface area such that each square meter of Earth’s surface gets less radiation at the poles. As Earth rotates, the incoming solar radiation is zonally spread along latitude lines.

Earth’s uneven heating by the sun due to the curvature of its surface NASA (Public Domain).

In this way, incoming solar radiation depends on latitude. The sun shines more directly on tropical regions at lower latitudes than at higher latitudes all year-round. Solar radiation adds heat to the Earth-atmosphere-ocean system, and thus lower latitudes get heated more than higher latitudes. This should be as expected because we know the tropics are warmer than the polar regions.

While Earth is continually heated by the sun, it is also continually losing energy by emitting outgoing longwave infrared (IR) radiation at all latitudes, and at all times, both on the light and the dark side of the globe. You’ll recall from Chapter 2 that IR radiation is emitted by Earth according to the Stefan-Boltzman law, which is highly dependent on temperature.

When averaged over the globe and over long time scales, incoming UV radiation exactly balances outgoing IR radiation. But, latitude by latitude, incoming UV and outgoing do not perfectly balance. More solar energy is received by the Earth in the tropics, and while the cooling by outgoing IR radiation helps to offset this, there is still a net gain of radiative energy in the tropics. However, near Earth’s poles, incoming solar radiation is less direct and too weak to offset the cooling by outgoing IR radiation, so there is net cooling at the poles. This causes warmer air at the equator, and cold air at the poles and drives Earth’s atmospheric general circulation.

Incoming solar radiation (yellow dashed) and outgoing infrared radiation (red solid) diagram (CC BY-NC-SA 4.0).

The above image illustrates this more directly. Incoming solar radiation is focused near the equator, while outgoing IR radiation is relatively evenly spread across all latitudes. This results in the below energy surplus near the equator and deficit toward the poles.

Heat transport balances the net radiative imbalances (CC BY-NC-SA 4.0).

Earth’s general circulation attempts to redistribute heat around the globe and rebalance the energy imbalances inherent in an unevenly heated, rotating planet. However, the general circulation cannot instantly balance global temperature, especially when the uneven heating is continuous. Therefore, a meridional temperature gradient always remains.

In an attempt to balance out Earth’s incoming and outgoing energy, warm air is transported toward the poles, while cool air flows back toward the equator. This seems simple enough. However, this seemingly simple flow is complicated by many factors, including Earth’s rotation, the position of continents, interactions with the oceans and many others. In order to build an understanding of this process, we’ll start with some simplified models and build complexity.