7.1: Differential Heating

Uneven Heating of Earth

Because the Earth is round, solar radiation is not evenly distributed across 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, as shown in Figure \(\PageIndex{1}\). 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, incoming solar radiation is distributed along latitude lines.

Figure \(\PageIndex{1}\): Earth’s uneven heating by the sun due to the curvature of its surface. Incoming sunlight provides equal amounts of energy across parallel beams, while Earth’s curved surface causes that energy to be spread over different surface areas. The illuminated portion of the planet receives the same total energy. Still, the curvature results in some regions receiving concentrated energy, while others receive the same energy distributed over a larger area. (Public Domain via NASA).

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, so lower latitudes are 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 constantly 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 Earth emits IR radiation according to the Stefan-Boltzmann law, which is highly dependent on temperature.

When averaged globally 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. The Earth receives more solar energy in the tropics, and while cooling by outgoing IR radiation helps offset this, there is still a net radiative energy gain 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.

Figure \(\PageIndex{2}\): Incoming solar radiation (yellow dashed) and outgoing infrared radiation (red solid) diagram (CC-BY-NC-SA 4.0; Roland Stull via Practical Meteorology).

Figure \(\PageIndex{2}\) 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 an energy surplus near the equator and a deficit toward the poles, as seen in Figure \(\PageIndex{3}\).

Figure \(\PageIndex{3}\): Earth gains more energy than it loses in low latitudes and loses more energy than it gains in high latitudes, creating an energy surplus near the equator and energy deficits toward the poles. The imbalance drives heat transport from surplus regions toward deficit regions. (CC-BY-NC-SA 4.0; Roland Stull via Practical Meteorology). Alternative Description of Image.

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. To build an understanding of this process, we’ll start with simplified models and gradually increase complexity.