Latitudinal Distribution of Solar, IR, and Net IR

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One last point we should make about radiation – how it is distributed globally is going to be quite important in the rest of this course. That is, we’ve been treating the Earth as a giant box for the most part – energy comes in, energy goes out. And that’s not a bad way to look at things when we talk about the Earth as a whole. But when we want to learn more about why the Sahara is so hot, the Arctic is so cold, why Cancun is such a popular spring break destination and Nome, Alaska isn’t, we need to start thinking about how radiation varies across the planet.

Shortwave Radiation Distribution:

We’ve already talked about this previously, so I won’t belabor it, but recall that we have more solar insolation at lower latitudes than higher ones. This is a fundamental result of the planetary tilt and the fact that the Earth is a sphere. Solar radiation is most intense near the equator, where sunlight strikes the Earth nearly perpendicular to the surface. As we move towards higher latitudes, the same amount of solar energy is spread over a larger area, resulting in less intense solar radiation. Consequently, regions near the poles receive considerably less solar energy than equatorial regions.

Longwave Radiation Distribution:

On the other end of the spectrum (no pun intended!), we have longwave radiation, which is emitted by the Earth's surface and its atmosphere. Recall that the Stefan-Boltzmann Law tells us that this emission is a direct consequence of Earth system temperatures – the hotter something is, the more longwave radiation it emits, the cooler something is the less longwave radiation it emits. Just as with solar radiation, the distribution of emitted longwave radiation varies with latitude. Warmer equatorial regions emit more longwave radiation, while cooler polar regions emit less.

Net Infrared Radiation Distribution:

So it seems like all the energy is going into and out of the tropics! That is true; low latitudes are critical for acting as the bank tellers for most of the Earth’s incoming and outgoing radiation. However, another way to look at this is through net radiation. Climate scientists love to create budgets, and one thing we can look at is the amount of energy in versus the energy out at each latitude. We know that the answer must be zero for the Earth system as a whole, but that doesn’t mean it has to be zero everywhere on the planet.

Graphs of annual mean absorbed solar radiation, OLR, and net radiation averaged around latitude circles.

Credit: Dennis L. Hartmann, Chapter 2 — The Global Energy Balance, Global Physical Climatology (Second Edition), 2016, Pages 25-48, ISBN 9780123285317